The origin of¹⁵NH₃ produced from the reaction of¹⁴NH₃ and¹⁵NO over vanadia-based SCR catalysts

The effects of H₂O and the vanadia content of the catalyst on the formation of¹⁵NH₃ during the reaction of¹⁵NO and¹⁴NH₃ in the absence of O₂ over V₂O₅-based catalysts have been determined by mass spectrometry and Fourier transform infrared spectroscopy. At 450°C, the contribution of¹⁵NH₃ to the total nitrogen-containing products remains constant at about 20% for water concentrations from 0 to 1.6%. The vanadia content also has little effect on the proportion of¹⁵NH₃ produced. Combination reactions producing¹⁴N¹⁵N and¹⁴N¹⁵NO consume surface oxygen species and oxygen mass balances indicate that the amount of¹⁵NH₃ formed is determined by the extent of these combination reactions. Small concentrations of O₂ (<300 ppm) were sufficient to prevent the formation of¹⁵NH₃. The reduction of NO by H₂ was also studied. Negligible amounts of NH₃ were formed under dry feed conditions, whereas, in the presence of 1.6% H₂O,¹⁵NH₃ represents about one third of the products. A mechanism involving reaction of an adsorbed N_s species with H₂O is used to account for these experimental observations.