FT-IR spectroscopic study of the reaction mechanism for selective reduction of NO over sol-gel prepared ln2O3-Ga2O3-Al2O3 catalysts

The catalytic activity of Ga2O3-Al2O3 for the selective reduction of NO with propene was inhibited by the presence of H2O, whereas the catalytic activity of In2O3-Ga2O3-Al2O3 was significantly promoted. Both Ga2O3-Al2O3 and In2O3-Ga2O3-Al2O3 promoted the formation of NO3-, acetate, formate, nitrile, isocyanate and amino species in the absence of H2O in the reaction gas mixture under the reaction conditions. Adsorption of organic nitro compounds, which are possible intermediates in the NO reduction, onto Ga2O3-Al2O3 and In2O3-Ga2O3-Al2O3 at the reaction temperature was detected as IR bands due to nitro, nitrite, carbonyl and isocyanate species, which were also observed in the NO reduction with propene. Surface NO3- species were highly reactive with propene, leading to the formation of the surface species of acetate, formate, isocyanate and amino species, as well as N-2 and CO2. On the basis of these findings, the following reaction mechanism was proposed: organic nitro compounds are first produced through the reaction of NO3- formed by NO oxidation on the catalyst surface with propene, and then decomposed to -NCO species, and the surface -NH species generated by hydrolysis of the -NCO species react with NO, species to produce N-2. Although the presence of H2O suppressed the formation of NO3- species as the initial reaction intermediate on Ga2O3-Al2O3 and In2O3-Ga2O3-Al2O3, the formation and subsequent decomposition (hydrolysis) of the -NCO species was promoted by H2O over In2O3-Ga2O3-Al2O3. Such contrasting behavior of the -NCO species is related to the different catalytic characteristics of Ga2O3-Al2O3 and In2O3-Ga2O3-Al2O3 for NO reduction by propene in the presence of H2O.