Selective Catalytic Reduction of NO by Methane over CeO2–Zeolite Catalysts—Active Sites and Reaction Steps

Abstract

The selective catalytic reduction of NO with methane over catalysts consisting of CeO2 and H-zeolites (physical mixtures or precipitates of the oxide onto the external surface of the zeolite) has been studied with the aim to establish the catalytic functions required and the basic features of the reaction mechanism. Methods employed include catalytic studies over various catalyst arrangements, e.g., experiments involving the SCR of NO2 or NO over the catalyst or its individual components (CeO2, zeolite), and over layered arrangements of these components (typically 1000 ppm NO (NO2), 1000 ppm CH4, 2% O2 in He, 10,000 h−1), studies of NO oxidation activity of catalyst components, and IR spectroscopy in the diffuse reflectance mode. It was found that the NO reduction includes a bifunctional interaction between redox sites at the CeO2 surface and zeolite Brønsted sites. The oxidation of NO to NO2, which is part of the reaction mechanism over these catalysts, takes place over the CeO2 surface, but it is not the only function of CeO2 in the reaction path that provides an extra activity of the mixed catalysts (as compared to the known activity of H-ZSM-5). From experiments with layered catalyst arrangements, it was concluded that the bifunctional interaction is not mediated by a long-distance transport step as would be gas-phase transport of NO2. The experimental evidence suggests that methane is activated on the CeO2 surface, possibly assisted by adsorbed NO2, and reacts with the latter to form a short-lived intermediate (nitromethane or nitrosomethane), which is detached from the surface. Upon further contact with a CeO2 surface, this intermediate becomes totally oxidized, releasing the nitrogen in oxidized form. Upon desorption into the zeolite it may be activated by Brønsted sites and undergo reactions resulting in the formation of N2, carbon oxides, and water.