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 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.