Analysis of in situEXAFS data of supported metal catalysts using the third and fourth cumulant

Abstract

X-Ray absorption spectra of supported Pt catalysts with various Pt cluster sizes were collected between 77 and 673 K, in inert and hydrogen atmospheres. When analyzing these spectra with the standard EXAFS equation, a Pt–Pt bond contraction and a large increase in the inner potential correction were observed with increasing temperature. These errors are up to 0.08 Å and 10 eV for clusters of 1 nm diameter. They were corrected by including the third and fourth cumulants as fit parameters. Fit guidelines were developed to analyze EXAFS data of supported metal catalysts collected at elevated temperatures, allowing for asymmetry and broadening or sharpening of the pair distribution function. These comprise fixing fit parameters, using different k-weightings and identifying trends in a series of experiments. By fitting the EXAFS spectra using these guidelines, it was determined that in small Pt clusters the Pt–Pt bond is 0.10 Å shorter than in bulk Pt. These contracted bonds relax to distances near that of bulk Pt upon hydrogen chemisorption.