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).