Effects of pressure and surface initiation efficiency on the flowing H2O2+ NO2+ CO + N2 chain reaction system

Abstract

The yields of CO₂ from the H₂O₂+ NO₂+ CO reaction system in flowing carrier gas at 298 K have been measured at total pressures (P) of 13.3, 40.0 and 93.3 kPa. The variations of the yield as functions of [NO₂] and [S](S = acetaldehyde or diethyl ether) have been shown to indicate that the efficiency of conversion of H₂O₂ to OH radicals in the heterogeneous initial step H₂O₂+ NO₂→ OH + HNO₃(1) is independent of total pressure for a particular surface activity. The results are consistent with the pressure dependence of k₂ for the propagation step CO + OH → CO₂+ H (2) indicated by other work. For the rate constant of the termination reaction OH + NO₂(+ M)→ HNO₃(+ M)(4) low apparent values (k′₄) are indicated compared with literature values of k₄. At P= 40.0 kPa, for three surfaces of apparent efficiencies of ca. 20%, ca. 53% and ca. 81% in converting H₂O₂ to OH in reaction (1), k′₄/k₄ values were ca. 0.45, ca. 0.65 and ca. 0.85, respectively. This trend is interpreted in terms of decreasing activity of these surfaces for adsorption of H₂O₂, producing decreasingly localized reaction zones: depletion of [NO₂] in the reaction zone depresses k′₄/k₄ while increased surface concentrations of H₂O₂ decrease the apparent H₂O₂ to OH conversion efficiency by inducing an additional route for destruction of the active centre involved in reaction (1).