The effect of polydispersity and inhomogeneity of supported bimetallic catalysts on the EXAFS analysis is investigated with some simple model calculations. These show that EXAFS is very insensitive to polydispersity. Polydispersity and inhomogeneous distribution of the metals over the particles however have only limited influence on the ability to distinguish between core-shell particles and particles with random distribution of both metals

Bazin, D.

Dexpert, H.

Lefferts, L.

Moonen, J.

Slot, J.

English

Moonen, J

Slot, JJM Han

Lefferts, L

Bazin, D

Dexpert, H

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The influence of polydispersity and inhomogeneity on EXAFS of bimetallic catalysts

Lefferts, Leonardus

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University of Twente Research Information

ELSEVIER Physica B 208&209 (1995) 689-690 The influence of polydispersity and inhomogeneity on EXAFS of bimetallic catalysts J. Moonen  a'*, J. Slot a, L. Lefferts a, D. Bazin b, H. Dexpert b aDSM Research, 6160 MD Geleen, Netherlands bLURE, Universitb de Paris-Sud, 91405 Orsay, France Abstract The effect of polydispersity and inhomogeneity of supported bimetallic catalysts on the EXAFS analysis is investigated with some simple model calculations. These show that EXAFS is very insensitive to polydispersity. Polydispersity and inhomogeneous distribution of the metals over the particles however have only limited influence on the ability to distinguish between core-shell particles and particles with random distribution of both metals. 1. Introduction We used EXAFS to study supported bimetallic catal- ysts with the aim to determine the size of the metal particles and the distribution of the metal in the particles. The size of the metal particles was determined from the number of neighbours found from EXAFS and compared with data obtained from the X-ray diffraction and elec- tron microscopy. The particle sizes as derived from EXAFS were much smaller than those found with line width analysis from the X-ray diffraction. In fact the sizes found with EXAFS indicated that no metal peaks should have been observed at all in the X-ray diffractograms. A possible explanation of the discrepancy was the poly- disperse character of the catalyst. To our surprise we found no literature at all that dealt with the influence of polydispersity on the analysis of EXAFS spectra. Therefore, we did some very simple calculations based only on the number of neighbours, being one of the prime parameters to be determined by EXAFS to find out how sensitive EXAFS is to polydis- persity. Since practical bimetallic catalysts are not only *Corresponding author. polydisperse but also are inhomogeneous with respect to the composition of the individual metal particles we also investigated the influence of inhomogeneity in order to find out if it was possible at all to say something about particle size and metal distribution of realistic bimetallic catalysts. 2. Polydispersity In all calculations that will be discussed FCC lattice symmetry and cubotahedral particle shape are assumed. In Table 1 the number of neighbours in the first four coordination shells (N1-N4) are given for a number of particle sizes. In the last row of the table the data for a bidispers system consisting of 32 wt% of particles of 13 atom and 68 wt% of particles of 1415 atoms are given. The numbers in this row are obtained by taking the weight average number of the relevant preceeding rows. From the numbers N1 to N4 of the second and the fourth row one can conclude that the monodisperse system and the bidisperse system are indistinguishable. Since the dispersity chosen here is large it can generally be stated that EXAFS is very insensitive to polydispersity. Large 0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD! 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 9 1 - 8  690 J. Moonen et al. /Physica B 208&209 (1995) 689 690 Table 1 Number of neighbours in first coordination shells as a function of particle size Nato,~ NI N2 N3 N4 Diameter [,A] 13 5.5 1.85 3.7 0.9 8.3 147 8.9 4.0 13.0 6.1 13.8 1415 10.5 5.0 18.5 9 .15  24.9 Bidispers 8.9 4.0 13.8 6.5 8.3 + 24.9 differences between particle size determined from the X-ray diffraction and EXAFS may well occur. 3. lnhomogeneity EXAFS can provide useful information on the distri- bution of the two metals A and B of a bimetallic catalyst AB. One can determine if one of the metals is preferen- tially in the core of the metal particle or if the metal atoms are distributed randomly. In a real bimetallic catalyst the metal particles will not only be polydisperse but they will also be in- homogeneous as far as their composit ion is concerned. Here the influence of both homogeneity and dispersity on the ability to distinguish between random particles and core-shell particles is investigated. The results are given in Table 2. The homogeneous and monodisperse system contains 147 a tom particles with 9.82 a tom% A atoms. The homogeneous, polydisperse systems consists of par- ticles of the same composit ion and two different sizes, 59.3 wt% of particles of 55 atoms and 40.7 wt% of par- ticles of 1415 atoms. The monodisperse, heterogeneous system consists of 147 atom particles; 50% of which contain 3.3 a tom% A atoms and 50% of which contain 16.3 a tom% A atoms. In the polydisperse, heterogeneous case both effects are combined. The procedure to obtain the numbers in the table is explained for the 9.8 a tom% A, 147 atom particles. An ensemble of 147 atoms of which 9.8 atom % of type A is taken. In the case of a random particle one atom is taken at random from this ensemble and placed in the centre of Table 2 Number of neighbours in the first coordination shell for different bimetallic catalysts Catalyst NAA NAB NaA Nan la 0.8 8.2 0.9 8.1 lb 0.7 6.5 0.7 8.5 Ic 5.5 6.5 0.7 8.0 lla 1.2 7.8 0.8 8.1 lib 1.0 6.2 0.7 8.5 llc 4.5 7.5 0.4 8.3 llla 0.7 9.0 0.8 8.2 ll lb 0.8 6.3 0.6 8.5 llIc 3.5 8.5 0.3 8.6 IVa 1.2 7.7 0.8 8.1 IVb 1.2 5.9 0.6 8.5 IVc 4.0 8.0 0.3 8.5 I: homogeneous, monodisperse. II: heterogeneous, monodis- perse. III: homogeneous, polydisperse. IV: heterogeneous, poly- disperse, a: random AB distibution, b: B core. c: A core. the particle. The second atom is taken at random from the remaining 146 and placed on a position in the second shell starting from the centre. This procedure is con- tinued until all 147 atoms are placed. Then the number  of contacts Nxv where X represents the absorbing a tom and Y represents the backscattering a tom are counted. This procedure is repeated 5000 times, and the numbers Nxv are averaged. For  core shell particles the particles are also filled from the inside, shell by shell. But in that case as many shells as can be filled completely are filled completely with the atoms that are in the core, then the random drawing procedure is used for the shell that consists of two types of atoms and subsequently the remaining shells are filled with the remaining atoms which are all from the second type. In the case of polydis- perse and/or  inhomogeneous particles the numbers obtained in this manner for the different particles are averaged in the appropriate way, thus resulting in the numbers given in Table 2. These calculations show that for core-shell particles and random particles can still be distinguished for inhomogeneous, polydisperse systems. 

Slot, J.J.M.

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https://doi.org/10.1016/0921-4526(94)00791-S

The influence of polydispersity and inhomogeneity on EXAFS of bimetallic catalysts

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