Formation of N2O greenhouse gas during SCR of NO with NH3 by supported vanadium oxide catalysts

Selective catalytic reduction of NO by NH3 over supported vanadium oxide catalysts has been studied for decades, but the studies mostly concentrated on the dominant N-2 product with much less attention paid to the formation of the undesired N2O product. In the present study, fundamental aspects of the N2O formation reaction were elucidated by a series of temperature-programmed surface reaction studies with isotopic labelled reactants. The surface vanadium oxide species on the TiO2 support are active sites for the N2O formation reaction, while tungsten species mainly function as promoters. Oxygen from NO, gaseous O-2 and catalyst oxygen all function as oxygen sources for formation of N2O (similar to 50%, similar to 30% and similar to 20%, respectively). The rate-determining-step for N2O formation involves breaking of the ammonia N-H bond. These new molecular level insights have the potential to guide the rational design of improved SCR catalysts for diesel engines with reduced N2O produced.