The effect of supports on the selective catalytic reduction (SCR) of NO by propane and hydrogen over Ag/alumina catalysts was investigated by using alumina supports modified by incorporation of various additives (Zn, Mg, Y, P and Na). The doping of Zn and Mg into alumina support enhanced the conversion of NO to N-2, while that of Na and P significantly decreased the NO conversion. The Zn-doping also increased the selectivity, while other additives decreased the selectivity. Comparing the rate of NO reduction and acid amount determined by NH3-TPD, the contribution of surface acid sites of alumina supports to the promotion of the SCR activity of Ag was indicated. As an exception, although the P-doping increased the acid amount of alumina, Ag/P-alumina only showed poor SCR activity. By using in situ FT/IR spectra, the poor SCR activity of Ag/P-alumina was explained by very low surface concentration of adsorbed nitrates and acetate under the reaction conditions. It was also suggested that the acid sites of alumina effectively stabilize Ag-n(delta+) clusters in the hydrogen containing atmospheres, which results in the high activity and selectivity on the SCR reaction. (C) 2007 Elsevier B.V. All rights reserved.