Intermediates in the Selective Reduction of NO by Propene over Cu-Al_2O_3 Catalysts: Transient in-Situ FTIR Study

摘要

The mechanism of the selective catalytic reduction (SCR) or NO by C_3H_6 on Cu-Al_2O_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_3H_6 + O_2 reaction, the acetate is produced via the partial oxidation of C_3H_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_2O_3 catalysts; it plays crucial roles in all the important steps, including the reaction of the acetate with nitrates.