Influence of O2 and H2 on NO reduction by NH3 over Ag/Al2O3: A transient isotopic approach

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 O2 and H2 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% H2 in Ar) at 373 K for 30 min. This observation was related to the creation of reduced Ag species, which catalyze O2 and NO dissociation, yielding adsorbed oxygen species. O2 is a better supplier of oxygen species. Oxygen species played a key role in NH3 dehydrogenation, yielding reactive NHx fragments that are important intermediates for nitrogen formation via a coupling reaction between NO and NH3. This reaction pathway predominated over direct NO decomposition to N2 in the presence of O2. 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 O2 and NO adsorption. Our findings suggest that the reduction of oxidized Ag is responsible for the boosting effect of H2 in the NH3-SCR reaction, and also that H2 helps decrease total N2O production.