Selective catalytic reduction of NO by methane over CeO2-zeolite catalysts- Active sites and reaction steps

The selective catalytic reduction of NO with methane over catalysts consist ing 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 re action mechanism. Methods employed include catalytic studies over various c atalyst arrangements, e.g., experiments involving the SCR of NO2 or NO over the catalyst or its individual components (CeO2, zeolite), and over layere d arrangements of these components (typically 1000 ppm NO (NO2), 1000 ppm C H4, 2% O-2 in He, 10,000 h(-1)), studies of NO oxidation activity of cataly st components, and IR spectroscopy in the diffuse reflectance mode. It was found that the NO reduction includes a bifunctional interaction between red ox sites at the CeO2 surface and zeolite Bronsted sites. The oxidation of N O to NO2, which is part of the reaction mechanism over these catalysts, tak es 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 (a s 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 tra nsport of NO2. The experimental evidence suggests that methane is activated on the CeO2 surface, possibly assisted by adsorbed NO2, and reacts with th e latter to form a short-lived intermediate (nitromethane or nitrosomethane ), which is detached from the surface. Upon further contact with a CeO2 sur face, this intermediate becomes totally oxidized, releasing the nitrogen in oxidized form. Upon desorption into the zeolite it may be activated by Bro nsted sites and undergo reactions resulting in the formation of N-2, carbon oxides, and water. (C) 2001 Academic Press.