A mechanistic scheme of N2O and N-2 formation in the selective catalytic reduction of NO with NH3 over a Ag/Al2O3 catalyst in the presence and absence of H-2 and O-2 was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N-2 and N2O are formed via direct or oxygen-induced decomposition of surface NH2NO species. These intermediates originate from NO and surface NH2 fragments. The latter NH2 species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N2O and O-2. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgOx particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N2O production in the presence of H-2 is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N2O decomposition. (C) 2008 Elsevier B.V. All rights reserved.