Kinetics of the sulfur oxidation on palladium: A combined in situ x-rayphotoelectron spectroscopy and density-functional studyKarin Gotterbarm, Nicola Luckas, Oliver Hfert, Michael P. A. Lorenz, Regine Streber et al. Citation: J. Chem. Phys. 136, 094702 (2012); doi: 10.1063/1.3687676 View online: http://dx.doi.org/10.1063/1.3687676 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v136/i9 Published by the American Institute of Physics. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

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THE JOURNAL OF CHEMICAL PHYSICS 136, 094702 (2012)

Kinetics of the sulfur oxidation on palladium: A combined in situ x-rayphotoelectron spectroscopy and density-functional study

Karin Gotterbarm,1,a) Nicola Luckas,2,a) Oliver Hfert,1 Michael P. A. Lorenz,1

Regine Streber,1 Christian Papp,1,b) Francesc Vies,2,b) Hans-Peter Steinrck,1,3,4

and Andreas Grling2,41Lehrstuhl fr Physikalische Chemie II, Universitt Erlangen-Nrnberg, Egerlandstr. 3,91058 Erlangen, Germany2Lehrstuhl fr Theoretische Chemie, Universitt Erlangen-Nrnberg, Egerlandstr. 3,91058 Erlangen, Germany3Erlangen Catalysis Resource Center (ECRC), Universitt Erlangen-Nrnberg, Egerlandstr. 3,91058 Erlangen, Germany4Interdisciplinary Center for Interface Controlled Processes, Egerlandstr. 3, 91058 Erlangen, Germany

(Received 29 November 2011; accepted 2 February 2012; published online 1 March 2012)

We studied the reaction kinetics of sulfur oxidation on the Pd(100) surface by in situ high resolutionx-ray photoelectron spectroscopy and ab initio density functional calculations. Isothermal oxidationexperiments were performed between 400 and 500 K for small amounts (0.02 ML) of preadsorbedsulfur, with oxygen in large excess. The main stable reaction intermediate found on the surfaceis SO4, with SO2 and SO3 being only present in minor amounts. Density-functional calculationsdepict a reaction energy profile, which explains the sequential formation of SO2, SO3, and eventuallySO4, also highlighting that the in-plane formation of SO from S and O adatoms is the rate limitingstep. From the experiments we determined the activation energy of the rate limiting step to be 85 6 kJ mol1 by Arrhenius analysis, matching the calculated endothermicity of the SO formation. 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3687676]

INTRODUCTION

Palladium is a highly important material in catalysis. It isan essential component of various commercial catalysts, suchas the three-way catalyst,14 nowadays in use for the treatmentof exhaust gases in automotives. Palladium is of common usealso in the petrochemical industry5 and for the large scale pro-duction of fine chemicals.6 Focusing on the three-way cat-alyst, sulfur-containing compounds can lead to catalyst poi-soning since they are omnipresent in the fuel feedstocks, i.e.,crude oil.7 Once the fuel is burned, atomic sulfur or SOx(x = 14) are formed and can attach to the catalyst palladiumphase, thus leading to the poisoning of the catalyst, reducingits lifetime and effectiveness.8, 9

Therefore, a major goal in the technical operation ofcatalysts is to minimize this poisoning.10, 11 One way toachieve this is by the removal of sulfur in the early pro-cessing of the used feedstocks by hydrodesulfurization meth-ods, even though a complete removal is, unfortunately, notpossible.12 Hence, minor amounts of sulfur are always leftin the produced fuels. The adsorption of these sulfur traceson the catalyst surface can lead to a substantial decrease ofits reactivity.10, 11, 1321 First endeavors to understand the ef-fects of sulfur on the electronic structure, the adsorption ca-pacity, and reactivity of palladium have been undertaken,22

a)K. Gotterbarm and N. Luckas contributed equally to this work.b)Authors to whom correspondence should be addressed. Electronic ad-

dresses: Christian.Papp@chemie.uni-erlangen.de and Francesc.Vines@chemie.uni-erlangen.de.

whereas the regeneration by means of sulfur oxidation hasnot been addressed so far from a fundamental point ofview.

A basic understanding of poisoning on the molecularlevel can be gained by combining ab initio calculations andin situ experiments on single crystal catalyst surfaces.2225

Low indexed surfaces are shown to be the major interfaceof metal nanoparticles in heterogeneous catalysts. Such sur-faces are thus expected to be a major part of a three waycatalyst for exhaust gas cleaning in automobiles. First in situstudies of the interaction of sulfur with one other major con-stituent of a three way catalyst, platinum, have been publishedlately allowing now for a wider view of the possible reactionsand reaction kinetics on the catalyst for a deeper understand-ing of the processes involved. Past studies have shown thatatomic sulfur adsorbs on four-fold hollow sites of the Pd(100)surface.23, 24 These sulfur atoms lower the surface activity byblocking the available adsorption/reaction sites on the sur-face due to sterical hindrance.19, 26, 27 Such blocking has beenfound to affect both hydrogen and oxygen adatoms27 whichare used to regenerate the catalyst through either a reductivemechanism (formation and desorption of H2S)12 or an oxida-tive mechanism (formation and desorption of SO2 or SO3).28

The only study on a thermal oxidation reaction is an earlytemperature programmed desorption study,29 which includedisothermal experiments. However, no experiments concerningthe reaction mechanism, the surface species formed during theprocess, and the reaction barriers were presented. This lack ofexperimental data and insight is due to the fact that the neces-sary in situ techniques were developed only lately.

0021-9606/2012/136(9)/094702/7/$30.00 2012 American Institute of Physics136, 094702-1

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http://dx.doi.org/10.1063/1.3687676http://dx.doi.org/10.1063/1.3687676http://dx.doi.org/10.1063/1.3687676mailto: Christian.Papp@chemie.uni-erlangen.demailto: Francesc.Vines@chemie.uni-erlangen.demailto: Francesc.Vines@chemie.uni-erlangen.de

094702-2 Gotterbarm et al. J. Chem. Phys. 136, 094702 (2012)

In this work, we provide a detailed description of the oxi-dation of sulfur atoms adsorbed on a Pd(100) surface througha combination of in situ x-ray photoelectron spectroscopy(XPS) and ab initio density-functional (DF) calculations. Theresults allow to clearly identify all reaction intermediates,such as SO2, SO3, and SO4, and to determine the relevantactivation energies of the reaction pathway.

EXPERIMENTAL

The XPS measurements were performed at the syn-chrotron facility BESSY II in Berlin at beamline U49/2PGM1 with a transportable setup30 equipped with an elec-tron energy analyzer (Omicron EA 125 U7 HR), a low energyelectron diffraction optics, a capillary array doser system, aquadrupole mass spectrometer for residual gas analysis, and athree-stage supersonic molecular beam.

The sample was cleaned from sulfur and other con-taminations (e.g., carbon) by exposure to O2 at 300 K,followed by heating to 970 K to desorb excess oxygen. Finalcleanliness was checked by XPS. S 2p spectra were collectedwith a total resolution of 110 meV at a photon energy of260 eV. The acquisition time was 9 s per spectrum. All XPspectra were recorded at normal emission. Small coveragesof atomic sulfur ranging from 0.01 to 0.03 ML were preparedby dosing H2S with background pressures ranging from1 to 3 109 mbar and at temperatures below 200 K.Subsequently the sample was flashed to 700 K to desorbhydrogen. Oxygen was dosed directly onto the sample by thesupersonic molecular beam at the denoted temperatures. Thethermal evolution of atomic sulfur coadsorbed with oxygenwas