Suppression of Water Formation over Stepped Pt(335) by Au

A partial monolayer of Au strongly modifies the kinetics of water adsorption and formation on the stepped Pt(335) surface. The effects are more than passivation. With a Au coverage of 0.7 ML as measured by Auger spectroscopy, saturation H and O adsorption are only 15% and 8.5%, respectively, of their values on bare Pt. Adsorption at step sites is nearly eliminated. Water formation is reduced to 5% of its bare Pt value: one-sixth of what would be expected if each Au atom blocked one site. Water formation on Pt has multiple reaction pathways. The partial Au layer eliminates only the low-temperature pathway. The H recombination temperature is reduced relative to bare Pt, so H recombination competes with water formation and reduces its efficiency. The partial Au layer also changes the desorption spectrum of water. At low water coverage, there is a single desorption peak, intermediate in temperature between those seen on Pt and Au.