Mechanism of the reaction of nitric oxide, ammonia, and oxygen over vanadia catalysts. 2. Isotopic transient studies with oxygen-18 and nitrogen-15

The mechanism of nitric oxide reduction with ammonia to form N₂, H₂O, and N₂O both in the presence and in the absence of O₂ over the following series of catalysts, unsupported V₂O₅, V₂O₅ on TiO₂, V₂O₅ on SiO₂/Al₂O₃, and V₂O₅ on Al₂O₃, has been investigated with the aid of labeled O₂ and labeled NH₃ at 400°C. The behavior of ammonia was studied both in the presence and in the absence of O₂. The presence of labeled O₂ gives extra information about the product distribution and the reaction mechanism. Evidence is given that ammonia does not react with O₂ or O from any source during the reaction, but that nitrogen and nitrous oxide were produced by a reaction involving all three species, NO, NH₃, and/or O₂. Nitrous oxide and water are both formed at two different sites of the catalyst. A series of transient tracing studies were performed in a plug-flow reactor using ¹⁵NH₃ and ¹⁸O₂. Both ¹⁵NN and ¹⁵NNO were produced on the unsupported V₂O₅, V₂O₅ on TiO₂, V₂O₅ on SiO₂/Al₂O₃, and V₂O₅ on Al₂O₃ with very high selectivities. The mechanism of the reaction of NO, NH₃, and O₂, proposed in a previous paper (ref 2), is further evaluated on the basis of this new experimental evidence.