Mechanistic differences in the selective reduction of NO by propene over cobalt- and silver-promoted alumina catalysts: kinetic and in situ DRIFTS study

Abstract

The distribution of gaseous products and the nature of the surface species generated during the selective catalytic reduction of NO with C 3H 6 in the presence of excess O 2 (i.e. C 3H 6-SCR) were studied over both a 0.4% Co/γ-Al 2O 3 catalyst and a sulphated 1.2% Ag/γ-Al 2O 3 catalyst. The results were compared with those previously reported for the C 3H 6-SCR over 1.2% Ag/γ-Al 2O 3 and γ-Al 2O 3. High concentrations of NO 2 were observed in the product stream of the SCR reaction over the 0.4% Co/γ-Al 2O 3 and sulphated 1.2% Ag/γ-Al 2O 3 materials. The results show that (as in the case of the γ-Al 2O 3 and also probably that of the 1.2% Ag/γ-Al 2O 3) the NO 2 was formed via an alternative route to the direct oxidation of NO with O 2. The yields of NO 2 were higher over the Co/γ-Al 2O 3 than over the other materials and in contrast to the other materials, no NH 3 was produced over the Co/γ-Al 2O 3 catalyst. Based on these results and those of in situ DRIFTS experiments, a global reaction scheme incorporating organo-nitrogen species as key intermediates is proposed. In this scheme, NO, propene and oxygen react to form organo-nitro and/or organo-nitrito adsorbed species, the reaction products of which combine to yield N 2. The results reported here suggest that Co preferentially promotes the formation of nitrito-compounds which can readily decompose to NO 2, whereas Ag preferentially promotes the formation of nitro-compounds (from reaction of strongly bound ad-NO x species) which can decompose to isocyanates and ammonia. The sulphation of the 1.2% Ag/γ-Al 2O 3 reduced the surface concentration of strongly bound ad-NO x species which were thought to react with the reductant or derived species to yield the organo-nitrogen species.