Identifying the reaction network complexity and structure sensitivity of selective catalytic oxidation of ammonia over Ag surfaces

Ammonia (NH3) emission has detrimental impacts on environment and human health, and the common method to remove NH3 is the selective catalytic oxidation of ammonia (SCO-NH3). Ag catalysts have shown good reactivity and selectivity for N-2 in this process, but the mechanisms and detail kinetics are still unclear. Herein we use density functional theory combined with microkinetic analysis to study SCO-NH3 on the two representative Ag surfaces, i.e. Ag(1 1 1) and Ag(2 1 1) surfaces. It is found that either the O-assisted or the OH-assisted pathway is preferred depends on the intermediates that are being dehydrogenated and the reaction temperatures. The favorable coupling processes are NH2 + NH or NH + NH on Ag(1 1 1) and NH + NH or N + N on Ag (211). Atomic O can occupy the Ag(1 1 1) surface at temperatures lower than 400 K, while both O and OH have obvious coverage at temperatures lower than 600 K on Ag(2 1 1). Oxygen dissociation is the rate-determining step at temperatures higher than 600 K on the two Ag surfaces, and NH + NH coupling is rate-determining at temperatures lower than 450 K on Ag(2 1 1). By comparing the energetics of the conversion routes of NH and N, we find that they may have higher possibility to be oxidized on Ag(2 1 1) than on Ag(1 1 1).