Mechanistic insight into N2O formation during NO reduction by NH3 over Pd/CeO2 catalyst in the absence of O2

Abstract

N2O is a major by-product emitted during low-temperature selective catalytic reduction of NO with NH3 (NH3-SCR), which causes a series of serious environmental problems. A full understanding of the N2O formation mechanism is essential to suppress the N2O emission during the low-temperature NH3-SCR, and requires an intensive study of this heterogeneous catalysis process. In this study, we investigated the reaction between NH3 and NO over a Pd/CeO2 catalyst in the absence of O2, using X-ray photoelectron spectroscopy, NH3-temperature-programmed desorption, NO-temperature-programmed desorption, and in-situ Fourier-transform infrared spectroscopy. Our results indicate that the N2O formation mechanism is reaction-temperature-dependent. At temperatures below 250 °C, the dissociation of HON, which is produced from the reaction between surface H• adatoms and adsorbed NO, is the key process for N2O formation. At temperatures above 250 °C, the reaction between NO and surface N•, which is produced by NO dissociation, is the only route for N2O formation, and the dissociation of NO is the rate-determining step. Under optimal reaction conditions, a high performance with nearly 100% NO conversion and 100% N2 selectivity could be achieved. These results provide important information to clarify the mechanism of N2O formation and possible suppression of N2O emission during low-temperature NH3-SCR.