The bifunctional reaction pathway and dual kinetic regimes in NO x SCR by methane over cobalt mordenite catalysts

Abstract

The pathway for selective reduction of NO x by methane over Co mordenite cataysts has been studied by comparing the rates of the individual reactions (NO oxidation, CH 4 oxidation, NO 2 reduction) with that of the combined reaction (NO + O 2 + CH 4). Co (+2) was exchanged into H-MOR and Na-MOR to give catalysts with different metal loading and number of support protons. Additionally, exchanged Co (+2) ions were precipitated with NaOH to produce dispersed cobalt oxide on Na-MOR. The NO oxidation rate is the same for ion exchanged Co (+2) ions in H-MOR and Na-MOR, but the rate of Co (+2) ions is much lower than that of cobalt oxide. NO oxidation equilibrium is obtained only for those catalysts with high metal loading, cobalt oxide or run at low GHSV. Under the conditions of selective catalytic reduction, methane oxidation by O 2 is low for all catalysts. The turnover frequency of Co on Na-MOR, however, is higher than that on H-MOR. The rate of NO 2 reduction to N 2 is directly proportional to the number of support acid sites and independent of the amount of Co. Comparison of the rates and selectivities for the individual reactions with the combined reaction of NO + O 2 + CH 4 indicates that there are two types of catalysts. For the first, the NO oxidation is in equilibrium and the rate determining step is reduction of NO 2. For these catalysts, the rate (and selectivity) for formation of N 2 is identical from NO + O 2 + CH 4 and NO 2 + CH 4. These catalysts have high metal loading and few acid sites. Nevertheless, the rate of N 2 formation increases with increasing number of protons. For the second type of catalyst, NO oxidation is not in equilibrium and is the rate limiting step. For these catalysts the rate of N 2 formation increases with increasing metal loading. Neither catalyst type, however, is optimized for the maximum formation of N 2. By using a mixture of catalysts, one with high NO oxidation activity and one with a large number of Brønsted acid sites, the rate of N 2 is greater than the weighted sum of the individual catalysts. The current results support the proposal that the pathway for selective catalytic reduction is bifunctional where metal sites affect NO oxidation, while support protons catalyze the formation of N 2.