Determination of thermal amplitude of surface atoms in a supported Pt catalyst by EXAFS spectroscopy

Abstract

A temperature sequence of EXAFS measurements was carried out on a highly dispersed Pt/SiO2 catalyst to determine the temperature dependence of atomic structure in the very small supported clusters. A one‐shell EXAFSmodel was fit to the first neighbor oscillations in the catalyst data to determine bond lengths and relative thermal motion (disorder) for an average cluster environment. A Debye model of disorder was employed in the EXAFS analysis. The disorder measured for the catalyst was determined to be 1.3 to 2 times the value determined for bulk Pt over all temperatures. Asymmetry of the radial distributions of nearest neighbors in the largely surface coordinated clusters and the increased atomic disorder lead to underestimates of the nearest‐neighbor distance R 1 at higher temperatures. The apparent bond distance contraction with temperature is manifest in the data as a retardation in the phase of the first neighbor oscillations. It is demonstrated that modelEXAFS functions employing asymmetric interatomic pair potentials (such as the Morse potential) exhibit very similar bond distance contractions to the catalyst data while maintaining similar estimates for the disorder. Finally, a two‐shell EXAFSmodel was fit to the catalyst data to separately determine the motion of the surface coordinated atoms in the small metal clusters.