plenary lectures - leibniz-institut für katalyse e.v.department of chemical engineering and...

Plenary Lectures

Operando III - Rostock-Warnemünde 2009 - Book of Abstracts Plenary Lectures

MRI: The Catalyst Working in the Reactor Environment

Lynn F Gladden Department of Chemical Engineering and Biotechnology,

University of Cambridge, Pembroke Street, Cambridge CB2 3RA, U.K.

Magnetic Resonance Imaging (MRI) is emerging as a measurement tool with unique capabilities in the field of reaction engineering because of its ability to quantify the physics and chemistry that is occurring at multiple length-scales within three-dimensional, optically opaque systems. This presentation gives a brief summary of the recent developments that have been made to make MRI a useful tool in this application. In particular, the ability to acquire images on a 20 ms timescale; to produce spatially resolved maps of chemical conversion without need for isotopic enrichment of 13C; and, most recently, to acquire velocity images of gas flows within reactors. All these developments have opened up a wealth of opportunities for us in the study of real catalytic processes. Brief case studies will include:

Images of liquid hold-up during trickle and pulsing flow

Velocity images of gas and liquid flows

Insights into the hydrodynamic transition between trickle and pulsing flow

Effect of periodic operation on liquid-catalyst contacting in a trickle-bed reactor

Tracking chemical conversion within a trickle-bed reactor

There will also be an introduction to some of the MR methods which can be used to monitor processes occurring within single catalyst pellets – in particular, mapping chemically-specific diffusion coefficients, and methods capable of probing composition and molecular mobility at the pore surface.

Operando III - Rostock-Warnemünde 2009 - Book of Abstracts Plenary PL1

Program Section: Preferred form of presentation:

CO and C2= caught Red-Handed

a Combined Reactor/Spectroscopy Study

Heiko Oosterbeek, Shell Technology Centre, Amsterdam,

P.O. Box 3800, 1030 BN Amsterdam, The Netherlands

Introduction and Objectives

Operando spectroscopy is not an obvious tool for research in industrial processes. Many hurdles have to be overcome, as there are large differences in pressure, material, shape, reactor size etc. Nevertheless, much can be learned in those cases that either those gaps can be closed or if rela-tions are known in e.g. up-scaling reactors/amount of catalyst.

This lecture tries to give examples where surface science techniques were used to elucidate reac-tion mechanisms and where clear industrial problems could be tackled.

Polarization Modulation Reflection Absorption Infra Red Spectroscopy (PM-RAIRS) served as the Operando tool in these studies. A high-pressure reactor, within the UHV system provided the necessary test results to complete the preparation/characterization/testing loop.

The co-polymerization of CO and ethene (Carilon) is studied, answering the question what the mechanistic difference is between the (homogeneous) liquid phase polymerization and the (het-erogeneous) gas phase polymerization. Secondly, the reason for the drawback of catalysts multi-layers is shown.

The very complex Fischer-Tropsch synthesis (CO hydrogenation to higher hydrocarbons) is also subject to this approach. Pressure/material/reactor/etc gaps were all met and (as much as possi-ble) closed. Via preparation/characterization/testing, a cobalt single crystal was modified such to resemble an industrial catalyst both in catalytic performance and in CO adsorption behaviour. On top of that, the catalytic site for chain growth was found. Ultimately a better catalyst could be de-signed, which was in the end the mere objective for the study.

Results and Discussion

Carilon polymerization

Carilon is a poly-keton produced by the perfect alternate co-polymerization of CO and C2=. It is a

homogeneously catalyzed reaction, using a Pd complex as the catalyst in methanol, the latter serving as liquid phase as well as initiator. Elaborate work was done to increase the catalysts ac-tivity by changing the ligands of this Pd complex. From NMR studies, it was revealed that the



reaction intermediate was a Pd/CO/C2=/CO metallocycle, where the oxygen of the second CO

was coordinating back (chelating) to Pd centre [1]. The resting state, however, was the open struc-ture as shown in figure 1.

Figure 1. Resting state in liquid phase polymerization of CO/C2=.

In gas phase polymerization, the Pd complex is anchored on seed polymer. From industry it ap-peared that the liquid phase catalyst was a poor gas phase catalyst. Optimisation of the catalyst pointed to a different mechanism (resting state). Gas phase processes cannot be studied with NMR, so PM-RAIRS was used to follow the reaction. For that, the Pd complex was deposited on a gold plated Si wafer, which served as the metal reflecting substrate.

From this PM-RAIRS study it was clear that in gas phase polymerization the metallocycle was the resting state, and moreover, CO was needed to assist the ring opening after which C2

= can be inserted.

Furthermore it appeared that going from the 5 membered to the 6 membered metallocycle, either the ligand or the polymer must undergo a 180° turn. This is a sterically hindered process. Using C13O/ C12O switching we were able to show that from the Pd clusters used in this study, only those at the very outside of the cluster could freely polymerize, whereas polymerization in the inner part of the cluster was severely sterically hindered. Using MeOH as a solvent, this could be postponed for some time as it acts as a plasticizer in the inner part. This phenomenon is the very cause for the lack of full utilization of catalyst if multilayers of catalyst are present on the seed polymer[2].

Fischer-Tropsch synthesis

FTS catalysts usually consist of a cobalt or iron on oxide catalyst, possibly promoted by a second oxide. For the presented study a cobalt catalyst is used. A catalytic test on several cobalt planes (the basal, open square and zig-zag plane) at one bar by using single crystals revealed that there was no preferential plane. Both activity and selectivity were similar. A PM-RAIRS study on the basal Co(0001) revealed that at high CO pressures and high temperature, a new site emerges, which could be identified as a CO induced step. Two CO molecules could be adsorbed at this site

R

O

O

PPd

PCO R

O

O

PPd

PCO R

O

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PCO

O

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PCO

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PPd

P CO

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COC2

= +

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PPd

P CO

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COC2

= +



(geminal dicarbonyl). By adding hydrogen to that system, the geminal dicarbonyl feature disap-peared. From post mortem STM we learned that the steps were still there, moreover it was a monoatomic island structure, where the dicarbonyls could form on the edges of the islands. From XPS it was deduced that after the dosage of hydrogen the amount of hydrocarbons on the surface was considerably increased. (See figure 2)

Figure 2 Finding the growing site in FTS After deliberate formation of these steps (defect sites) it was shown that the occurrence of dicar-bonyl sites was strongly increased, moreover the activity of such a crystal was concomitantly higher. From this we concluded that the growing site in FTS is a step site, which is formed by the corrosive nature of CO itself.

The selectivity, however remained very poor, compared of that of an industrial catalyst. As main cause, the reactor and reactor conditions were blamed. To test this, the industrial catalyst was tested in the reactor. 50 Microgram of catalyst was mounted on a Ag(poly) crystal. Activity and selectivity showed that the reactor/conditions were exactly as would be predicted by those condi-tions, proving that there was a material gap rather than a reactor gap. From PM-RAIRS it was learned that the adsorption of CO on an industrial catalyst was much different from a single crys-tal. It could best be mimicked by a partially oxidised Co(poly) crystal. Testing such an oxidised crystal did not show the high chain growth probability, which was expected. Post-mortem XPS

10 L CO10 L CO

2200 2150 2100 2050 2000 1950 1900 1850 1800 Wavenumber (cm-1)

a)

b)

c)

d)

100 mbar CO100 mbar CO100 mbar CO100 mbar CO

100 mbar CO220 °C

100 mbar CO220 °C100 mbar CO220 °C

100 mbar CO220 °C

100 mbar CO/200 mbar H2

220 °C

100 mbar CO/200 mbar H2

220 °C

Co CoC

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O



analyses learned that the cobalt crystal was simply reduced to the metallic state in the CO/H2 mixture.

To cover the cobalt with cobalt oxide, which does not reduce, ZrO2 or SiO2 was brought onto the crystal by means of physical vapour deposition in the submonolayer regime. Up-to a half monolayer the chain growth probability increases monotonically. The best results were obtained with half a monolayer of either ZrO2 or SiO2 on Co(poly), which intuitively resembles the best a ZrO2- Co/SiO2 catalyst [3]. (see figure 3).

Figure 3. Selectivity increase by partial coverage of Co(poly) with non-reducible oxides

This knowledge made it possible to make a new catalyst with unsurpassed activity and selectiv-ity.

Conclusions

By using an integrating approach using UHV surface science techniques, combined with an in-situ high pressure reaction cell and PM-RAIRS as an Operando spectroscopic tool both mecha-nistic and industrial questions could be answered.

References [1] see e.g. Francis C. Rix, Maurice Brookhart and Peter S. White. J. Am. Chem. Soc., 1996, 118 (20), pp 4746–4764

[2] Mul, W. P.; Oosterbeek, H.; Beitel, G. A.; Kramer, G.; Drent. E. Angew. Chem., Int. Ed. 2000, 39, 1858−1851

[3] Heiko Oosterbeek, Phys. Chem. Chem. Phys., 2007, 9, 3570

0

0,5

1

1,5

2

2,5

3

0 2 4 6 8

Carbon number

no

rmal

ised

Lo

g (

Yie

ld)

ZrO2/Co(poly)ZrO2/Co(poly)

Co(poly)Co(poly)SiO/Co(poly)

**

*

SiO/Co(poly)

**

*

**

*CatalystCatalyst

Co(0001)

ASF plot

1 bar H2/CO=2


Program Section: Homogeneous catalysis Preferred form of presentation: Oral

Combining Operando Spectroscopy with Experimental Design,

Signal Processing and Advanced Chemometrics.

State-of-the-Art and a Glimpse of the Future.

Marc Garland

Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore, 627833


Modern organic syntheses rely to a large extent on metal-mediated strategies, both stoichiometric as well as catalytic. Homogeneous catalysis using transition metal complexes as catalyst precursors is widely used in academia as well as industry to obtain increased rates of re-action, but equally importantly, to control chemo-, regio-, and stereo-selectivities. Homogeneous catalytic strategies are particularly attractive due to their increased atom economy.

The use of operando spectroscopic techniques has greatly increased understanding of ho-mogeneous catalysis at the molecular level. Indeed, vibrational spectroscopies such as FTIR and Raman as well as NMR, have been widely used to identify new transient and non-isolatable or-ganometallic species which are present during the catalysis. Subsequently, some but not all of these new species, can frequently be implicated as possible intermediates in the catalytic trans-formations. Operando spectroscopic techniques have also provided a means to better evaluate the time-dependent rates of formation of the primary and secondary organic products. The ability to identify species, and in some cases to evaluate concentrations and rates, has had an enormous positive impact on better mechanistic understanding of homogeneous catalysis [1].

The amount of spectroscopic data acquired during a catalytic run or during a set of cata-lytic runs can be enormous, and the proper interpretation of these spectra presents a serious hur-dle. Moreover, there is normally a considerable amount of information imbedded within these data sets, which are not extractible by conventional means.

The present talk focuses on the state-of-the-art for various signal processing techniques and other chemometric methods in order to extract more information. In particular, three goals are discussed at considerable length (1) pure component spectral reconstruction of the constitu-ents present (even down to sub-ppm concentration levels) (2) the subsequent use of these spectral estimates in order to obtain reliable quantitative information, such as concentrations and rates and (3) the development of kinetic expressions. Examples using vibrational spectroscopy are pre-sented. It is emphasized that experimental design is extremely important, and the normal ways of conducting experiments in homogeneous catalysis are often not really adequate for further de-tailed numerical analysis.



Results

Experimental Design Mass transfer considerations are important pre-requisites for ob-taining representative spectroscopic measurements [2]. Also, small sets of data are often not ade-quate for really detailed numerical analysis. A semi-batch experimental design is often much more useful than the normal experimental approach of starting a batch reaction and then just monitoring it. By performing numerous reagent, temperature and pressure perturbations during a semi-batch experiment, the experimentalist can more easily cover a wider range of spectroscopic observations. In addition, semi-batch experimentation frequently helps to avoid the wide spread problem of co-linearity in chemical experimentation. The resulting numerical analysis of the spectroscopic data is often much more informative [3]

BTEM and Quantitative Analysis Band-target entropy minimization (BTEM) is an al-gorithm developed in our laboratory, which analyzes sets of spectroscopic data (primarily vibra-tional spectra) in order to enumerate the number of observable species present as well as deter-mine the corresponding individual pure component spectra [4]. BTEM achieves this without the use of any a priori information such as libraries. With a few hundred or thousands of spectra as input, BTEM is capable of reconstructing the spectra of a large number of solutes, even trace constituents (i.e. species at ppm or sub-ppm concentration levels) [5]. Such trace constituents of-ten contribute 1 part per 1000 or less of the total measured spectroscopic signal. Nevertheless, signal-to-noise ratios of 25:1 or better are typically achieved for the corresponding spectral esti-mates. BTEM has been used by our group to identify dozens of new organo-rhodium complexes present during catalytic syntheses. We have also developed a set of algorithms in order to deter-mine the properly scaled pure component spectra. This allows calibration of the system, and hence, determination of concentrations and rates as well as kinetic expressions [6].

Future A possible future direction for homogeneous catalytic research involves the ex-perimental determination of the individual physico-chemical and thermo-physical properties of the solutes present during catalysis. When fully developed, this approach should allow the identi-fication and evaluation of individual solute dipole moments, solute-solute interactions (i.e. vol-umes of interaction) etc. Such information should help to provide further mechanistic understand-ing of changes in rate and selectivity patterns due to molecular recognition processes.

References [1] Mechanisms in Homogeneous Catalysis: A Spectroscopic Approach; B. Heaton (Ed), Wiley-VCH, 2005[2] M. Garland, Transport Effects in Homogeneous Catalysis, Encyclopedia of Catalysis, I.T. Horvath (Ed), Wiley

2002[3] E. Widjaja, C. Li, and M. Garland, M. Organometallics 2002, 21, 1991. [4] a) W. Chew, E. Widjaja and M. Garland, Organometallics, 2002, 21, 1882. b) M. Garland, Processing Spectro-

scopic Data. In Ref [1] [5] a) C. Li, E. Widjaja, M. Garland, J. Am. Chem. Soc. 2003, 125, 5540. b) C. Li, L. Chen, M. Garland, J. Am. Chem. Soc. 2007, 129, 13327. c) C. Li, E. Widjaja, W. Chew, M. Garland, Angew. Chem. Int. Ed. Engl. 2002, 41,3785.[6] E. Widjaja, C. Li, M. Garland, J. Catal. 2004, 223, 278.



Combining diffuse reflectance infrared spectroscopy (DRIFTS) with transmis-sion based synchrotron techniques for the time resolved study of working cata-lysts.

Mark A. Newton 1a, Marco Di Michiela, Marcos Fernandez-Garciab aThe European Synchrotron Radiation Facility,6, Rue Jules Horowitz, BR-220, Grenoble,

France.

bInstituto de Catalisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28049, Madrid, Spain.

Session 4: New reactor cells and coupling techniques Introduction and Objectives

The synchronous combination of Diffuse reflectance infrared spectroscopy (DRIFTS) and mass spectrometry (MS) with either transmission based X-ray absorption (EXAFS)1-4 and hard X-ray diffraction (HXRD)5 techniques using essentially the same experimental methodology is de-scribed. As such we demonstrate a single experimental methodology that may be applied to the study of problems in catalysis and that permits detailed structural information on a wide range length scales to be obtained at the same time as infrared based information regarding adsorbate formation and reaction, and an assessment of the net performance of the catalyst/catalytic process under study via MS or chromatography.


The experimental considerations and modifications required to a standard DRIFTS experiment to, couple DRIFTS to transmission EXAFS, and secondly, DRIFTS to HXRD are discussed.5 The numerous practical benefits of using hard X-rays (ca. 90 KeV) to make such diffraction/PDF measurements on working catalysts systems are also outlined. The potential of this approach to obtain a far more holisitc view of fundamental structural-reactive aspects of catalyst operation is given, using the dynamic structural lability of supported Pd nanoparticles during CO/NO redox situations6,7 as an example. The potential for this DRIFTS based approach to be made more ge-neric in its utility, to include for instance combinations with diffraction in dispersive mode, SAXS, and other techniques based X-ray transmission and/or X-ray backscattering, are also dis-cussed.



Conclusions

A new multi-technique experiment that permits the in situ and time resolved study of working catalysts using a synchronous combination of infrared spectroscopy in DRIFTS mode, mass spec-trometry, and either transmission EXAFS or hard X-ray diffraction is demonstrated, and its pos-sible extensions and further potential utility discussed.

References [1] M. A. Newton, B. Jyoti, A. J. Dent, S. G. Fiddy, J. Evans, Chem. Comm., 2004, 2382. [2] M. A. Newton, A. J. Dent, S. G. Fiddy, B. Jyoti, J Evans, Catal. Today, 2007, 126, 64. [3] M. A. Newton, Topics in Catalysis, accepted, Special issue on Operando techniques edited by M. Banares. [4] A. J. Dent, J. Evans, S. G. Fiddy, B. Jyoti, M. A. Newton, M. Tromp., Angewandte Chemie, 2007, 46, 5356. [5] M. A. Newton, M. Di Michiel, M. Fernandez-Garcia, manuscripts in preparation. [6] M. A. Newton, C. Belver-Coldeira, A. Martínez-Arias, M. Fernández-García, Nature Materials, 2007, 6, 528-532. [7] M. A. Newton, C. Belver-Coldeira, A. Martínez-Arias, M. Fernández-García., Angew. Chem. Intl. Ed., 2007, 46, 8629-8631.


Program Section: Combining operando spectroscopy and theoretical studies Preferred form of presentation: plenarylecture

Spectroscopic and Molecular Modeling Approaches to MechanisticStudies of Surface Reactivity

Zbigniew Sojka and Piotr Pietrzyk

Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland

Elucidation of the surface reaction mechanism with molecular resolution is the most ele-gant and powerful way for rational design of the catalyst performance. However, inherent hetero-geneity of catalytic surfaces, pronounced speciation of active sites, related to their different loca-tion and the variety of the coordination states, along with usually complex surface reaction dy-namics may severely restrain an in-depth insight into the reaction course. Even so, many impor-tant mechanistic issues may be explored successfully by combination of molecular modeling with computational spectroscopy. This provides a bottom-up approach allowing for reliable state of the art interpretation of experimental data, concerning the nature of postulated active sites and reaction intermediates, molecular description of the elementary reaction steps and quantificationof their energetics as well.

In our methodology we com-bine computational spectroscopy with molecular modeling and reactivity studies using functional model systemscapable of mimicking desired func-tionalities of real catalysts (Fig. 1). In the case of heterogeneous catalysis, in contrast to homogeneous systems, the structure of active sites and surface in-termediates is not distinctly defined.Therefore, judicious selection of ap-propriate molecular models and ade-quate calculation schemes are of great importance for obtaining sensible re-sults. Spectroscopic parameters of the

postulated sites and reactive species can be calculated to be directly compared with available ex-perimental data (EPR and NMR parameters, IR frequencies and intensities, etc.). Owing to the relativistic DFT methods nearly quantitative reproduction of many magnetic parameters is now possible with reasonable computing costs. Such approach provides not only a quantitative bridgebetween the spectroscopic fingerprints of the investigated species, their molecular structure andthe reactivity, but it also can be used for guiding molecular modeling of catalytic reactions, pro-

Fig. 1 Research methodology combining spectroscopic investi-gation, reactivity studies and DFT modelling used in this work.


Program Section: Combining operando spectroscopy and theoretical studies Preferred form of presentation: plenarylecture

viding useful check points, justifying the adequacy of both the adopted model and the method as well.

When monitored with spectroscopic techniques (IR, Raman EPR, NMR) both in static or in operando conditions, heterogeneity of the catalyst gives rise to pronounced speciation of thesurface species, resulting in complicated multi-component spectra. This severely restrains therecognition of the individual spectral features, which is especially important for magnetic reso-nance spectra where information is unevenly distributed over all spectral range. As a conse-quence, a great deal of significant chemical information about the investigated system can be ex-tracted only with the help of advanced computer simulation, supported by quantum chemical cal-culations of spectral parameters. Such approach, for instance, allowed for discovery of novel di-oxovanadium(IV) radical produced within the channels of dealuminated BEA zeolite upon ther-mal treatment (Fig. 2).

The aim of this contribution is to provide the molecular insight, through DFT modeling and spectroscopic investi-gations, into the mechanism of deNOx and deN2O reactions. Binding, activation and catalytic turn-over of small molecules suchas NOx, N2O, CO or hydrocarbons on vari-ous surface transition-metal ions of differ-ent electron configuration and spin multi-plicity, dispersed in porous materials such as zeolites, pillared clays, mayenite andnanostructured oxides catalysts are dis-

cussed in terms of structure-property and structure-reactivity relationships. The elementary eventssuch as coordination of reactants, charge and spin redistributions during bond breaking and mak-ing, which are the principal molecular constraints of an efficient decomposition of nitrogen ox-ides (N2O and NO) are discussed. Particular attention is paid to dynamics of the N–O bond cleavage in N2O molecule through electron and oxygen atom transfer routes, evaluation of pref-erable coordination modes of NO, discrimination between inner- and outer-sphere mechanism of N–N bond formation, and influence of the spin and electronic redistribution within the active complex on O–O bond formation (spin catalysis). We will illustrate all above mentioned mecha-nistic aspects of deNOx and deN2O processes using selected examples coming from our labora-tory and from literature.

Fig. 2 Two types of vanadium(IV) complexes along withthe characteristic EPR spectra, assigned basing on relativ-istic DFT calculations of their magnetic parameters.

AcknowledgementsFinancial support by the Ministry of Science and Higher Education (MNiSW) of Poland, grant no. N N204 239334 isacknowledged.


Keynote Lectures

Operando III - Rostock-Warnemünde 2009 - Book of Abstracts Keynote Lectures

Program Section: Bridging the gap between model and technical conditions Preferred form of presentation: Oral

Insights into NOx Storage-Reduction Mechanisms by Vibrational Spectroscopic Techniques

Atsushi Urakawa, Eva Rödel, Nobutaka Maeda, Holger Hesske, and Alfons Baiker

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, HCI, 8093 Zurich, Switzerland


The NOx storage-reduction (NSR) system is one of the most promising technologies in automo-tive catalysis for the abatement of NOx under oxidative atmosphere. [1] NSR is based on an un-steady-state operation, utilizing periodic switching between fuel-lean (oxidative atmosphere) and fuel-rich (reductive atmosphere) conditions of the engines. During the lean periods NOx gases are oxidized over previous metals and stored on alkali/alkali earth metal components in the form of nitrates, and subsequently these nitrates are reduced to N2 in the rich periods. Typical NSR cata-lyst compositions are Pt-Ba/Al2O3 and Pt-Ba/CeO2.

The aim of this study was to investigate the mechanistic aspects of NSR by vibrational spectro-scopic techniques, namely IR (DRIFTS and PM-IRRAS) and Raman spectroscopies. A plug-flow cell was designed for a combined DRIFTS-Raman study to achieve good time-resolution and space-resolution along the catalyst bed and also to minimize the gas mixing within the cell (Fig. 1). Besides, the temporal behaviour of catalyst structures and evolved gas species were investi-gated simultaneously by PM-IRRAS [2] using both model and actual powder catalysts (Fig. 1). To our knowledge, this is the first application of in situ PM-IRRAS for powder samples. In addi-tion, ab initio IR and Raman spectra of the involved barium bulk materials were obtained by DFT calculations in order to assist reliable assignments of the observed IR and Raman bands.

Fig. 1 Schematic measurement configurations of the combined DRIFT-Raman spec-troscopy (left) and PM-IRRAS of powder catalysts (right).

Operando III - Rostock-Warnemünde 2009 - Book of Abstracts Keynote KL1



Fig. 2 (left, middle) shows the temporal evolutions of surface and bulk Ba species observed by DRIFT-Raman spectroscopy. The dynamic processes of the surface and bulk species could be followed by DRIFTS and Raman spectroscopy, respectively, by their different local sensitivity. Their temporal evolutions at different catalyst bed positions in comparison to those of the gas species detected by MS and CLD yielded insights into the NSR mechanisms, elucidating the key factors leading to the final NSR performance. Fig. 2 (right) shows the temporal evolutions of gas, surface, and bulk Ba species during NSR, observed all simultaneously by PM-IRRAS. The appli-cation of PM-IRRAS to powder samples resulted in complete transmission through sample near outer surfaces and hence more bulk information was obtained compared to DRIFTS. Furthermore, theoretical calculations gave detailed information about the nature of vibrational modes and con-firmed the assignment of experimentally observed bands.

Conclusions

Two in situ methods, DRIFT-Raman spectroscopy and PM-IRRAS, were applied in the mecha-nistic investigation of NSR. Considerable insight into the involved active sites and species, their temporal evolutions, and the nature of chemical gradients along the catalyst bed was gained.

References [1] W. S. Epling, L. E. Campbell, A. Yezerets, N. W. Currier, and J. E. Parks II, Catal. Rev., 46, 163 (2004) [2] A. Urakawa, T. Bürgi, H.-P. Schläpfer, and A. Baiker, J. Chem. Phys. 124, 054717 (2006)

Fig. 2 Time-resolved DRIFT (left) and Raman (middle) spectra at the front position of the Pt-Ba/CeO2 catalyst bed and the evolutions of gas phase, surface, and bulk species ob-served by PM-IRRAS (right) using Pt-Ba/Al2O3 catalyst during NSR (NO + O2 vs H2,both in He)

0 50 100 150 200 250

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rich lean

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Abs

orba

nce

/ a. u

.

surface Ba-nitrite

surface Ba-nitrate

surface Al-nitrate

Time / s

bulk Ba-nitrate

gas phase NO2


Program Section:2 Preferred form of presentation: Oral

X-Ray Scattering and Spectroscopy Studies of Model Nanocatalysts

Stefan Vajdaa,b, Sungsik Leea, Byeongdu Lee a, Sönke Seifert a, Jeffrey Elama, Michael Pellina,Randall E. Winans a, Kristian Sell c,Ingo Barke c, Armin Kleibert c, Viola von Oeynhausenc,Karl-Heinz Meiwes-Broer c, Sonja A. Wyrzgol d,, Xuebing Li d and Johannes A. Lercher d

aArgonne National Laboratory, USA, bYale University, USA, cUniversität Rostock, Germany, dTechnische Universität München, Germany


The elucidation of size/composition/shape and function correlation along with the determina-tion of the nature of catalyst under reaction conditions are central for addressing fundamental aspects of catalysis and for the development of new classes of catalytists. Highly uniform catalysts dispersed on technologically relevant supports are prerequisites for such studies.[1]

The objective of the presented study is to apply synthesis techniques allowing for the control of the size and composition of catalyst at the few-atom to few tens of nm size range and to use highly sensitive surface X-ray probes, grazing incidence small angle X-ray scattering (GISAXS) and X-ray spectroscopy (GIXANES, GIXAFS) combined with temperature-programmed reactivity (TPR) for the characterization of the nanocatalysts under working conditions. The powerful combination of these techniques will be illustrated on select exam-ples of sub-nanometer clusters and larger nanocatalyst in various reactions.


Uniform Ag nanocatalyst were fabricated by generating clusters in an ACIS cluster source and using a quadrupole deflector to preselect the desired cluster size for deposition. [2] The evolu-tion of the morphology of the catalyst under working conditions was monitored by GISAXS [3], the activity in production of propylene oxide vs. acrolein from propene by molecular oxy-gen followed by mass spectrometry. Typical GISAXS patterns are shown in Figure 1. As seen from changes from 1a to1b, the mor-phology of the catalyst changes upon the inlet of the reactants at room temperature and further evolves with temperature. The strongly size-dependent activity and selectivity of Ag catalyst (6 nm to 25 nm size range) will be discussed and compared with the performance of size-preselected sub-nanometer Au6-10 catalysts stabilized against sintering. [4]

To produce ultrafine Pt clusters on p-type Si substrate, polymer-protected Pt nanoparticles were prepared and spin-coated on the semiconductor. Poly(N-vinyl-2-pyrrolidone) (PVP) was used as the stabilizing agent to avoid particle agglomeration before coating. Subsequently, the

Figure 1. GISAXS patterns of size selected silver catalysts obtained: a) at 30 °C and 1 atm pressure of helium; b) at 30 °C and under 1 atm pressure after the inlet of the mixture of propene and oxygen in helium and c) at 200 °C , exposed to the reactants.


Program Section:2 Preferred form of presentation: Oral

polymer was removed by cold oxygen plasma. Pt L3 edge XANES spectra were recorded on a set of Pt samples is shown in Figure 2. PVP-protected clusters consist of fully reduced Pt while oxidized Pt species appear after oxygen plasma treatment, indicated by high absorption intensity.[5] Further reduction at 100°C for 0.5 h under H2 regenerates Pt0. The metal was completely reduced without particle sintering on the surface. In operando GIXANES, the oxidized Pt sample was treated with hydrogen at 100°C, and absorption spec-tra were recorded during the thermal treatment and hydrogen environment. Fully reversible re-duction/oxidation was observed during cycles with H2 and O2.

GIXANES studies of size-preselected Pt10

clusters, as well as GISAXS/GIXANES/TPR studies of size-preselected Ni4, Co4, Co17-20,Co27-32 clusters on supports with variable chemistry will be presented, e.g. in reactions with CO/H2. GIXANES reveals changes in catalyst oxidation state induced by reaction environment and temperature. The onset of the binding energy shift for Co27-32 (Fig.3), at about 200 °C is in agood agreement with the FT temperatures reported in the literature 6.

Conclusions

A combination of methods of precise catalyst synthesis and surface techniques for catalyst characterization was presented. These methods allow for studies of ultrasmall amounts of nanocatalyst under realistic reaction conditions for rational catalyst design.

References [1] Bell, A. T., Science 299, 1688 (2003) [2] Methling, R.-P., Senz, V., Klinkenberg, E.-D., Diederich, Th., Tiggesbäumker, J., Holzhüter, Bansmann, G J and Meiwes-Broer, K.H., Europ.Phys. J D 16, 173 (2001)[3] Lee, B., Seifert, S., Riley, S. J., Tikhonov, G.Y., Tomczyk, N., Vajda, S. and Winans, R. E. J. Chem. Phys.

123, 074701 (2005) [4] S. Lee, L. M. Molina, M. L, María J. López, J. A. Alonso, B. Hammer, B. Lee, S. Seifert, R. E. Winans, J.

W. Elam, M. J. Pellin, and S. Vajda, Angew. Chemie. Int. Ed, in Press[5] Ramallo-Lopez, J. M., F. G. Requejo, A. F. Craievich, J. Wei, M. Avalos-Borja, E. Iglesia, J. Mol. Catal. A

228, 299. (2005) [6] J Van De Loosdrecht., B. Balzhinimaev, J. A. Dalmon, J. W. Niemantsverdriet, S. V. Tsybulya, A. M. Saib,

P. J. Van Berge, J. L. Visagie, Catalysis Today, 123, 293 (2007),

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Figure 3. XANES on Co27-32 supported on amor-phous Al2O3 (left) and ultra-nanocrystalline diamond (right) during reaction with H2 and CO.


Program Section: New applications in liquid-phase processes Preferred form of presentation: oral

Pd at work during liquid-phase alcohol oxidation: combined ATR-IR, XANES, FTIR and UV study

Davide Ferria, Cecilia Mondellib, Jan-Dierk Grunwaldtc, Alfons Baikerb

aEmpa, Lab. for Solid State Chemistry and Catalysis, CH – 8600 Dübendorf, bETH, Inst. for Chemical and Bioengineering, CH – 8093 Zurich, Switzerland, c DTU, Dept. of Chemical and

Biochemical Engineering, DK – 2800 Kgs. Lingby, Denmark


The dehydrogenation mechanism proposed for the selective aerobic oxidation of aliphatic and aromatic alcohols to ketones and aldehydes is based on the insight gained using UHV techniques and electrochemical methods. The experimental conditions under which such measurements have been performed are not necessarily the conditions the catalyst experiences during reaction. Al-though the reaction mechanism is generally accepted, some crucial aspects of the reaction, like the nature of active sites and the effect of promotion by for example Bi still need to be investi-gated under conditions more relevant to the environment into which the catalyst is immersed. Therefore, we present an in situ/operando investigation of the selective alcohol oxidation using attenuated total reflection infrared (ATR-IR) spectroscopy and X-ray absorption spectroscopy shedding light on these aspects. The investigation is completed by the determination of catalytic activity using online FTIR spectroscopy and by measurements in a batch reactor cell.


A number of species evolves during reaction in cyclohexane at 50°C on the surface of 5 wt% Pd/Al2O3 followed in a flow-through ATR-IR cell. Beside benzaldehyde, adsorbed CO from product decarbonylation and benzoate species have been observed both under dehydrogenation and oxidative conditions and cause catalyst deactivation. The particular pattern of adsorbed CO, appearing at 1852 cm-1, suggests the possibility to distinguish sites responsible for decarbonyla-tion from active sites involved in dehydrogenation. Site selective blocking of the Pd surface by CO in cyclohexane [1] and successive admission of an Ar-saturated solution of benzyl alcohol to simulate dehydrogenation conditions, indicates that CO adsorbed on defects and eventually (100) planes disappears and produces a new feature (1850 cm-1) likely originating from CO on Pd hol-low sites simultaneously to the development of catalytic activity. The spectroscopic data suggest that alcohol dehydrogenation is rather structure insensitive, whereas decarbonylation is a struc-ture sensitive reaction [2] and occurs preferentially on extended surfaces like (111) terraces.


Program Section: New applications in liquid-phase processes Preferred form of presentation: oral

The presence of the Bi promoter significantly affects the distribution of species evolving during the liquid-phase reaction both under dehydrogenative and oxidative conditions. ATR-IR meas-urements on Bi-promoted Pd/Al2O3 show no formation of CO during selective oxidation, indicat-ing that the sites were decarbonylation occurs (hollow sites) are preferentially covered by Bi. The promoter effect is found to increase selectivity, product decarbonylation and hydration being in-hibited by Bi. Moreover, in situ XANES measurements reveal that both Bi and Pd are reduced during benzyl alcohol oxidation. The catalyst can be re-oxidized only when admitting alcohol-free O2-saturated solvent. Importantly, Bi is found to re-oxidize prior to Pd and catalysis is less efficient on the re-oxidized catalyst [3].

The combination of data from ATR-IR spectroscopy and XAS indicates that Bi prevents Pd oxida-tion by better regulating the oxygen diffusion to Pd (less benzoic acid formed, Bi re-oxidizes before Pd). The suppression of product decar-bonylation indicates that Bi blocks the Pd sites responsible for this side-reaction.Finally, the results have been cor-roborated by measurements in a batch reactor cell operated at ambi-

ent conditions, in which the surface of the Pd-catalyst is monitored by ATR-IR and the product evolution by UV-spectroscopy.

Conclusions

The combination of ATR-IR, XAS and online FTIR provides rich information on the nature of active sites and the promoter effect when the catalyst is studied under working conditions, taking the liquid-phase selective oxidation of benzyl alcohol on Pd as an example. Moreover, the data have been transferred from a flow- to a batch-reactor cell.

References [1] D. Ferri, C. Mondelli, F. Krumeich, A. Baiker J. Phys. Chem. B 110 (2006) 22982. [2] R. Shekhar, M.A. Barteau, Surf. Sci. 319 (1994) 298. [3] C. Mondelli, D. Ferri, J.D. Grunwaldt, F. Krumeich, R. Psaro, A. Baiker J. Catal. 252 (2007) 77.

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Program Section: 4- New reactor cells and coupling techniques Preferred form of presentation: Oral

The power of quantitative kinetic studies of adsorbate reactivity

by operando/in situ spectroscopy.

F.C. MEUNIERa Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 Bd Maréchal

Juin, F-14050 Caen

E-mail : [email protected]

The utilisation of operando techniques to investigate catalytic reactions is receiving an increasing attention. Such investigations can provide clues on the nature and concentration of surface inter-mediates and the structure of the catalyst “at work”. Examples of quantitative studies based on Raman and transmission FTIR spectroscopy investigating alcohol oxidation1, alkene oxidation2

and syngas conversion to methanol3 will be briefly discussed to introduce the subject.

Diffuse reflectance FT-IR spectroscopy (DRIFTS4,5) is increasingly being used as a means to in-vestigate the reactivity of surface species under reaction conditions, but it is usually considered only as a qualitative technique. However, it was demonstrated that DRIFTS spectroscopy can be an accurate quantitative tool for operando studies, providing that an appropriate analytical trans-formation of the diffused intensity is used (i.e. in most cases the pseudo-absorbance rather than the Kubelka-Munk function6) and that a calibration curve relating band intensity to adsorbate concentration is available7. We also showed that an appropriately modified DRIFTS cell reactor8

led to reaction rates identical to those measured in a linear quartz tube plug flow reactor7.DRIFTS reactors are particularly suited to operando investigations since the catalyst powder can be used as such, whereas FTIR-transmission techniques require pressing wafers, which can lead to mass-transport limitations and catalyst modifications.

Spectrsoscopic studies are even more powerful when combined with isotopic transient methods (SSITKA9,10), which allow operating at the chemical steady-state. The operando DRIFTS-SSITKA method described here relies on using a single catalytic bed, which allows the charac-terisation by DRIFT spectroscopy of the surface of the very same catalyst particles that are re-sponsible for the catalytic activity measured at the exit of the cell by gas-chromatography or mass-spectrometry11. This methodology is similar to that developed earlier for transmission FTIR by Chuang et al.12,13. These techniques derived from the so-called “isotopic jump” technique of Tamaru et al.14, which relied on a two-bed IR cell. A typical SSITKA-DRIFTS result is shown in Fig. 1, in which the exchange of a surface species (i.e. formate) is compared to that of the reac-tion product (i.e. CO2). The exchange of CO2 is markedly faster than that of the formate, indicat-ing that the latter is mostly irrelevant as a reaction pathway. In some cases even when the time constant of exchange of CO2 and formate is similar, a full quantitative analysis based on the com-parison of the CO2 production rate and the formate decomposition rate has revealed that the for-



mate (at least the one seen by IR) is still irrelevant (Fig. 2). While we proposed that these IR-observable species were essentially spectators15,16 in the case of our experiments, others have suggested these species may be crucial intermediates. The origins of the discrepancy between these views will be discussed.

References

[1] W.M. Zhang, S.T. Oyama, J. Phys. Chem. 100 (1996) 10759. [2] V.A. Matyshak, O.V. Krylov, Catal. Today 25 (1995) 1. [3] K.T. Jung, A.T. Bell, Catal. Lett. 80 (2002) 63-68. [4] V. Dal Santo, L.C. Dossi, A. Fusi, R. Psaro, C. Mondelli, S. Recchia, Talanta 66 (2005) 674. [5] M.M. Schubert, T.P. Haring, G. Brath, H.A. Gastiger, R.J. Behm, Appl. Spectrosc. 55 (2001) 1537. [6] J. Sirita, S. Phanichphant, F.C. Meunier, Anal. Chem. 79 (2007) 3912. [7] F. C. Meunier, D. Reid, A. Goguet, S. Shekhtman, C. Hardacre, R. Burch, W. Deng, M. Flytzani-Stephanopoulos, J. Catal., 247 (2007) 277. [8] F.C. Meunier, A. Goguet, S. Shekhtman, D. Rooney, H. Daly, Appl. Catal A: Gen. 340 (2008) 196. [9] S. L. Shannon and J. G. Goodwin, Chem. Rev. 95 (1995) 677. [10] Y. Yang, R.S. Disselkamp, J. Szanyi, C.H.F. Peden CHF, C.T. Campbell, J.G. Goodwin, Rev. Sci. Inst. 77 (2006) 094104. [11] A. Goguet, D. Tibiletti, F.C. Meunier, J.P. Breen, R. Burch, J. Phys. Chem. B 108 (2004) 20240. [12] M.W. Balakos, S.S.C. Chuang, G. Srivinas, J. Catal. 140 (1993) 281. [13] R.W. Stevens, S.S.C. Chuang, J. Phys. Chem. B 108 (2004) 696. [14] A. Ueno, T. Onishi, K. Tamaru, Trans. Farad. Soc. 66 (1970) 756. [15] D. Tibiletti, F.C. Meunier, A. Goguet, R. Burch, M. Boaro, M. Vicario, A. Trovarelli, J. Catal. 244 (2006) 183. [16] F. C. Meunier, A. Goguet, C. Hardacre, R. Burch, D. Thompsett, J. Catal. 252 (2007) 18 [17] R. Leppelt, B. Schumacher, V. Plzak, M. Kinne, R.J. Behm, J. Catal. 244 (2006) 137. [18] G. Jacobs, B. H. Davis, Appl. Catal. A: General 333 (2007) 192. [19] F.C. Meunier, D. Tibiletti, A. Goguet, S. Shekhtman, C. Hardacre, R. Burch, Catalysis Today, 126 (2007) 143.

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Program Section: 6 Preferred form of presentation: ORALApplication of reporter moleculesto image catalytic activity

Zeolites in the act: A combined micro-spectroscopic approach

Marianne H.F. Kox, Eli Stavitski and Bert M. Weckhuysen

Inorganic Chemistry and Catalysis Group,Department of Chemistry, Utrecht UniversityEmail address: [email protected]


Zeolites are one of the most widely used catalysts in industry due to their unique combination ofacid properties and well defined framework. Therefore, a fundamental understanding of its cata-lytic activity is of great importance. For this purpose, in-situ spectroscopic techniques are com-monly employed to unravel useful structure-function relationships. However, in many cases theobtained information is averaged over the entire catalyst sample, therefore lacking importantspace-resolved information. In recent years, micro-spectroscopic techniques have proven to be avery valuable tool for probing distinct areas of catalysts particles, e.g. revealing the correlationbetween the crystals morphology, pore geometry and its catalytic activity.

Here, we demonstrate the space and time-resolved catalytic activity in large coffin-shaped zeoliteZSM-5 crystals (100x20x20 μm) using in-situ optical and fluorescence micro-spectroscopy.[1, 2]

The acid-catalyzed oligomerization of different styrene derivatives has been used as a model re-action, since the carbocationic intermediates formed on the Brønsted acid sites show strong ab-sorption in the visible region and therefore can act as reporter molecules for catalytic activity. Forboth optical and (confocal) fluorescence measurements the same in-situ spectroscopic cell hasbeen used to fully ensure the same reaction conditions.


After exposure of the zeolite crystals to (4-methoxy)styrene at elevated temperature (373 K), anon-uniform coloration behaviour can be observed. Furthermore, as revealed by optical micro-spectroscopy, the edge and main body of the crystals show a distinct product distribution, i.e.mainly dimeric carbocations are present in the edges of the crystals whereas dimeric and trimericcarbocations are the predominant species in the main body of the crystal (Figure 1, Scheme 1).

� ~ 585 nm � ~ 635 nm

Figure 1.Optical microphotograph of the zeolite crystal afteroligomerization with 4-methoxystyrene at 373K and corre-sponding optical absorption spectra of the edge and mainbody of the crystal at different styrene concentrations.

Scheme 1. Simplified schematic representation of thereaction pathway towards the formation of dimericand trimeric carbocation reporter species.

edge center


Program Section: 6 Preferred form of presentation: ORALApplication of reporter moleculesto image catalytic activity

As the ZSM-5 crystals, having a 900 rotational intergrowth structure,[3] appear to accommodatethe carbocation reaction intermediates aligned within the straight pores, as shown by optical mi-croscopy using polarized light (Figure 2), the non-uniform catalytic behaviour can be explainedby the distinct pore geometry in different regions of the ZSM-5 crystals; pore blockage at theedge mainly limits to product formation to dimeric carbocation whereas in the main body the ac-cess of monomers via sinusoïdal pore openings to the straight pores of the crystal appears to al-low further oligomerization to higher oligomers.

In addition, differently substituted styrene derivatives have been measured with the micro-spectroscopic techniques, all clearly showing a non-uniform catalytic activity. Time-resolved ab-sorption measurements allow determination of the reaction rate constants for the formation ofdimeric carbocation intermediates (Figure 3). From this, a different catalytic activity can be ob-served for distinct styrene compounds, e.g 4-methoxy>4-ethoxy and 4-bromo>4-chloro>4-fluoro,due to geometry and carbocation stabilization effects, respectively. Finally, confocal fluorescencemeasurements have been performed, allowing visualization of the three-dimensional product dis-tribution inside one ZSM-5 crystal during the catalytic reaction (Figure 4).

Conclusions

The combined in-situ micro-spectroscopic approach is a very valuable tool for visualization ofspace -and time-resolved catalytic activity in three dimensions: Different pore geometries lead todistinct products, which are aligned within the straight pores and different styrene derivates leadto differences in the catalytic reactivity.

References[1] M. H. F. Kox, E. Stavitski, B. M. Weckhuysen, Angew. Chem. Int. Ed. 2007, 46, 3652.[2] E. Stavitski, M. H. F. Kox, B. M. Weckhuysen, Chem. Eur. J. 2007, 13, 7057.[3] E. Stavitski, M. R. Drury, D. A. M. de Winter, M. H. F. Kox, B. M. Weckhuysen, Angew. Chem. Int. Ed.

2008, 47, 5637.

Compound K [mol-1s-1]

4-methoxystyrene 0.054-ethoxystyrene 0.0044-methylstyrene 0.0054-bromostyrene 0.054-chlorostyrene 0.0094-fluorostyrene 0.0075

Figure 2. Optical images ofthe ZSM-5/4-methoxystyrenecrystals taken with a) non-polarized light b,c) polarizedlight (indicated by the ar-rows).

Figure 3. Time evolution profile of the optical absorptionband due to formation of the dimeric carbocation (Scheme1) during the oligomerization of 4-methoxystyrene at 373 K.The table shows reaction rate constants for differently sub-stituted styrene compounds.

Figure 4. Confocal fluorescence im-ages of a ZSM-5/4-methoxystyrenecrystal. Indices a),b),c) correspond tothe upper horizontal plane, the inter-mediate horizontal plane and thevertical intermediate plane, respec-tively.

a)

b)

c)


Oral Communications

Operando III - Rostock-Warnemünde 2009 - Book of Abstracts Oral Communications

Program Section: Increasing time and space resolution of operando methodsPreferred form of presentation: Oral Presentation

Spatio-temporally resolved fluorescence measurements of single catalytic turnovers

Maarten B.J. Roeffaersa, Rob Amelootb, Gert De Cremerb, Bert F. Selsb, Dirk E. De Vosb, Johan Hofkensa

a Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001, Leu-ven, Belgium, b Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Bel-

gium, email: [email protected]


Catalytic crystals contain crystal faces, edges and defects with varying activity, and progress in industrial heterogeneous catalysis critically depends on better, quantitative correlations between the surface features and the reaction kinetics.[1] As a result of the sensitivity and limitations of each spectroscopic approach, several problems are still unaccounted for (e.g. pressure gap, mate-rials gap and phase gap). These shortcomings can be solved by applying new spectroscopic tech-niques.[2] Our group has introduced fluorescence microscopy as a sensitive tool to monitor chemical reactions at the single crystal level and even at the single molecule level.[3-5] Further-more the scope of this technique can be extended by using specific probes specifically targetingcertain regions or chemical groups inside the catalyst particle.[6] As such it is possible to establish spatially resolved structure-activity relationships at the level of individual features of catalysts, such as crystals, crystal faces or even individual catalytic sites. However due to light diffractionthe spatial resolution that can be attained with optical techniques is limited to several hundreds of nanometres.[7] This is clearly insufficient to monitor catalytic activity at the single site level. Re-cently we have extended our work towards visualizing the activity of single site catalysis. By doing so, we expect to build more straightforward correlations between the catalytic activity and the local properties.


In surface organometallic catalysis well-defined ‘single site’ surface species are generated which can be studied in detail. This has resulted in a better understanding of the functioning of such in-dividual sites. Such an approach is highly compatible with fluorescence microscopy, since this technique operates with single molecule sensitivity. Instead of visualizing catalytic activity by making use of the transformation of a non-fluorescent into a fluorescent molecule, the switchable fluorescence of a catalytic species can be read out with single molecule sensitivity and high time resolution. We have chosen for a modular building scheme in which the catalytically active site is connected via a linker to a reporter fluorophore.[8] This fluorophore reports on the binding state of the catalytic group by changes in fluorescence intensity. In particular our system consist of a ter-tiary amine base, serving as a strong base which in an unbound state quenches the fluorescence of

Operando III - Rostock-Warnemünde 2009 - Book of Abstracts Oral O1-1

Program Section: Increasing time and space resolution of operando methodsPreferred form of presentation: Oral Presentation

the reporter chromophore by photo-induced electron transfer. When bound to a reagent this elec-tron transfer is stopped resulting in the restoration of the fluorescence signal (see Figure 1a). By coupling such a system to the glass cover glass, the catalytic activity of one single site can be ob-served.

A) B)

Figure 1. Design and use of a fluorescent single-site catalyst. a) Schematic representation. b) Representative time trace showing the strong difference in fluorescence signal from the quenched and unquenched form of the fluorescent single-site catalyst.

Figure 1b shows a typical time trace of such a fluorescent single-site catalyst showing an almost perfect ‘digital’ behaviour. This strong difference in fluorescence intensity between the two states allows an easy analysis of the recorded fluorescence traces. However it is not always possible to design or isolate such fluorescent single site catalysts. This was the major driving force to also apply the newest developments in high-resolution microscopy.[7]

Conclusions

The proposed method using a fluorescent single-site catalyst is not limited to amine-catalysed reactions. With the present probe molecule also binding/unbinding and redox changed of metal complexes come within reach. Furthermore small changes in the design of the used fluorescent catalyst make it possible to study of a whole range of chemical reactions.

References[1] B. M. Weckhuysen, Chemical Communications 2002, 97.[2] M. B. J. Roeffaers, J. Hofkens, G. De Cremer, F. C. De Schryver, P. A. Jacobs, D. E. De Vos, B.

F. Sels, Catalysis Today 2007, 126, 44.[3] M. B. J. Roeffaers, B. F. Sels, H. Uji-i, F. C. De Schryver, P. A. Jacobs, D. E. De Vos, J. Hofkens,

Nature 2006, 439, 572.[4] M. B. J. Roeffaers, G. De Cremer, H. Uji-i, B. Muls, B. F. Sels, P. A. Jacobs, F. C. De Schryver, D.

E. De Vos, J. Hofkens, Proceedings of the National Academy of Sciences of the United States of America 2007, 104, 12603.

[5] M. B. J. Roeffaers, B. F. Sels, H. Uji-i, B. Blanpain, P. L'Hoest, P. A. Jacobs, F. C. De Schryver, J. Hofkens, D. E. De Vos, Angewandte Chemie-International Edition 2007, 46, 1706.

[6] M. B. J. Roeffaers, R. Ameloot, M. Baruah, H. Uji-i, M. Bulut, G. De Cremer, U. Müller, P. A. Jacobs, J. Hofkens, B. E. Sels, D. E. De Vos, Journal of the American Chemical Society 2008,130, 5763.

[7] S. W. Hell, Nature Biotechnology 2003, 21, 1347.[8] R. Ameloot, M. Roeffaers, M. Baruah, G. De Cremer, B. Sels, D. De Vos, J. Hofkens, submitted.


Program Section: 1 Preferred form of presentation: Oral

Dealumination of Y-zeolite investigated by in situ Al K-edge XANES and parametric Rietveld refinement of time resolved XRPD patterns

G. Agostinia, C. Lambertia, L. Palinb, M. Milanesiob, N. Danilinac; B. Xuc, J. A. van Bokhovenc aDept. of Chemistry IFM and NIS centre of Excellence Univ. of Torino (I), b Dept of Science and

Adv. Technol. Univ. A. Avogadro, Alessandria (I), cICB, ETH Zurich, HCI E127, Zurich (CH)


Ultra Stable Y zeolites (USY) are largely employed as industrial catalysts in an important number of reactions running at high temperature. They are obtained from H+-Y or NH4

+-Y via a partial dealumination process performed upon steaming at high temperature.[1] To understand the processes that occur during steaming of a zeolite, in situ structural analysis is required. Here, we present time-resolved X-ray powder diffraction (XRPD)[2] and in situ X-ray absorption spectroscopy (XAS) at the alumimum K edge.[3] XRPD provides the long-range structure and XAS the aluminum coordination. Because both methods can be performed under in situ conditions, a complete picture of the evolution of the aluminum species and their framework and extra-framework nature can be determined.

Experimental, Results, Discussion and Conclusions

Al K-edge XANES spectra have been collected at LUCIA, SLS. XRPD have been collected with the translating image plate device[4] available at GILDA BM8, ESRF, see Fig.1a. Both experiments have been performed in situ by heating a NH4

+-Y zeolite in H2O/N2 flux up to 873K.

Figure 1. (a): Scheme of the in situ setup available at GILDA BM8 (adapted from ref. [4a]). (b): Evolution of the XRPD patterns as a function of T during a ramp up (similar data have been collected in the ramp down).

Rietveld refinement of the whole series of patterns (Fig. 1b) has been performed with TOPAS code[5] implemented by the parametric approach, developed by Stinton and Evans,[6] to minimize



the correlation between scale factor and the presence of extra-framework atoms in the channels. We found three sites for water molecules, and two for NH4

+ cations, that leave as NH3 at higher T then the H2O molecules (Fig. 2a). Starting from 720 K, aluminium starts to occupy extra-framework sites (spheres in Fig. 2a). A more important population of the extra framework Al3+ sites occurs during the cooling step at the temperature when water starts to populate again the cavities. Al K-edge XANES shows changes in the aluminium coordination at high temperature and after cooling data supporting the Rietveld refinement. The starting sample showed the characteristics features of tetrahedrally coordinated Al: the whiteline at 1560 eV and a broad feature 15 to 20 eV above the whiteline. The spectrum that is measured at high temperature showed only minor changes. Upon cooling, the spectra changed and a loss of intensity of the whiteline and appearance of intensity above the absorption edge occurred, which indicates a change in coordination. A sudden change in coordination occurred in spectra that were recorded below 120°C in the ramp down, which showed a dramatic decrease in tetrahedral whiteline and a large increase in intensity above the absorption edge, which now dominates the spectrum. The broad feature at 15 to 20 eV above the edge also decreased in intensity. Ex-situ 27Al MAS NMR confirms the picture emerged from the in situ XRPD and XANES experiments.

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Figure 2. (a): Evolution along the heating ramp of the refined occupancy factors (converted in atoms/molecules per unit cell) for water molecules (3 sites: squares, left ordinate axis) ammonium counterions (2 sites: triangles, left ordinate axis) and extra-framework Al3+ species (circles, right ordinate axis). (b) From bottom to top, evolution of the Al K-edge XANES spectra collected in situ along the both heating up and cooling down rumps.

References [1] R. A Beyerlein, C. Choi-Feng, J. B. Hall, B. J. Huggins, G. J. Ray, Top. Catal. 4 ( 1997) 27. [2] P. Canton, P. Riello, C. Meneghini, A. Benedetti, X-Ray Diffraction and Scattering in “In-Situ Spectroscopy of

Catalysts”, B. M. Weckhuysen Ed., American Scientific Publishers (2004), p.293-320. [3] J. A. van Bokhoven, A. M- J. van der Eerden, D. C. Koningsberger, J. Am. Chem. Soc., 125 (2003) 7435. [4] a) C. Meneghini, G. Artioli, A. Balerna, A. F. Gualtieri, P. Norby, S. Mobilio, J. Synchrotron Radiat., 8 (2001)

1162. b) M. Milanesio, G. Artioli, A. F. Gualtieri, L. Palin, C. Lamberti, J. Am. Chem. Soc., 125 (2003) 14549. [5] A. A. Coelho, Journal of J. Appl. Crystallogr., 38 (2005) 455. [6] G. W. Stinton, J. S. O. Evans, J. Appl. Crystallogr., 40 (2007) 87.


Program Section: New reactor cells and coupling techniques Preferred form of presentation: oral

Sub second liquid transient ATR FT-IR micro flowcell for in-situ analysis of sorption phenomena and surface kinetics

T.J.A. Renckensa, A.R. Almeidab, M.R.Damena, G.Mulb, M.T.Kreutzera

a Product and Process Engineering, DelftChemTech, Delft University of Technology, b Catalysis

Engineering, DelftChemTech, Delft University of Technology


In catalyst kinetics measurements, steady state experiments are the norm. However, a lot more information can be obtained from transient experiments where the catalyst is subjected to chang-ing conditions and the response is followed in time. Transient experiments (e.g. TAP) [1] have been extensively performed for gas-phase catalysis, where low pressures and end of pipe analysis provide useful information on sorption phenomena and reaction mechanism. In liquid phase ca-talysis, the situation is further complicated because a step change in concentration is blurred by axial dispersion along the length and slow diffusion across the width of the channel. Currently liquid phase transients take longer than many surface dynamics to complete, making quantifica-tion near impossible. The objective of this work is to develop a device that allows transient analysis of surface chemistry of real catalysts in the liquid phase for processes that occur at the second scale.

We approach the challenge outlined above by choosing a spectroscopic technique that allows for operando measurements and by developing a flow cell with controlled fluidic properties so we can ensure a rapid transient. In order to obtain operando chemical information, we have chosen Attenuated Total Reflection (ATR) FT-IR spectroscopy [2]. This technique allows IR measure-ments to be performed on powder and liquids outside the crystal, showing both bulk and ad-sorbed species.

The problem of shallow gradients is addressed using segmented flow (see Figure 1), by the introduction of an immiscible phase that sepa-rates the starting and final condi-tions. Segmented flow has a double function – it reduces both axial dis-persion (mixing along the length of the reactor) and reduces the diffu-

sional boundary on the crystal, allowing for faster diffusion along the width of the channel. Available literature [4] allows prediction of the diffusional boundary. To obtain the 1 second

Figure 1: The old liquid bulk in the channel is pushed out by the gas slug, leaving a thin film layer near the channel wall. The gas slug is pushed out by the replacing liquid, which diffuses across the film layer.



switch time outlined in our objective, channel height must be on the order of 100 micron. Micro-fabrication techniques were used to produce a flowcell with the proper dimensions.

Adsorption desorption measurements were performed relevant for the selective photo oxidation of cyclohexane, where slow product desorption is a possible cause of catalyst deactivation [3].


Figure 2: The concentration of cyclohexanone in cyclohexane during a segmented switch from 0 to 0.05 M cyclohexanone in an empty channel

Figure 3: The cyclohexanone coverage in time is shown during desorption of cylohexanone in cyclo-hexane from titania. The bulk concentration of cyclohexanone is shown in the inset.

When using segmented switching over an uncoated channel, a characteristic time of switching of 0.6 s is obtained (Figure 2). When a titania coating is introduced in the micro flowcell, switch time increases to 3.3 seconds (see inset Figure 3). Steady state adsorption behaviour of cyclohex-anone in cyclohexane on titania was determined to be of the Langmuir type, with an overall ad-sorption constant K=26±5 m3/mol. The desorption behaviour has a characteristic desorption time that was shorter than 3.3 seconds (Figure 3).

Conclusions

Segmented switching works as predicted based on literature in bare channels, but the catalyst in-creases the switch time. Useful conclusions were obtained for selective photocatalytic oxidation of cyclohexane, and will be discussed in the presentation.

References [1] Berger RJ et al. - App Catal A 342 3 (2008) [2] Burgi T, Baiker A - Adv Catal 50 227 (2006) [3] Almeida AR, Moulijn JA, Mul G - J Phys Chem C 112 1552 (2008) [4] Kreutzer MT, Kapteijn F, Moulijn JA, Heiszwolf JJ - Chem Eng Sci 60 5895 (2005)



Space- and Time-resolved operando DRIFT and Raman spectroscopic study on NOx storage-reduction

Nobutaka Maeda, Atsushi Urakawa, Alfons Baiker

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, HCI, 8093 Zurich, Switzerland


NOx storage-reduction (NSR) has gained considerable attention due to its great ability of NOx

removal in oxygen-rich gas streams and complex nature, triggering global challenges in the elu-cidation of its underlying mechanism [1]. The NSR concept was build upon a periodic fuel lean-rich operation of the engine, where NOx is stored on Ba species under lean conditions and the stored NOx is, subsequently, released and reduced to N2 during a short rich period. However, elementary reaction steps during storage and reduction processes are still a matter of controversy and unknown to a large degree. In light of this, we have developed a combined operando DRIFT and Raman spectroscopy with space- and time-resolution to investigate dynamic surface-bulk processes and their gradients along the catalyst-bed during NSR.


Figure 1A shows the combined DRIFTS-Raman setup configuration. The plug-flow cell design allows rapid exchange of gaseous atmosphere between lean (0.42 % NO/3.3 % O2/He) and rich (3.3 % H2/He) periods, and the detection perpendicular to the axial direction of the catalyst bed allows gradient-profiling and identification of chemical species along the bed. 100 mg (6 mm in length) of Pt-Ba/CeO2 catalyst was placed into the cell. IR light and 785 nm-Raman excitation laser were focused onto the sample spot of the catalyst bed through a ZnSe window. Simultane-ous IR-Raman detection is possible, and the cell positioning with less than 1 μm accuracy can be achieved. The different local sensitivity [2, 3] between DRIFTS (surface-sensitive) and Raman spectroscopy (bulk-sensitive) gives both surface and bulk information simultaneously. The lean-rich cycles were repeated several times until reaching steady-state conditions, and the last three spectra were averaged to increase the S/N ratio, leading to a time-resolution as high as 100 ms.

Surface plots of DRIFT and Raman spectra during lean-rich cycles taken at the three catalyst-bed positions, i.e. front, back and middle positions (0.5, 3.0, 5.5 mm distant from the bed front) are shown in Figure 1B-G. DRIFT spectra (B-D) revealed that the formation of nitrites and ni-trates was considerably delayed at the middle and back positions, at which the significant de-crease in the band intensities was also observed. The most pronounced feature, besides the band intensities, is the gradient-difference between nitrites and nitrates along the catalyst bed. Nitrates



were formed from the beginning of lean periods, being independent of the bed positions, which is rationally considered to arise from the more complete oxidation of NO to NO2 towards the back position. Raman spectra (E-G) also proved the clear gradient along the bed as in DRIFT spectra. However, only Ba nitrates were confirmed as a bulk component during lean periods, indicating that nitrites, observed in DRIFT spectra, are merely a surface intermediate that undergoes further oxidation to produce surface nitrates and bulk Ba nitrates.

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Figure 1. Schematic illustration of the cell applicable to space- and time-resolved DRIFTS-Raman experi-ments (A). Time-resolved DRIFT (B-D) and Raman (E-G) spectra during NSR (673 K) at the front, middle, and back positions of the catalyst bed. The units are milli-absorbance in DRIFT spectra and relative intensity in Raman spectra.

Conclusions

Time-resolved DRIFT and Raman operando spectroscopy with space-resolution was demon-strated to be a powerful tool to investigate dynamic surface-bulk processes during NOx storage-reduction and to enable a gradient-profiling of reaction intermediates along the catalyst bed.

References [1] N. Takahashi, H. Shinjoh, T. Iijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Suzuki, N. Miyoshi, S. Matsumoto,

T. Tanizawa, T. Tanaka, S. Tateishi, K. Kasahara, Catal. Today 1996, 27, 63-69. [2] E. Roedel, A. Urakawa, S. Kureti, A. Baiker, Phys. Chem. Chem. Phys. 2008, DOI: 10.1039/b808529c.[3] H. Harima, Microelectron. Eng. 2006, 83, 126-129.


Program Section: Increasing Time and Space Resolution of Operando Methods Preferred form of presentation: Oral

Nanoscale Chemical Imaging of a Working Fischer-Tropsch Catalyst by insitu Scanning X-ray Transmission Microscopy

Emiel de Smita,*, Ingmar Swarta, J. Fredrik Creemerb, Gerard H. Hovelingc, Mary K. Gillesc,

Tolek Tyliszczakc, Patricia J. Kooymand, Henny W. Zandbergene, Cynthia Morina, Bert M.

Weckhuysena,* & Frank M.F. de Groota

* E-mail: [email protected]

a Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht

University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands, bDIMES-ECTM, Delft

University of Technology, P.O. Box 5053, 2600 GB Delft, The Netherlands, cDEMO, Delft

University of Technology, P.O. Box 5031, 2600 GA Delft, The Netherlands, dAdvanced Light

Source, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720,

USA, eDelftChemTech and National Centre for High Resolution Electron Microscopy, Delft

University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands, fKavli Institute of

NanoScience, National Centre for High Resolution Electron Microscopy, Delft University of

Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands.


Microscopy is a widely applied tool in the characterization of heterogeneous catalyst. Frominfrared light to electron beams, contemporary microscopists apply the full wavelength spectrumto image catalytic materials on different microscopic scales. Modern microscopes often integratespectroscopy with imaging techniques to simultaneously obtain morphological and quantitativechemical information on the sample (e.g. STEM-EELS [1], Infrared Microscopy [2], UV-VISMicroscopy [3] and Fluorescence Microscopy [4]). In the field of chemical microspectroscopy,there is a continuous drive in increasing the spatial resolution of the images while maximizingchemical information content under chemically relevant (i.e. operando) conditions.

Scanning X-ray Transmission Microscopy (STXM), with its current 15 nm spatial resolution andhigh chemical speciation potential by X-ray Absorption Spectroscopy (XAS), is a recentpromising contender in the field. The technique uses a focussed soft X-ray (200 – 2000 eV) beamto make a two dimensional raster scans of a sample. X-rays are absorbed by the sample andtransmitted signal is detected. By changing the energy of the incident X-ray beam, images can beacquired over a range of energies, yielding a full absorption spectrum per measured point.

The main experimental challenge is to overcome the strong attenuation of soft X-rays in gaseousatmospheres, which makes the technique difficult to apply under realistic reaction conditions.Here, we use a specially designed nanoreactor [5] to overcome this challenge and study thedistribution and chemical identity of iron and carbon species in an Fe/CuO/K2O/SiO2 Fischer-Tropsch Synthesis (FTS) catalyst [6] during reduction in H2 at 350oC and FTS in CO/H2 at250oC.


Program Section: Increasing Time and Space Resolution of Operando Methods Preferred form of presentation: Oral


To minimize the interaction between X-ray light and gas phase molecules, we applied a speciallydesigned nanoreactor which minimizes the attenuation of X-rays by gas phase reactants by

reducing the gas path length to 50 μm. The nanoreactor consists of a reactor chamber of about

500 x 500 x 50 μm which is supplied with reactant gasses by micrometer-sized gas channels andis fitted with a resistive Pt heater element, allowing heating up to 500oC.

Studying the catalyst material during reduction and FTS revealed 35 nm spatial variations in theiron valence and its metal/carbide/oxide nature. After reduction, the catalyst material consisted

mainly of iron (II) silicates, iron oxide and small amounts of α-Fe (Fig. 1). During FTS more ironoxide was converted into iron (II) silicate species and Fe0. Carbon species were observed to be

almost exclusively located near Fe0 species indicating the conversion of α-Fe into iron carbides.

Fig. 1: Spatial distribution map of iron species over the SiO2 support (white) and Fe L2,3-edge XAS spectra ofa Fe/CuO/K2O/SiO2 catalyst after reduction in H2 at 350oC for 2 h.

Conclusions

STXM was applied under in situ conditions for the first time, to study the physicochemicalproperties of a complex iron-based FTS catalyst. It was illustrated that STXM is a versatile toolin the characterization of catalytic solids. Its nanometer resolution combined with powerfulchemical speciation by XAS and the ability to image materials under realistic catalytic conditionsopens up opportunities to study many chemical processes taking place on solids – including, butcertainly not limited to heterogeneous catalysis.

References[1] T. W. Hansen, J. B. Wagner, P. L. Hansen, S. Dahl, H. Topsoe, C. J. H. Jacobsen, Science 294 (2001) 1508.[2] E. Stavitski, Marianne H. F. Kox, I. Swart, Frank M. F. de Groot, Bert M. Weckhuysen, Angew. Chem. Int.

Ed. 47 (2008) 3543.[3] M. H. F. Kox, E. Stavitski, B. M. Weckhuysen, Angew. Chem. Int. Ed. 46 (2007) 3652.[4] M. B. J. Roeffaers, B. F. Sels, H. Uji-i, F. C. De Schryver, P. A. Jacobs, D. E. De Vos, J. Hofkens, Nature

439 (2006) 572.[5] J. F. Creemer, S. Helveg, G. H. Hoveling, S. Ullmann, A. M. Molenbroek, P. M. Sarro, H. W. Zandbergen,

Ultramicroscopy 108 (2008) 993.[6] E. de Smit, B. M. Weckhuysen, Chem. Soc. Rev. In Press (2008) doi:10.1039/B805427D.



Decoupling of the deactivation mechanisms in Co-based Fischer-Tropsch cata-lysts by a wide range of in situ spectroscopies at realistic working conditions

Magnus Rønninga, Alexey Voronov a, Nikolaos Tsakoumis a, Øyvind Borgb, Erling Ryttera,b, An-ders Holmena

aDepartment of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7401 Trondheim, Norway

bStatoilHydro R&D, Research Centre, NO-7005 Trondheim, Norway

Introduction

The Fischer-Tropsch (FT) synthesis is currently being widely studied as a step in the gas-to-liquids (GTL) technology. Supported cobalt is the favourable catalytic material for synthesis of long-chain hydrocarbons from synthesis gas produced from low-sulphur natural gas. Cobalt is usually chosen as the active component for its high activity, high selectivity to linear paraffins, and low water-gas shift activity.

We have previously combined XAS and XRD in situ to study the deactivation of unpromoted and Re-promoted Co/Al2O3 catalysts in FT synthesis [1]. However, the study was limited to ambient pressure and diluted feed. Due to the heavy wax products, in situ studies of the FT reaction is challenging. However, this can be solved either by tuning the reaction conditions to methanation (high H2/CO ratio) or by heating the output line from the reactor to keep the wax in liquid state until it is collected. In order to obtain realistic working conditions the latter is preferred, since the partial pressures of steam, H2 and CO in the reactor are crucial for the selectivity and deactivation of the catalysts. Hence, the reaction should be run at high pressure (10-20 bar) and at relatively high conversion (50%) to expose the catalyst to a realistic working environment, closely resem-bling an industrial reactor.

Catalyst deactivation is a major challenge in Fischer-Tropsch synthesis [2]. Deactivation effects are observed for catalysts on all commonly used supports. At present, there is not sufficient knowledge to explain and to distinguish between the proposed deactivation mechanisms. The suggested mechanisms include cobalt surface oxidation [3-5], sintering [6,7] and solid state reac-tions leading to inactive cobalt phases [6,8]. Recent studies suggest that sintering accounts for a fraction of the deactivation and that carbon deposits also play a crucial role [8,9].


In the present work, Fischer-Tropsch catalysts have been studied using XAS in combination with XRD at conditions close to industrial operation coupled with online product analysis. The infor-mation from the two techniques is highly complimentary and thus provide a unique possibility to decouple the potential deactivation mechanisms: XAS is able to give information about the oxi-dation states of Co during the reaction whereas XRD gives information about particle size and



morphology and hence sintering [10,11]. In addition, in situ Raman and IR spectroscopy have been carried out at similar conditions in order to monitor the oxide species on the catalysts. Ra-man spectroscopy has been shown to be able to detect inactive Co phases at relatively low con-centrations [12]. The vibrational spectroscopies are also able to detect carbonaceous deposits on the catalysts and in favourable cases even identify some of the deposited components.

We have recently installed a reactor feeding system at BM01B at the European Synchrotron Ra-diation Facility (ESRF) which allows for working at pressures up to 20 bars with undiluted syn-gas in the feed. An online MS is used for monitoring the conversion level. The in situ cell makes it possible to combine XRD, XAS and Raman spectroscopy. A similar setup is constructed in our home laboratory where in situ measurements are being performed in a combined FTIR-Raman instrument.

The results from the various techniques are discussed in connection with long-term deactivation studies in a laboratory-scale reactor. A detailed description of the differences between fresh and used catalysts will be presented.

Conclusions

A wide range of characterisation techniques are necessary in order to identify the various deacti-vation mechanisms in Co Fischer-Tropsch catalysts. Furthermore, the techniques have to be em-ployed at realistic working conditions in terms of partial pressures, temperature and conversion level, since these parameters have a significant influence on the deactivation mechanisms.

References [1] Ø. Borg, M. Rønning, S. Storsæter, W. van Beek, A. Holmen, Stud. Surf. Sci. Catal. 163 (2007) 255. [2] J. van de Loosdrecht, B. Balzhinimaev, J.-A. Dalmon, J.W. Niemantsverdriet, S.V. Tsybulya, A.M. Saib,

P.J. van Berge, J.L. Visagie, Catal. Today, 123 (2007) 293 [3] D. Schanke, A. M. Hilmen, E. Bergene, K. Kinnari, E. Rytter, E. Ådnanes, A. Holmen, Catal. Lett. 34

(1995) 269. [4] D. Schanke, A. M. Hilmen, E. Bergene, K. Kinnari, E. Rytter, E. Ådnanes, A. Holmen, Energy Fuels 10

(1996) 867. [5] G. Jacobs, P. M. Patterson, T. K. Das, M. Luo, B. H. Davis, Appl. Catal. A 270 (2004) 65. [6] G. Jacobs, P. M. Patterson, Y. Zhang, T. Das, J. Li, B. H. Davis, Appl. Catal. A 233 (2002) 215. [7] T. K. Das, G. Jacobs, P. M. Patterson, W. A. Conner, J. Li, B. H. Davis, Fuel 82 (2003) 805. [8] M.J. Overett, B. Breedt, E. du Plessis, W. Erasmus, J. van de Loosdrecht, Prepr. Pap.-Am. Chem. Soc.,

Div. Petr. Chem. 2008, 53, 126-128 [9] D.J. Moodley, J. van de Loosdrecht, A.M. Saib, H.J.W. Niemantsverdriet, Prepr. Pap.-Am. Chem. Soc.,

Div. Petr. Chem. 2008, 53, 122-125 [10] G. Jacobs, T. K. Das, P. M. Patterson, J. Li, L. Sanchez, B. H. Davis, Appl. Catal. A 247 (2003). [11] Ø. Borg, N. Hammer, S. Eri, O.A. Lindvåg, R. Myrstad, E.A. Blekkan, M. Rønning, E. Rytter, A. Hol-

men, Catal. Today (2008) accepted [12] X.X. Gao, C.J. Huang, N.W. Zhang, J.H. Li, W.Z. Weng, H.L. Wan, Catal. Today, 131 (2008) 211


Program Section: Bridging the gap between model and technical conditions Preferred form of presentation: oral

On line Raman investigations of the catalytic behavior of Wells-Dawson heter-opolycompounds in the oxidation of propene

E. Arendt, E.M.Gaigneaux*

Unité de catalyse et chimie des matériaux divisés. Université catholique de Louvain.

Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium * [email protected]


Thanks to their potential multi-functionality, heteropolyoxometal acids and salts appear as mate-rials of growing importance in catalysis. When their surrounding is modified (nature of support, temperature, gaseous atmosphere, nature of solvent, …), heteropolycompounds tend to undergo structural changes and rearrangements. As a result, the compounds can act as precursors of the desired active species which truly make the catalytic job. Therefore, working under conditions adequate to maintain the most efficient active species could be a most promising way to take ad-vantage of HPAs.

With the objective to explore this hypothesis, a setup combining a fixed bed reactor, a Raman spectrometer and online analysis of gaseous products by GC and MS was used to investigate the behavior of the ammonium phosphomolybdic Wells-Dawson salt (NH4)6P2Mo18O62 in the course of the propene oxidation. As a preliminary study to these operando experiments, the solid-state thermal rearrangement of the Dawson anion into a Keggin-type containing phase has been exam-ined by in situ Raman spectroscopy under different treatment conditions.


In the first part of this study, the thermal stability of the Wells-Dawson sample was examined under both reducing and oxidizing conditions and its molecular evolution was characterized using in situ Raman spectroscopy. In these experiments, the sample was treated in different gaseous atmospheres and at different temperatures. Under oxidizing conditions, it is shown that the struc-ture of the Wells-Dawson sample remains intact up to 175°C, while at higher temperature, struc-tural changes and rearrangements are observed. Indeed, above 175°C, the Wells-Dawson com-pound rearranges with the formation of a phase containing the Keggin-type unit [PMo12O40]3- and above 350°C, there is the formation of a phase mixture containing the Keggin-type unit and MoO3. In these experiments, the rearrangement of the sample is influenced by both temperature


Program Section: Bridging the gap between model and technical conditions Preferred form of presentation: oral

and thermal treatment. In order to study the effect of the temperature alone on the solid-state rear-rangement, a series of experiments were carried out in oxidizing conditions. In these experiments, the Wells-Dawson sample was heated during 10 hours up to (i) 200°C, (ii) 250°C and (iii) 300°C. By treating the sample at 200°C during 10h, there is no modification in the polyoxoanion struc-ture, while solid-state rearrangement occurs in the two other cases. Indeed, during the increase of temperature up to both 250 and 300°C, the sample acquires the Keggin features. Maintaining for longer time these temperatures, no further modification is observed during 10 hours at 250°C while after 4 hours at 300°C, MoO3 consecutively appears.

In the second part of this study, solid-state rearrangement of the ammonium phosphomolybdic Wells-Dawson salt (NH4)6P2Mo18O62 was investigated with on line Raman spectroscopy in the course of the oxidation of propene. The reaction was run between 300 and 450°C and the cata-lytic activity was measured stepwise every 50°C with a staying time of 120 min at each step. Three different gaseous conditions were used: (i) 20%vol. O2, 10 % propene and 70%vol He (ii) 10%vol. O2, 20 % propene and 70%vol He and (iii) 10%vol. O2, 10 % propene and 80%vol He. The total flow rate was set at 30mL min-1 . Whatever the gaseous conditions, the catalyst exhibits a weak activity (below 5% of propene conversion) and the on line Raman analysis reveals that the Wells-Dawson sample undergoes a thermal rearrangement above 300°C to give the Keggin type anion and MoO3 at higher temperature (400-450°C).

Conclusions

The present investigation shows that both the nature of the gaseous atmosphere (namely its oxido-reduction strength) and the thermal treatment can be used to influence the reorganisation of the Wells-Dawson sample into a Keggin-type anion and oxide species. In addition, similar cata-lytic results and thermal rearrangements were observed whatever the gaseous atmosphere. It seems that in these cases, the oxido-reduction strengths of the gaseous atmosphere chosen in this study were not enough marked. Determining the appropriate catalyst with the most desired en-hanced performances in oxidation processes should deserve further investigation with more pro-nounced oxido-reduction strength of the gaseous atmosphere.


Program Section: 2 Preferred form of presentation: oral

An operando XPS-MS study of the oscillations

in the propane oxidation over nickel

Vasily V. Kaicheva,*, Alexey Yu. Gladkya, Igor P. Prosvirina, Valerii I. Bukhtiyarova,

Raoul Blumeb, Michael Häveckerb, Axel Knop-Gerickeb, Robert Schlöglb

aBoreskov Institute of Catalysis of SB RAS, Novosibirsk, Russia, E-mail: [email protected] der MPG, Berlin, Germany


Many heterogeneous catalytic reactions exhibit rate oscillations under certain conditions [1]. To-day approximately 70 oscillating heterogeneous catalytic system are known. Among them is theoxidation of light alkanes over transition metals, which are intensively studied over the past sev-eral years. For example, the regular self-sustained oscillations were observed in the methane oxi-dation over Ni, Co and Pd supported and unsupported catalysts in oxygen-deficient conditions atambient pressure [2]. Similar oscillations were also observed in the ethane oxidation over Ni andCo foils. In our previous work, it was found that the propane oxidation over Ni can proceed in aself-oscillation regime as well [3,4].

The characteristics of these oscillations are sufficiently similar to suggest a common origin forthe oscillatory behaviour. The stable and repeatable oscillations appear after an induction periodof tens minutes, when the catalysts demonstrate very low activity. It points out that the reactionkinetics alone cannot be responsible for the oscillations. The mechanisms of oscillations based onUHV studies [1] also cannot be simply extrapolated to the high-pressure reactions. In order toelucidate the oscillation mechanism in the oxidation of light alkanes over transition metals, it isnecessary to apply some operando techniques. Unfortunately, self-oscillations in the methane andethane oxidation have been observed only at atmospheric pressure, where most of the surface-sensitive techniques, including XANES and XPS, can not be used. Therefore, as a case reaction,we chose the catalytic oxidation of propane over Ni, where regular oscillations with periods ofseveral minutes can be observed in mbar pressure range [3,4].


In this study, we used time-resolved X-ray photoelectron spectroscopy (XPS) in situ, i.e., whileoscillations takes place, simultaneously with mass-spectrometry (MS) for monitoring gas-phasereaction components. In situ XPS is one of the most useful tools to investigate both the surfacecomposition and the nature of adsorbed species on the surface of heterogeneous catalysts. Whenthe in situ XPS is coupled with mass-spectrometry, it becomes a particularly effective operandotechnique, which makes it possible to correlate the surface properties with the catalytic perform-ance. The experiments were carried out at the ISISS beam line at BESSY in Berlin. Total pres-sure during in situ experiments was 0.5 mbar. The Ni foil was used as a catalyst.



The relaxation oscillations in the propane oxidation were observed at temperature 550-650°C andfor gas mixtures with propane/oxygen ratios from 1:1 to 20:1. Usually, the catalyst stayed in aninactive state for the most of the time with occasional evolution of H2, CO and H2O. It should benoted that the concentration of CO2, which is one of the possible products, is difficult to deter-mine by MS due to masking the CO2-originated signals by intense signals with the same m/z ra-tios from the propane fragmentation pattern. However, in our previous experiments with isotope-labelled oxygen 18O2, the small amounts of CO2 had been detected to evolve synchronously withH2O [4]. Such product distribution indicates that both the partial and total oxidation of propaneoccurred over Ni during the active half-period. The periodic changes in the reactant concentrationwere accompanied by synchronous fluctuations of the catalyst temperature.

The period of oscillations for the propane/oxygen ratio of 3:2 was about 900 seconds that wasenough for collection of XPS core-level spectra directly during both active and inactive half-periods. Typical times for measurement of one XPS scan were about 40 and 30 seconds forNi2p3/2 and O1s spectra, respectively. Moreover, the regular character of oscillations made it pos-sible to collect a number of scans, which were averaged for increasing the signal-to-noise ratio ofresulting spectra. Correspondingly, the use of the XPS-MS operando technique allowed us to di-rectly detect the chemical state of the Ni surface during both active and inactive half-periods ofoscillations. The Ni2p3/2 spectrum from the inactive surface showed the characteristic pattern ofNiO with the main line at 855 eV and two satellites at higher (by ~1.5 and ~7 eV) binding en-ergy. Also we observed the strong O1s peak at 529.9 eV. In contrast, the Ni2p3/2 spectrum fromthe active surface consisted of the only sharp feature at 853 eV, which corresponded to Ni in themetal state. In this case, we observed only a weak O1s feature at 530 eV.

It means that during oscillations, the periodic reduction and reoxidation of upper layers of the Nifoils occurs. The highly active state is associated with metallic Ni, whereas the low-activity stateis characterised by the presence of NiO on the catalyst surface. Taking into account the mean es-cape depth of electrons at kinetic energy of 200 eV, the lower bound of thickness of the layertransformed from Ni to NiO can be estimated as 5-10 nm.

Conclusions

Finally, we can conclude that self-oscillations in the propane oxidation over Ni originate due tothe periodic oxidation and reduction occurring on the catalyst surface. To the best of our knowl-edge, it is the first direct experimental evidence for the redox mechanism of triggering rate in theoxidation of light alkanes over a transitional metal in the self-oscillation regime.

This work was supported in part by the Russian Foundation for Basic Research (09-03-00791).

References[1] R. Imbihl, G. Ertl, Chem. Rev. 95 (1995) 697-733.[2] X. Zhang, C.S.-M. Lee, D.O. Hayward, D.M.P. Mingos, Catal. Today 105 (2005) 283-294.[3] A.Yu. Gladky, V.K. Ermolaev, V.N. Parmon, Catal. Lett. 77 (2001) 103-106.[4] A.Yu. Gladky, V.V. Kaichev, V.K. Ermolaev, V.I. Bukhtiyarov, V.N. Parmon, Kinet. Catal. 46 (2005) 251-259.



SAPO Methanol to Olefin Catalysts Under Working Conditions

David Wragga,d, Poul Norbya,d, Helmer Fjellvåga,d, Arne Grønvoldb,d, Terje Fuglerudb,d, Jasmina Hafizovicc,d, Ørnulv Vistadc,d, Duncan Akporiayec,d

aCentre for Materials Science and Nanotechnology, University of Oslo, bIneos Chlorvinyls, cSINTEF Materials and Chemistry, d InGAP Centre for Research Based Innovation

E-mail: [email protected]


Methanol to olefin (MTO) conversion over zeotype catalysts allows us to produce some of the most versatile chemical building blocks from a cheap, renewable and readily available starting material. The silicoaluminophosphate framework SAPO-34 is an effective catalyst for this proc-ess [1] and we have used a combination of in situ synchrotron XRD, Raman spectroscopy and mass spectrometry to study it under real working conditions. Despite much study of the mecha-nism of the process [2] there is still little information on the actual behaviour of the catalyst. Car-rying out in situ experiments in a capillary based flow cell at temperatures of 440°C and at ele-vated pressure we aim to understand the physical and chemical changes in the catalyst during MTO conversion and relate these to ex situ and mechanistic data.


Initial data collected at 440°C with methanol vapour in a helium carrier gas flowing over the catalyst bed showed a significant expansion in the unit cell of the catalyst in the c-axis direction with a rapid increase in axis length slowing gradually until a final value is reached after 120 min-utes. The Raman spectra collected during this period show the build up of a broad band attributed to coke. An experiment at room temperature under the same gas feed illustrates the differences between the changes due to MTO reaction intermediates and methanol adsorption, with the for-mer producing a far greater expansion of the framework (figure 1). Subsequent experiments have shown that this expansion is reversible on calcination of the catalyst under flowing oxygen and helium and that the catalyst may be used after calcination for a further cycle of MTO conversion without apparent ill-effects.

Using Fourier analysis the number of electrons in the cages of the catalyst during the reaction was determined and a plot of these values against time gives a curve remarkably similar to that of the c axis variation in figure 1. This suggests that the asymmetric expansion of the catalyst is driven by the build up of intermediates.



0 20 40 60 80 100 120 14014.90

14.95

15.00

15.05

15.10

15.15

15.20

15.25

15.30

15.35

c-axis MTO c-axis methanol adsorption

c-ax

is M

TO (Å

)

Time (Minutes)0 20 40 60 80 100 120 140 160 180 200

14.92

14.94

14.96

14.98

15.00

15.02

15.04

c-ax

is (Å

)

Dataset

Figure 1 (left) C-axis variation for SAPO-34 in the MTO process (black curve) and methanol ad-sorption at room temperature (red curve). Figure 2 (right) c-axis variation during an experiment

using short pulses of methanol instead of a continuous feed.

Difference Fourier maps show the increasing level of electron density in the cages during the re-action clearly and display intriguing suggestions of benzene derivatives in their shape. A similar analysis of high resolution PXRD data from SAPO-34 with adsorbed methanol has allowed us to determine the positions of the methanol molecules in the cages.

In order to study the strain caused in the framework by the structural changes and internal pres-sure from the intermediates we analysed the widths of the PXRD peaks during the process. A clear expansion in peak width (indicating increased strain) follows a similar pattern to the in-crease in the c-axis length but with obvious peaks at the point where the maximum value of c is reached. This appears to correlate to the (smaller) variation observed in the a-axis. Experiments using pulses of methanol shed further light on this with the c axis expanding after each pulse and rapidly contracting (though never to less than the maximum value of the previous pulse (figure 2). This seems to suggest a highly active and large initial species after pulsing and prior to the peak working value of the c-axis which is large enough to cause expansion in both the a and c-axis directions. So far all the increases in peak width which we have observed have been reversi-ble.

Conclusions

We have observed major physical changes in the structure of SAPO-34 during the MTO process caused by the build up of reaction intermediates in the cages which cause considerable strain; and yet also show that these are reversible.

References [1] Z. Liu and J. Liang, Current Opinion in Solid State & Materials Science 1999, 4, 80-84. [2] M. Bjorgen, F. Bonino, S. Kolboe, K. Lillerud, A. Zecchina, and S. Bordiga, J. Am. Chem. Soc., 2003, 125,

15863-15868.



In situ determination of hydrogen inside a catalytic reactor using Prompt Gamma Activation Analysis

Detre Teschnera, János Borsodia,b, Zsolt Révayb, Tamás Belgyab, László Szentmiklósib, Zoltán Kisb, Marc Armbrüsterc, Kirill Kovnirc, Matthias Friedrichc, Manfred Swobodaa, Malte Behrensa

Axel Knop-Gerickea, Robert Schlögla

a Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany b Institute of Isotopes, Hung. Acad. Sci., POB 77, Budapest, H-1525, Hungary c Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, D-01187 Dresden, Germany

[email protected]


Prompt gamma activation analysis (PGAA) is a rapidly developing chemical analytical method utilizing nuclear techniques. It is based on the radiative neutron capture of nuclei, in other words, the (n, ) reaction. Each isotope of every chemical element (with the exception of 4He) is capable of absorbing a neutron, followed by relaxation in the form of radiation. The emitted prompt radiation is characteristic; i.e., the detected energies identify the nuclides, and the intensities are proportional to their amounts in the analyzed sample. PGAA has been used for the analysis of a great variety of materials in many scientific fields, from archaeology through geology to material science determining trace elements especially when no sample preparation is possible. The method is also capable of resolving isotopic composition. The highest-performance PGAA facili-ties are located at neutron guides of high-power research reactors and use high-purity germanium detectors for the collection of the gamma spectra.

The determination of reacting components during a catalytic reaction has been a great challenge for analytical chemists for many years. This issue is further intensified when the hydrogen con-tent of hydrogenation catalysts has to be quantified operando. Mechanistic studies on heteroge-neous hydrogenation reactions suggest that the hydrogen content of catalysts play an essential role in defining activity and particularly selectivity. Because most of the spectroscopic methods are not sensitive for hydrogen (or cannot be applied in situ), we developed the PGAA into an in situ technique by placing a continuous-flow reactor instead of a normal specimen into the neutron beam (1). The necessary modifications on a standard PGAA facility will be presented in great details.


First, the formation of palladium -hydride was studied to validate the accuracy of the technique in quantifying hydrogen. A fresh Pd black sample was introduced into flowing H2 at room tem-



perature, and the H content was determined to be 0.75 H per Pd atom. This is in perfect agree-ment with the Pd/H phase diagram at 1 bar hydrogen that indicates -hydride at this condition with a stoichiometry of PdH0.73.Subsequently we applied PGAA in investigating the mechanism of 1-pentyne hydrogenation over palladium catalysts (2). The PGAA experiments show that, unselective total hydrogenation ex-clusively occurs on saturated -hydride. On the other hand, during selective hydrogenation the hydrogen content has no correlation with the activity and reflects rather the prehistory of the sample. Selective hydrogenation is only possible after decoupling bulk properties from the sur-face events. Additionally, these mechanistic insights have been validated for other selective al-kyne hydrogenation reactions and have been contrasted to alkene hydrogenation that share many similarities to total hydrogenation of alkynes (3).Based partly on the above conclusions, a family of palladium intermetallic compounds have been designed, prepared and characterized as promising candidates for superior acetylene hydrogena-tion catalysts (4, 5). Indeed, palladium-gallium intermetallic compounds show excellent selectiv-ity and long term stability in this process. As a proof of concept, intermetallic samples have been investigated using the PGAA facility. All results indicate that hydrogen does not dissolve in the intermetallic samples, and hence the concentration of subsurface hydrogen detrimental to the se-lective hydrogenation should be very low.

Conclusions

A new method, in situ PGAA, has been developed and applied successfully to determine the hy-drogen content of palladium catalyst during hydrogenation reactions. The results show clear cor-relation between hydrogen content and reaction selectivity indicating the importance of methodo-logical developments and the relevance of studying catalytic reactions in situ.

References [1] Zs. Révay, T. Belgya, L. Szentmiklósi, Z. Kis, A. Wootsch, D. Teschner, M. Swoboda, R Schlögl, R. Zepernick,

Anal. Chem., 2008, 80, 6066. [2] D. Teschner, J. Borsodi, A. Wootsch, Zs. Révay, M. Hävecker, A. Knop-Gericke, S. David Jackson, R. Schlögl,

Science, 2008, 320, 86. [3] D. Teschner , Zs Révay, J. Borsodi, A. Knop-Gericke, R. Schlögl, D. Milroy, S. David Jackson, D. Torres, P.

Sautet, Angew. Chem. Int. Ed., 2008. DOI: 10.1002/anie.200802134 [4] J. Osswald, R. Giedigkeit, R.E. Jentoft, M. Armbrüster, F. Girgsdies, K. Kovnir, T. Ressler, Y. Grin, R. Schlögl,

J. Catal., 2008, 258, 210. [5] J. Osswald, K. Kovnir, M. Armbrüster, R. Giedigkeit, R.E. Jentoft, U. Wild, Y. Grin, R. Schlögl, J. Catal., 2008,

258, 219.


Program Section: Bridging the gap between model and technical conditions. Preferred form of presentation: Oral

Dynamic structure of supported platinum catalysts during oxidation of carbon monoxide

Jagdeep Singha, Evalyn M.C. Alayona, Moniek Trompb, Olga V. Safonovac, Pieter Glatzelc,Maarten Nachtegaald, Ronald Frahme, Jeroen A. van Bokhovena

aInstitute of Chemical and Bioengineering, ETH Zurich, Switzerland, bSchool of Chemistry, Uni-versity of Southampton, UK, cEuropean Synchrotron Radiation Facility, Grenoble, France, dPaulScherrer Institute, Villigen, Switzerland, eDept. of Physics, University of Wuppertal, Germany.


The oxidation of carbon monoxide is one of the simplest catalytic reactions, which is studied ex-tensively in the field of catalysis. The preferential oxidation of carbon monoxide in hydrogen feed gas in fuel cells, being an industrially important reaction has been studied on various noble metal (e.g. platinum) catalysts [1]. Also, the removal of carbon monoxide traces from industrial and automotive exhausts is of considerable interest as far as clean environment is concerned [2]. Knowledge of the structure of the catalytically active sites in these solid catalysts is essential to understand the functioning of solid catalysts and chemical processes. Despite the effort and re-search in this field, the fundamental questions about the active species and mechanism of the re-action remain disputed. Most researchers claim that metallic platinum is the active surface species [3]. Recent surface x-ray diffraction studies on surfaces of platinum single crystals [4] on the other hand, indicate that the rate of oxidation of carbon monoxide is higher when the surface is oxidized. It has also been suggested that the active structure is a combination of metallic and oxi-dic phases on the supported metal catalysts [5]. Ertl and co-workers showed that on single-crystals under low-pressure conditions, varying reconstruction of the platinum surface, which oc-curs after the adsorption of carbon monoxide, leads to carbon monoxide-rich and oxygen-rich domains that have different reaction rates [6]. Here the objective is to bridge both the pressure and material gaps by determining the structure of a real alumina-supported platinum catalyst un-der real conditions during oxidation of carbon monoxide by combining in situ, time-resolved, and high-energy resolution fluorescence x-ray spectroscopy (HERFD XAS) [7] with kinetic measurements. Bridging these gaps is essential to translate results from single crystal studies and understand catalytic process under realistic conditions.


Similar to results on single crystals [3b, 4] we observed a low and a high activity state of the cata-lyst. At oxygen to carbon monoxide ratios greater than stoichiometric, the catalyst showed two different reaction regimes respectively with low and high reaction rates. When heating, at a par-ticular temperature there was a sudden increase in activity to the high-activity regime. This so-


Program Section: Bridging the gap between model and technical conditions. Preferred form of presentation: Oral

called ignition occurred at lower temperature with increasing oxygen concentration. HERFD spectra, which were obtained every two minutes, showed large changes between spectral features taken below and above the ignition temperature. Below ignition, a whiteline of low intensity with a double feature was observed, which is characteristic of the platinum particles with adsorbed carbon monoxide [8]. This adsorbed carbon monoxide is in general agreement with infra-red data[9]. In contrast to the single crystals, at a higher temperature than the ignition temperature, the spectra showed a strong increase in the intensity of the whiteline while the edge energy shifted to lower energy, which is characteristic of oxidized platinum [8]. This suggests that the high-activity regime is characterized by the presence of large amounts of oxidic platinum. During the ignition, the spectra recorded with a time resolution of 0.5 secs, using Quick Extended X-ray Absorption Fine Structure (QEXAFS) and quick XANES measurements showed increasing amounts of plati-num oxide which autocatalytically increased the conversion. These data suggest that oxidized platinum plays an active role in generating high activity. In contrast to single crystals that show 0.5 ML of chemisorbed oxygen such as adsorption of oxygen, nano-sized particles undergo oxi-dation.

Conclusions

The catalyst shows different structure in a low- and high-activity regime: adsorbed carbon mon-oxide on platinum in the low-activity regime, that poisons the surface, and partially oxidic plati-num in the high-activity regime as observed with in situ HERFD XAS. The fast changes in the activity during the ignition are related to the rapidly changing structure of the catalyst as deter-mined from QEXAFS. High temperature and a high oxygen concentration are required to obtain the more active partially oxidic platinum catalyst.

References [1] (a) J.N. Armor, Appl. Catal. A 176 (1999) 159; (b) C.D. Dudfield, R. Chen, P.L. Adcock, Int. J. Hydrogen En-

ergy 26 (2001) 763. [2] J.T. Kummer, J. Phys. Chem. 90 (1986) 4747; (b) R. J. Farrauto, R. M. Heck, Catal. Today 51 (1999) 351. [3] (a) T.H. Lindstrom, T.T. Tsotsis, Surf. Sci. 150 (1985) 487; (b) J.A. Anderson, J. Chem. Soc. Faraday Trans. 88

(1992) 1197; (c) P.-A. Carlsson, L. Oesterlund, P. Thormählen, A. Palmqvist, E. Fridell, J. Jansson, M. Skoglundh, J. Catal. 226 (2004) 422.

[4] M.D. Ackermann et al., Phys. Rev. Lett. 95 (25) (2005) 255505. [5] (a) R. Burch, P.K. Loader, Appl. Catal. B 5 (1994) 149; (b) S. Yang, A. Maroto-Valiente, M. Benito-Gonzalez, I.

Rodriguez-Ramos, A. Guerrero-Ruiz, Appl. Catal. B 28 (2000) 223. [6] (a) G. Ertl, P. R. Norton, J. Ruestig, Phys. Rev. Lett. 49 (1982) 177; (b) G. Ertl, Surf. Sci. 287-288 (1993) 1. [7] (a) F.M.F. de Groot, Coord. Chem. Rev. 249 (2005) 31; (b) O.V. Safonova, M. Tromp, J.A. van Bokhoven,

F.M.F. de Groot, J. Evans, P.Glatzel, J. Phys. Chem. B 110 (2006) 16162. [8] J.A. van Bokhoven, C. Louis. J.T. Miller, M. Tromp, O.V. Safonova, P. Glatzel, Angew. Chem. Int. Ed. 45

(2006) 4651. [9] (a) P.T. Fanson, W.N. Delgass, J. Lauterbach, J. Catal. 204 (2001) 35; (b) P.-A. Carlsson, L. Oesterlund, P.

Thormaehlen, A. Palmqvist, E. Fridell, J. Jansson, M, Skoglundh, J. Catal. 226 (2004) 422.


Program Section: 2 Preferred form of presentation: Oral presentation

New Insights into Hydroformylation Kinetics

Mario Basedaa, Klaus-Diether Wieseab, Dieter Hessb, Robert Frankeb, Michael Veitha

aUniversity of Applied Sciences, Gelsenkirchen/Recklinghausen, August-Schmidt-Ring 10, D-45665 Recklinghausen; bEvonik Oxeno GmbH, Paul-Baumann-Str. 1, D-45772 Marl


Hydroformylation is the most notable example of how homogeneous catalysis can be used as a technology. For simple short-chain terminal olefins, especially propylene, ligand-modified rho-dium catalysts represent the state of the art because of their high regio- and chemoselectivity. The hydroformylation of higher olefins and internal and branched olefin mixtures is steadily gaining favor, but problems like the loss of expensive rhodium, the degradation of the expensive ligands, and the unresolved recycling technology of the catalyst systems have made the use of such ole-fins difficult. Up to now, this field has been dominated by modifications to the old cobalt high-pressure process [1].

Another problem is the complex kinetics of ligand-modified rhodium catalysts. The reaction sys-tem is complex, because isomerisation and terminal and internal hydroformylation have to be considered. The ligand effects are unclear, too. Few researchers have done any work in this area. This work, however, aims to provide a deeper insight to the kinetic problem by applying Oper-ando techniques under technical hydroformylation conditions.


Hydroformylation experiments where performed in a pressure vessel equipped with an IR- and Raman cell and an automated probing system for GC-analysis of the products. The ligand for rhodium was a bulky monophosphite. The following olefins were used alone and in a mixture as a model for more complex technical mixtures:

3,3-dimethylbut-1-ene as a terminal olefin without the problem of isomerisation

cyclooctene as a model for internal double bonds

With Raman, it is possible to follow the gas uptake of carbon monoxide and hydrogen and their respective concentrations. So instead of assumptions about mass transfer, the real concentrations in the course of the reaction are accessible.

Prior to the experiments, DFT calculations where performed to obtain the infrared spectra of pos-sible catalyst complexes. To extract these from the complete spectrum, we used the method of “band target entropy minimization” BTEM developed by Marc Garland [2].



Three major rhodium species were observable, which are in equilibrium: Rh6(CO)12, the free complex H-Rh(CO)3(Ligand) with an equatorially bound ligand, and the corresponding acyl com-plex of 3,3-dimethylbut-1-ene. The concentrations were changed systematically as the hydrofor-mylation proceeded.

At the beginning of the reaction, a quick build up of the acyl complex can be observed, and the conversion switches to a zero order. At the end of the reaction, the acyl complex disappears, and the rate of reaction turns back to a first order law. If 3,3-dimethylbut-1-ene is mixed with cylooc-tene, the reaction of 3,3-dimethylbut-1-ene proceeds undisturbed, whereas the reaction of cyclooctene is completely inhibited. Only when the terminal olefin is consumed and the acyl complex has disappeared, does the reaction of cyclooctene start with the same kinetic behavior as in the absence of the terminal olefin.

The observations suggest that the acyl complex of a terminal olefin is a resting state that con-sumes a significant part of the rhodium. Mathematical evaluation results in models of the Eley-Rideal type, with a strong inhibition of the reaction of both olefins by the terminal olefin. We tested the model with PRESTO®, using the complete set of experimental data for mixtures of 3,3-dimethylbut-1-ene with cyclooctene, including the temperature dependence. The fit was excel-lent, and the model even describes the changes of the catalyst species very well.

Conclusions

Through in-situ-spectroscopy and new deconvolution methods (BTEM) of the observable catalyst species in hydroformylation, and in combination with the kinetics of the reaction, it was possible to develop kinetics with a mechanistic foundation. This leads to a better understanding of the complex hydroformylation kinetics of technical olefin mixtures. Moreover, the methodology should be applicable to other technical important homogeneous catalyzed reactions and be able to provide a rational explanation of reaction mechanisms.

References [1] Wiese, K.-D.; Obst, D.: Hydroformylation. Topics in Organometallic Chemistry, 18 (ed. M. Beller); Springer-

Verlag Berlin Heidelberg 2006; 1–33.

[2] Krummel, Karl I.; Garland, M.: Advances in mechanistic studies of phosphine modified hydroformylation : Detection and infrared characterization of phosphine modified rhodium acyl. Abstracts of Papers, 231st ACS National Meeting, Atlanta, GA, United States (2006)


Program Section: 2 or 4 Preferred form of presentation: Oral

HP-NMR Investigations on Rhodium Catalysts for Hydroformylation

Detlef Selenta, Wolfgang Baumanna, Armin Börnera, Klaus-Diether Wieseb

aLeibniz-Institut für Katalyse e. V. an der Universität Rostock, Albert-Einstein-Straße 29a, 18059 Rostock; bEvonik Oxeno GmbH, Paul-Baumann-Str. 1, 45772 Marl

e-mail: [email protected]


Rhodium-catalyzed hydroformylation of olefins affording aldehydes, is one of the most important homogeneously catalyzed reactions. It is used for large scale production as well as for the synthesis of fine chemicals. Both fields at present are characterized by contrary demands for the formation of n- and iso-products, respectively. With the help of the appropriate modifying ligand applied the desired linear or branched enantiomeric aldehydes are formed predominantly. Also for n-regioselective reactions, ligands which possess elements of chirality have been employed. Here, the performance of diastereomeric catalysts is hardly to be predicted, though they are expected to influence differently the outcome of the hydroformylation reaction. Widely accepted concepts developed for the description of regioselectivity rely on the combination of structural information on the catalytically active hydride rhodium complexes obtained from spectroscopic in situ measurements, X-Ray data as well as quantum chemical calculations together with the effects observed for selected substrates. Thus, for example, the often cited P-Rh-P bite angle approach is valuable to describe the influence of the coordination geometry around the rhodium center on the regioselective hydroformylation of terminal olefins, if rigid xanthene based diphosphines are used as ligands.1 Herein we will give evidence, that the application of such concepts to more flexible biaryl based diphosphites can lead to an misinterpretation of the structure-reactivity relationship.

Results and Dissussion

Three diastereomic diphosphites derived from 2,2-dihydroxybinaphthol were prepared from enantiopure building blocks. The selenium adducts of the (R,R,R)-, (S,R,S)- as well as the newly prepared (S,S,R)-isomer by their 1J(31P-77Se) coupling constants of 1483…1485 Hz point toward a similar electron donor potential of these ligands.2 HP-NMR spectroscopic investigations under syngas pressure as well as semi-empirical calculations performed for diastereomeric complexes [HRh(diphosphite)(CO)2] verified a preferred bis-equatorial coordination of the bidentate ligand, with P-Rh-P angles approaching 120° for all complexes. In mutual accordance to these findings and results in literature obtained for other ligands, 1-octene hydroformylation resulted in high regioselectivities of 96% n-nonanal. Interestingely, hydroformylation of internal octenes, and an detailed HP-NMR investigation of precatalyst to catalyst transformation in contrast gave very individual results. Aldehyde selectivities between 63…86% were observed, together with an almost complete inhibition of reaction


Program Section: 2 or 4 Preferred form of presentation: Oral

with the (S,S,R)-isomer or racemic ligand mixture applied. HP-NMR, which was performed in an NMR flow cell allowing full control of gas saturation and CO/ H2 partial pressures,3

confirmed a distinct catalyst formation pathway for each diastereomer.

The results show, that the structure-selectivity correlation found for diphosphite based rhodium catalysts seems to exist for one selected substrate only, and has to be verified. Some important data found for the diastereomeric ligands, and hydride rhodium catalysts, respectively are comparable and fit to the similar regioselectivities observed in hydroformylation. However, the intrinsically different character of the catalysts is hidden and becomes obvious when the substrate is changed by a double bond shift to an internal olefin. Our HP NMR methodology was proved to allow for a detailed and controlled study of catalyst preformation under conditions representative for an advanced lab scale reactor. Details on the respective, now commercialized equipment are also given in our contribution.

Conclusions

Obviously, the established methodology of catalyst characterization seems not to be sufficient to predict the reaction outcome when the substrate is changed slightly. For example, the subtle steric and energetic differences to be expected for conformers of corresponding diastereomeric catalytic intermediates will influence the individual population as well as reactivity. These differences are not nessecarily represented by analytically accessible resting state structures as are hydride complexes, but are important for catalysis. Therefore the geometric or quantum chemical comparison of catalysts should be extended by other geometric concepts, as for example is the cone angle profile,4 and always accompanied by an detailed kinetic investigation.

References

[1] L. A. Van der Veen, P. H. Keeven, G. C. Schoemaker, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. van Leeuwen, M. Lutz, A. L. Spek, Organometallics 2000, 19, 872. [2] D. Selent, W. Baumann, K.-D. Wiese, A. Börner, Chem. Commun. 2008, 6203. [3] D. Selent, W. Baumann, A. Börner, DE 103 33 143 A1 (2005).[4] J. M. Smith, B. C. Taverner, N. J. Coville, J. Organomet. Chem. 1997, 530, 131.

Figure 1. Flow cell and gas recirculating device used for HP NMR investigations

Rh

H

CO

CO

P

P

O

O

O O

OOO

H

CO / H2*

*

*

Scheme 1. Hydroformylation with diphosphite modified rhodium(I) catalysts


Program Section: New applications in liquid-phase processes including homogeneous catalysis and catalyst synthesis Preferred form of presentation: oral presentation

Kinetic and mechanistic investigations on rhodium complexes using operando UV/Vis-spectroscopy

Christian Fischer, Torsten Beweries, Angelika Preetz, Hans-Joachim Drexler,

Uwe Rosenthal*, Detlef Heller*

Leibniz-Institut für Katalyse an der Universität Rostock e.V., Albert-Einstein Strasse 29a, 18059 Rostock, [email protected]


Fibreoptical optrodes afford the possibility to apply UV/Vis spectroscopy for the determination of rate and equilibrium constants under anaerobic conditions. Implementation of this method in investigations on ligand exchange processes with rhodium complexes will be presented.


Application of cationic rhodium complexes containing a diolefin as precatalyst can cause distinc-tive induction periods in the asymmetric hydrogenation of prochiral olefins. Such induction peri-ods, which lead to a decreased activity, are caused by the fact that parts of the catalyst concentra-tion are blocked for the actually interesting hydrogenation of the prochiral olefin. That is, these parts are inactive due to coordination of the diolefin. Therefore, we developed a method applying UV/Vis spectroscopy for the quantitative determination of rate constants in the stoichiometric hydrogenation of rhodium diolefin complexes in polar, coordinating solvents, preferably metha-nol, yielding solvate complexes.[1]

In contrast, the corresponding hydrogenation of the diolefin in apolar, aprotic solvents, such as dichloromethane leads to arene bridged dimeric species. These complexes are analogue to [Rh(DPPE)]2

2+ firstly described by Halpern et al.[2] Recently, we isolated similar complexes with the ligands DIPAMP and BINAP and characterized them by X-ray (Fig. 1, left).[3] After explora-tory and kinetic investigations in our group we concluded that such dimeric species are key com-pounds in different applications, e. g. in the reductive C-C bond formation discovered by Krische.[4]

Fig. 1: X-ray structures of [Rh(DIPAMP)]22+ (left), [Rh(Et-DuPHOS)(mesithylene)]+ (centre)

and [Rh(Me-DuPHOS)(benzene)]+ (right).


Program Section: New applications in liquid-phase processes including homogeneous catalysis and catalyst synthesis Preferred form of presentation: oral presentation

The performance of hydrogenations in aromatic solvents e. g. benzene, can lead to a decreased activity induced by the formation of 6 arene complexes (Fig.1, centre, right). The deactivaton results owing to the higher stability of the arene complex compared to substrate complexes.[5]

To the best of our knowledge, in the literature only one publication with a few stability constants for rhodium complexes with the achiral ligand DPPE exists. As mentioned above, stability con-stants are the key to the understanding of reactivity. Thus, we were interested in the quantitative description of the corresponding equilibrium as a function of ligand, temperature and solvent. De-termination of these desired rate and stability constants has been carried out in situ and data were collected by UV/Vis spectroscopy (Fig.2).

360 370 380 390 400 410 420 430 440 450

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Fig. 2: UV/Vis spectra for the reaction of [Rh(DIPAMP)]2(BF4)2 (0.0002829 M) with a high ex-cess of MeOH; data range 360-450 nm; cycle time 60 sec; 25.0.°C

Conclusions

With the kinetic and mechanistic studies and calculations we have a methodology at hand that affords stability and rate constants concerning the systems described above.

References [1] A. Preetz, H.-J. Drexler, C. Fischer, Z. Dai, A. Börner, W. Baumann, A. Spannenberg, R. Thede, D. Heller,

Chem. Eur. J. 2008, 14, 1445-1451. [2] J. Halpern, D. P. Riley, A. S. C. Chan, J. S. Pluth, J. Am. Chem. Soc. 1977, 99, 8055-8057. [3] A. Preetz, W. Baumann, C. Fischer, H.-J. Drexler, T. Schmidt, D. Heller, Organometallics,2008 submitted. [4] a) H.-Y. Jang, R. R. Huddleston, M. J. Krische, J. Am. Chem. Soc. 2002, 21, 15156-15157. b) H.-Y. Jang, M. J.

Krische, Acc. Chem. Res. 2004, 37, 653-661. [5] D. Heller, H.-J. Drexler, A. Spannenberg, B. Heller, J. You, W. Baumann, Angew. Chem. 2002, 114, 814-817.

Angew. Chem. Int. Ed. 2002, 41, 777-780.



The Catalytic Binuclear Elimination Reaction: Confirmation from In-situ FTIR studies of Homogeneous Rhodium Catalyzed Hydroformylation.

Chuanzhao Li, Li Chen, Effendi Widjaja, Marc Garland

Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore, 627833


In the early 1960’s, Heck and Breslow proposed the possible catalytic formation of organic prod-ucts from the reaction of two mononuclear intermediates [1]. Subsequently, studies were con-ducted on stoichiometric binuclear eliminations [2] and attempts were made to confirm the exis-tence of catalytic binuclear elimination [3]. In the past few years, we have conducted in-situ FTIR spectroscopic studies on both rhodium-manganese and rhodium-rhenium systems which exhibit pronounced synergistic effects in alkene hydroformylation. The combination of in-situ FTIR, signal processing, multivariate analysis and modelling have conclusively shown that the attack of HM(CO)5 [M = Mn, Re] on RCORh(CO)4 is the primary contribution to aldehyde formation in these systems [4].

In the present contribution, the search for catalytic binuclear elimination is reviewed, and the detailed findings for both the rhodium-manganese and rhodium-rhenium hydroformylation sys-tems are discussed.


The experimental system consisted of a high pressure autoclave, a high pressure hermetically sealed membrane pump and a high pressure infrared cell, all connected in a recycle configuration. In a typical experiment, the initial solution consisted of n-hexane, Rh4(CO)12, and alkene under CO. Then either (1) n-hexane, HMn(CO)5 and H2 or (2) n-hexane, HRe(CO)5 and H2 were rap-idly added. Circa 300 FTIR spectra for each catalytic batch reaction were collected. Numerous batch reactions were performed, with varying initial concentration of solutes (namely Rh4(CO)12,CO, H2, alkene, HMn(CO)5 or HRe(CO)5).

The catalytic system consisting initially of n-hexane, Rh4(CO)12, cyclopentene, CO, H2,and HRe(CO)5 at circa 290 K is illustrated below. Figure 1a shows the pronounced synergism observed with increasing HRe(CO)5. At circa a 1:1 molar ratio of Rh:Re, the rate of aldehyde formation has increased by a factor of circa 10. Figure 1b shows the corresponding time-dependent concentrations for the solutes from one of the experimental runs. It is seen that the precursor Rh4(CO)12 is rapidly converted to RCORh(CO)4. In addition, a new dinuclear species RhRe(CO)9 is also rapidly formed. Only a very brief induction period is seen in the formation of the organic product (aldehyde).



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Fig 1a (left) shows the strong synergism observed with increasing initial HRe(CO)5. Fig 1b (right) shows the time-dependent concentration of the organometallic species and aldehyde for a single run.

Thousands of spectra were measured. Extensive signal processing and multivariate analy-sis allowed detailed kinetic modelling of (1) the interconversion of the observable organometallic species and (2) the two terms responsible for aldhyde formation. Fig 2 shows the proposed cata-lytic mechanisms for the rhodium-manganese and rhodium-rhenium systems. These diagrams show the simultaneous existence of both mononuclear and dinuclear intermediates. The bimol-ecular reaction of mononuclear intermediates is the largest contribution to aldehyde formation.

Fig 2. The proposed catalytic binuclear reaction systems for rhodium-manganese hydroformylation (Fig 2a) and rho-dium-manganese hydroformylation (Fig 2b).

Conclusions

In-situ FTIR measurements allowed detailed modelling of rhodium-manganese and rho-dium-rhenium systems and lead to the confirmation of catalytic binuclear elimination [5].

References [1] Breslow, D. S.; Heck, R. F. Chem. Ind. (London) 1960, 467. [2] Kovacs, I.; Ungvary, F.; Marko, L. Organometallics 1986, 5, 209. [3] Mirbach, M. F. J. Organomet. Chem. 1984, 265 (2), 205. [4] a) C. Li, E. Widjaja, M. Garland, J. Am. Chem. Soc. 2003, 125, 5540; b) C. Li, E. Widjaja, M. Garland, Organometallics 2004, 23, 4131; c) C. Li, L. Chen, M. Garland, J. Am. Chem. Soc. 2007, 129, 13327. d) C. Li, L. Chen, M. Garland Adv. Synth. Catal. 2008, 350, 679. [5] Catalysis A to Z, 4th edition, 2009.


Program Section: 1 Preferred form of presentation: oral presentation

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Specific insight of active sites by operando resonance Raman spectroscopy :the case of new catalysts efficient for selective oxidation of isobutane

Stéphane Loridant, Quyen Huynh, Jean-Marc M. Millet

Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR5256 CNRS-Université Claude Bernard Lyon 1, 2 av. Einstein, F-69626 Villeurbanne Cedex, France


The main purpose of this study was to use operando resonance Raman spectroscopy to character-ize polyoxometallate catalysts during selective oxidation of isobutane. Resonance Raman spec-troscopy enables to enhance Raman bands of molecules or crystals showing an electronic transi-tion in the UV or visible range by adjusting the energy of the laser exciting line with the energy of this transition. It allows observation of diluted scatterers at the surface of materials. New cata-lysts constituted of Keggin-type phosphomolybdic anions with H+, Cs2+, Te4+ and (VO)2+ as counter-cations have been recently developed for selective oxidation of isobutane [1,2]. The characterization by various techniques of these catalysts which are both stable and efficient showed that they exhibited the structure of the cesium heteropolysalts with Te4+ and (VO)2+ pre-sumably capping the anions. Operando resonance Raman spectroscopy has been used to confirm these results and deeper characterize the new catalysts.


UV-Vis spectra of these catalysts after reaction showed two bands at 680 and 850 nm assigned to Mo5+-Mo6+ charge transfers. The existence of these transitions was used to get resonance Raman spec-tra. As expected, an impressive change of spectra was obtained varying the laser exciting line (Fig.1). Then, an exciting line at 647 nm was systemati-cally used. The enhanced bands are close to those observed for reduced bi-capped (PMo14O42) anions confirming that Te4+ and (VO)2+ are present as capping cations. Furthermore, a comparison of the relative intensity of bands due to these chromopho-res in various samples has shown that it increases roughly with the yield in methacrylic acid and hence, we propose that the reduced capped anions are the active sites.

Fig.1 : Raman spectra of a catalyst achieved after re-action with exciting lines at 458, 514, 647 et 785 nm.



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Feed without H2O276°C

Feed without H2O373°C

Operando experiments were achieved by coupling resonance Raman spectroscopy and catalytic measurements in a home-made cell. The characterization of samples with and without vanadium under various gas feeds has demonstrated the efficiency of the methodology.

For instance, the bands due to capped anions were already detected just after preparation for vana-dium-containing samples using resonant exciting line (Fig.2). Interestingly, they vanished when the temperature was raised up to 360°C under air but reappeared when it was decreased indicating a change of the electronic state of capped anions. On the contrary, the bands were observed under He and gas feed whatever the investigated tem-perature. Furthermore, their relative intensities increased with time on stream when the catalysts were activated at 370°C suggesting that capped anions are stabilized by the gas feed. A band at 343cm-1 was particularly intense when the cata-lysts were activated around 370°C under feed without water vapour. This band has been as-signed to reduced but not capped Keggin ani-ons. This suggests that water plays a role in keep-ing capped the anions maintaining a high activity. Finally, it should be noticed that no bands corre-sponding to species formed when Keggin anions are decomposed were observed [3]. This corre-lates with the high stability of catalytic properties observed for these new catalysts.

Fig.2: Resonance Raman spectra of a vanadium-containing catalyst achieved at various temperatures under air and He and during operando experiments under i-C4H10/O2/H2O/He:27/13.5/10/49.5 and i-C4H10/O2/He:27/13.5/59.5 feeds.

Conclusions

Resonance Raman spectroscopy has shown to be very powerful to characterize active sites of bulk oxidation catalysts with low surface areas that are hardly detectable with other techniques. The operando methodology applied to this technique has helped to understand the influence of parameters such as the composition of the gas feed on the structure and properties of catalysts.

References [1] Q. Huynh, Y. Schuurman, P. Delichere, S. Loridant, J.M.M. Millet, J. Catal., submitted. [2] Q. Huynh, P. Lacorre, J.M.M. Millet, French Patent 0601284 (2006). [3] G. Mestl, T. Ilkenhans, D. Spielbauer, M. Dieterle, O. Timpe, J. Kröhnert, F. Jentoft, H. Knözinger, R.

Schögl, Appl. Catal. A 210 (2001) 13.



In situ control of the crystal growth of γγγγ-Bi2MoO6 catalyst in the presence of cobalt by combination of XRD, XAS and Raman spectroscopy

Chanapa Kongmarka, Vladimir Martisb, Annick Rubbensa, Caroline Pirovanoa, Axel Löfberga, Gopinathan Sankarb, Rose-Noëlle Vanniera, Elisabeth Bordes-Richarda

aUnité de Catalyse et de Chimie du Solide, UMR CNRS 8181, Université des Sciences et Technologies de Lille, ENSCL, BP 90108, 59652 Villeneuve d’Ascq Cedex, France

bDavy Faraday Research Laboratory, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK

Introduction

The activity/selectivity of an oxidic catalyst may be strongly affected by its crystal morphology. Controlled microstructures are therefore preferred in catalytic applications, which requires a

careful control of the synthesis conditions. γ-Bi2MoO6 is known to be a good catalyst in the selective oxidation or ammoxidation of hydrocarbons [1, 2], and its properties are strongly enhanced in the presence of cobalt, nickel, iron in the so-called multicomponent molybdates. The

formation of γ-Bi2MoO6 particles in the presence or not of Co was studied by combined in-situ X-ray diffraction, XANES and time-resolved Raman spectroscopy. The experiments were carried out on the SNBL line at the European Synchrotron Radiation Facilities in Grenoble. A specially designed in-situ hydrothermal cell was used (Fig. 1).

Fig. 1. Schematic diagram of the combined XAS/XRD and Raman scattering set-up used at station SNBL of the ESRF.

γ-Bi2MoO6 was prepared by hydrothermal route [3] from aqueous solution of (NH4)6Mo7O24·4H2O in ammonia which was slowly added into a solution of Bi2O3 dissolved in HNO3 (BiMo). A solution of (NH4)6Mo7O24·4H2O with cobalt nitrate (CoMo) was also prepared and mixed with the BiMo solution to get the final stoichiometry: Bi2MoO6/CoMoO4 = 70/30 and 90/10. The pH was adjusted to 6 before loading of the mixture in the hydrothermal synthesis cell



for 20 h. X-ray diffraction patterns (λ = 0.5 Ǻ) and XANES spectra at the Mo Kedge (19.9 to 20.5 keV) were collected sequentially, every 6 and 7 minutes, respectively. Raman spectra were

collected every 132 sec (λ = 532 nm). The study was carried out at 160, 170 and 180°C.

Results and discussion

Whatever the used technique, two steps in the crystal growth of Bi2MoO6 were clearly evidenced. XANES at the Mo edge as well as Raman spectroscopy showed that the oxygen environment of molybdenum is tetrahedral at first. After 30 minutes of experiment at 180°C, it

becomes octahedral as in Bi2MoO6. During the first minutes, XRD revealed the formation of β-

Bi2O3 (fluorite structure) which disappears while γ-Bi2MoO6 is formed The structural model proposed by Theobald et al. [4] was used for the data refinement. An anisotropic size broadening was observed, which was modeled with linear combinations of spherical harmonics [5]. This model revealed that, at first, crystallites of 5-nm-thick platelets of Bi2MoO6, with a 15-nm-long latin cross shape are formed, and that they progressively grow to a diamond shape, the size of which remains smaller than 50 nm after 3 hours of experiment at 160°C. The presence of cobalt

is responsible for variations of temperature of both allotropic γ-γ”-γ’transitions and melting of γ-

Bi2MoO6 (TGA/DTA). Similar modifications were also observed in the case of V2O5/TiO2 [6] or NiMoO4/MoO3 [7]. Moreover, Co also modifies the shape of Bi2MoO6 crystallites.

Conclusion

The combination of in-situ XRD, XANES and time-resolved Raman spectroscopy has proven

to be a powerful tool to understand the growth of γ-Bi2MoO6 particles in the presence or not of

cobalt molybdate. From XRD the evolution with time of the shape of γ-Bi2MoO6 nanocrystallites

was evidenced. The crystal growth rate increased at increasing temperature. Such experiments may afford the possibility for preparing catalysts with controlled microstructure as well as provide help to understand synergetic effects in multicomponent molybdates.

The Alliance program (Franco-British Partnership program) for financial support and ESRF for synchrotron radiation beam time are gratefully acknowledged.

References

[1] R.K. Grasselli, J.D.Burrington, Adv. Catal. 30 (1981) 133. [2] G. Sankar, M.A. Roberts, J.M. Thomas, G.U. Kulkarni, N. Rangavital, C.N.R. Rao, J. Solid State Chem. 119 (1995) 210. [3] A.M. Beale, G. Sankar, Chem. Mater. 15 (2003) 146. [4] F.R. Theobald, A. Laarif, A.W. Hewat, Ferroelectrics 56 (1984) 219. [5] M. Jarvinen, J. Appl. C. (1993) 527. [6] P. Courtine, E. Bordes, Appl. Catal. A: Gen., 157 (1997) 45. [7] O. Lezla, E. Bordes, P. Courtine and G. Hecquet, J. Catal., 170 (1997) 346.



Differences in nitrite reduction mechanism over Pt/Al2O3 and Pd/Al2O3 asfound by ATR-IR spectroscopyB.L. Mojet*, S.D. Ebbesen, L. Lefferts

Catalytic Processes and Materials, Faculty of Science and Technology, MESA+, University ofTwente, P.O. box 217, 7500 AE, Enschede, The Netherlands,

*[email protected]


Groundwater pollution by nitrite and nitrate is a widespread problem which has risen inthe recent years throughout the world. Since high nitrite and nitrate concentrations in drinkingwater are dangerous to human health, technologies for their removal have to be developed andimplemented. It is an important and developing area in environmental research. The mostpromising technique for nitrate and nitrite removal is the catalytic denitrification by noble metalcatalysts [1]. Selectivity towards nitrogen is a key point in order to minimize the formation ofammonia, which is also toxic for humans, but deep insight in the mechanism is lacking .

Supported noble metal catalysts possess activity towards nitrite removal, where platinumand palladium were found to be the most active and selective towards nitrogen. It has been shownthat the catalytic reduction of nitrate proceeds via nitrite as an intermediate. In addition, adsorbedNO was suggested to be the key intermediate on the noble metal surface [2], but so far no NOads

was reported. As a first step in unraveling the nitrate hydrogenation mechanism, we investigatedthe adsorption and hydrogenation of nitrite over Pt/Al2O3 and Pd/Al2O3 by Attenuated TotalReflection Infrared Spectroscopy (ATR-IR).


Thin catalyst layers of either Pt/Al2O3 or Pd/Al2O3 were immobilized on ZnSe InternalReflection Elements (IRE). After calcination and reduction, the catalyst film was ready to beused. Catalyst details are given in Table 1. Attenuated Total Reflection Infrared (ATR-IR) spectrawere recorded with a Tensor FTIR spectrometer (Bruker) at room temperature using a home builtstainless steel flow through ATR-cell [3].

Table 1. Catalyst detailsCatalyst Metal content Metal dispersion (%) Layer thickness (μm)

Pd/Al2O3 4.96 wt% Pd 0.45a 5.0 ± 0.5

Pt/Al2O3 5.0 wt% Pt 0.75b 3.5 ± 0.25a Pd dispersion was determined by CO chemisorption assuming CO:Pd = 1.b Pt dispersion was determined by H2 chemisorption assuming H:Pt = 1.



During flow of NO2-(aq) over hydrogen covered H-Pt/Al2O3, infrared bands evolved as shown in

Figure 1. A variety of peaks develops as function of time as a result of reaction of NO2-(aq) with

the hydrogen covered platinum surface. The adsorption and conversion of nitrite results in thehydrogenation products NH4

+, "HNO"(ads) (i.e. a surface species containing NO and al least one

H-atom), N2O and-11580 cm

(ads)NO , and adsorbed NOx- species. Subsequent hydrogenation resulted in

ammonia and an additional adsorbed NO-species at 1620 cm-1.Remarkably, similar experiments overPd/Al2O3 showed that adsorption and re-duction of nitrite results in different sur-face intermediates compared to Pt/Al2O3.While on Pt/Al2O3, "HNO"(ads) is detectedin the reaction sequence to ammonia, onPd/Al2O3, only NH2(ads) was observed.These observations indicate that differentsteps in the formation of ammonia are ratelimiting over platinum and palladium.Differences and similarities between ad-sorption and hydrogenation on both cata-lysts will be discussed and an optimizedreaction scheme will be presented.

ConclusionIt is shown that in-situ ATR-spectroscopy is a valuable tool to identify adsorbed species duringthe hydrogenation of nitrite, which were not observed before. Clearly, the nitrite mechanism iscomplex and involves a number of pathways, some of them connected via adsorbed species, andleading to ammonia and N2O. The study clearly revealed that although the reaction pathways ofon platinum and palladium seem rather similar, the rate determining steps are definitely different,resulting in different reaction selectivity.

References[1] Kapoor, A., Viraraghavan, T., and Viraraghavan, T. J. Environ. Eng. 123, 371 (1997).[2] Wärnå, J., Turunen, I., Salmi, T., Maunula, T. Chem. Eng. Sci. 49, 5763 (1994).[3] Ebbesen S.D.; Mojet, B.L.; Lefferts, L., Langmuir 22, 1079 (2006).

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Figure 1. Water corrected ATR-IR spectra while flowing asolution of NO2

-(aq) (4.3·10-4 mol/L) over H-Pt/Al2O3.


Program Section: Preferred form of presentation: ORAL

Catching the red-ox state of Cu sites isolated in a nanoporous P4VP polymeric

matrix during gas phase reactions

E. Groppo*, M. J. Uddin, S. Bordiga, C. Lamberti, G. Spoto and A. Zecchina

Department of Inorganic, Physical and Materials Chemistry, NIS Centre of Excellence and

INSTM Centro di Riferimento, University of Turin, Via P. Giuria 7, I-10125 Torino, I


In the recent years metal-containing polymers are becoming very attractive owing to theirpossible applications as catalysts for organic synthesis.[1] The development of easy methods toimmobilize organometallic species into macroporous polymeric matrices, where a permanentopen texture is obtained by 20-40% cross-linking levels, represents a new frontier in the field,because it will allow their application to catalytic reactions also in the absence of a swellablesolvent.[2] An example of this class of systems are the nitrogen-containing polymers, likepoly(vynilpyridine) (PVP), which play important roles as basic catalysts,[3] and are extensivelyused to generate metal complexes with transition metals.[4] PVP has been used as a good supportfor immobilization of CuCl2

[5] in the liquid phase oxidative carbonylation of methanol todimethylcarbonate (DMC).[6] The adoption of a high cross-linked PVP system will allow toperform the same reaction in the gas phase that, to the best of our knowledge, has never beeninvestigated.

In this work CuCl2 has been immobilized inside a commercial poly(4-vinlypyridine) (P4VP)matrix, 25% cross-linked with divinylbenzene. The aim of the work is threefold: (i) to understandthe local structure and coordination geometry of Cu sites isolated inside the nanovoids of thismatrix; (ii) to investigate the red-ox activity of the Cu sites upon contact with reagents from thegas phase; (iii) to follow the red-ox behaviour of Cu sites during reaction with ethylene in the gasphase, in operando conditions. To this aim several in-situ spectroscopic techniques (FTIR, UV-Vis DRS, XANES and EXAFS) are adopted.


We show that the inclusion of CuCl2 inside P4VP nanoporous polymer strongly perturbs thevibrational properties of the matrix related with the pyridyl groups,[7] demonstrating that the Pyunits act as preferential anchoring sites for Cu2+.[8] The Cu2+ cations, even if characterized by asterically encumbering coordination sphere, are able to adsorb NO. UV-Vis and XANESspectroscopies (Figure 1a) reveal that, upon treatment in H2 at 180°C, Cu2+ cations are reduced toCu+ species; EXAFS analysis (Figure 1b) demonstrates that, on average, the Cu+ sites are linkedto only one Py ring and one Cl ion.[7] The so obtained Cu+ species are able to reversibly adsorb


Program Section: Preferred form of presentation: ORAL

CO (Figure 1c) and are converted again into Cu2+ upon exposure to O2.[7] A similar red-ox

behaviour is observed during reaction with ethylene in the gas phase, as followed by DRIFT inoperando conditions.

1 2 3 4

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Figure1. a) Effect of degassing at room temperature (blue) and progressive reduction in H2 at 450 K (green andorange) of the CuCl2/P4VP sample, as monitored by XANES (main part) and UV/Vis (inset) spectroscopy; b) k3-weighted, phase-uncorrected FT of the EXAFS spectra, for both modulus (main part) and imaginary parts (inset). c)CO adsorption at room temperature on the Cu+ sites at increasing coverage (Pmax=50 Torr, black), as monitored byFTIR spectroscopy.

Conclusions

From the complete knowledge of the structure of the Cu sites in all steps of the process, itemerges that the flexibility of the entire polymeric structure is the key factor in the reversibilityof the red-ox process. In the absence of a swellable solvent, the accessibility of the active sites bya reactant in the gas phase is not straightforward, and a highly cross-linked polymeric matrix isnecessary for a reagent to reach the Cu sites in the gas phase.

References[1] Leadbeater, N. E. et al. Chem. Rev. 2002, 102, 3217.[2] a) Hodge, P. Chem. Soc. Rev. 1997, 26, 417; b) Barbaro, P. Chem.-Eur. J. 2006, 12, 5666.[3] a) Acar, N. et al. Eur. Polym. J. 2001, 37, 1599-1605; b) Yarapathi, R. V.; Kurva, S.; Tammishetti, S. Catal.

Commun. 2004, 5, 511-513.[4] a) McCurdie,et al. Polymer 1999, 40, 2889; b) Belfiore, L. A. et al. Polymer 2001, 42, 9995; c) Wu, K. H. et al.

Polym. Degrad. Stabil. 2003, 79, 195; d) Pardey, A. J. et al. Polyhedron 2005, 24, 511-519.[5] Takahashi, I., Kojima, H., JP Patent 08325204, 1996.[6] Rivetti, F., Romano, U., EP Patent 534545, 1993.[7] Groppo, E., et al. J. Phys. Chem. C, 2008, 112, 19493–19500.[8] Groppo, E., Uddin, M. J., Bordiga, S., Zecchina, A., Lamberti, C. Angew. Chem.-Int.Ed. 2008, 47, 9269.


Program Section: 4 (New reactor cells and coupling techniques) Preferred form of presentation: Oral

Caught in the Act: The Self-Assembly of Nanoporous Aluminophosphates Ob-served Using a Multi-technique Operando Approach

Andrew M. Bealea, Matthew G. O’Briena, David S. Wraggb, Russell E. Morrisb,

Bert M. Weckhuysena

aInorganic Chemistry and Catalysis, Debye Institute for NanoMaterials Science, Utrecht Univer-

sity, Sorbonnelaan 16, Utrecht, CA 3584, The Netherlands

bSchool of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, UK


Understanding the factors that influence how porous crystalline materials such as zeolites andaluminophosphates are formed from a precursor gel under hydrothermal conditions, is of consid-erable fundamental interest since it could lead to a more rational approach towards their designand synthesis. A very powerful but still relatively unexplored way of probing these heterogene-ous processes is to perform real-time operando studies during crystallisation under real hydro-thermal conditions. [1] However the application of one or two techniques to study crystallisationmeans that the characterisation is often partial and unable to yield a full understanding of theformation behaviour. Therefore to overcome these limitations, it is often beneficial to performmultiple operando measurements using a variety of analytical techniques and, where possible, tointelligently combine complementary techniques into one experimental setup; an approach whichoften yields new insight. [2, 3]

Ideally such a combination(s) should be able to provide information at various levels of structuralcomplexity ranging from the atomic level up to the macroscale, if a truly detailed insight is to beobtained. Furthermore, these techniques should focus beyond the assembly of just the porousframework formation and should therefore probe the role of both the template in creating the po-rosity and also to understand the influence of the solvent. For this very purpose, we have recentlycoupled the technique of Raman Spectroscopy to our recently developed bespoke combination ofX-ray Absorption Spectroscopy (XAFS), Small Angle X-ray Scattering (SAXS) and Wide AngleX-ray Scattering (WAXS) to obtain more insight into the formation of Zn2+ and Co2+ substitutedAlPO-34 frameworks. Crucially, since these data are obtained from one experiment, much moredetailed mechanistic insight than has previously been possible could be obtained.


Shown in Figure 1 is a schematic of the combined mutitechnique setup complete with the time-resolved data collected during the crystallisation of ZnAPO-34 at 70 °C. Analysis of this ‘one-shot’ data revealed that the following changes took place (starting from the outside dataset in the


Program Section: 4 (New reactor cells and coupling techniques) Preferred form of presentation: Oral

top right hand side): a change in the intensities of the peaks in the Raman spectra at 664 and 674cm-1 corresponding to two conformer states of the TEAOH template, [4] the appearance of a fea-ture at ca. 9.677 keV in the Zn K-edge XANES, indicative of structural ordering around Zn2+, thegrowth of two features in the SAXS data around Q = 0.01 and 0.04 Å respectively due to two dif-ferent nanoparticle populations and finally in the WAXS stack data plot (top left hand side) theappearance of Bragg peaks at 12.27 ° (110), 13.88 ° (-111), 15.44 ° (111), 17.69 ° (-120/-210),26.10 ° (-311/130/310) typical for ZnAPO-34, could be observed. Information regarding the keystages of crystallization can be gleaned from cross-correlating these observations. For example,SAXS suggested that the amorphous signal in the WAXS data was caused by the presence ofnanoparticulate AlPO gel particles approximately ca. 32 nm in size. XANES revealed that thesenanoparticles contained tetrahedrally coordinated Zn2+ species (i.e. in the coordination state ofthe final crystalline material). Furthermore the appearance of Bragg peaks in the WAXS coin-cided with the evolution of a second feature in the SAXS (ca. 14-16 nm), a change in the Ramanpeak intensities and finally, the formation of a crystalline environment around Zn2+ (XANES).

Conclusions

These results allow us to propose that theformation of ZnAPO-34 from a reactant gelin the presence of TEAOH can be brokendown into 3 general stages: Stage 1, the for-mation of an initial amorphous nanoparticu-late containing gel, Stage 2; the growth un-der hydrothermal conditions of thesenanoparticles until Stage 3: the crystallisa-tion of the final ZnAPO-34 material. How-ever, it was interesting to note that immedi-ately after the appearance of Bragg peaks/crystalline nanoparticles for the final ZnAPO-34 mate-rial, an apparent ‘pause’ in the crystallisation process occurred. At the same time changes in theRaman and Zn K-edge XANES data continued during this time, hinting at some sort of importantrearrangement/equilibration process before crystallisation proceeds. Similar behaviour was alsoobserved during the crystallization of CoAPO-34. In summary, these time-resolved experimentsrepresent the most comprehensive combined operando study of the critical stages leading to theformation of a nanoporous material to date.

References[1] Francis, R. J., O’Hare, D. J. Chem. Soc. Dalton. Trans. 3133, 1998.[2] Beale, A. M., & Weckhuysen, B. M. et al. J. Am. Chem. Soc. 128, 12386, 2006.[3] Tinnemans, S. J. & Weckhuysen, B. M. et al. Catal. Today 113, 3, 2006.[4] O’Brien, M. G. & Weckhuysen, B. M. et al. J. Am. Chem. Soc. 128, 11744, 2006.

Figure 1. Setup schematic complete with an illustration of the data re-corded during ZnAPO-34 crystallisation.



Multidimensional insight into the synthesis of molybdate catalyst precursors by linking simultaneous in situ WAXS/SAXS/Raman

with Raman/ATR/UV-vis spectroscopy

Ursula Bentrupa, Jörg Radnika, Jork Leitererb, Franziska Emmerlingb, Angelika Brücknera

aLeibniz-Institut für Katalyse e.V. an der Universität Rostock, Außenstelle Berlin, Richard-Willstätter-Str. 12, D-12489 Berlin, Germany, bBAM, Bundesanstalt für Materialforschung

und -prüfung, Richard-Willstätter-Str. 11, D-12489 Berlin, Germany


During synthesis of mixed oxide catalyst precursors different parameters, such as synthesis method, nature of used components, and synthesis conditions play an important role. However, the influence of these variable parameters is in many cases not fully understood and often not systematically investigated. For a deeper understanding in which manner different synthesis pa-rameters affect the structure and crystallinity of the precursors and, consequently, the structure and performance of the final catalysts, sophisticated methods for the on-line monitoring of the synthesis process are necessary. Against this background a new setup at the μ-spot beamline at BESSY (Berliner Elektronenspeicherring Gesellschaft für Synchrotronstahlung) was installed which allows simultaneous wide- and small-angle X-ray scattering (WAXS/SAXS) and Raman spectroscopic measurements. In parallel simultaneous Raman/ATR/UV-vis experiments were performed in a simple laboratory setup to check the potential of this arrangement regarding com-prehensive information about structural changes. For checking the comparability of these two setups Raman spectroscopy was used as linker. In this way we have studied the precipitation of certain phases in the liquid phase synthesis of molybdate catalyst precursors containing Ni, Bi, Fe, and P. The aim was to elucidate structural changes of complex anions during the precipitation process on molecular scale together with changes of nanoscopic properties of respective precipitate such as agglomeration and crystalliza-tion.


In a first example the precipitation of Ni-molybdate by adding a Ni-nitrate solution to a solution of ammonium heptamolybdate (AHM) was monitored at 40°C. While the precipitate is formed immediately, only after 40 min a crystalline phase indicated by the development of sharp Bragg reflexes of the known Ni-molybdate phase (NH4)4H6NiMo6O24·5H2O could be observed by WAXS/SAXS measurements. In accordance with these results the respective bands in Raman and ATR spectra become more structured and intensive with progressive reaction time which is obviously caused by the increasing crystallinity of the precipitate. The UV-vis spectra were measured in reflection mode. In this way, the continuous formation and increasing amount of the



precipitate can be followed characterized by the shift of the absorption edge of the Mo(VI) Ocharge transfer band to lower wavelength. WAXS/SAXS measurements in the more complex system AHM/Ni,Fe,Bi/HNO3 have shown that a crystalline precipitate is immediately formed indicated by the respective Bragg reflexes (I in Fig. 1). The precipitate alters after addition of H3PO4 (II in Fig. 1), loses its crystallinity and partly dissolves, respectively, at higher temperature (III in Fig. 1). This process can also be fol-lowed by the continuous shift of the absorption edge to higher wavelength in the UV-vis spectra. Changing band positions of molybdate and phosphate species were observed in the ATR and Raman (Fig. 2) spectra pointing to an increasing condensation degree of the molybdate ions. This is favoured by the incorporation of phosphate forming Keggin-type [PMo12O40]3- anions with specific band positions. Fitting of the scattering curves at small angles (III in Fig. 1) using differ-ent models based on Keggin anions the formation of which was deduced from Raman spectra suggests the formation of aggregated Keggin ions in a cylinder-like structure occurring mainly in the liquid phase.

Fig. 1. X-ray scattering curves collected during Fig. 2. Raman spectra of the synthesis solution synthesis process in the system AHM/Ni,Fe,Bi/HNO3. obtained after respective reaction steps.

Conclusions

The combination of WAXS/SAXS/Raman with Raman/ATR/UV-vis spectroscopy allows the si-multaneous investigation of solution and precipitates during the preparation of mixed oxide pre-cursors in liquid phase under realistic synthesis conditions, realtime and in situ. The combined evaluation of Raman and WAXS/SAXS data allows to discriminate between different molybdate species appearing in solution and precipitate, respectively. Moreover, these molybdate species could be assigned to separate phases of different crystallinity. The implementation of UV-vis and ATR spectroscopy into the SAXS/WAXS/Raman setup, which is planned for the future, would be a suitable completion concerning the monitoring of precipitate formation.


Program Section: 3 Preferred form of presentation: poster

In situ monitoring of the drying process during the preparation of complex Mo-based oxides in levitated droplets with synchrotron X-ray scattering

J. Radnika, U. Armbrustera, U. Bentrupa, , J. Leitererb, F. Emmerlingb. A. Brücknera

aLeibniz-Institut für Katalyse e.V. an der Universität Rostock, Außenstelle Berlin, Richard-Willstätter- Str. 12, 12489 Berlin, Germany, bBAM, Bundesanstalt für Materialforschung

und -prüfung, Richard-Willstätter-Str. 11, 12489 Berlin, Germany


In the last years, complex Mo-based oxides gain more and more interest because of their potential for the selective oxidation of alkanes, but the complexity of the synthesis of such oxides hinder their industrial application. Numerous steps in the process, like the precipitation, the drying and conditioning and, as last step, the calcination, could influence the performance of the final cata-lysts [1]. To optimise the synthesis of such complex oxides, particularly in terms of a better and reproducible performance of the catalysts, a better insight into these processes during the synthe-sis is necessary. This aim can only be achieved with suitable in situ methods allowing a look into the processes during the preparation. Only few investigations of the influence of the drying methods on the catalytic performance were known. For MoVTeNb oxides a significant influence of the drying method on the catalytic per-formance were reported [2], but further information about the early stages and the crystallisation during the drying is still missing. In this contribution, the early stages of crystallisation of such complex oxide precursors were for the first time monitored during drying of the slurry by syn-chrotron X-ray scattering. The use of an acoustic levitator allows to avoid any influences from solid container walls on the droplet of the slurry (Fig. 1). These results were compared with the drying of a droplet positioned on polyethylene foil.

Fig. 1. Experimental setup at the μ-spot beamline at BESSY integrates the acoustic levitator for X-ray scattering (SAXS and WAXS) measurements. The levitated sample is monitored and remote controlled during the whole experiment.


Program Section: 3 Preferred form of presentation: poster


A solution of Bi(NO3)3/HNO3 was added to a Ni(NO3)2 / Fe(NO3)3 solution. This mixed nitrate solution was added to an ammonium heptamolybdate (AHM) solution under continuous stirring for 60 min at ambient temperature. Concentrated H3PO4 was added, stirred for 30 min at ambient temperature and further 60 min at 50°C. One droplet of the formed slurry with a volume of ap-proximately 4 μl was directly hand-injected into the ultrasonic levitator and irradiated by a mono-chromatic X-ray beam with a diameter of 100 μm and an energy of 11.94 keV. The same experiment was repeated on a droplet deposited on a PE foil. Fig. 2 shows the recorded scattering curves of the slurry in the levitated droplet and in the droplet on the PE film. Both series of scattering curves are similar but the crystallisation starts significant faster on the PE foil. An estimation of the crystallite size using Scherrer equation shows a con-tinuous increase of the crystallites from 40 to 80 nm for the levitated droplet, whereas for the droplet on the film only crystallites between 80 and 90 nm could be found. In both cases shifts of the Bragg reflexes indicate a crystallisation of the slurry beginning at the water-air interface and moving to the inner part of the droplets. The Bragg reflexes of both samples correspond to the cubic compound like (NH4)3[PMo12O40]·xH2O (PDF 43-315) with Keggin structure.

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eFig. 2 Scattering curves of the slurry in a levitated droplet collected every 20 s (left side) and in a droplet positioned on PE foil collected every 80 s (right side).

Conclusions

A new experimental approach using an ultrasonic levitator and synchrotron X-ray scattering al-lows a detailed insight into the crystallisation of complex Mo-based oxide precursors during the drying process. The comparison between the drying in a droplet levitating in the ultrasonic trap and positioned on PE foil shows, that the crystallite growth is influenced by the different drying method, but not the structure. An integration of other methods like Raman spectroscopy into this setup is possible and promising.

References[1] F. Schüth, Chem.Ing.Tech. 2005, 77, 1399-1416. [2] M. M. Lin, Appl. Catal. A 2003, 250, 287-303.


Program Section: 4 New reactor cells and coupling techniques Preferred form of presentation: oral

Shining new light on nucleation and growth processes of nanoparticles during chemical synthesis via coupled in situ SAXS/XANES

Jörg Polte a, Friedmar Delißen a, Torsten T. Ahner b, Sergey Sokolov b, Ralph Kraehnert b,Martin Radtke a, Uwe Reinholz a, Heinrich Riesemeier a, Franziska Emmerling a,

Andreas F. Thünemann a, Ulrich Panne a

a BAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Str. 11, 12489 Berlin, Germany

b LIKAT - Leibniz Institute for Catalysis, Richard-Willstätter-Str. 12, 12489 Berlin, Germany

Nanoparticles of noble metals are nowadays among the most studied nanoscale material due tonumerous prospective applications in areas such as medicine, biotechnology and catalysis 1, 2.Gold nanoparticles, as one of the most common materials, can be prepared via various synthesisroutes including chemical, sonochemical or photochemical paths, where the usual chemical routeis precipitation from a dissolved metal precursor solution by a reducing agent3, 4 . In general forcontrolling the synthesis, i.e. size and shape of metal nanoparticles, a detailed understanding ofthe mechanism and kinetics of precursor reduction and particle growth is essential. As in the case of the frequently studied gold nanoparticle synthesis via citrate method, a coherent mechanisticexplanation for the evolution of particles has not been provided yet for many systems.

A decisive factor for this limited mechanistic understanding rests in the fact that data have oftenbeen obtained by ex-situ methods requiring sample preparation techniques (SEM, TEM, XRD) 5,

6, or via indirect characterization techniques. In the first case, sample preparation can induce dry-ing artifacts, and a time resolution under 1 min is hard to achieve, whereas in the second ap-proach the size of the particles is deduced indirectly from size-dependent optical properties (UV). A quantitative interpretation of such UV spectra is complicated, since the precursors peak widthand positions shifts with pH, and the particle plasmon resonance is also affected by size andnumber of particles, particle shape, ligand shell and small interparticle distances. Thus, directlyderived in-situ information on the chemical and morphological state of reactants and particles isdesirable.

In the present contribution we report on a new method to determine in parallel the oxidation state of the dissolved and particulate metals as well as the size, number and polydispersity of formedparticles analysing the original colloid without further sample preparation in-situ or operando.The method relies on time-resolved XANES and SAXS measurements carried out on sampledroplets exposed to synchrotron radiation.

For analysis, liquid droplet samples were held in position by an acoustic levitator. Applying thiscontact-free method, solid and liquid samples can be positioned in a gaseous environment bymeans of a stationary ultrasonic field. Evaporation of the solvent during the experiment was pre-


Program Section: 4 New reactor cells and coupling techniques Preferred form of presentation: oral

vented by ‚feeding’ the suspended droplet with solvent injected via piezo syringes. A sketch ofthe setup is given in the Figure below.

To illustrate the capabilities of the developed analytical device the method-coupling was appliedto the monitor the chemical preparation of gold colloids. Supplemented by the conventional tech-niques of in-situ UV-Vis and SEM, the evolution of gold nanoparticles during chemical reduction and particle formation was explored using either Pluronic or sodium citrate as reducing agent.From the data, a mechanistic scheme is developed explaining the different phases of particle for-mation and growth, thus providing a basis for improved control over the synthesis process.

References

1. Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T., Synthesis of monodisperse spherical nanocrystals. Angewandte Chemie-International Edition 2007, 46, (25), 4630-4660.

2. Daniel, M. C.; Astruc, D., Gold nanoparticles: Assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. ChemicalReviews 2004, 104, (1), 293-346.

3. Zhang, J. L.; Du, J. M.; Han, B. X.; Liu, Z. M.; Jiang, T.; Zhang, Z. F., Sonochemical formation of single-crystalline gold nanobelts. Angewandte Chemie-International Edition 2006, 45, (7), 1116-1119.

4. Sakai, T.; Alexandridis, P., Mechanism of gold metal ion reduction, nanoparticle growth and size control in aqueous amphiphilic block copolymer solutions at ambient conditions. Journal of Physical Chemistry B 2005, 109, (16), 7766-7777.

5. Pong, B. K.; Elim, H. I.; Chong, J. X.; Ji, W.; Trout, B. L.; Lee, J. Y., New insights on the nanoparticle growth mechanism in the citrate reduction of Gold(III) salt: Formation of the au nanowire intermediate and its nonlinear optical properties. Journal of Physical Chemistry C 2007,111, (17), 6281-6287.

6. Kimling, J.; Maier, M.; Okenve, B.; Kotaidis, V.; Ballot, H.; Plech, A., Turkevich method for gold nanoparticle synthesis revisited. Journal of Physical Chemistry B 2006, 110, (32), 15700-15707.



Fig. 1 : Structure of Re/K03 catalyst

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TiO2-supported oxorhenate catalysts:

A multi-techniques operando study

Elise BERRIERa, Xavier SECORDELa, Sylvain CRISTOLa, Mickaël CAPRONa, Anne-Sophie

MAMEDEa, Christophe DUJARDINa, Edmond PAYENa, Angelika BRÜCKNERb

a Unité de Catalyse et de Chimie du Solide, Université des Sciences et Technologies de Lille;

Bât C3 Cité Scientifique 59655 Villeneuve d’Ascq Cedex, France

bLeibnitz institute for Catalysis at the University of Rostock, branch Berlin;

Richard-Willstaetter-Str. 12 12489 Berlin, Germany

[email protected]

Introduction and Objectives The unique performance of anatase TiO2-supported rhenium oxide catalysts in methanol conver-sion to dimethoxymethane (DMM) has been emphasized by Iwasawa and coworkers in 2000 [1]. This peculiarity is believed to be due to the redox capability of Re oxides leading to selective me-thanol oxidation to formaldehyde, coupled with an appropriate acidity allowing the acetalization of formaldehyde with methanol to form DMM [1], [2]. The present study aims at elucidating the structure-reactivity relationship by comparing 2 supports and 2 synthesis pathways. We have in-vestigated a series of characteristics like the oxidation state and the reducibility of the active phase, the mono- or polymeric structure of the rhenium oxide, the nature of intermediates and the role played by the support. In an attempt to solve these questions, a large spectrum of in situ and operando techniques were called together to achieve an as complete as possible depiction of the catalytic system. Consequently, we have thoughtfully drawn premises on operando visible Ra-man and FTIR results, completed with in situ UV-visible absorption, quasi-in situ X-Photoelectron spectroscopies and MET images. The first catalyst preparation is the incipient wet-ness impregnation of anatase supports provided by Sachtleben, namely HOMBIKAT K03 (SBET=93 m².g-1) and F01 (SBET=254 m².g-1), by a solution of perrhenic acid. A subsequent calci-nation in in neutral atmosphere (N2) was applied. The other strategy has consisted in finely grind-ing metallic Re with the above-mentioned support materials followed by a calcination step in pure oxygen to obtain a white powder embedding oxidized rhenium oxide.


The structures of the activated catalysts were investigated by in situ Raman spectroscopy and the spectra of fresh and acti-vated K03-supported catalyst achieved by the grind-ing/calcination method are presented in Fig. 1. The most sur-prising trend of the catalyst’s structure is the observation of broad lines around 700-950 cm-1. These features were as-signed to Re-O stretching modes in Re-O-Ti and Re-O-Re



groups. The observation of these frequencies can positively derive from a signal enhancement due to resonance effects providing part of the rhenium stands as ReVI. The latter assumption is fairly confirmed by the Re 4f7/2 peak energy and intensity in the XPS spectra of catalysts, reveal-ing presence of ReVI reaching 20%. The corresponding catalyst prepared by the impregnation method exhibit similar properties, while small differences are detected when the active phase is supported onto a higher specific area support, F01.

Reduction of the catalyst in CH3OH/He flow gave rise to ReIV as suggested by XPS and UV-visible spectra (Fig. 2) while the Raman intensity is severely lowered, so that the anatase bands are almost completely vanished. This is in good agreement with formation of colour centres due to ReIV. In addition, we have underlined the close linkage be-tween rhenium reducibility and selectivity in methylal. Concerning the F01 supported catalyst, a coke layer has been observed. The removal of such an organic film by oxygen flow was not efficient in recovering the initial

structure and activity of the material. More precisely, the reducing step was found to durably de-activate the acid function of the active phase.

Upon realistic reaction conditions (MeOH/He/O2), the properties of Re/K03 catalysts were shown to be hardly modified, indicating a good degree of re-versibility and accounting for the high conversion and selectivity (>80%) in methylal with regards to the reaction. The DRIFT spectra recorded upon re-duction and reaction feed have suggested the metha-nol is essentially adsorbed onto rhenium sites when O2 is allowed in the feed, whereas the support also took part to the methanol adsorption upon CH3OH/He flow.

Conclusions

The combination of techniques we present here appears to be most relevant in understanding the reduction of supported oxide materials. Our results suggest the active redox couple in methanol conversion to methylal is ReVI-ReIV and the electronic energy levels in TiO2 are of major influ-ence in Re-O-Ti linkage strength, and hence, on the catalyst’s activity.

References

[1] Y. Yuan, T. Shido and Y. Iwasawa, Chem. Commun., 2000, 1421-1422 [2] J. M. Tatibouët, Appl. Catal. A: Gen., 148 (1997), 213

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Fig. 2 : visible absorption upon various reducing treatments



AGIR: new tool combining

Analysis by Gravimetry and IR spectroscopy

A. ALENDA, P. BAZIN, F. THIBAULT-STARZYK

Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 Bd Maréchal

Juin, F-14050 Caen

Introduction

Infrared spectroscopy is a technique allowing the characterisation of the surface of materials under vacuum (in situ analysis) and/or the determination of the nature and evolution of surface species on a solid sample (catalyst), while at the same time reproducing real working conditions of the catalyst (operando analysis under gas flow). Thermogravimetric analysis (TGA) allows monitoring weight changes in the sample. AGIR is a new tool designed in the Laboratoire Catalyse et Spectrochimie combining these two approaches, both the qualitative of IR and the quantitative measurement of TGA.


Figure 1 shows the prototype design. We modified a SETARAM SETSYS microbalance to follow mass changes (μg accuracy) so as to combine it with a catalytic reactor equipped with infrared windows. The new system has been first used to measure molar extinction coefficients by two different methods.

AGIR in situ. With in situ analysis, under vacuum, we have measured the molar absorption coefficients of the ν(CN) vibration band in various chlorinated acetonitrile molecules (deuterated, mono-, di- and trichloro-acetonitrile) adsorbed on a MHI zeolite (Si/Al=10.75). The substitution of one hydrogen/deuterium atom by a chlorine atom strongly modifies the intensity of the infrared response.

AGIR Operando. With operando analysis (under flow), we have measured molar absorption coefficients for δ(H2O) and δ(NH4

+) at 1640 and 1540 cm-1 for water and ammonia adsorbed on a HY zeolite with low Si/Al ratio. We have shown the influence of coverage on the infrared response of adsorbed species in zeolites. Adsorption sites are changing with coverage, and bands are shifted and their shape and intensity are modified. Other interesting facts were observed: water modifies strongly the aspect of the δ(NH4

+) vibration band in ammoniated zeolites, without changing the absorption coefficient.



Conclusions

Measuring the sample mass together while at the same time recording its infrared spectrum showed us the key importance of conditions under which the measurement is done. The presence of co-adsorbed species (water in particular) strongly modifies the spectrum of surface species. Under reaction condition, this new technique will be especially important for a correct assignment of infrared features and catalytic behaviour. This work will be continued with the analysis of various materials, in the field of catalysis as well in other parts of material science, where gravimetry is a traditional tool that has never been before combined with in situ infrared.

Acknowledgements

SETARAM is deeply acknowledged for their help with the gravimetric measurement, and the design of the sample holder. The authors are indebted to SOMINEX for their help in the final design and machining of the reactor-cell.

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Program Section: 4. New reactor cells and coupling techniques. Oral presentation.

Identification of hydrocarbon deposits present on methane reforming catalysts.

Ian Silverwood1, Neil G. Hamilton1, R. Mark Ormerod2, Hayley Parker2, John Staniforth2, Stew-art F. Parker3, Chris Frost3 and David Lennon1*.

1. Department of Chemistry, Joseph Black Building, University of Glasgow, Glasgow, G12 8QQ, UK. 2. School of Physical & Geographical Sciences, Keele University, Staffs, ST5 5BG, UK. 3. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK.


With oil reserves dwindling and becoming increasingly difficult to access, methane from natural gas offers an alternative both as a fuel and a chemical feedstock. Methane is relatively cheap and abundant. This presentation concentrates on the dry reforming process, where CH4 is combined

with CO2 over supported nickel catalysts to produce synthesis gas (CO + H2):

CH4 + CO2 → 2CO + 2H2 (1)

Catalyst deactivation for this reaction is a major problem, with carbon laydown leading to a pro-gressive decrease in activity as a function of time. This work seeks to use primarily vibrational spectroscopy in a variety of guises to identify the nature of the hydrocarbonaceous layer present over catalysts active for the ‘dry’ reforming of methane. In addition to using a combination of infrared spectroscopy and Raman spectroscopy to interrogate these materials, inelastic neutron scattering (INS) is also employed to characterize the nature of the overlayers that form during reaction and which favour the syngas production process. To this end, novel INS cells have been constructed that can be directly coupled to a gas manifold/mass spectrometer arrangement that facilitates acquisition of the INS spectra of the working catalysts. Comparisons of catalytic per-formance between conventional micro-reactor studies and the INS reaction studies show excel-lent correspondence despite a scale-up factor of ca. 1000!

Fig.1 Fig. 2.

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Program Section: 4. New reactor cells and coupling techniques. Oral presentation.


INS experiments were performed at the ISIS facility at the Rutherford Appleton Laboratory, U.K.

Approximately 20 g of various Ni/α-Al2O3 catalysts were loaded in to INS cells and operated un-der dry reforming conditions for approximately 6 hours prior to acquisition of the INS spectra. Mass spectrometric analysis of the product stream established comparable reaction profiles to those encountered in parallel micro-reactor studies (catalyst charge ca. 20 mg). Figure 1 shows

the temperature programmed oxidation profile for the reference Ni/α-Al2O3 catalyst, which al-most exactly matches that seen with the micro-reactor.

The infrared spectra for the standard Ni/α-Al2O3 catalyst and a gold modified analogue post-reaction are presented in Figure 2. The spectra are similar in both cases presenting strong fea-tures at 3400 and 2900 cm-1, which are respectively assigned to support hydroxyl groups and C-H stretching modes for hydrocarbon fragments bound to the metal surface. The latter indicates the presence of aliphatic and aromatic entities.

Figure 3 (see figure left) shows the INS spectrum for the Ni/Al2O3 catalyst and shows similar features to the infrared spec-trum. However, whilst exhibiting inferior resolution to the optical spectroscopic meas-urement (Figure 2), the INS spectrum con-veys the distinct advantage of not being hin-dered by complex coverage dependent opto-electrical cross sections, enabling quantita-tive measurements to be undertaken. Thus, the surface hydroxyl group concentration is estimated to be 600 ppm, whilst the unre-solved C-H stretch corresponds to a hydro-carbonaceous overlayer of only 200 ppm.

These surface concentrations are rationalised within mass balance measurements, which collec-tively guide the development of a reaction mechanism for this economically important reaction.

Conclusions

This work will describe how new INS catalyst cells recently designed and constructed at the cen-tral facility can be used to obtain the vibrational fingerprint of the working catalyst. Crucially, this provides a semi-quantitative estimation of relevant surface moieties, the concentrations of which can then be used to guide mechanism development strategies.

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Program Section: New reactor cells and coupling techniques Preferred form of presentation: Poster

Operando Measurements of Nanoparticle Monolayers by Non-Linear Optical Spectroscopy

Robert M Riouxa, Sasha Kweskinb, Katie Bratlieb, Kyriakos Komvopoulosc, Gabor A Somorjaib

aDepartment of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, bDepartment of Chemistry, University of California and Lawrence Berkeley National

Laboratory, Berkeley, CA 94720, cDepartment of Mechanical Engineering, University of Califor-nia, Berkeley, CA 94720


We demonstrate the operando application of a non-linear optical spectroscopy technique to the study of catalytic materials. The technique, sum frequency generation (SFG) surface vibrational spectroscopy is surface-specific, and therefore ideally suited for the study of adsorbates under high pressure conditions of working catalysts. SFG has been used extensively to study surface chemistry relevant to catalysis on extended metal single crystal surfaces for greater than a dec-ade1. While SFG has been successful in determining the identity of adsorbates and their absolute concentration under high-pressure reaction conditions, these are in-situ studies rather than oper-ando studies due to the nature of the catalytic surface. In this paper, we utilize total internal re-flection (TIR)-SFG to study a two-dimensional monolayer-based nanoparticle catalyst2,3,4 during high-pressure catalysis.


SFG is a nonlinear optical spectroscopy technique based on the simultaneous absorption of two incident photons of different wavelength with the generation of a third photon having a frequency at the sum of the two incident photons. The technique is inherently surface specific; SFG photons are generated where a lack of inversion symmetry exists. Clean and adsorbate-covered interfaces are inherently SFG active. The z-dimension of extended

single crystal surface is greater than the wavelength of incident light, and SFG is only created at the exposed interface; however, for nanoparticles whose size in the z-dimension is smaller than the wavelength of light can lead to a cancellation of the SFG signal due to destructive interference between the two sides of the nanoparticle. For monolayers of Pt nanoparticles, this limitation is overcome with SFG measured in a total internal reflection geome-

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Figure 1. SFG intensity versus time of the oxidation of CO on a Pt nanoparticle mo-nolayer at 300 K.


Program Section: New reactor cells and coupling techniques Preferred form of presentation: Poster

try2. The first demonstration of TIR-SFG from a monolayer of Pt NPs was the adsorption and oxidation of carbon monoxide (Figure 1)2. During the low-temperature oxidation of carbon monoxide, the surface population of CO was followed by SFG, and the accumulation of carbon dioxide by gas chromatography (GC). We have also demonstrated the application of SFG-GC as an operando technique for the hydrogenation of pyridine over catalysts composed of monolayer Pt nanoparticles supported on oxidized silicon. It is proposed that a reaction intermediate, pyridinium cation (C5H5NH+) is observed by TIR-SFG under atmospheric pressure conditions3.

In the last example, we demonstrate that the adsorption of C2H4 can be followed by SFG. On Pt nanoparticle cubes, ethylidyne, a com-mon intermediate of ethylene surface chemis-try that requires 3-fold sites for its formation is found on the surface of the cubes (the ideal surface of a cube contains only 4-fold sites) (Figure 2)4.

The operando-based SFG analysis demon-strates the surface of the Pt cube either recon-structs to the pseudo-hexagonal surface or is covered with a true (111) oriented Pt overlayer. It cannot be confirmed whether the adsorption of ethylene induced reconstruction or if the surface of the cube is reconstructed before the adsorp-tion of ethylene. The application of TIR-SFG is an emerging field in operando studies in cataly-sis, and should have a significant impact on both gas and liquid phase operando studies.

Conclusions

The application of a nonlinear optical spectroscopy for the adsorption of small molecules (carbon monoxide and ethylene), the oxidation of carbon monoxide and the hydrogenation of pyridine on nanoparticle monolayers has been demonstrated. This work demonstrates that nonlinear spectro-scopic techniques can be employed in operando measurements of catalytic behaviour -- and when successfully implemented -- has many advantages over operando measurements based on linear spectroscopic techniques.

References [1] G. A. Somorjai, G. Rupprechter, J. Phys. Chem. B 103 (1999) 1623-1638. [2] S. J. Kweskin, R. M. Rioux, S. E. Habas, K. Komvopoulos, P. Yang, G. A. Somorjai, J. Phys. Chem. B 110

(2006) 15920-15925. [3] K. M. Bratlie, K. Komvopoulos, G. A. Somorjai. J. Phys. Chem. C 112 (2008) 11865-11868 [4] R. M. Rioux, S. J. Kweskin, K. R. McCrea, S. E. Habas, K. Komvopoulos, P. Yang, G. A. Somorjai. Submitted

to J. Am. Chem. Soc. (2008).

Figure 2. Model of Pt cubic nanoparticle based on operando SFG observations of high pressure ethylene adsorption.



A new reaction cell for the FTIR operando study

of monolith-supported catalysts.

F.C. MEUNIER, P. BAZIN, V. BLASIN-AUBE, O. MARIE, M. DATURIa Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 Bd Maréchal

Juin, F-14050 Caen


Operando spectroscopy has become an essential tool in the investigation of catalytic reactions [1]. The philosophy of operando techniques is to carry out the spectroscopic study using condi-tions as close as possible to those of the actual application [2]. The vast majority of the investiga-tions are usually carried out on model catalysts, facilitating data analysis. However, an importantquestion arise: are the conclusions obtained by such studies relevant to the behaviour of the actualcommercial material? Obviously the answer may vary on a case by case basis, even when the sci-entists took care working on the actual active phase of the catalyst. As a matter of fact, it is well known that catalyst modifications can result from a restructuring or contamination of the active phase induced by binders or solvents used for shaping, slip casting, wash-coating and the subse-quent thermal treatments [3,4]. For these reasons we have decided to undertake the challenge of studying a catalyst in the form of a wash-coated monolith, which may represent the ultimate chal-lenge in the complete understanding of a real catalytic system at work.

The NOx Storage Reduction (NSR) technique was used here as a model reaction, because this technology has been studied by our laboratory for over a decade and therefore represents an ideal starting point to evaluate more complex monolith-based work. In addition, improved understand-ing of NSR technology is needed to treat Diesel and lean-burn engine exhaust gases [5,6]. Dieseland lean-burn engines offer valuable fuel saving properties and will therefore help abating CO2

emissions. While numerous studies have been published dealing with the use and characterisationof NSR materials (including operando studies), there is currently no spectroscopic data investi-gating the behaviour of NSR materials present on commercial monoliths, which are typically used a catalyst carrier in real applications.

Studying a real shaped NSR material is crucial to understand structure-activity relationship be-cause the structure of the deposited material is known to be affected by the deposition process, as discussed above. In addition, the properties of the washcoat obtained (i.e. thin layer of active ma-terial and binders at the surface of the monolith channels) are typically different from those of the active phase powder. This is furthermore important in the present case, as the adsorption of NOx may also occur on part of the binder normally though as of being unimportant, thereby modifying



the overall NOx storage and release dynamics of the system. We have now carried out experi-ments in our laboratory with a monolith (6 cm long, 400 CPSI) located in a newly custom-madecell placed in a FTIR spectrometer fitted with a traditional IR source and MCT detector. This study is actually the first IR investigation, to our knowledge, of a monolith core through the main axis (surface analysis of the external monolith surface have been reported).


The typical conditions of operation of NSR materials were simulated by alternating a lean feed (500 ppm of NOx + 8 % O2, held for 2 min) followed by a rich feed (2 % CO + 1 % H2 held for 20 s), in the presence of large concentrations of water and CO2 and traces of hydrocarbons (as typically found in real exhaust) inside a custom-made operando cell reactor. The results were compared to data collected over wafers of the active phase and crushed monoliths, bearing in mind that crushing and pressing wafers may alter the structure of catalysts and may potentially account for differences [7]. The spectra obtained via the two techniques appeared to be somewhat similar. The interaction mode of the IR beam is thought to be a combination of transmission (rays passing through the very edges of the washcoat surface), reflection (rays being reflected by metal particles and large crystallite faces at the washcoat surface) and diffusion (rays being reflectedand transmitted once or more through the washcoat).

Conclusions

A reactor for the investigation of monolithic cores was built and tested for NOx storage-reductionprocesses. While signals related to the various materials and adsorbates present over the carrier could be recorded, the corresponding signal/noise ratio was low and will require further im-provement of the optics lay-out and/or source power. Additional tests are in progress to assess the possibility of deriving quantitative information from the spectra obtained.

References

1 F. Meunier, M. Daturi, Catal. Today, 113 (2006) 1.2 I.E. Wachs, Catal. Commun., 4 (2003) 567.3 P. Gélin, T. Des Courières, Appl. Catal., 72 (1991) 179.4 N. Kubicek, F. Vaudry, B.H. Chiche, P. Hudec, F. Di Renzo, P. Schulz, F. Fajula, Appl. Catal. A, 175 (1998) 159.5 T. Lesage, C. Verrier, P. Bazin, J. Saussey, S. Malo, C. Hedouin, G. Blanchard, M. Daturi, Topics Catal., 30-31(2004) 31.6 J. P. Breen, C. Rioche, C. Hardacre, R. Burch, F.C. Meunier, Appl. Catal. B, 72 (2007)178.7 B. S. Klose, R. E. Jentoft, A. Hahn, T. Ressler, J. Kröhnert, S. Wrabetz, X. Yang, F. C. Jentoft, J. Catal, 217 (2003) 487.



Combined DFT modelling and operando techniques for better structure and reactivity understanding

Karim HAMRAOUI, Gautier LAVAL, Elise BERRIER, Sylvain CRISTOL,

Jean-François PAUL, Edmond PAYEN

Unité de Catalyse et de Chimie du Solide, Université des Sciences et Technologies de Lille;

Bât C3 Cité Scientifique 59655 Villeneuve d’Ascq Cedex, France

[email protected]


Operando techniques are nowadays considered as essential in looking at the reaction-induced structural changes. However, experimental approaches frequently fail to provide a comprehensive point of view. Combination of several techniques can offer a wider scope, but in most cases, di-rect interpretation remains challenging when not unachievable. The present work aims at showing the benefits of a fully collaborative theory-experimental study of TiO2 supported oxmolybdates catalysts for methanol oxidation to formaldehyde.


Two catalysts were studied: 1wt % MoO3/TiO2 and 5wt%MoO3/TiO2. The νS (Mo=O) vibration gave rise to a band at respectively 950 cm-1 and 970 cm-1 in the Raman spectra of fresh, hydrated catalysts. This shift is in line with presence of monomeric entities in the case of 1wt % MoO3/TiO2 and polymeric entities in the case of 5wt%MoO3/TiO2. At the same time, structure modelling by DFT calculations led to structures which vibration frequencies were found to ex-hibit a similar shift. The complete dehydration of both catalysts was predicted to occur around 250°C and a shift of the νS (Mo=O) vibration to higher frequencies was accordingly calculated in agreement with experimental findings. Methanol was used as a chemical probe for redox activity following two lines: reduction of the catalyst without oxygen or methanol conversion in presence of oxygen, where a catalytic cycle is expected. The Raman spectra were recorded while operating and the products were analysed in real time by a gas chromatograph (GC). The adjunct of 1wt % MoO3 onto the TiO2 surface led to no significant change by comparison to the conversion level of TiO2 and the catalyst was not se-lective. Conversely, the 5wt%MoO3/TiO2 was found to reach high level of conversion and a good selectivity to formaldehyde (>90%). The reduction of the catalyst by a methanol/helium mix was found to lead to very different struc-tural changes according to the Mo loading as presented in Fig. 1. Concerning the 1wt % MoO3/TiO2 catalyst, besides vanishing of the νS (Mo=O) frequency, very tiny changes occur at



moderate temperatures. At 300°C, large bands were detected at 1300 and 1600 cm-1 due to coke formation.

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Conversely, the whole Raman spectrum of 5wt%MoO3/TiO2 is dramatically and rapidly modified. Noticeable is the emergence of broad bands in the frequency range 700-900 cm-1 and the progres-sive loss of anatase spectrum. In addition, the formation of a coke layer at the surface is very fast. Therefore, a He flow was flushed through the catalyst when changing the temperature. We have assigned these changes to an efficient reduction of MoVI to MoV. Jointly, the DFT modelling has concluded the non reducibility of monomeric molybdenum oxides. Reduction of polymeric en-tities is found to be easier yielding MoV as evidenced by the spin density located on one Mo atom presented on figure 2. The good agreement between the proposed model and the experimental allows the calculation of the whole reaction pathway for methanol oxidation to formaldehyde. This confirms that the rate determining is the C-H bond scission. Furthermore, the computed activation energy is reduced by a factor of two when going from a monomeric to a polymeric entity.

Conclusions

The dual theory / operando approach has allowed us to better understand the Raman spectra of reduced and intermediate oxide structures and propose relevant assignments for MoxOy clusters. At the same time, the experimental control in real time allows better level of modelling.



Role of exposed metal sites in adsorptive properties of CPO-27-M (M= Mg, Zn, Co and Ni): combined use of ab-initio modeling and experimental results

C. Lamberti, E. Groppo, L. Valenzano, B. Civalleri, S. Chavan, F. Bonino, J. G. Vitillo, A. Zecchina and S. Bordiga

Department of Inorganic, Physical and Materials Chemistry and NIS Centre of Excellence, Università di Torino, Via P. Giuria 7, 10125 Torino, Italy and INSTM Centro di Riferimento


Metallorganic Frameworks (MOFs, also known as “Coordination Polymers”) are crystalline nanoporous materials comprised of metal containing clusters connected three-dimensionally by poly-functional organic ligands. The ligands act as spacers, creating an open porous three-dimensional structure, with very high pore volume and surface area. This hybrid architecture opens the possibility to design and synthesize a great variety of new porous materials, which are in principle able to display novel functionalities that are potentially exploitable for a number of applications in catalysis, ion-exchange, non linear optics, as sensors, in gas separation and/or storage.1-12 Significant work is ongoing by several groups involving the 2,5-dihydroxyterephalic acid linker. This linker is known to form MCPs based on both the tetraanionic form 2,5-dioxido-1,4-benzenedicarboxylate (DOBDC), where both the aryloxide and carboxylate moieties act as ligands to the metal and the dianionic form 2,5-dihydroxy-1,4-benzene-dicarboxylate (DHBDC), where only the carboxylate moieties act as ligands to the metal and the alcohol remains protonated. By using DOBDC as ligand, CPO-27-M13, also known as MOF-7414, has been synthesized with a large variety of divalent metal ions (M= Mg, Zn, Co and Ni)13,15,16. Each of these materials is composed of MII ions generating linear, infinite-rod secondary building units bound by DOBDC resulting in a hexagonal, 1D pore structure (Figure 1). The pores of the as-synthesized material are lined with solvent molecules, typically H2O or DMF, that complete the coordination sphere of the M(II) ions and are removed upon evacuation to generate coordinately unsaturated metal sites. Very recently it has been published that Mg substituted material has extremely interesting adsorption properties towards CO2,16 while the Ni homologue showed extremely good performances towards H2.15,17 An extensive experimental study has been performed on most of the materials of this series, showing some relevant differences in term of the activities towards the molecules considered (CO, NO, H2, CO2…).15-19 In this contribution the results obtained on Ni, Mg and Co homologues are compared, describing the nature of the different reactivity, also with the contribution of ab initio molecular modelling, performed on the periodic structures, with the CRYSTAL code,20 that already successfully used to simulate MOF-5 properties.21 This approach is able to describe accurately the structural (obtained by XRD and by EXAFS), vibrational (IR and Raman) and energetic (microcalorimetry) features of CPO-27-M



interaction with gaseous molecules such as CO2 and CO. The different behaviour of CPO-27-M materials will be compared with available data coming from literature related to pure oxides, oxidic solid solutions and on zeolites exchanged with the same cations.

Figure 1. Pictorial representation at different magnification grades of a dehydrated CPO-27-M sample. Part a): the coordination sphere of M(II). Part b): bidimensional view of the honeycomb structure. The C atoms are reported in gray, H atoms in white, O in red, and M(II) in green.

O OHs

O OHs O cu O cs

O cs

References [1] Ferey, G. Chem. Mater. 2001, 13, 3084. [2] Stein, A. Adv. Mater. 2003, 15, 763. [3] Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403. [4] Yaghi, O. M.; Davis, C. E.; Li, G. M.; Li, H. L. J. Am. Chem. Soc. 1997, 119, 2861. [5] Yaghi, O. M.; Jernigan, R.; Li, H. L.; Davis, C. E.; Groy, T. L. J. Chem. Soc.-Dalton Trans. 1997, 2383. [6] James, S. L. Chem. Soc. Rev. 2003, 32, 276. [7] Janiak, C. Dalton Trans. 2003, 2781. [8] Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334. [9] Schüth, F.; Sing, K. S. W.; Weitkamp, J., Handbook of porous solids; Wiley-VCH: Weinheim, Germany, 2002; Vol. 2. [10] Bordiga, S.; Lamberti, C.; Ricchiardi, G.; Regli, L.; Bonino, F.; Damin, A.; Lillerud, K. P.; Bjorgen, M.; Zecchina, A. Chem. Commun. 2004, 2300. [11] Bordiga, S.; Vitillo, J. G.; Ricchiardi, G.; Regli, L.; Cocina, D.; Zecchina, A.; Arstad, B.; Bjorgen, M.; Hafizovic, J.; Lillerud, K. P. J. Phys. Chem. B 2005, 109, 18237. [12] Hafizovic, J.; Bjorgen, M.; Olsbye, U.; Dietzel, P. D. C.; Bordiga, S.; Prestipino, C.; Lamberti, C.; Lillerud, K. P. J. Am. Chem. Soc. 2007, 129, 3612. [13] Dietzel, P. D. C.; Morita,Y.; Blom, R.; Fjellvag, H. Angew. Chem., Int. Ed. 2005, 44, 6354; Dietzel, P. D. C.; Panella, B.; Hirscher, M.; Blom, R.; Fjellvag, H. Chem. Commun. 2006, 959. [14] Rosi, N. L.; Kim, J.; Eddaoudi, M.; Chen, B. L.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 1504 [15] Zhou, W.; Wu, H.; Yildirim, T; J. Am. Chem. Soc., 2008, 130, 15268. [16] Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. , J. Am. Chem. Soc., 2008, 130, 10870 [17] Vitillo, J. G.; Regli, L.; Chavan, S.; Ricchiardi, G.; Spoto, G.; Dietzel, P. D. C.; Bordiga, S.; Zecchina, A J. Am. Chem. Soc. 2008, 130, 8387. [18] Bonino, F.; Chavan, S.; Vitillo, J. G.; Groppo, E.; Agostini, G.; Lamberti, C.; Dietzel, P. D. C.; Prestipino, C.; Bordiga, S. Chem. Mater. 2008, 20, 4957; Chavan, S.; Vitillo, J. G.; Groppo, E.; Bonino, F.; Lamberti, C.; Dietzel, P. D. C.; Bordiga, S.; J. Phys. Chem. C 2009, 113, in press. [19] Dietzel, P. D. C.; Johnsen, R. E.; Fjellvag, H.; Bordiga, S.; Groppo, E.; Chavan, S.; Blom, R. Chem. Commun. 2008, 5125. [20] Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; Zicovich-Wilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, Ph.; Llunell, M. CRYSTAL06, 2006, Universita' di Torino, Torino; web-page: http://www.crystal.unito.it [21] Civalleri, B.; Napoli, F.; Noël, Y; Roetti C.; Dovesi, R. CrystEngComm, 2006, 8, 364.

b)

a)


Program Section: Combining operando spectroscopy and theoretical studies Preferred form of presentation:oral

On the mechanism of gas-phase methylation of phenol catalyzed by MgO: a combined computational, in-situ spectroscopic and reactivity approach

Nicola Ballarinia, Fabrizio Cavania, Luca Masellia, Sauro Passeria and Johannes A. Lercherb

aDipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM Università

di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy. INSTM, Research Unit of Bologna; a

partner of NoE Idecat (FP6). bTechnische Universität München, Department Chemie, Lichtenbergstr. 4, D-85747 Garching,

Germany.

Introduction

The ring methylation of phenol to o-cresol is industrially carried out with methanol as alkylating agent over basic catalysts. One major problem of the industrial process is the decomposition of methanol and, consequently, a large excess of methanol is usually fed, in order to reach an ac-ceptable per-pass conversion of phenol. In this context it is surprising that the potential role of methanol surface chemistry and eventual decomposition during the alkylation of phenols is hardly addressed. Bases catalyze dehydrogenation of methanol to formaldehyde that in turn pri-marily alkylates the carbon atoms of the aromatic ring [1]. The further catalytic chemistry seems to depend then on the availability of formaldehyde under reaction conditions. In the present work, we explore the catalysis with MgO as typical catalyst with the aim of establishing the mechanism of catalysis alkylation of aromatic compounds with nucleophilic functionalization.


The catalytic gas-phase methylation of phenol with methanol over MgO yields o-cresol and 2,6-xylenol as the principal reaction products. Tests carried out with variation of temperature and residence time showed that at low temperatures, i.e., below 300°C, the primary products are sali-cylic aldehyde, anisole and o-cresol. With increasing conversion, salicylic aldehyde is reduced to o-cresol. Thus, o-cresol is produced via a direct and an indirect pathway via formation of salicylic aldehyde.

The optimized models for the adsorption of phenol on three different sites of a (MgO)12 cluster representing MgO surface sites were calculated. In order to account for differences in the coordi-nation of Mg and O, the sites for adsorption considered are those corresponding to the corner, step, and terrace positions. For adsorption on terrace sites, the O-H group interacts associatively. Upon adsorption on coordinatively unsaturated sites the OH group dissociates. It should be noted that adsorption of phenol on MgO leads to phenolate species and surface OH groups. The ener-getically most favored adsorption configuration is that one in which the adsorbed molecule


Program Section: Combining operando spectroscopy and theoretical studies Preferred form of presentation:oral

adopts an orthogonal orientation with respect to the MgO surface. This orientation is caused by the repulsion of the aromatic ring by the electron rich surface oxygen. The calculated IR spectra for phenol adsorbed over the different defective sites of MgO were compared with the experi-mental one. The only relevant difference between calculated spectra was the frequency of C-O stretching band at 1300 cm-1 when phenol is adsorbed in the corner position, but at considerably lower values for the step and the terrace positions. The correspondence between the experimentaland the calculated spectrum for the phenol adsorbed on the corner site was very good. Calculated spectra were used for the assignment of bands experimentally recorded in-situ during reactivity tests.

Figure 1 compiles the IR spectra recorded in-situ while feeding methanol and phenol at 250°C over the high-surface-area MgO, after subtraction of the MgO spectrum. The main time depend-ent features of the spectra included (i) the progressive increase of the band at 1444 cm-1 with timebeing attributed to the CH3 bending vibration in o-cresol, (ii) the concomitant decrease of the in-tensity of the aldehydic C-H stretching band between 2700 and 2800 cm-1 and (iii) the asynchro-nous variation of the two components in the band at 1627 cm-1, also attributed to salicylic alde-hyde.

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Figure 1. In-situ spectra recorded at 250°C while feeding methanol and phenol over MgO.

Conclusions

The combination of catalytic tests, IR spectroscopy of adsorbed reactants, products and possible intermediates together with a computational study as well as in situ IR spectra recorded during reaction allows giving a full description of the reaction network in the methylation of phenol. Under conditions favorable for methanol dehydrogenation to formaldehyde, o-cresol is the only reaction product. The reaction between adsorbed phenolate and formaldehyde likely generates salicylic alcohol via hydroxymethylation, which is rapidly dehydrogenated to salicylic aldehyde. The aldehyde is transformed into o-cresol via reduction by formaldehyde.

References[1] Ballarini, N.; Cavani, F.; Maselli, L.; Passeri, S.; Rovinetti, S. J. Catal. 2008, 256, 215


Program Section: Combining operando spectroscopy and theoretical studies Preferred form of presentation: Oral

Resonance Raman spectroscopic and theoretical study of alumina sup-ported vanadium oxide catalyst

Hack-Sung Kima, Stanislaus Zygmuntb, Larry A. Curtissc, and Peter C. Staira

aDepartment of Chemistry, Center for Catalysis and Surface Science and Institute for Cataly-sis and Energy Processes, Northwestern University, Evanston, IL 60208, USA; Chemical Sci-

ences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA, bDepartment of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383, USA,

cMaterials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA aE-mail address of the presenting author: [email protected]


The core bonds in supported metal oxide catalysts that affect the heterogeneous catalytic reac-tions are the M=O, M-O-M, and M-O-S bonds (M= V, Mo, W, etc, S= Al, Si, Ti, etc).1-3 Sup-ported vanadium oxide is the most extensively studied catalyst among supported metal oxide catalysts.4 The M-O vibrational bands are typically weak or undetected by IR or by normal (non-resonance) Raman spectroscopy. Resonance Raman spectroscopic application to the solid oxide catalysts is rare, but can be remarkably useful. With resonance enhancement, the detection sensitivity of Raman spectroscopy increases enormously. For example, intense V-O stretching bands5 can be observed by resonance Raman spectroscopy for the supported VOx

catalysts with low VOx densities under proper resonance conditions. Besides the enhanced fundamental bands, overtone and combination bands can be observed which provide addi-tional information. It is also a powerful tool for assigning the electronic transitions observed in UV-VIS absorption spectra because of the direct link to the specific stretching vibrations that are enhanced.

Moreover, the density functional theory (DFT) calculations for monomeric VOx structures on both the dehydrared and hydrated alumina surface were compared with the Raman spectra for the VOx catalyst with a very low VOx coverage. At the coverage, monomeric VOx appear to predominantly exist on alumina surface. The calculated V=O stretching frequencies for three monomeric structures are in good agreement with the three V=O frequencies observed by Raman spectroscopy. This suggests that the three monomeric structures could exist at the de-hydrated condition.

Here we present a combined experimental and theoretical study and an extensive analysis of resonance Raman spectra for alumina-supported vanadium oxide catalyst.


The Raman spectra for -Al2O3 and vanadia supported on -Al2O3 (Figure 1) make possible the identification of VOx-associated overtones and combinations as well as fundamentals. The


Program Section: Combining operando spectroscopy and theoretical studies Preferred form of presentation: Oral

resonance Raman spectra excited at 220 nm are in general agreement with previously pub-lished spectra5 excited at 244 nm and 488 nm, but the new spectra include additional features such as overtones, combinations, and OH stretching. The Raman spectra in the OH stretching region for supported metal oxides have never been reported, to our knowledge. The UV Ra-man results (Fig. 1) suggest that vanadia binds predominantly to an aluminum site where the terminal OH has been coordinated. From the analysis of the resonance Raman spectral fea-tures, several vibrational parameters including anharmonic constant, bond dissociation en-ergy, and bond length change in the excited state for V=O and V-O were obtained. Compared with the Raman spectra excited at other wavelengths, the resonance Raman spectra excited at 220 nm for dehydrated vanadia samples are characterized by the selective enhancement of V=O stretching bands, which allows us to assign the higher-energy charge transfer band in the UV-VIS spectra to the V=O transition. The assignment is significant because the higher-energy charge transfer band has never been identified.

Figure 1: Resonance Raman spectra obtained at dehydrated conditions for -Al2O3 and X V/nm2 on -Al2O3 (X= 0.16 and 1.2)

Conclusions

This work is an extensive resonance Raman spectroscopic analysis applied to a solid metal oxide catalyst. We present examples of selecting the excitation wavelength for resonance Ra-man spectroscopy of alumina-supported vanadium oxide catalyst: 1) Assignment of UV-VIS absorption bands. 2) Estimation of the V=O bond dissociation energy which could be signifi-cant for catalytic bond-breaking and -making processes. 3) Estimation of anharmonic con-stants. 4) Estimation of V=O and V-O bond length change in the excited electronic state.

References [1] Stencel, J. M. Raman spectroscopy for catalysis; van Nostrand Reinhold: New York, 1990 [2] Busca, G. Journal of Raman Spectroscopy 2002, 33, 348 [3] Wachs, I. E. Catalysis Today 2005, 100, 79 [4] Weckhuysen, B. M.; Keller, D. E. Cataysis Today 2003, 78, 25 [5] Wu, Z. L.; Kim, H. S.; Stair, P. C.; Rugmini, S.; Jackson, S. D. Journal of Physical Chemistry B 2005, 109,2793.


Program Section: 5 Preferred form of presentation: ORAL

Potential of the in situ FTIR for analysis of the oscillating system: Solving the riddles of non-steady state behavior of N2O decomposition over Fe/Pt-FER

Z. Sobalika, K. Jíšaa, A. Vondrováa, D. Kauckýa, B. Bernauer b

aJ. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague 8, Czech Republic. e-mail: [email protected]

bFaculty of Chemical Technology, Institute of Chemical Technology, ICT, Czech Republic


Regular periodic changes in the rate of N2O decomposition over metallo-exchanged zeolites has been reported by several authors [1], and evidence high complexity of the N2O interaction with these catalysts. Moreover, quite specific dependences of the oscillation behavior on the reaction conditions and their specific features over individual metallo-zeolites, pointed into decisive role of the structural parameters of the active sites in the individual zeolite structures. This obviously opens questions on a detail mechanism of this process, but also provides an opportunity for in-sight into mechanism of this reaction, moreover with general implications for understanding of the catalytic activity over metallo-zeolites. Among the studied catalysts, the oscillation behavior over Fe- and Fe/Pt-FER, one of the most active catalysts for this reaction [2], has been shown as highly specific among the studied metallo-zeolites. Accordingly, potential of the in situ FTIR time-resolved technique in combination with a mathematical model of the catalytic process, in-cluding the diffusion, sorption, and kinetic parameters for the elementary reactions studied, has been demonstrated. The analysis of the structural implications of the data has been supported by the results of QM modelling of the local structure of the iron-zolites [3].The aim of the in-situ FTIR study was to analyze parameters of the oscillatory behavior over well defined Fe- and Fe/M- ferrierite catalysts, establish the role of zeolite structure in the oscillatory phenomenon, and the process of its perturbation by NO.


Thin pellet (ca 80 m) of Fe- and Fe/Pt/FER samples with Fe/Al up to 0.15 and with defined iron cation positions in the zeolite structure were exposed to a stream of N2O/He mixture (> 100 ml/min), in some cases with NO pulses, and followed by time-resolved FTIR (Nexus 670, Ther-moNicolet) combined with UV-Vis Avantes Fiber Optic Spectrometer. Measurements were car-ried at temperatures between 200 and 450 oC using a high temperature catalytic micro-reactor (ISRI, U.S.) adapted for parallel measurements in both optical regions. The gases were fed into


Program Section: 5 Preferred form of presentation: ORAL

the reactor using a PC-controlled mass flow controlled system providing for complex concentra-tion perturbations on a time scale below 1 sec.

The observed changes in the IR bands intensities, indicating changes in the oxidation state of the iron ions, concentration of iron nitrosyl, as well as changes in the of NOx surface species, were simulated using a mathematical model of a one-pellet FTIR reactor. The used one-dimensional dynamic model, accounting for diffusion and reaction in catalytic thin slab, takes into account the extra- and intracrystalline transport phenomena, and allow for determination of the diffusion, sorption, and kinetic parameters of the elementary reactions. The calculations were performed by simultaneous solving the partial differential equations with a FORTRAN program based on Athena Visual Studio tools [4]. The kinetic parameters of the process site density, frequency fac-tor, and activation energy, were determined by their variation aiming into the best possible fit be-tween model prediction and FTIR time-resolved results, i.e. instantaneous mean concentrations of adsorbed species. It has been shown that such approach has a potential for analysis of a complex dynamic behavior during N2O decomposition over Fe-FER, and for providing a direct experimental evidence for the role of NOx(ads) formation for the oscillation behavior during N2O decomposition. Thus, the regular variations in the N2O decomposition were directly connected to variations in the surface coverage by the NOxads species, acting as a co-active center, and triggering the changes in per-formance of the whole system.

Conclusions

High potential of the in-situ FTIR study accompanied by simulation of the complex processes inside the in-situ reactor has been demonstrated on analysis of the oscillation behavior during N2O decomposition. This stressed the importance of the approach combining the in situ experi-ments with mathematic simulation as necessary for solving complex structural problems of the catalytic activity of metallo-zeolites.

References [1] P. Ciambelli, A. Di Benedetto, R. Pirone, G. Russo, Chem. Eng. Sci. 54 (1999) 4521; T. Turek, Catal. Today 105 (2005) 275; Z. Schay, L. Guczi, G. Pál-Borbély, A.V. Ramaswamy, Catal. Today 84 (2003) 165.; El-M. El-Malki, R. A. van Santen, W. M. H. Sachtler, J. Catal. 196 (2000) 212.; D. Kaucký, K. Jíša, A. Vondrová, J. Novák-ová and Z. Sobalík, J. Catal. 242 (2006) 270 [2] K. Jíša, J. Nováková, M. Schwarze, A. Vondrová, and Z. Sobalík , J. Catal. 2008, submitted.[3] S. Sklenak et al., in preparation.[4] W.E. Stewart, M. Caracotsios, Computer-Aided Modeling of Reactive Systems, J.Wiley & Sons, N.Y. 2008.


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When the nature of the reactant controls the structure of the catalyst: hydro-genation of alkynes on palladium

Philippe Sauteta, Daniel Torresa, Detre Teschnerb, Axel Knop-Gerickeb, Robert Schlöglb

a University of Lyon, Laboratory of Chemistry, Ecole Normale Supérieure de Lyon and CNRS, 46Allée d’Italie, F-69364 Lyon Cedex 07, France, b Fritz-Haber-Institut der Max-Planck-

Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany


Heterogeneous catalysis involves two partners: a chemical reaction with reactants, inter-mediate and products and a solid catalyst with a surface and specific sites where the catalytic re-action occurs. In textbooks, the catalyst is viewed as a more or less rigid substrate, presenting aheterogeneity of structures (terraces, steps, defects). On some of these surface structures (the ac-tive site) the catalytic reaction takes place with a large rate. The active site might be designed bya complex chemical procedure, such as the deposit of small clusters on a specific support, or theconstruction of a well-defined organometallic complex attached to the solid substrate. In anycase, in this view, the active site is constructed a priori, before the surface meets the reactants,and is unchanged during the catalytic act.

This approach has been challenged for many years already as being to simplistic. Severalcatalytic reactions require an activation time, during which the catalytic activity is progressivelyattained and the surface structure of the catalysts is changed. In this case one should not speak ofa catalyst, but of a catalyst’s precursor, the active site being constructed in situ during the reac-tion. Such an approach is appealing but extremely complex to characterize, since in situ (or oper-ando) spectroscopic techniques are required.

Theoretical chemistry is an adequate approach to obtain insights in these two complemen-tary faces of heterogeneous catalysis, although it has been until now mainly used to understandhow the structure of the surface site can control the catalytic reaction. In this paper, DensityFunctional theory will be combined with in Situ XPS (at pressure of 1 mbar) in order to study thenear-surface region of palladium under various hydrogenation conditions for a set of reacting al-kynes and alkenes on a palladium surface.


Under the reduced pressure conditions of 1 mbar, alkynes are hydrogenated to alkeneswhile alkene feeds are transformed to alkanes, just as at more realistic conditions. However thestructure of the catalytic surface is strongly dependent on the nature of the reactant. We show thata PdC carbide surface phase is formed during alkyne hydrogenation, while this PdC phase absentfor alkenes. Density functional theory allows to understand the stability of the PdC phase on a



model Pd(111) surface, as a function of the carbon chemical potential. The carbon chemical po-tential in turn is controlled by the type of reactant, which is the source of carbon, and by the tem-perature and pressure. The stable surface termination is hence dependent in a subtle way on thegas phase composition: For acetylene subsurface carbon formation is thermodynamically fa-vored, while this is not the case for ethene. Substituents on the molecules and the respectivemolecule/hydrogen pressure ratio allow modulating this effect.

This PdC surface phase has a strong influence on the hydrogenation activity and selectiv-ity. Experiment shows that the alkyne hydrogenation is selective towards alkene only in condi-tions where the PdC phase is present. From the calculations we show that the hydrogen and hy-drocarbon adsorptions are strongly affected, hence explaining the observed selectivity.

Conclusions

All the data clearly indicate that the (near-)surface state of the palladium catalyst is astrong function of the experimental hydrogenation conditions. Understanding such interplays willallow designing heterogeneous catalysts to a desired reaction. The present study clearly demon-strates the significance of combining theory and experimentation in bringing new insights on thestructure of active site in realistic catalytic conditions and on their specific reactivity.

References[1] D. Teschner , Zs Révay, J. Borsodi, A. Knop-Gericke, R. Schlögl, D. Milroy, S. David Jackson, D. Torres and

P. Sautet, Angew. Chem. Int. Ed. in press 2008.[2] D. Teschner, J. Borsodi, A. Wootsch, Zs. Révay, M. Hävecker, A. Knop-Gericke, S. David Jackson, R. Schlögl,

Science 2008, 320, 86.


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Program Section: 6 Preferred form of presentation: ORAL Resolving the Contributions of Surface Lewis and Bronsted Acid Sites

during NOx/NH3 SCR: An Operando TP-IR Spectroscopic Investigation

Kevin Douraa, Irène Malpartidab, Marco Daturib, Israel E. Wachsa

aOperando Molecular Spectroscopy and Catalysis Laboratory, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015 USA,E-mail: [email protected]

bLaboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 Bd Maréchal Juin, F-14050 Caen, France

Objectives

The selective catalytic reduction of NOx with NH3 over supported V2O5-WO3/TiO2 catalysts has been investigated for almost 30 years, yet a fundamental understanding of the relative contributions of the surface Lewis and Brønsted acid sites for the SCR reaction is still being debated. To resolve this long standing debate, the objective of this investigation was to perform an operando Temperature Programmed-IR spectroscopy investigation (FT-IR analysis of the surface NH3 and NH4

+ species with simultaneous IR and MS analysis of the gas phase products) over a model supported V2O5-WO3/TiO2/SiO2 catalyst.

Experimental

The model supported catalyst was initially prepared by the incipient-wetness impregnation technique using isopropanol solutions of titanium isopropoxide inside a glovebox under a continuously flowing N2 environment on the SiO2 support (~15 nm particles) due to the moisture sensitivity of the titania precursor. HR-TEM analysis revealed that the resulting TiO2

nanoparticles (NPs) are 3-5 nm. Tungsten oxide was subsequently added to the 30% TiO2/SiO2

catalyst by incipient-wetness impregnation of aqueous solutions of ammonium metatungstate ((NH4)10W12O41·5H2O) and calcined in flowing air at 450 C for 4 h. Raman analysis demonstrated that the supported 5% WO3 phase was 100% dispersed as surface WOx species (square-pyramidal mono-oxo structure) that preferentially self-assembled on the TiO2 NPs in the supported 5% WO3/30% TiO2/SiO2 catalyst. Vanadium oxide was introduced in the finalpreparation step by the incipient-wetness impregnation of isopropanol solutions of vanadium tri-isopropoxide (VO[CHO(CH3)2]3 inside a glovebox with continuously flowing N2. Raman analysis revealed that the supported 1% V2O5 phase was 100% dispersed as surface VOx species (trigonal mono-oxo VO4 structure). The in situ surface and gas phase IR spectra for NH3

chemisorption on the supported 1%V2O5-5%WO3/30%TiO2/SiO2 catalyst were collected during NH3 adsorption at 150°C while simultaneously monitoring the gas phase species by massspectrometry (MS). Subsequent surface and gas phase IR spectra were collected during the surface reaction between gas phase NO/O2 and adsorbed NH3 while simultaneously monitoring the gas phase species by MS during the temperature ramp to 400°C (5oC/minute).


Chemisorption of NH3 on the supported V2O5-WO3/TiO2/SiO2 catalyst gave rise to IR peaks characteristic of surface NH3 species (~1610 and 3100-3500 cm-1) on Lewis acid sites and surface ammonium NH4

+ species (~1420 and 2600-3000 cm-1) on surface Brønsted acid sites. It was found that nearly all 700 ppm of gas phase NH3 sent to the catalysts was adsorbed onto the catalysts surface, as shown in Figure 1a, and that the rate of chemisorption on Lewis and


Program Section: 6 Preferred form of presentation: ORAL Brønsted sites are comparable, as shown in Figure 1b. Operando TP-IR-MS spectroscopyrevealed that both adsorbed surface NH3 and NH4

+ react with gas phase NO on the model supported 1%V2O5-5%WO3/30%TiO2/SiO2 catalyst to produce N2 (Tp~295 and 392°C), as shownin Figures 2 and 3. Between 295 and 320°C, the surface NH4

+ species are predominantly consumed whereas the surface NH3 species concentration remains relatively constant. Interestingly, the concentration of surface NH4

+ species initially slightly increases at ~290oC by the transformation of some surface NH3 species to surface NH4

+ species. The slight increase in the concentration surface NH4

+ species coincides with the conversion of surface NH4+ to gaseous

N2 and H2O products. This suggests that some surface NH3 species become converted to surface NH4

+ species in the presence of moisture. Both types of surface species are continuously consumed in the temperature range of ~320-390oC. In the 390-400oC temperature window, only surface NH3 species remain and are mostly responsible for N2 formation in the SCR reaction in this temperature range.

Conclusions

In the lower SCR reaction temperature regime of ~295-320oC, the surface NH4+ species on

Brønsted acid sites are preferentially consumed and some surface NH3 species on Lewis acid sites are converted to surface NH4

+ species in the presence of moisture. In the intermediate temperature regime of ~320-390oC, both surface NHx species are consumed in the production of N2. At the highest temperature regime of ~390-400, the surface NH3 species are predominantly responsible for N2 formation. The combined operando TP-IR-MS spectroscopy experiment was able to distinguish between the different reactivity of the surface NH4

+ species on Brønsted acid sites and the surface NH3 species on Lewis acid sites during the SCR reaction, and revealed the higher reactivity of Bronsted acid surface sites.

2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0

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NH4+

1420 cm-1

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

Ban

d Ar

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.u)

T im e ( m in )

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Figure 1: a) Operando IR-TPSR profile of NH3 gas adsorbing on 1%V2O5-5%WO3/30%TiO2/SiO2 b) Chemisorption profile of Lewis and Bronsted sites during adsorption.

a)

b)

Figure 2: Operando MS-TPSR spectra of 1%V2O5-5%WO3/30%TiO2/SiO2 during the SCR of NOx with NH3

Figure 3: Consumption rate of NH3 and NH4+ during

Operando IR-TPSR

3 9 2

N H 4+

( 1 4 2 0 c m -1 )

1 5 0 3 1 31 8 01 5 01 5 01 5 01 5 0 1 5 0 1 5 0 4 0 04 0 04 0 03 9 0

A r F lo w (A c t iv a t io n )

3 6 11 5 0

T e m p . R a m p S ta r te d

2 4 61 5 01 5 0

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d A

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N H 3c o o r d in a te d

( 1 6 1 0 c m -1 )

2 9 5

1.00E-010

1.50E-010

392 N2 (m/e=28)

H2O (m/e=18)

396 400388356236174150 399390

Inte

nsity

(a.u

)

Temp (C)303

NO (m/e = 30)295

303



FT-IR studies on Ga2O3 and Pd-Ga2O3: reactivity for (reverse) water-gas shift

K. Föttinger a, A. Haghofer a,b, W. Jochum c, B. Klötzer c, A. Knop-Gericke b,

R. Schlögl b, G. Rupprechter a

aInstitute of Materials Chemistry, Vienna University of Technology, Austria, bFritz-Haber-Institute of the MPG, Berlin, Germany,

cInstitute of Physical Chemistry, University of Innsbruck, Austria.

email: [email protected]


Vibrational FT-IR spectroscopy is a versatile tool for investigating metal as well as oxide sur-faces, on the one hand by adsorbing various probe molecules, on the other hand the technique is very well suited for operando measurements. It allows determination of the properties of sup-ported metal nanoparticles as well as the surface chemistry and adsorption sites on oxide sur-faces. Under reaction conditions, in-situ FTIR spectroscopy provides information on population of sites and on their role for catalytic reactions, and allows for identification of reaction interme-diates.

Pd-Ga2O3 attracted interest due to its unique catalytic properties. The excellent selectivity of Pd-Ga2O3 for methanol synthesis was attributed to PdGa alloy formation [1] or to the support being responsible for the reaction with Pd accelerating the hydrogenation of carbonate- and carboxy-intermediates on the support [2]. It was observed that also Ga2O3 on its own possesses significant catalytic activity for e.g. (reverse) water-gas shift reaction [3] and methanol synthesis [2]. Surface hydrides Ga +–H ( <2) were reported to be crucial for the reaction of hydrogenated oxycarbona-ceous intermediates [4]. In addition, reactivity of the Ga2O3 support can influence significantly the product distribution e.g. during methanol synthesis or steam reforming, because it can cata-lyze (un)wanted side reactions.

This contribution aims at a detailed understanding of the mechanism of the (reverse) water-gas shift (WGS and RWGS) reaction on Ga2O3. We apply FT-IR spectroscopy for

(i) studying the interaction with CO, CO2, H2 and H2O, which are reactants and products in (R)WGS, and can act at the same time as probe molecules and

(ii) for operando studies during (R)WGS both on the pure oxide and the metal promoted Ga2O3.




Commercial Ga2O3 was used in this work. FT-IR measurements were performed in transmission mode. For operando studies we use a flow cell with the outlet connected to a gas chromatograph.

Ga2O3 without metal promotion shows significant catalytic activity for water-gas shift (WGS) and reverse water-gas shift (RWGS).

We have followed the interaction of Ga2O3 with hydrogen by IR and TPR. Ga2O3 can be reduced during heating in hydrogen, which leads to the appearance of Ga-H vibrational bands at tempera-tures > ~ 473 K. Further heating to higher temperatures (> 550 K) induces oxygen vacancy for-mation [5]. Both oxygen vacancies and Ga-H species are very sensitive to traces of water and can be easily re-oxidized with water, even under strongly reducing conditions. This was confirmed by deliberate quenching experiments following the disappearance of Ga-H bands by IR and detec-tion of evolved hydrogen by mass spectrometry.

Furthermore, the interaction with CO was investigated in detail by IR and TPD studies. Ga2O3 is reduced by heating in CO, forming CO2 and oxygen vacancies. Carbonate-type species appear via re-adsorption of produced CO2.

Based on these results we proposed a reaction scheme for the interaction with H2, H2O, CO and CO2 [3]. The key steps are formation of oxygen vacancies with CO or H2 and replenishment with CO2 or H2O. Formation of formates is also considered.

The proposed mechanism was evaluated by applying operando studies for RWGS reaction. Cata-lytic activity sets in at about 550 K. In this temperature range formation of oxygen vacancies as well as decomposition of formates was observed.

Conclusions

By applying operando FT-IR on Ga2O3 we proposed a reaction scheme for (R)WGS reaction. Via a systematic study of the interaction with each reactant/product we obtained a detailed under-standing of the fundamental processes. We observed two pathways occurring in parallel, a formate- and a vacancy-assisted mechanism, depending on the applied conditions and tempera-ture range.

References [1] Iwasa, N., Mayanagi, T., Ogawa, N., Sakata, K., Takezawa, N., Catal. Lett. 54 (1998), 119. [2] Collins, S.E., Baltanas, M.L., Bonivardi, A.L., J. Catal. 226 (2004), 410. [3] Jochum, W., Penner, S., Kramer, R., Föttinger, K., Rupprechter, G., Klötzer, B., J. Catal. 256 (2008), 278. [4] Collins, S.E., Baltanas, M.L., Garcia Fierro, J.L., Bonivardi, A.L., J. Catal. 211 (2002), 252. [5] Jochum, W., Penner, S., Föttinger, K., Kramer, R., Rupprechter, G., Klötzer, B., J. Catal. 256 (2008), 268.


Program Section: 6. Application of reported molecules to image catalytic activity. Oral Presentation.

The observation of equilibria present in stepwise gas phase hydrogenation re-actions.

Andrew McFarlane 1, Liam McMillan 1, Ian Silverwood 1, Neil Hamilton 1, David T. Lundie 2

and David Lennon 1*.

1. Department of Chemistry, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ,U.K. 2. Hiden Analytical, Hiden Analytical Ltd., 420 Europa Boulevard, Warrington, WA5 7UN, U.K.


Olefin chemistry on the industrial scale is closely linked with increased refinery capacity and

covers a wide range of chemical processes. Diolefins, or dienes, are also industrially significant,

with C4 and C5 1,3-dienes featuring strongly. Here, the double bonds are conjugated and the

molecules are more reactive than their monoene counterparts. Of the higher homologues, 1,3-

pentadiene presents an interesting case, with recent studies from this laboratory using infrared

spectroscopy to follow the sequence of stepwise hydrogenation steps that culminate in the forma-

tion of the alkane. This work involves using an adapted infrared cell as a batch reactor, where the

infrared beam samples the gas phase present over an alumina supported palladium catalyst. Cali-

bration routines therefore enable determination of the reaction profile for the reacting gases that

are in direct equilibrium with the catalyst surface. Interestingly, these studies provide informa-

tion on the thermodynamic pathways available to the reaction system, which can contrast with

studies performed under flowing conditions, which can often only feature the formation of kinetic

products. Thus, such studies can be used to propose new routes to emphasize particular products.

Specifically, in the case of 1,3-pentadiene, a previously un-reported isomerisation reaction is ob-

served alongside a stereoselective hydrogenation step. Enticingly, these transformations provide

the opportunity to produce valuable partial hydrogenation products from stereoisomeric mixtures

of the conjugated diene.


Figure 1 shows the infrared spectra corresponding to the hydrogenation of cis-2-pentene over a 1% Pd/Al2O3 catalyst at 303 K as a function of time. The intensity of the CH=CH wags at 969 and 688 cm-1 can be used to discriminate between respectively trans-2-pentene and cis-2-pentene. Following this approach, Figure 2 shows the reaction profile for this reaction. Whereas trans-2-pentene shows a simple single stage direct conversion to pentane (not shown here), Figure 2 shows the hydrogenation of cis-2-pentene to proceed via a consecutive sequence where firstly the


Program Section: 6. Application of reported molecules to image catalytic activity. Oral Presentation.

cis-2-pentene is converted to trans-2-pentene, which then is subsequently hydrogenated to form pentane.

Fig.1 Fig. 2

This is a surprising result, indicating that under these experimental conditions a barrier exists for the hydrogenation of the cis-alkene but this barrier is absent for the trans-alkene. A further bene-fit of this approach is that the measured reagents/products conform to Beer’s law, so quantifica-tion of the reacting species is possible, with no complications over surface-moderated molar ex-tinction coefficients. The resulting mass balance establishes complete conversion of the cis-2-pentene but, upon completion of reaction, the pentane only accounts for 80% of the initial con-centration of hydrocarbon content. One scenario that could account for these observations is that a fraction of the cis-isomer is retained by the catalyst at special sites, the binding energy of which is sufficient to mitigate against subsequent hydrogenation, even in the presence of a large excess of hydrogen.

Fig. 3.In order to test the generality of these con-cepts, a technical mix of 1,3-pentadiene (con-taining both cis- and trans-isomers) was exam-ined. The resulting reaction profile (not shown) is indeed consistent with trans-2-pentene being the only monoene active for the second stage hydrogenation process. These observations can be rationalised within the fol-lowing reaction scheme, Figure 3.

Conclusions

A simple arrangement of a gas infrared cell operating as a batch reactor can provide valuable in-formation on equilibria active at the gas/solid interface. The realisation of thermodynamically feasible pathways can then be used to emphasize particular product distributions that favour the formation of valuable olefins, trans-2-pentene in this case.


Program Section: 6 (Application of reporter molecules…) Preferred form of presentation: Oral (keynote?)

Toward hypersensitive NMR of heterogeneous catalytic hydrogenations

Igor V. Koptyug, Kirill V. Kovtunov, Vladimir V. Zhivonitko, Ivan V. Skovpin, Renad Z. Sagdeev

International Tomography Center, SB RAS, 3A Institutskaya St., Novosibirsk 630090, Russia


Nuclear Magnetic Resonance (NMR) is a powerful spectroscopic technique with a broad range of applications that include in situ studies of homogeneous and heterogeneous catalytic reactions. However, sensitivity in many cases limits the ability of NMR to detect important species, for in-stance the short-lived reaction intermediates. Nuclear spin isomers of molecular hydrogen (e.g., parahydrogen with the total nuclear spin I=0) can serve as useful reporters that can boost the de-tection sensitivity of NMR by several orders of magnitude and reveal fine details of reaction mechanisms. Parahydrogen-induced polarization of nuclear spins (PHIP, or PASADENA) has become an established spectroscopic tool for the mechanistic studies of homogeneous hydrogena-tion reactions catalyzed by transition metal complexes in solution. Observation of PHIP requires that a pairwise addition of the two H atoms of the same H2 molecule to a double or a triple bond of an unsaturated substrate takes place. As a result, it was never before considered in the context of heterogeneous hydrogenations. It is the aim of this work to develop PHIP as a powerful tool for the in situ/operando studies of heterogeneously catalyzed hydrogenation reactions.


Many transition metal complexes are known to be efficient and selective catalysts for homogene-ous catalytic reactions. In the ongoing attempt to bridge the gap between homogeneous and het-erogeneous catalysis, such complexes are being attached to or immobilized on various solid sup-ports. Numerous literature studies demonstrate that heterogenized transition metal complexes can be useful for heterogeneous hydrogenations. We demonstrate that, similar to their homogeneous counterparts, heterogenized transition metal complexes (e.g., [RhCl(PPh3)2PPh2(CH2)2]–SiO2)are able to produce strong NMR signal enhancements when parahydrogen is used in the liquid-solid (e.g., styrene in solution) or in the gas-solid (e.g., propylene or propyne) hydrogenation re-actions [1-3]. It is always assumed that the mechanism of hydrogenation reaction remains un-changed upon the heterogeneization of metal complex, i.e., that the reaction proceeds via the formation of metal dihydride species. The observation of PHIP represents a direct proof that addi-tion of the two hydrogen atoms, first to the active center and then to the substrate, is pairwise, and thus allowed us to verify the reaction mechanism. PHIP was also used in combination with the imaging modality of NMR (MRI) to visualize active zones in operating catalytic reactors [4].

Heterogeneous hydrogenations with supported metal catalysts (e.g., Pt/Al2O3, Pd/Al2O3) are characterized by an entirely different reaction mechanism, with dissociative hydrogen chemisorp-


Program Section: 6 (Application of reporter molecules…) Preferred form of presentation: Oral (keynote?)

tion on the metal surface followed by rapid migration of H atoms on the surface, their dissolution in the metal lattice and spillover onto the support. All this should prevent pairwise hydrogen ad-dition to the substrate, and thus the observation of PHIP should be impossible. We have found that, unexpectedly, it is possible to observe PHIP in heterogeneous hydrogenations catalyzed by supported metal catalysts [3,5]. In the preliminary study, the contribution of the pairwise addition route to the overall reaction mechanism was estimated and the structure-reactivity relationship was investigated. In particular, it was established that the NMR signal enhancement in the reac-tion product changes as the metal particle size is varied systematically in the range 0.7-11 nm. Apart from the particle size effects, the nature of the support also has a pronounced influence on the signal enhancement. Among the porous supports investigated (ZrO2, Al2O3, C, SiO2, TiO2),TiO2 which is known to have the strongest perturbation of the electronic structure of supported metal clusters leads to the largest NMR signal enhancement.

Conclusions

The results obtained demonstrate that PHIP-based NMR signal enhancement can be employed to develop a powerful and highly sensitive spectroscopic tool for the operando studies of heteroge-neous hydrogenation reactions. In particular, detection of reaction intermediates, identification of mechanistically important surface species among the numerous spectators, and revealing the structure-reactivity relationships should be possible with the use of parahydrogen as a powerful reporter molecule, with the added advantage that PHIP is produced only when parahydrogen molecule enters the reaction cycle. Besides, PHIP-based hypersensitive NMR detection of reac-tion products can reveal other chemical transformations such as dehydrogenation, double bond migration and isomerization, and address the issues of reaction stereo- and chemical selectivity. Extension to the operando studies of other catalytic and non-catalytic reactions is also feasible.

Acknowledgments. This work was supported by the grants from RFBR (08-03-00661, 08-03-91102, 07-03-12147), the program of support of leading scientific schools (NSh-3604.2008.3) and CRDF (RUC1-2915-NO07). We thank our colleagues (A. Pines, UC Berkeley; S.R. Burt, BYU; L.-S. Bouchard, UCLA; M.S. Anwar, LUMS, Pakistan; V.I. Bukhtiyarov and I.E. Beck, BIC SB RAS, Novosibirsk) for a fruitful collaboration.

References [1] I.V. Koptyug, K.V. Kovtunov, S.R. Burt, M.S. Anwar, C. Hilty, S. Han, A. Pines, R.Z. Sagdeev. J. Amer. Chem.

Soc., 129, 5580-5586 (2007). [2] L.-S. Bouchard, K.V. Kovtunov, S.R. Burt, M.S. Anwar, I.V. Koptyug, R.Z. Sagdeev, A. Pines. Angew. Chem.

Int. Ed., 46, 4064-4068 (2007). [3] K.V. Kovtunov, I.V. Koptyug, in: "Magnetic Resonance Microscopy. Spatially Resolved NMR Techniques and

Applications". S. Codd and J.D. Seymour, eds., 2009, Wiley-VCH, Weinheim, 101-115. [4] L.-S. Bouchard, S.R. Burt, M.S. Anwar, K.V. Kovtunov, I.V. Koptyug, A. Pines. Science, 319, 442-445 (2008). [5] K.V. Kovtunov, I.E. Beck, V.I. Bukhtiyarov, I.V. Koptyug. Angew. Chem. Int. Ed., 47, 1492-1495 (2008).



Haloperoxidase reaction events monitored at single molecule level:Comparison between a Haloperoxidase enzyme and its inorganic biomimic

Gert De Cremer,a Maarten Roeffaers,b Virginia Martinez Martinez,b Dirk De Vos,a Johan Hofkens,b Bert Selsa

aDepartment of Microbial and Molecular Systems, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium; [email protected] bDepartment of

Chemistry, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium


Recently, several strategies have been developed for monitoring catalytic activity using single molecule fluorescence spectroscopy (SMFS), yielding insights at the molecular level of catalytic properties that were previously hidden in ensemble averaged experiments.1 The most common scheme relies on fluorogenic substrates that become fluorescent upon catalytic action. While a large assortment of such fluorogenic probes is available for bulk activity measurements, only a few of them are suitable for SMFS. Exploration of single molecule events with such probes in catalytic systems has thus so far been limited.2

This work introduces a simple assay to monitor individual oxidation events at varying distances from a haloperoxidase biocatalyst (Curvularia verruculoasa bromoperoxidase) and its inorganic biomimic (WO4

2--LDH).3 In the presence of bromide salts and H2O2, these haloperoxidases form hypobromite (HOBr, reaction 1) which can either rapidly brominate organic compounds (HA in reaction 2) or can decompose H2O2 into 1O2 and water (reaction 3).

H2O2 + H+ + Br- HOBr+H2O (1)

HOBr + HA ABr + H2O (2)

HOBr + H2O21O2 + H2O + Br- + H+ (3)

HOBr is expected to migrate into the reaction medium where it performs the bromination. How-ever, it cannot be excluded that strong interactions with the catalyst, e.g., via electrostatic interac-tion of OBr- with the LDH or via specific binding with the protein matrix of the enzyme, confine the actual halogenation to the active site’s close surroundings. Although under debate, the latter confinement may be the origin of stereo- and regioselective bromination of natural compounds.We here introduce a nonfluorescent fluorescein derivative, aminophenyl fluorescein or APF, to localize the bromination. This probe reacts with a high specificity and high rate with hypohalitesto form the strongly emissive fluorescein, with only a limited sensitivity to other reactive oxygen species (ROS) (See scheme below).4




As can be seen in the figure below, the fluorescence intensity time transients recorded at the cata-lyst’s position consist a low background level with very intense spikes, each spike corresponding to a single turnover. The right figure shows the distance profile of HOBr reactivity with respect to the catalyst. By focussing the laser at increasing distances from the catalyst center, it was found that for both the enzyme and its biomimic a considerable amount of HOBr is released into the solution where it reacts with APF over distances up to 1 μm from the catalyst center.5 For these catalytic systems the secondary reaction (2) thus takes place in solution and not in/on the catalyst. More results are highlighted in ref 5.

Conclusions

We could demonstrate that APF is a powerful single-molecule probe for monitoring single reac-tion events by in situ generated HOBr. For the two studied systems, Curvularia verruculosa bro-moperoxidase and WO4

2--exchanged LDH crystals, the formed HOBr is released into the solution where it brominates the APF probe over distances up to 1 μm from the catalyst center.

References[1] Roeffaers, M. B. J., De Cremer G. et al. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 12603.[2] Velonia, K., Flomenbom, O. et al. Angew. Chem., Int. Ed. 2005, 44, 560; De Cremer, G., Roeffaers, M. B. J. et

al. J. Am. Chem. Soc. 2007, 129, 15458; English, B. P., Min, W. et al. Nature Chem. Biol. 2006, 2, 87.[3] Sels, B., De Vos, D. et al. Nature 1999, 400, 855.[4] Setsukinai, K., Urano, Y. et al. J. Biol. Chem. 2003, 278, 3170.[5] Martinez, V.M., De Cremer G. et al. J. Am. Chem. Soc. 2008, Accepted (in press); doi: 10.1021/ja804606m


plenary lectures - leibniz-institut für katalyse e.v.department of chemical engineering and...

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