Interaction of oxygen with supported Ag-Au alloy catalysts

The effects of alloying silver with gold on the oxygen adsorption properties of Ag are studied over a set of 15% (Ag-Au)/alpha-Al2O3 catalysts of variable alloy composition, using microgravimetric and temperature desorption techniques. Three adsorbed oxygen species are observed on the alloy surfaces at elevated temperatures, namely molecular, atomic and subsurface. It is found that geometric and electronic alterations induced by the presence of Au influence the adsorption characteristics differently for each species. When the Au content of the surface increases, the population of atomic oxygen decreases, its activation energy of adsorption increases, and the Ag-O bond weakens, while the enthalpy of adsorption decreases linearly from approximately 170 kJ/mol over pure Ag to about 70 kJ/mol over surfaces containing 24 at.% Au. Molecular oxygen adsorption on Ag is favored by the presence of Au, as indicated by the decrease of its activation energy of adsorption with increasing Au content. Finally, subsurface oxygen diffusion is strongly inhibited by the presence of gold, and the activation energy of this process increases from 100 kJ/mol for pure Ag to 400 kJ/mol for samples containing 24 surface at.% Au. These results are discussed evoking geometric and electronic alterations induced by the presence of Au on the catalytic surfaces. (C) 1996 Academic Press, Inc.