Determination of the metal particle size of highly dispersed Rh, Ir, and Pt catalysts by hydrogen chemisorption and EXAFS

Hydrogen-to-metal (H/M) ratios exceeding unity for Pt and Rh and exceeding 2 for Ir were measured for highly dispersed Pt, Rh, and Ir catalysts supported on Al2O3 and SiO2. Since the coordination of hydrogen to metal atoms is unknown for such highly dispersed catalysts, the metal surface area of these catalysts cannot be calculated from the hydrogen chemisorption values. Therefore EXAFS (extended X-ray absorption fine structure) measurements were performed to determine the metal particle size and thereby to calibrate hydrogen chemisorption results. The H/M ratio determined by hydrogen chemisorption is a linear function of the average metal coordination number determined by EXAFS. This linear relationship is independent of support but varies with the metal with the H/M ratio increasing in the order Pt <Rh <Ir. Several hypotheses for the high H/M values are discussed. Spillover and subsurface hydrogen are excluded as explanations and only multiple adsorption of hydrogen on metal surface atoms is shown to be capable of explaining all experimental observations. The H/Msurface stoichiometry differs among Pt, Rh, and Ir in the order H/Pt <H/Rh <H/Ir, analogous to the order of stability of corresponding metal polyhydride complexes and of theoretical expectation.