Mechanism and kinetics of the chain reaction in H2O2+ NO2+ CO systems
Abstract
The mechanism and kinetics of the chain reaction in H2O2+ NO2+ CO mixtures at total pressures ≈ 10 kPa and 292 K have been investigated by measuring the reproducible final yields of CO2 for conditions of initial [H2O2] corresponding to ⩽ 20% of the saturated vapour pressure and excess [NO2]. The reduction of yields of CO2 upon addition of n-butane results from the additional termination step OH + n-C4H10→ H2O + C4H9(8) competing against the propagation step CO + OH → CO2+ H. (2) Literature values of k8 and k2 are applied to indicate unity stoichiometry for the heterogeneous initiation step H2O2+ NO2→ OH + HNO3(1) for Pyrex glass or boric acid-coated surfaces showing maximal CO2 production efficiencies. The variations in the yield of CO2 with initial [NO2] or added [NO] yield rate constants for the termination reactions OH + NO2(+M)→ HNO3(+M)(4), OH + NO(+M)→ HNO2(+M)(5)k4(M = CO)=(2.5 ± 0.4)× 109 dm3 mol–1 s–1 at 13.3 kPa, k5(M = CO)=(8.2 ± 1.2)× 108 dm3 mol–1 s–1 at 13.3 kPa and k4(M ≈ N2)=(1.2 ± 0.3)× 109 dm3 mol–1 s–1 at 7.32 kPa; these indicate that the reaction proceeds under conditions of efficient diffusive mixing rather than boundary layer formation, by comparison with corresponding literature values.
A mechanism is advanced to explain the variation of CO2 yields as a function of small initial [NO2] for low [H2O2].