Addition of H-2 to a NO/NH3/O-2/H2O feed for selective catalytic reduction of nitrogen oxide over Ag/Al2O3 catalysts causes an unusual enhancement of activity, e.g., the marginal activity (< 10%) of 1 wt% Ag impregnated on γ-Al2O3 or mesoporous Al2O3 modi. cations is boosted to nearly 100% over a broad temperature range from 200 to 550 &DEG;C at a space velocity of 30,000 cm(3) g(-1) h(-1)). Contrary, silver on SiO2 or α-Al2O3 shows no improvement of activity in the presence of H-2. The effect is tentatively attributed to a higher percentage of intermediary nano-sized Ag clusters on high-surface area Al2O3 in the presence of hydrogen. This promotes oxygen activation and hence NO oxidation to reactive intermediate nitrite species. The required dispersion of Ag cannot be stabilized on SiO2 or α-Al2O3.