CATALYTIC REDUCTION OF NITROGEN-OXIDES BY PROPENE IN THE PRESENCE OF OXYGEN OVER CERIUM ION-EXCHANGED ZEOLITES .2. MECHANISTIC STUDY OF ROLES OF OXYGEN AND DOPED METALS

The mechanism of catalytic reduction of NOx by propene in the presence of oxygen (NO + C3H6 + O-2 reaction) over Ce-ZSM-5 was studied, emphasizing on the roles of the coexisting oxygen and the doped metal ions. Na-, Sr-, and Cu-ZSM-5 were also studied for comparison. Catalysts having high activity for the oxidation of NO to NO2 (Ce- and Cu-ZSM-5) were also very active for the NO + C3H6 + O-2 reaction. In contrast, Na- and Sr-ZSM-5, which have little activity for NO oxidation, were inactive for NO + C3H6 + O-2 reaction. Even the latter catalysts, however, became active in the case of the NO2 + C3H6 + O-2 reaction. These results indicate that the initial step of the NO + C3H6 + O-2 reaction is the oxidation of NO to NO2. NO2 thus formed reacts with propene very rapidly to form N-containing organic compounds, as NO, rapidly disappeared even at a very low contact time for the NO2 + C3H6 + O-2 reaction, and small quantities of C2N2 and HCN were formed, together with the imbalances of N, C, and O. It was further found for several ion-exchanged zeolites that the ratio between N-2 and N2O produced by the reduction of NO was similar to those observed in the oxidations of nitromethane and trimethylamine for each catalyst, indicating that these two reactions proceed via similar reaction intermediate(s). On the basis of these results, the mechanism of the reduction of NO was deduced as follows. The overall reaction is divided into three steps: (1) NO is oxidized to NO2 by oxygen, which is accelerated by Ce ion, (2) NO2 reacts rapidly with propene to form organic nitro- or nitrite-compounds, and (3) these intermediates decompose to N-2 (probably in several parallel and consecutive steps) by the reaction with NO and/or O-2, where Ce ion also acts as a catalytic center. [GRAPHICS] (C) 1994 Academic Press, Inc.