Experimental study and modeling of the reaction H+O-2+M -> HO2+M (M = Ar, N-2, H2O) at elevated pressures and temperatures between 1050 and 1250 K

The H + O-2 + M --> HO2 + M reaction was investigated at temperatures between 1050 and 1250 K and pressures from 7 to 152 bar behind reflected shock waves in gas mixtures of H-2, O-2, NO, and bath gases of Ar, N-2 and H2O. Narrow linewidth laser absorption of NO2 at 472.7 nm was used to measure quasi-steady NO2 concentration plateaus in experiments designed to be sensitive only to the H + O-2 + M --> HO2 + M and the relatively well-known H + NO2 --> NO + OH and H + O-2 --> OH + O reaction rates. The pressure dependence of the reaction was studied by measuring the fall-off of the reaction for M = Ar over a 10-152 bar pressure range. A simple modified Hindered-Gorin model of the transition state is used in an RRKM analysis of the results to facilitate comparisons of this work with measurements from other researchers at lower pressures. The RRKM calculations can also be described, using the simple functional form suggested by Troe, with the following: k(infinity)/cm(3) molecule(-1) s(-1) = 4.7 x 10(-11) (T/300)(0.2); k(0)(Ar)/cm(6) molecule(-2) s(-1) = 2.0 x 10(-32) (T/300)(-1.2); k(0)(N-2)/cm(6) molecule(-2) s(-1) = 4.4 x 10(-32) (T/300)(-1.3); k(0)(H2O)/cm(6) molecule(-2) s(-1) = 3.4 x 10(-31) (T/300)(-1.0); F-c = 0.7 for Ar and N-2 and 0.8 for H2O. Measured values of the reaction rate for M = Ar in the highest pressure experiments fall below both simple RRKM analysis and the more sophisticated treatment of Tree using an ab initio potential energy surface. Collision efficiencies of N-2 and H2O relative to Ar at 1200 K are 3.3 and 20 respectively.