On the role of gas-phase reactions in the mechanism of the selective reduction of NOx

Abstract The selective reduction of NO over Cu-, Co- and H-mordenite was studied using CH 4 , C 2 H 4 and CH 3 OH, and compared with the same reaction in the gas phase. Co- and H-mordenite proved to be active and selective for the three reducing agents, while Cu-mordenite was so only when C 2 H 4 and CH 3 OH were used. Besides being reduced in the presence of O 2 , NO acted as a homogeneous catalyst in the oxidation of CH 4 and C 2 H 4 . A free-radical mechanism similar to the one occurring in the methane partial oxidation reaction could explain that behavior. Interestingly, on Co-mordenite, the conversion of nitric oxide to nitrogen began to occur at similar temperatures for both CH 4 and CH 3 OH. For both Co-M and Cu-M the NO to N 2 conversion started after the CH 3 OH conversion reached 100%. In the gas-phase reaction no NO conversion was observed when CH 4 or C 2 H 4 were used as reducing agents, but a 30% of the nitric oxide conversion was obtained when CH 3 OH was the reducing reactant at 600°C. Very similar results were observed with or without O 2 in the feed stream. When catalysts were present, the presence of oxygen noticeably increased the rate of NO to N 2 conversion.