PULSE-RADIOLYSIS OF GASEOUS AMMONIA-OXYGEN MIXTURES

The effect of oxygen concentration on the decay rates of the radicals NH2 and NH was studied using the well-known electronic absorption bands of the two species. We also observed the OH radical being formed with the yield going through a maximum with increasing oxygen concentrations. Due to the possible role of OH as a chain carrier in the system, we investigated the important reaction OH + NH3 → H2O + NH2 and found k(OH+NH3) = (1.6 ± 0.2) × 108 M-1 s-1 at 300 K and an activation energy of 0.87 ± 0.08 kcal/mol. H atoms are consumed by the reaction H + O2 → HO2, and the transient yield of OH is explained consistently by the reaction H + HO2 → 2OH with k(H+HO2) = 2 × 1010 M-1 s-1 in a computer-simulated model. The decay of NH2 was found to be independent of the oxygen partial pressure in the range 0-420 mbar at 349 K. The invariance of the NH2 decay upon replacement of H by HO2 implies that k(HO2+NH2) = k(H+NH2) whereas we estimate a surprisingly low upper limit for the rate constant of the direct reaction, k(NH2+O2) ≤ 5 × 106 M-1 s-1. The half-life of NH was found to decrease in the characteristic range of oxygen concentrations where the replacement of H by HO2 takes place. On the basis of computer simulations, we explain the change in the NH half-life by the reaction HO2 + NH (k = 4.3 × 1010 M-1 s-1) whereas the direct reaction NH + O2 appears to be unimportant, k(NH+O2) ≤ 2 × 107 M-1 s-1. A transient species absorbing at 2300 Å is formed in NH3-O2 mixtures. The kinetic features show that the precursor is HO2 reacting with NH3 to form either a hydrogen-bonded complex NH4O2 or, by water elimination, NHOH. A model for high-dose-rate radiolysis of pure gaseous ammonia is proposed. The computer-simulated model, which incorporates N2H3 as an important intermediate species, is consistent with the observed kinetic features and with the experimental product yields. On the basis of this model we obtained the following rate constants: k(NH2+NH2) = k(H+NH2) = 1.6 × 1010 M-1 s-1, k(NH+NH2) = 7 × 1010 m-1 s-1, and k(N2H3+N2H3) = 1011 M-1 s-1 at 349 K. © 1979 American Chemical Society.