Reaction intermediates in the selective catalytic reduction of NO_x over Fe/ZSM-5

For the selective catalytic reduction of NO_x with alkanes to N₂ over Fe/ZSM-5 catalysts, prepared by sublimation, we had previously shown that NO is oxidized over Fe-oxo ions to NO₂ which forms chemisorbed NO_ycomplexes. Subsequent reaction of the NO_y groups with alkanes results in N-containing deposits that react with different adsorbed NO₂ molecules to form N₂. At low temperature, the reactivity of n-butane exceeds that of i-butane. Quantitative evaluation of the reaction kinetics for the deposits from not only propane and butanes but also adsorbed model compounds, including ammonia, amines, nitriles, and isocyanates, provide information on the reaction steps on the catalyst surface. Reaction of NO₂ with the catalyst deposit follows Langmuir-Hinshelwood kinetics. The primary intermediates which contain N atoms at a high valence state have to be transformed into deposits containing amino groups. Nitrogen-nitrogen bonds are formed when these groups react with adsorbed NO₂. The chemical reactions, transforming the primary intermediate into a deposit containing N atoms in a low oxidation state, are easy if an H atom is available in the α position with respect to the NO₂ group; the absence of such H atoms in the primary product from i-butane is the cause for its lower reactivity.