Mechanistic aspects of the selective reduction of NO by propene over alumina and silver-alumina catalysts

The selective catalytic reduction of NO with C3H6 in the presence of a larg e excess of O-2 (i.e., C3H6-SCR (selective catalytic reduction)) was studie d over gamma-Al2O3, 1.2% Ag/gamma-Al2O3, and 10% Ag/gamma-Al2O3 catalysts. The gamma-Al2O3 and the low-loading silver material exhibited high conversi ons to N-2 whereas the high-loading sample predominantly yielded N2O. Surpr isingly, a comparison of actual NO2 yields to thermodynamically predicted y ields of NO2 showed that the formation of NO2 during the C3H6-SCR of NO ove r gamma-Al2O3 was not achieved through the direct oxidation of NO with O-2 An alternative mechanism involving the formation of organo-nitrite species followed by their decomposition/oxidation was suggested to be the main rout e for the formation of NO2, The promoting role of low loadings of silver on alumina on the activity for N-2 production was attributed to the higher ra te of formation of inorganic ad-NOx species (e.g., nitrates) as evidenced b y in situ DRIFTS and thermogravimetric analyses. It was proposed that these inorganic ad-NOx species further react with the reductant or a derived spe cies to form various organo-NOx compounds. In particular, organo-nitro and organo-nitroso compounds and/or their derivatives (e.g., isocyanate, cyanid e, amines, and NH3) were suggested to react with NO or the organo-nitrite a nd/or its derivative NO2 to yield N-2. When no reductant was present, the l ow-loading Ag/gamma-Al2O3 material was poisoned by strongly bound nitrates and its activity for NO2 formation was similar to that observed over the al umina. (C) 1999 Academic Press.