Influence of O 2 and H 2 on NO reduction by NH 3 over Ag/Al 2O 3: A transient isotopic approach

Abstract

Mechanistic aspects of low-temperature (423–723 K) selective catalytic reduction of NO with NH 3 (NH 3-SCR) over an Ag(1.7 wt%)/Al 2O 3 (2Ag/Al 2O 3) 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/Al 2O 3 showed very low activity in the NH 3-SCR reaction. The activity increased tremendously after ex situ reduction of 2Ag/Al 2O 3 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 NH 3 dehydrogenation, yielding reactive NH x fragments that are important intermediates for nitrogen formation via a coupling reaction between NO and NH 3. This reaction pathway predominated over direct 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 NH 3-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 NH 3-SCR reaction, and also that H 2 helps decrease total N 2O production.