Theoretical aspects of H/N/O-chemistry relevant to the thermal reduction of NO by H₂

Kinetic modeling of existing data on the reduction of NO by H₂, initiated photolytically or thermally, with a comprehensive reaction mechanism established by means of ab initio quantum-chemical and statistical rate-constant calculations, allows us to identify several key elementary processes which are important in different temperature and NO-concentration regimes. At T < 900 K, the reduction of NO by H₂ and other hydrides induced photolytically occurs primarily by the bimolecular reaction HNO + HNO → cis/trans-(HNO)₂ under NO-lean conditions, and by the termolecular process HNO + 2NO → HN₂O + NO₂, followed by the fast redox reaction, HN₂O + NO → HN₂ + NO₂ and N₂ + HONO under NO-rich conditions. In the temperature range of 900 K < T < 1500 K, the reduction of NO by H₂, initiated by the H₂ + NO → H + HNO reaction, occurs readily and the global reduction rate is dominated by the HNO + NO → N₂O + OH reaction. At temperatures higher than 1500 K, commonly heated by shock waves, the rate of NO reduction is controlled almost exclusively by the H + NO → N + OH reaction. The N atom thus formed generates efficiently the O atom by the fast N + NO → N₂ + O process. These two NO reduction reactions are greatly enhanced in this temperature regime by the abundance of H atoms, produced by the fast chain processes: O + H₂ → H + OH and OH + H₂ → H + H₂O. The rate constants as well as the mechanisms of these key elementary processes involving H/N/O-species have been interpreted in terms of the theoretical results derived from ab initio quantum-chemical and statistical-theory calculations.