Catalysis of radical recombination

Chain termination may occur due to catalysis of atom and free radical recombination by the succession of general steps 

When A and B are both H atoms, the catalyzed recombination occurs by way of the nitroxyl radical, through reactions (xxviii) and (xxix), viz. 

At high H atom concentrations, when 2 9 -2s[M], the decay of H atoms is first order in H, and is controlled by the rate of reaction (xxviii). These are the conditions which have been used in studies of the catalyzed recombination in discharge-flow systems [253—256]. Values of 2 8 for Hj, He, Ar, CO2, N2O, SF and H2O are summarized by Baulch et al. [257], who recommend the expression 

At low H atom concentrations ( 29 fe 2 8M) reaction (xxviii) is equilibrated, the H atom decay becomes second order in the radical concentration, and the overall rate is controlled by reaction (xxix). With the assumption of partial equilibration of reactions (i), (ii) and (iii) of the hydrogen—oxygen system in the recombination region of rich, atmospheric pressure H2/N2/O2 flames, and with the addition of reaction (xxx) 

Data on reactions (xxvi) when A is O or OH are reviewed by Baulch et al. [257]. 

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