Catalytic reduction of nitrogen oxides by propene in the presence of oxygen over Cerium ion-exchanged zeolites .3. Effects of carriers

The mechanism of catalytic reduction of NO, by propene in the presence of oxygen, NO (or NO2) + C3H6 + O-2 reaction, over Ce-supported catalysts was studied, focusing on the relative activities of catalysts and carriers for several relevant reactions. The rate of N-2 formation in the NO + C3H6 + O-2 reaction was very much dependent on the kinds of carriers, the rate being in the order of Ce-ZSM-5 > Ce-mordenite much greater than Ce-Y zeolite > Ce/SiO2. This order was very different from the order observed for the rate of NO + 1/2O(2) --> NO2, that is, Ce-mordenite > Ce-ZSM-5 approximate to Ce-Y zeolite approximate to Ce/SiO2, although the latter reaction was the initial and indispensable step for the NO + C3H6 + O-2 reaction [C. Yokoyama and M. Misono, J. Catal. 150, 9 (1994)]. However, the former order was the same as that for the formation of N-2 in NO2 + C3H6 + O-2 reaction over the carriers alone (Na-ZSM-5 > Na-mordenite > Na-Y zeolite approximate to silicalite > SiO2). It was confirmed that NO + C3H6 + O-2 reaction did not proceed on the carriers alone as they had little activity for NO oxidation. These activity orders were reasonably explained by the important role of carriers in the formation of N-2 from the reaction between propene and NO2, which is the step subsequent to the oxidation of NO to NO2 in the NO + C3H6 + O-2 reaction. The rates of NO2 + C3H6 + O-2 reaction over the carriers (Na-zeolites, silicalite, and silica) were proportional to the amounts of propene adsorbed on the carriers at room temperature. On the other hand, the amount of NO2 adsorbed was significantly greater and comparable with the Na content. On the basis of these results, it is proposed that the active sites of these carriers for this reaction are Na ions which can adsorb propene in addition to NO2. Accordingly, it is suggested that the active sites of Ce zeolites are Ce ion exchanged at these active Na sites. (C) 1996 Academic Press, Inc.