Differences in the reactivity of organo-nitro and nitrito compounds over Al₂O₃-based catalysts active for the selective reduction of NO_x

The reactivity of nitromethane and tert-butyl nitrite, used as "models" of possible intermediates of the SCR of NO by hydrocarbons, was studied over γ-Al₂O₃, 1.2wt%Ag/Al₂O₃, 10wt%Ag/Al₂O₃, and 0.4wt%Co/Al₂O₃. DRIFTS measurements revealed the presence of formate, cyanide, and isocyanate species on the surface of γ-alumina, 1.2wt%Ag/Al₂O₃, and 0.4wt%Co/Al₂O₃ upon nitromethane adsorption, while nitrate species arose on these materials following the adsorption of tert-butyl nitrite. The oxidation of nitromethane over alumina, 1.2wt%Ag/Al₂O₃, and 0.4wt%Co/Al₂O₃ yielded NH₃ as the main primary product of reaction, while NO and NO₂ were formed at low temperatures during the oxidation of tert-butyl nitrite over the same materials. The mechanism derived from this study and from the observation that the formation of NO₂ during the C₃H₆-SCR of NO over γ-Al₂O₃ and 0.4wt%Co/Al₂O₃ was not achieved through the direct oxidation of NO by O₂ was therefore suggested. The formation of N₂ over γ-Al₂O₃, 1.2wt%Ag/Al₂O₃, and 0.4wt%Co/Al₂O₃ was proposed to occur through the reaction of organo-nitro and nitrito compounds and their derivatives. The presence of a low loading of silver appeared to favour the formation of organo-nitro compounds, while cobalt seemed to promote the formation of organo-nitrite compounds.