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, empha sizing on the roles of the coexisting oxygen and the doped metal ions. Na-, Sr-, and Cu-ZSM-5 were also studied for comparison. Catalysts ha ving high activity for the oxidation of NO to NO2 (Ce- and Cu-ZSM-5) w ere 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 inac tive for NO + C3H6 + O-2 reaction. Even the latter catalysts, however, became active in the case of the NO2 + C3H6 + O-2 reaction. These res ults 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 disappe ared 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-exchan ged zeolites that the ratio between N-2 and N2O produced by the reduct ion of NO was similar to those observed in the oxidations of nitrometh ane and trimethylamine for each catalyst, indicating that these two re actions proceed via similar reaction intermediate(s). On the basis of these results, the mechanism of the reduction of NO was deduced as fol lows. The overall reaction is divided into three steps: (1) NO is oxid ized 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 i on also acts as a catalytic center. [GRAPHICS] (C) 1994 Academic Press , Inc.