Mechanistic Aspects of the Selective Reduction of NO by Propene over Alumina and Silver–Alumina Catalysts

Abstract

The selective catalytic reduction of NO with C3H6 in the presence of a large excess of O2 (i.e., C3H6-SCR (selective catalytic reduction)) was studied over γ-Al2O3, 1.2% Ag/γ-Al2O3, and 10% Ag/γ-Al2O3 catalysts. The γ-Al2O3 and the low-loading silver material exhibited high conversions to N2 whereas the high-loading sample predominantly yielded N2O. Surprisingly, a comparison of actual NO2 yields to thermodynamically predicted yields of NO2 showed that the formation of NO2 during the C3H6-SCR of NO over γ-Al2O3 was not achieved through the direct oxidation of NO with O2. An alternative mechanism involving the formation of organo-nitrite species followed by their decomposition/oxidation was suggested to be the main route for the formation of NO2. The promoting role of low loadings of silver on alumina on the activity for N2 production was attributed to the higher rate of formation of inorganic ad-NOx species (e.g., nitrates) as evidenced by in situ DRIFTS and thermogravimetric analyses. It was proposed that these inorganic ad-NOx species further react with the reductant or a derived species to form various organo-NOx compounds. In particular, organo-nitro and organo-nitroso compounds and/or their derivatives (e.g., isocyanate, cyanide, amines, and NH3) were suggested to react with NO or the organo-nitrite and/or its derivative NO2 to yield N2. When no reductant was present, the low-loading Ag/γ-Al2O3 material was poisoned by strongly bound nitrates and its activity for NO2 formation was similar to that observed over the alumina.