Isotopic evidence for direct conversion of NO to NH₃ in the absence of O₂ over V₂O₅/TiO₂ and α-Cr₂O₃ selective catalytic reduction catalysts

The products of the reaction between ¹⁵NO and ¹⁴NH₃ over V₂O₅/TiO₂ and α-Cr₂O₃ catalysts in the absence of oxygen have been determined by mass spectrometry and Fourier transform infrared spectroscopy. With both catalysts ¹⁴N¹⁵N comprises approximately two-thirds of the nitrogen-containing products. Very surprisingly, ¹⁵NH₃ makes up about one-half the remainder. This has significant implications for the likely mechanism of the SCR reaction in the absence of oxygen. Formation of ¹⁵NH₃ by direct exchange of both hydrogen and oxygen between ¹⁴NH₃ and ¹⁵NO molecules can be ruled out since no ¹⁴NO is produced. It seems that ¹⁵NO is broken down to separate ¹⁵N and oxygen species on reduced sites and that ¹⁵NH₃ can then be made by reaction of the ¹⁵N species with water. It is possible that the nitrogen-containing species is an NH group. With V₂O₅/ TiO₂ the only remaining product is ¹⁴N¹⁵NO. However, this is largely replaced by doubly labeled products, ¹⁴N₂, ¹⁵N₂, and ¹⁵N₂O, when using α-Cr₂O₃.