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].