Catalytic efficiency of gas phase and alumina-supported bimetallic AgnAum (n + m = 2-4) alloy clusters is studied using density functional methods As a pre-requite, adsorption of O-2 and CO molecules, and co-adsorption of both molecules on these clusters are studied in detail. O-2 and CO are co-adsorbed on nearby sites on the gas phase tetramer clusters Ag2Au2 and Ag3Au. But their catalytic efficiency is hindered by large barriers (1.55 eV and 1.44 eV, respectively) to the breaking of O-O bond. Among the deposited clusters, Ag2Au and AgAu2 have O-2 and CO co-adsorbed on nearby locations. Of these two, Ag2Au has a lower kinetic barrier for subsequent CO2 formation. Thus Ag2Au looks the most promising candidate. (C) 2017 Elsevier B.V. All rights reserved.