Modification of formate stability by alloying: the Cu(100)-c(2 x 2)-Pt system

Room temperature deposition of Pt on Cu(100) has been studied by LEED, AES and desorption spectroscopy indicating that sub-monolayer Pt growth at 300 K leads to formation of a poorly ordered c(2 x 2) surface alloy co-adsorbed with Pt microclusters. In contrast to the Cu(100)-c(2 x 2)-Pd system, a high degree of crystalline perfection requires thermal activation which leads to surface Pt atoms switching to second layer sub-surface sites to form a c(2 x 2) underlayer below a copper monolayer. The presence of Pt in the second layer leads to electronic perturbation of the outermost copper monolayer. Formic acid adsorption on the Cu(100)-c(2 x 2)-Pt sub-surface alloy leads to formation of a formate intermediate with reduced stability relative to clean Cu(100) signalled by a similar to 30 K downward shift in the simultaneous CO2/H-2 evolution. The formate decomposition on the Cut 100)-c(2 x 2)-CuPt underlayer follows first order decomposition kinetics and the decomposition activation energy is reduced from 119 kJ mol(-1) (Cu(100)) to 110 kJ mol(-1) on the CuPt underlayer alloy. Thicker Pt films with Pt coverages of 1 ML up to 2.5 ML containing significant quantities of Pt in the outermost layer in a local c(2 x 2) environment lead to an additional downward shift in the formate decomposition temperature to 406 K corresponding to a decomposition activation energy of 104 kJ mol(-1). This activation energy corresponds to the formate stability on a mixed CuPt bimetallic site. Thus, the surface chemistry of Pt in Cu3Pt alloys differs significantly from that of pure Pt resulting in a much increased stability for the formate intermediate.