Theoretical Interpretation of XAFS and XANES in Pt Clusters

Abstract

This paper first briefly summarizes the dramatic progress over the past decade both in fundamental theory and in the interpretation of XAFS and XANES. These developments have led to several ab initio codes such as FEFF for simulating XAFS and XANES, together with compatible analysis codes which permit an interpretation of the spectra in terms of geometrical and electronic properties of a material. As an example of relevance to catalysis, we discuss recent work which interprets the Pt L-edge XANES of PtX clusters based on the self-consistent FEFF8 code. For pure Pt clusters, we find that self-consistency is important in determining the variation of XANES with cluster size. For PtCl clusters, we show that the presence of a Cl–Pt bond leads to a “hybridization peak,” i.e., a peak in the Cl d-density of states (dDOS) mixed with Pt d-states, which can be used as a measure of Cl content. For Pt–H clusters, we show that hydrogen addition is well correlated with the growth of a broad shoulder above the white line. We find that this feature can be attributed largely to AXAFS, i.e., to a change in the atomic background absorption. We also analyze the effect of a support, in terms of model calculations for a realistic Pt₆ cluster within a zeolite-LTL pore.