Active sites of H-ZSM5 catalysts for the oxidation of nitric oxide by oxygen

The catalytic activities of H-ZSM5-18, H-ZSM5-150, Li-ZSM5-18, and H-Mag (numerical suffixes mark the Si/Al ratios of zeolites, H-Mag is the proton exchanged form of the layered sodium silicate, magadiite) were compared for the oxidation of NO by O₂ at different ratios of reactants at reaction temperatures from 25 to 600°C. H-ZSM5-18 typically has activity maxima near 25 and 400°C at most O₂/NO reactant ratios. Regardless of the partial pressures of reactants, NO±1/2O₂ NO₂ equilibria are attained at 400°C and above. The H-ZSM5-150 and Li-ZSM5-18 zeolites are only active at temperatures near 25°C. H-Mag is practically inactive at the reaction conditions used. Results indicate that Lewis acidic lattice aluminium ions and silanol hydroxyls are not active in the oxidation of NO to NO₂ over H-ZSM5 zeolites. Brønsted acidic bridging hydroxyls are probably active sites for this reaction at temperatures above 200°C.