Intermediates in the Selective Reduction of NO by Propene over Cu−Al₂O₃ Catalysts:  Transient in−Situ FTIR Study

Abstract

The mechanism of the selective catalytic reduction (SCR) of NO by C 3 H 6 on Cu−Al 2 O 3 catalysts, which consist of highly dispersed Cu 2+ ions in the surface aluminate phase, are investigated by in-situ FTIR spectroscopy. During NO + C 3 H 6 + O 2 reaction, the acetate is produced via the partial oxidation of C 3 H 6 and becomes the predominant adspecies in the steady-state condition at 473−623 K. The acetate, which is stable in NO, is quite reactive with NO + O 2, leading to the formation of isocyanate species (Cu−NCO) on the surface and N 2 and CO 2 in the gas phase. The rate of acetate reaction in NO + O 2 is close to the steady-state rate of NO reduction over wide range of temperature, indicating that the acetate is an intermediate in the SCR and takes part in the rate-determining stage. A mechanism is proposed; the acetate and nitrates, formed by NO + O 2, react to generate the Cu−NCO species, then Cu−NCO reacts with nitrates or NO to produce N 2 and CO 2 . This mechanism explains the role of oxygen in facilitating SCR. Cu 2+ ion is the principal active component in Cu−Al 2 O 3 catalysts; it plays crucial roles in all the important steps, including the reaction of the acetate with nitrates.