An examination of active sites on Au-Ag bimetallic catalysts based on CO oxidation over Au/Ag2O and a comparison to Ag-contaminated Au powder

Abstract

There are two theories regarding the origin of the remarkable synergistic effect observed in Au-Ag bimetallic catalysts when applied to various oxidative reactions. One is based on the importance of the contact interfaces between AgOx regions and the surface of the bulk Au as active working sites, while the other holds that charge transfer from Ag to Au in a surface Au-Ag alloy causes the catalytic activity. One key point in examining these theories and determining the origin of the synergy involves determining whether or not Ag exists as an oxide or as a metallic alloy on the Au surface. To confirm that enhanced activity results from contact between Ag2O and Au nanoparticles (NPs), a comparative study of catalytic CO oxidation over Au/Ag2O and Ag2O was performed in the present work, using a closed recirculation reaction system. A reaction mixture consisting of a stoichiometric composition of CO and O2 (CO/O2 = 2/1) was supplied to both catalysts and the resulting pressure decrease rates were tracked, from which the amounts of gas consumed as well as the quantity of CO2 produced were determined. The steady state reactions of both Au/Ag2O and Ag2O did not lead to any meaningful difference in the rate of pressure decrease during the oxidation. The pressure decrease over both catalysts was attributed to the reduction of surface lattice O on Ag2O by CO. The results obtained for Au/Ag2O are in good agreement with previous data resulting from the use of Ag-contaminated Au powder (Ag/Au-b) having an oxidized surfaces. This finding suggests that the perimeters between AgOx zones and the bulk Au surface may not function as active sites during CO oxidation. A review of previous results obtained with Ag/Au-b specimens having so-called steady state surfaces indicates that AgOx species in such materials are reduced to the 0 state to form a Ag-Au alloy that provides the active sites.