Silver clusters on theta-Al2O3 support catalyze highly chemoselective reduction of a nitro group for the reduction of substituted nitroaromatics such as nitrostyrene. These catalysts show higher selectivity than conventional platinum-group metal-based heterogeneous catalysts. Systematic studies on the influence of the metal particle size and support oxides show that the intrinsic activity increases with decrease in the silver particle size, and acid-base bifunctional supports such as Al2O3 give higher activity than acidic or basic supports. Kinetic and in situ infrared studies provide a reaction mechanism which explains fundamental reasons of these tendencies. Cooperation of the acid-base pair site on Al2O3 and the coordinatively unsaturated Ag sites on the silver cluster is responsible for the rate-limiting H-2 dissociation to yield a H+/H- pair at metal/support interface, while the basic site on oxides acts as an adsorption site of nitroaromatics. High chemoselectivity could be attributed to a preferential transfer of the H+/H- pair to the polar bonds in the nitro group. (C) 2009 Elsevier Inc. All rights reserved.