Isotopic labeling studies of the reaction between 15NO and 14NH3 have been performed over a range of vanadiatitania-based SCR catalysts (pure V2O5 and catalysts containing 1.4-23.2 wt % V2O5) for the extended temperature range of 200-500 °C. For temperatures less than 350 °C, 14N15N is always the major product. At higher temperatures, however, product distributions are very sensitive to vanadia content; ammonia oxidation to 14NO is particularly dominant for pure V2O5 and, at 500 °C, accounts for more than 70% of the nitrogen-containing products. Pure V2O5 also produces significantly more 14N15NO, and at much lower temperatures, than that observed for a 1.4 wt % V2O5/TiO2 catalyst. On the basis of these results it is clear that ammonia oxidation to 14NO is the major reason for the observed decrease in the NO conversion over vanadia-based catalysts at temperatures greater than 400 °C. Ammonia oxidation to nitrogen and nitrous oxide is less significant; 14N2O and 14N2 each comprise less than 10% of the total products for both the pure and supported vanadia catalysts. Addition of 1.6% water decreases the amount of nitrous oxide (largely 14N15NO) produced over the supported catalyst at 450 °C by over 90%. A simultaneous increase in the amount of 14N15N is also observed. The presence of water also suppresses the 14NH3 oxidation to 14N2, 14N2O, and 14NO, even at 500 °C. By contrast, for pure V2O5 at 500 °C, water has a relatively minor effect on the product distribution, and the major product remains 14NO. In general, high temperatures, dry feed gas conditions, and high vanadia contents favor both the production of 14N15NO relative to 14N15N and the ammonia oxidation reaction producing 14NO. Results from this and previous studies suggest that there is a relationship between N2O formation and NH3 oxidation capability.
|Number of pages||9|
|Journal||Journal of Physical Chemistry|
|Publication status||Published - 1994|