Kinetic Studies of Hydroxyl Radicals in Shock Waves. V. Recombination via the H+O2+M→HO2+M Reaction in Lean Hydrogen—Oxygen Mixtures

Abstract

Recombination in highly diluted, lean hydrogen—oxygen—argon mixtures with H₂/O₂ ratios of 0.50 and 0.33 has been investigated with a shock tube over the temperature range 1150°—1850°K for reaction pressures of 0.85–5.41 atm. An OH ultraviolet line absorption technique was used to follow the progress of the reaction after substantial equilibration of the rapid bimolecular reactions which form OH initially. It was determined that under these conditions the recombination process is third order and its rate, R_rec, obeys the empirical relationship $$R_{\mathrm{rec}}{\mathrm{=k}}_{\mathrm{eff}}[\mathrm{H}\mathrm{][}{\mathrm{O}}_2\mathrm{][}\mathrm{Ar}\mathrm{].}$$ k_eff was found to decrease slightly with increasing temperature and to depend to a small extent upon the composition of the reacting mixture. The experimental results indicate that the reaction $$\mathrm{H}+{\mathrm{O}}_2+\mathrm{M}\to {\displaystyle {lim}_k}{\mathrm{HO}}_2+\mathrm{M}$$ dominates the recombination process. Further analysis of the data made it possible to obtain from k_eff a value of k(M = Ar) = 1.42×10¹⁵ cc²/mole² sec at 1500°K and an estimate of k(M = H₂O)/k(M = Ar) of about 30. This value of k(M = Ar) has been combined with other experimental determinations to obtain the expression $\mathrm{k(}\mathrm{M}=\mathrm{Ar}{\text{)=8.6×10}}^{14}\exp \text{(1280±90/RT)\hspace{0.17em}}{\mathrm{cc}}^2/{\mathrm{mole}}^2\sec$ which describes the data to within 20% over the entire temperature range from 225° to 1850°K.