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 H2 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 H2 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 (∼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.