Mechanistic aspects of low-temperature (423-723 K) selective catalytic reduction of NO with NH3 (NH3-SCR) over an Ag(1.7 wt%)/Al2O3 (2Ag/Al2O3) catalyst in the presence and absence of O-2 and H-2 were studied using a transient low-pressure (peak pressure < 10 Pa) technique, the temporal analysis of products (TAP) reactor, in combination with isotopic traces. Preoxidized 2Ag/Al2O3 showed very low activity in the NH3-SCR reaction. The activity increased tremendously after ex situ reduction of 2Ag/Al2O3 in a hydrogen flow (5 vol% H-2 in Ar) at 373 K for 30 min. This observation was related to the creation of reduced Ag species, which catalyze O-2 and NO dissociation. yielding adsorbed oxygen species. O-2 is a better supplier of oxygen species. Oxygen species played a key role in NH3 dehydrogenation, yielding reactive NH., fragments that are important intermediates for nitrogen formation via a Coupling reaction between NO and NH3. This reaction pathway predominated overdirect NO decomposition to N-2 in the presence of O-2. In addition to generation of active oxygen species. gas-phase oxygen accelerated transformation of Surface N-containing intermediates into gas-phase reaction products. The role of hydrogen in the NH3-SCR reaction is to transform oxidized Ag species into reduced species that are active sites for O-2 and NO adsorption. Our findings suggest that the reduction of oxidized Ag is responsible for the boosting effect of H-2 in the NH3-SCR reaction, and also that H-2 helps decrease total N2O production. (c) 2006 Elsevier Inc. All rights reserved.