Asymmetric pair distribution functions in catalysts

The structural parameters, i.e., coordination numbers, bond distances and d isorder obtained from the analysis of EXAFS spectra may sometimes be signif icantly influenced by errors introduced due to the inadequacy of the analys is method applied. Especially in the case of heterogeneous catalysts it has been realized that often there is a need to use an improved EXAFS data ana lysis compared to the simple harmonic approach which works well for well-de fined bulk structures. This is due to the fact that catalysts contain highl y dispersed or disordered structures with pair distribution functions quite different from well-crystalline bulk materials. In addition, the increased interest for in situ studies, which typically imply high temperatures, mak es the influence from anharmonic vibrations on the shape of the pair distri bution a significant factor. In this paper, we discuss the importance of as ymmetric pair distribution functions for nano-sized particles and how they influence the structural parameters obtained from the standard data analysi s. An alternative method, which takes into account deviations from the Gaus sian pair distribution function typically used in the analysis of EXAFS spe ctra, will be described. The method is based on an analysis of the pair dis tribution functions derived from molecular dynamics simulations of small me tal particles and its reliability is demonstrated by comparing structural p arameters obtained from independent X-ray diffraction experiments.