Kinetic and thermodynamic measurements of the reactions of the positive ions, Mn+ and MnOH+, formed by adding manganese to fuel-rich flames of either H2 + O2 or C2H2 + O2

Abstract

Mass spectrometric sampling of flat, fuel-rich flames of H2 or C2H2, burning at 1 atm. and at 1800–2650 K, has revealed that the ions MnOH+ and Mn+ are present in such flames, when seeded with amounts of manganese less than 1 p.p.m. Free electrons were the only negatively charged species. The concentrations of these two positive ions were shown to be linked by the reaction MnOH+ + H = Mn+ + H2O being close to equilibrium downstream of the reaction zone of a flame, provided it was hotter than 2100 K. The equilibrium constant of this reaction was measured in flames differing in temperature; the values showed that the reaction's forward step is exothermic by 233 ± 20 kJ mol−1. This magnitude indicates that the ionisation energy of MnOH is 763 ± 35 kJ mol−1. In addition, the rate constant of the forward step was measured and found to have an activation energy of 112 ± 25 kJ mol−1.In a simple flame of H2 + O2 + N2, ions were produced from the major species containing manganese, i.e. MnO and atomic Mn, by chemi-ionisation in: MnO + H → MnOH+ + e− and Mn + OH → MnOH+ + e−. Free atoms of Mn were roughly three times as abundant as molecules of MnO. The kinetics of these chemi-ionisation reactions were measured; the reactions resulted in elevated levels of ionisation because of a flame having concentrations of the free radicals H and OH in excess of those for equilibrium.In flames of C2H2, their significant natural ionisation was transferred to the metallic additive by: H3O+ + MnO → MnOH+ + H2O and also H3O+ + Mn → Mn+•H2O + H, followed by Mn+•H2O → Mn+ + H2O. These processes led to the level of ionisation early in a hydrocarbon flame being well above that for equilibrium, which was approached by ions recombining in MnOH+ + e− → MnO + H or Mn + OH, as well as in the three-body reaction: Mn+ + e− + M → Mn + M. Recombination coefficients were measured for these processes, as well as for MnOH+ + Cl− → Mn + OH + Cl and Mn+ + Cl− + M → Mn + Cl + M.