Studies of reactions of atoms in a discharge-flow stirred reactor. Part 4.—The O + NH3+ NO/CO system

Abstract

The reaction between O(³P) and NH₃ has been studied at 350 K and a total pressure ⩽ 0.2 kPa in a discharge-flow stirred-reactor system using chemiluminescent emission (O + NO, O + CO) intensities to measure concentrations. For small NH₃ additions ([NH₃]/[O]≈ 10) a stoichiometry of –Δ[O]/Δ[NO]= 3.8 ± 0.2 and a rate constant for the initial step of (3.2 ± 0.3)× 10⁵ dm³ mol^–1 s^–1 were established, both in reasonable agreement with previous measurements. When NO was added to the NH₃+ O system at concentrations at which, on the basis of rate constant data in the literature, the NO would be expected to compete with O atoms for any NH₂ or NH radicals in the system, no NO consumption was observed. On the other hand, when high ratios of [NH₃]/[O](⩽ 100) are used so that NH₂ is generated by the secondary reaction OH + NH₃→ NH₂+ H₂O net consumption of NO is observed on NO addition. These findings suggest that NH₂ and NH are not primary products of the attack of O atoms on NH₃, contrary to previously postulated mechanisms.

Addition of CO (⩽ 60% replacement of the N₂ carrier gas) decreased the oxygen-atom consumption rates and stoichiometry, reflecting competition for OH produced between the reactions CO + OH → CO₂+ H O + OH → O₂+ H. The observed effects corresponded to the generation of 1.3 ± 0.5 OH radicals per NH₃ reacted at low [NH₃]/[O].

To explain these findings it is suggested that the first step in the reaction between O and NH₃ is associative leading to NH₃O, which may rearrange to NH₂OH. The latter is attacked by O atoms to produce H₂O and HNO which appear to be the major immediate reaction products. In conjunction with the subsequent steps O + HNO → OH + NO O + OH → O₂+ H this mechanism leads to stoichiometries in acceptable agreement with those determined here and in previous studies.