J. BOURDON (Editor)Growth and Properties of Metal Clusters, pp. 493-503 493© 1980 Elsevier Scientific Publishing Company - Printed in The Netherlands

THE STRUCTURE AND SELECTIVITY OF SUPPORTED METAL PARTICLES+ ,,~ ++

~y J.M. Dominguez and M. JOSe YacamanInstituto Mexicano del Petr6leo, Av. de Los Cien Metros and

++Instituto de Fisica, UNAM, Apartado Postal 20-364, Mexico 20, D.F.

1. INTRODUCTION

In modern catalysis studies one of the most important aspects is the corre-lation between shape and crystallography of small metallic particles andkinetic data. At the present time very little is known about the detailledstructure of supported metal catalyst. The main reason being the difficultyto apply conventional electron and X-ray diffraction methods to nanometersize particles. However some recent developments on weak beam electron mi-croscopy (ref. 1 - 2) and small area diffraction (ref. 3) seem to make pos-sible the full characterization of the particles.

In the present paper we characterize particles of Pt supported on graphiteusing weak beam electron microscopy. We correlate the particle shape withthe kinetics data for the conversion of neopentane. This reaction is of the"demanding" type (ref. 4) ie; the activity is influenced by the particlesize and more general by the mode of catalyst preparation. This system ap-pears to be very convinient for the correlation sought because the reactionhas the metal as sole site of catalytic activity (ref. 5).

2. EXPERIMENTAL TECHNIQUES

a) Sample Preparation.

The preparation of Pt/C catalyst was made using materials of high purity.The support was a graphite LONZA - LT10, type ex-anthracite, with 99.9% ofcarbon, as reported by the supplier. The surface area determined by B.E.T.adsorption of N2 equals 18 m2/g and the density equals 2.2 g/cm3. The metalPt, was deposited on the support from chloroplatinic acid (H2PtCI 6, 6H 20)using the method of Bartholomew and Boudart (ref. 5).

The 10% in weight Pt/C catalyst was obtained from a solution of 10 mI. ofethanol by 1 gr. of support. The mixture was shaken during 24 hrs. and thenthe excess of solvent was eliminated by heating the powder at 100°C in anevaporator during several hours. The sample was then air dried at 80°C dur-ing 12 hrs. The obtained powder was placed in a quartz cell and heated at850°C in the presence of an hydrogen flow. Thus the sample was left 16 hrs.in a static atmosphere of hydrogen and then was purged with Argon (99.99%)until room temperature was reached.

b) Kinetic Measurements.

The activity data of the neopentane conversion over a Pt/C catalyst was col-lected in a differential reactor flow system coupled with a Varian Aerograph

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1200 chromatograph. The gas coming from the reactor was analyzed by pas-sing it through a column 6 m. long, filled with chromosorb W (80 mesh) im-pregnated with dimethyl-sulfolane (20%). The chromatographic analyses werEperformed at a reactor temperature of 300 and 360°C respectively and at atotal pressure of 1 atmosphere. The reagent gas flowing through the reactcwas a mixture of neopentane (Fluka, 99.9% purity) diluted in helium and hy-drogen (Air Liquide, 99.95%) with an hydrogen to neopentane ratio equal to10. These conditions were kept constant during the experiment and are si-milar to the ones described by Boudart and Ptak (ref. 7). The products de-tected were methane, ethane, propane, isobutane and isopentane.

Initial reaction rates were measured after 5 minutes of reaction. The per-cent of isomerization (selectivity) gives the amount of neopentane transfoIed in isopentane (0 ~Si ~ 100%).

c) Electron Microscopy.

Samples were mounted on 200 Mesh grids covered with a layer of carbon whicrwas evaporated from standard electrodes at a vacuum of 10- 6 Torr.

The Pt/C powder was ultrasonically dispersed in ethanol and mounted on thegrid, where a drop of the suspension was then air dried.

Observations were carried out in a Jeol 100-C electron microscope fittedwith a high resolution top entry goniometer stage. Dark field images wereobtained by tilting the electron beam so as to make the diffraction spotspatially coincident with the current center of the microscope. Astigmatiswas corrected using the phase contrast features of the carbon film. Latticimages of Mn203 whiskers were used to calibrate the magnification.

3. SHAPE AND CRYSTALLOGRAPHY OF THE SMALL Pt PARTICLES

When observed in bright field microscopy the Pt clusters show two main pro-files, hexagonal and elongated hexagonal. These types are shown in figure1. A systematic study was made to obtain the correspondent 3-D shape forthese particles. Dark field images using a reflection close to the Braggcondition showed the complete hexagonal protile (figure 2). That indicatedthat the particles were not multiple twinned icosahedra (ref. 8) which alsohave and hexagonal bright field profile but in dark field they only showpart of the particle iluminated.

The regular hexagonal particles (H1) are observed only in (220) - type ofreflections. The elongated hexagons are observed in (111), (200) and (220)type of reflections.

Some micro diffraction experiments were performed using a STEM attachmentfor the microscope. The diffraction patterns for regular hexagonals wereconsistenly indexed as corresponding to an FCC crystal with a < 111> zoneaxis. A typical pattern of this type is shown in figure 3.

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The patterns of particles of the HZ type indicated an FCC in a < 110 > zoneaxis. This data suggested the 3-D shape of the particles was a cubo-octa-hedron bounded by {111} and < 110 >faces. The particle HI will correspond toa cubov oc t.ah ed r on sitting on a < 111 > face. The HZ particle will corres-pond to a cubo-octahedron sitting in a < 110 >face and is produced probablyby 9 particle growth in contact with some special type of sites on the sub-strate. This model was comfirmed by weak beam thickness fringes (ref. Z)which give a direct indication of the 3-dimensional shape. An example ofthickness fringes for an HZ particle which is slighty tilted is shown in fi-gures 4 a) and b). The profile correspond exactly to the cubo-octahedronwhen observed along a <110> axis.

Fig. Sa) thickness fringe profile for a H1 particle in bright and dark field(fig. Sa) and 5b)). The dark field image shows a triangular fringe at thetop to the hexagonal particle which shows that the {111} faces of the cubo-octahedron are triangular rather than hexagonal as shown in the model in fi-gure 6. A further information is that the cubo-octahedron is truncated tosome extent. The exact amount of truncation can be obtained from the numberof fringes present and their total size. We have observed that the trunca-tion tend to be larger for smaller particles.

An additional confirmation for the cubo-octahedron model is the relativeorientation between the gr vector corresponding to a (ZZO) planes for a<111> oriented cubo-octahedron and their corresponding dark field imagewhich is unique for this figure as described elsewhere (ref. 8).

4. ORIENTATION RELATIONS PARTICLE-SUBSTRATE

A important point in order to understand the mechanism of growth for the par-ticles is to determine the orientation relationship between the particlesand the substrate. In order to do that the following experiment was perform-

oed; a region of graphite of the order of 1500 A containing a large numberof particles was examined. This graphite had a surface corresponding to abasal plane i e <0001> zone axis with the corresponding hexagonal sym-metry. Dark field images of HI particles where taken in several positionsof the objective aperture. In these images the rotation of the cubo-octa-hedron with respect toa < OOZO > graphite reflection was measured. Thisgives the orientation of the particles with respect to the substrate orien-tation. It was confirmed by examining the diffraction pattern that theorientation of the graphite did not change along the area observed. Thatallow us to make some statistical measurements of the particle orientation.A plot of the relative frecuencies vs. the particle rotation is shown inFig. 7. As it is apparent the particles are not located at random but theypresent well defined preferred orientations.The behavior'shown in figure 7 indicates that particles grow epitaxially andthey might be able to rotate on the substrate to acomodate in a minimum ener-gy configuration this point is relevant with respect to the sintering be-havior of the particles.

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Figure 1. Bright field image of a Pt - Graphite catalyst showing regularhexagonal particles HI and elongated hexagonal particles HZ.

Figure 2. Dark field image using a (ZZO) reflection of an HI particle. Thecomplete hexagonal profile is observed.

s. KINETIC MEASUREMENTS

The corresponding data to the isomerization of neopentane are shown in Ta-ble 1. This Table shows the kinetic data which correspond to the two paral-lel reactions, hydrogenolysis and isomerization of neopentane at 300 and360° C respectively. The data collected in table Z are those taken at in-itial conditions, as mentioned above. These figures show that the isomeri-zation rather than hydrogenolysis is favored by Pt/C catalyst at both 300and 360°C with the selectivity being equal to 87.Z and 8Z.8% of the totalconversion, respectively. Another important aspect is that the distributionof hydrogenolysis products: CH4 and iso- C4H 10 are more favored than in-termediates CZH6 and C3H8; ·which means the breaking of only one bond of theneopentane molecule is more likely. The quantity of iso-butane formed de-creases with an increase in the temperature from 300 to 360°C. Is possiblethat in latter case the secondary reactions become important, such as the

497

Figure 3. Micro-diffraction pattern of an hexagonal particle showing < 111 >

zone axis. Some graphite reflections are observed.

Figure 4a). Bright field image of an HZ particle.

Figure 4b). Weak beam dark field image corresponding to the particle infigure 4a). Thickness fringes and moire patterns are observed.

iso- CSH1Z transformation into light hidrocarbons giving CH4 as the princi-pal final product. It has been suggested by Boudart et al. (ref. 4) thatatom arrays of 3 sites (B3 type sites) could favour the tri-adsorption ofneopentane molecule (precursor of isomer). This mechanism is squematicallyrepresented in figure 8 in which the different steps of the reactions are

498

indicated. B3 sites are present in{ 111 } crystal planes. Foger andAnderson (ref. 11) have correlated the percent of atoms in{ 111} planes witselectivity (Si). Those authors assumed a cubo-octahedral and octahedralshapes.

A B

Figure Sa) Bright field image of an hexagonal particle showing a triangulcfringe.

Figure Sb) Weak beam dark field image corresponding to the particle infigure Sa) showing thickness fringes.

Figure 6. Cubo-octahedron shaped cluster showing the different type ofsites which are present on the surfaces.

In view of these ideas and the results of the present work a calculationwas made of the number of B3 sites (ref. 12) for a regular cubo-octahedronas a function of the average particle diameter. The results are plotted infig. 9 which includes some experimental selectivity points. The Si valueswere obtained from references (ref. 4 and 11) and from the present measure-ments. In fig. 9 we have not normalized the curves as proposed by Andersor.

499

50

010203040ROTATION ANGLE

Figure 7. Plot of the orientations. of the hexagonal particles with respectto the graphite substrate.

neo C,Hrz

~~ ~~: :: 1 I !Ii I:! I I ,. :!;

~&Figure 8. Squematic representation of model for isomerization of neo penta-

ne

(Si = 90% for a 100% < 111> oriented film) because a value of Si > 90% hasbeen reported for a Pt/ y -Al Z03 catalyst (Si = 100%) treated at hightemperatures (ref. 13). As can be seen from the figure the best experimen-tal agreement is found for larger sizes. It should noted that discre-pancies exist with the data reported by the authors of (ref. 4 and 11),who used different preparation conditions and different supports from theones in the present work. Indeed there is no evidence that the particlesin those catalysts were cubo-octahedrons.

500

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Figure 9. Change on the percent of 3 - atom arrays in {111 } surfaces of aperfect cubo-octahedral particle (continuos line) as a functionof diameter of the particle. Selectivity points are shown bysquares. Point 1.Z and 3 are from ref. 10, and point 5 is fromref. 4.

6. DISCUSSION AND CONCLUSIONS

The data plot in figure 9 represents a very encouraging correlation betweenthe particle shape and crystallography with selectivity for isomerizationdata. It seems that indeed the electron microscopy is a very powerfull tech-nique to obtain such data.

The platinum that have been characterized are cubo-octahedron having {111 }and {100 }facets. It is worth to mention that in the case of large singlecrystals of Pt Leed experiments (ref. 14-15) show that the (100) surfaceis reconstructed to an hexagonal array. At the present time there is noexperimental evidence of that reconstruction also occurs in small metalclusters. However a theoretical considerations indicate that those effectsshould be more dramatic in the case of small finite crystalls. If re-construction of (100) surfaces is present the number of B3 sites will beincreased. We are currently investigating these effects by matching ex-perimental and computer calculated images for a number of particle sizes.

The experimental evidence shows that the shape of the HI and HZ particles isa cubo-octahedron in <111> and <110> orientation. The figures being eithercomplete or truncated. Although hexagonal Pt particles have been reportedin other works (ref. 16 and 17). This is the first time that an unambigousdetermination of the reol shape is made. The 'Pill-box 'shape reported byBO~ker et al. (ref. 17) migth correspond to a truncated cubo-cotahedron par-ticle of figure 5. All the particles observed were single crystalline incontrast with the findinds of Chen and Schmidt (ref. 16) who reported for

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TAB L E No.

Activity of Pt!C catalyst in neopentane reaction at300 and 360°C, HZ!neopentane = 10 , 1 atm.

IProduct (300°C) (360°C specific rate specific rateat 300°C at 360°C(x 8 -1-1 (x 8 - 1 - 110 moles-s -g ) 10 moles-s -g )

HYdrogenolysis Total specific activities of hydro-genolysis reported to 1 gram of Pt

~ethane 43 50.7

ethane 13.5 14.9 7.6 14.9

propane 1Z. 3 16.5

'sobutane 31.1 17.8

isomerization specific rate of isomerizationselectivity

'sopentane 87.2 82.8 51 .8 72.1

Pt in 51°2 about of 20% of twinned particles. In adition the particles werefound in an epitaxial orientation with respect to the rubstrate. These twofacts suggest that there is an strong interaction between the support andthe clusters. This interaction is likely to play an important role in thecatalitic properties of the Pt-C system.

The present results also indicate that the lattice parameter of the Pt clus-ter remains very close to the bulk value. That is in agreement with theEXaF5 measurements of Moraweck et. al. (ref. 18) that found an interatomicdistance for Pt clusters very close to the bulk value.

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REF ERE N C E S

(1) M. Jose Yacaman and 1. Ocana. Phys. Stat. Sol. a) !Z.., 571 (1977).(2) M. Jose Yacaman , K. Heinemann and H. Poppa Chemistry and Physics

of Solid Surfaces III Edited by R. Vanselow. CRC. Press.(3) J.B. Warren. In introduction to analytical electron microscopy

Edited by J.J. Hren, J. Goldstein and D. Joy. Plenum Press (1979).{4) M. Boudart, A.W~ Aldag, L.D. Ptak and J.E. Benson. J. of Catal.

D, (1 968) 35.(5) J. R. Anderson and N.R. Avery. J. Catal. ~, 542-544 (1963).(6) C.H. Bartholomew and M. Boudart J. Catal 25, (1972) 173.(7) M. Boudart and L.D. Ptak. J. of Catal. 16 (1970) 90.(8) M. Jose Yacaman, K. Heinemann, C. Yang and H. Poppa. J. of Cryst.

Growth.(9) M. Jose Yacaman and J.M. Dominguez. Surface Science in Press.(10) A. G6mez and D. Dingley. To be published.(11) K. Foger and J.R. Anderson. J. of Catal. ~,318 (1978).(12) R. Van Hardeveld and A. Van Montfort. Surf. Sci. i (1966) 369-430.(13) J. M. Dominguez. Ph.D. Thesis (1977). Claude Bernard University

Lyon.(14) K. Muller, P.Heilmann, K. Heinz and K. Muller. Surface Science 83

(1979) 487.(15) K. Muller, P. HeImann, K. Heinz and G.G. Waldecker. Vakuum. Rech.

~ (1976) 227.(16) M. Chen and L.D. Schmidt. J. Catal. ~, (1978) 348.(17) R.T. Baker, E.B. prestridge and R.L. Garten J. Catal. ~ (1979) 390.(18) B. Moraweck, G. Clugnet and A.J. Renouprez. Surf. Sci. ~, (1979)

L631.

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DISCUSSION

J.M. DOMINGUEZ, J. YACfu~

Renouprez - Down to what particle size can you obtain a diffraction pattern? What is theinfluence of diffraction by the support on the quality of the metal diffraction pattern ?

Yacaman - By using dynamical diffraction theory, one can find orientations in which thebeams coming from the particles will travel in the substrate without being diffracted orsuffering stronganormalous adsorption. In those conditions the substrate influence isreduced to a minimum. With the tilting stage on the microscope, we can reproduce thoseconditions in practice.

Wynblatt - 1°/ Are the particular sets of orientations you observe, for particles ongraphite, consistent with coincidence type models of a (OOOI)-graphite/metal interface

2°/ How difficult is it to distinguish between the thickness fringes you ob-tain and, say, interfacial dislocations?

Yacaman - 1°/ Yes, all the orientations that we observed are predicted by the coinci-dence model.

2°/ Interfacial dislocations will not interact with thickness fringes. Ascan be seen in figure Sb) moire fringes and thickness fringes can coexist. Moire fringeson the other hand will be very difficult to distinguish from interfacial dislocation.That is why we prefer not to use the moires for quantitative measurements.

Gillet - Are the shapes you observe equilibrium shapes? If so, what is the influenceof the substrate ? Are those shapes taken during the growth or are they due to diffe-rent treatments during catalytic processes ?

Yacaman - We have observed the particles after and before the reaction and the shape isnot altered. Moreover, the treatment undergone by the particles leads them to theirequilibrium shapes. The fact that the particles are in a well defined relation withthe substrate suggests a strong particle-substrate interaction, which has to be con-sidered when calculating the equilibrium shape.