Selective Catalytic Realaction of Nitric Oxide with Hydrocarbons in Oxygen-rich Atmosphere

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Selective reduction of No by CH_4 on zeolite catalysts has been studied. After investigation on the catalytic features of various cation-exchaged zeolites, we found that Ga-ZSM-5 and In-ZSM-5 were highly active and selective for NO reduction by CH_4. We also showed that this reaction was moderately promoted even by H-form ZSM-5, mordenite, and ferrierite. From the catalytic performance of Ga-and In-ZSM-5in NO-CH_4-O_2, NO_2-CH_4-O_2, and NO-O_2 reactions, it is conuluded that the selective reduction of NO on these catalysts proceeds in two stages : (1) NO is oxidized to NO_2 on zeolite acid sites, (2) NO_2 and CH_4 react on gallium or indium sites. The highly selective features of Ga and In in zeolite for NO_2-CH_4 reaction seem to be attributed to the coordinatively unsaturated nature of these sites which adsorb both of these reactants on the same site. Effect of water vapor on NO conversion was also investigated. It was found that Ga-ZSM-5 was strongly inhibited by steam, while In-ZSM-5 was fairly active even in the presence of 10% steam.