Hydrogen Adsorption and Absorption with Pd-Au Bimetallic Surfaces

Pd-Au bimetallic catalysts have shown promising performance in numerous reactions that involve hydrogen. Fundamental studies of hydrogen interactions with Pd-Au surfaces could provide useful insights into the reaction mechanisms over Pd-Au catalysts, which may, in turn, guide future catalyst design. In this study, the interactions of hydrogen (i.e., adsorption, absorption, diffusion, and desorption) with Pd/Au(111) model surfaces were studied using temperature-programmed desorption (TPD) under ultrahigh-vacuum conditions. Our experimental results reveal Pd-Au bimetallic surfaces readily dissociate H-2 and yet also weakly bind H adatoms, properties that could be beneficial for catalytic reactions involving hydrogen. The presence of contiguous Pd sites, characterized by reflection-absorption infrared spectroscopy using CO as a probe molecule (CO-RAIRS), was found to be vital for the dissociative adsorption of H-2 at 77 K. The H adatom binds to Pd-Au alloy sites more strongly than to Au(111) but more weakly than to Pd(111) as indicated by its desorption temperature (similar to 200 K). With hydrogen exposure at slightly higher temperatures (i.e., 100-150 K), extension of a low-temperature desorption feature was observed, suggesting the formation of subsurface H atoms (or H absorption). Experiments using deuterium indicate that H-D exchange over the Pd-Au bimetallic surface obeys Langmuir-Hinshelwood kinetics and that H/D adatoms are mobile on the surface at low temperatures.