Numerical Simulation of the Platinum L III Edge White Line Relative to Nanometer Scale Clusters D. Bazin,* ,² D. Sayers, ‡ J. J. Rehr, § and C. Mottet | LURE, Ba ˆ t. 209 D, UniVersite ´ Paris-Sud, 91405 Orsay Cedex, France, Department of Physics, North Carolina State UniVersity, Raleigh, North Carolina 27695-8202, Department of Physics, Washington UniVersity, Seattle, Washington 98195, and CRMC 2 , Campus de luminy, Case 913, 13288, Marseille Cedex 9, France ReceiVed: December 2, 1996; In Final Form: May 2, 1997 X From an experimental point of view and more particularly in heterogeneous catalysis, the L III white line is at the center of electronic charge transfer either between either the nanometer scale metallic particle and the support or between the two metals which are present inside the cluster. In this work, we show that a strong correlation exists between the intensity of the white line and the size of the cluster. Thus, at least two physical phenomenon can affect the intensity of the white line: the size of the cluster, which can be considered as an intrinsic effect (density of state of nanometer scale platinum cluster are far from the bulk one), and a possible charge transfer between the cluster and the support, which can be considered as an extrinsic one. If the first results obtained with the FeFF program are encouraging, it is clear that to go further in the analysis, the detailed geometric configurations present in the cluster surface have to be integrated very precisely in order to obtain quantitative effects that would be more clearly related to the characteristics of the density of states. 1. Introduction At the L III X-ray absorption edge (XANES), a phenomenon discovered a long time ago 1 and called the “white line” is often observed. It is interpreted as due to electronic transitions from a core level, 2p 3/2 for the L III edge, to vacant d states of the absorbing atom and is thus distinct from the extended X-ray absorption fine structure. 2-9 Quickly recognized as a powerful tool to characterize in situ d-electron configuration in cata- lysts, 10,11 it has motivated numerous works performed on monometallic or multimetallic systems to determine charge transfer between either the metallic particle and the support or the molecule adsorbed at the surface of the particle, 12-14 or between the two metals present inside the particle. 15,16 Because this approach can be made in situ, 17 experiments are made either after 18 or during the chemical reaction. 19,20 In this work, we consider nanometer scale sized metallic clusters which are classically used in numerous industrial processes such as reforming or postcombustion. For this kind of material, we try to define an experimental approach using the different techniques related to synchrotron radiation. In particular the necessity to use X-ray absorption spectroscopy as well as anomalous diffraction in order to obtain a fine structural description has already been underlined. 21-27 More- over, using new codes liked FEFF based on the development of the scattering series 28-30 or CONTINUUM, 31 it has been shown that the XANES of K absorption edge can be used to obtain information regarding the size of the metallic particle. 32,33 Let us now consider the L III edge and more particularly the so-called “white line” to evaluate the effect of different parameter on this particular feature. We will focus here on nanometer scale platinum clusters with the FCC structure (O h symmetry), namely, cuboctahedra with 13 and 55 atoms. 2. Comparison between FEFF6 and FEFF7 Numerous works deal with the simulation of the XANES spectrum of the platinum metallic foil that exhibits the so-called white line. As pointed by Zabinsky et al., 34,35 this edge is generally reproduced in the framework of a muffin-tin multiple scattering approach by using large clusters of few hundreds of atoms in which the absorbing atom is always at the center of the cluster. The result is compared to the experimental spectrum for a 147-atom cluster (Pt 147 ) in Figure 1, showing an overall agreement concerning both the white line and the other structures. However, the intensity of the line is not the same for a surface atom, which has to be taken into account in particular if one considers smaller clusters, such as Pt 13 (see Figure 2). More- over, in the latter case, the edge is shifted (E ) 11 560 eV), which is better reproduced by using the new version of the FEFF code (FEFF7) than the previous one (FEFF6). Therefore, in the following all the XANES spectra will be calculated using the FEFF7 code. 3. Nanometric Platinum Cluster In the following, we will consider clusters of increasing size built on an FCC lattice by adding to a central atom (labeled * To whom correspondence should be addressed. ² Universite ´ Paris-Sud. ‡ North Carolina State University. § Washington University. | CRMC 2 . X Abstract published in AdVance ACS Abstracts, June 15, 1997. Figure 1. Comparison between the white line associated with the platinum foil calculated with FeFF6 (dots) and experimental (line). 5332 J. Phys. Chem. B 1997, 101, 5332-5336 S1089-5647(96)03949-1 CCC: $14.00 © 1997 American Chemical Society