Isotopic labeling studies of the effects of temperature, water, and vanadia loading on the selective catalytic reduction of NO with NH3 over Vanadia-Titania catalysts

Isotopic labeling studies of the reaction between [sup 15]NO and [sup 14]NH[sub 3] have been performed over a range of vanadia-titania-based SCR catalysts (pure V[sub 2]O[sub 5] and catalysts containing 1.4-23.2 wt% V[sub 2]O[sub 5]) for the extended temperature range of 200-500[degree]C. For temperatures less than 350[degree]C, [sup 14]N[sup 15]N is always the major product. At higher temperatures, however, product distributions are very sensitive to vanadia content; ammonia oxidation to [sup 14]NO is particularly dominant for pure V[sub 2]O[sub 5] and, at 500[degree]C, accounts for more than 70% of the nitrogen-containing products. Pure V[sub 2]O[sub 5] also produces significantly more [sup 14]N[sup 15]NO, and at much lower temperatures, than that observed for a 1.4 wt % V[sub 2]O[sub 5]/TiO[sub 2] catalyst. On the basis of these results it is clear that ammonia oxidation to [sup 14]NO is the major reason for the observed decrease in the NO conversion over vanadia-based catalysts at temperatures greater than 400[degree]C. Ammonia oxidation to nitrogen and nitrous oxide is less significant; [sup 14]N[sub 2]O and [sup 14]N[sub 2] each comprise less than 10% of the total products for both the pure and supported vanadia catalysts. 36 refs., 6 figs., 3 tabs.