Selective catalytic reduction of NOx with methane over metal exchange zeolites

Catalytic reduction of NOx with methane in an oxidizing atmosphere was studied over metal exchanged zeolites. We found that the combinations of Co2+, Mn2+ and Ni2+ with certain types of zeolites, e.g., ZSM-5 and mordenite, are active catalysts for this reaction. For a Co-ZSM-5 catalyst, the nitric oxide conversion displays a volcano-shape curve as temperature increases, which is reversible upon decreasing temperature. The nitric oxide reduction activity is proportional to the level of Co2+ exchanged into ZSM-5, but excess amounts of cobalt do not contribute to the activity. Mn-ZSM-5 is very similar to Co-ZSM-5 in the nitric oxide reduction activity, and Ni-ZSM-5 has slightly lower activity compared to Co-ZSM-5. Under an oxidizing conditions, Cu-ZSM-5, however, is ineffective for the nitric oxide reduction. Co-Y, which has much more Co2+, is much less active compared to Co-ZSM-5, or Co-mordenite. The amount of nitric oxide adsorbed, measured by TPD, on Co-Y is extremely small (NO/Co=0.06) compared to Co-ZSM-5 (NO/Co > 1.1) and Co-mordenite (NO/Co=0.8).