Differences in the Reactivity of Organo-Nitro and Nitrito Compounds over Al2O3-Based Catalysts Active for the Selective Reduction of NOx

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.