Mixtures of N20, Hz, 0 2 , and trace amounts of NO and NO2 were photolyzed at 213.9 nm, at 245'-328'K, and at about 1 atm total pressure (mostly H2). HO2 radicals are produced from the photolysis and they react as follows:
(la)
Reaction (lb) is unimportant under all of our reaction conditions. Reaction (la) was studied in competition with reaction (3) from which it was found that = 6.4 X exp 1-(1400 f 500)lRTI ~m ~/ ~/ s e c * / ~.
If k3 is taken to be 3.3 X cm3/sec independent of temperature, k l , = 1.2 X lo-" exp 1-(1400 f 500)lRTI cm3/sec. Reaction (2a) is negligible compared to reaction (2b) under all of our reaction conditions. The ratio kZb/kl = 0.61 f 0.15 at 245ยฐK. Using the Arrhenius expression for kl, given above leads to k2b = 4.2 X cm3/sec, which is assumed to be independent of temperature. The intermediate HOzNOz is unstable and induces the dark oxidation of NO through reaction (-2b), which was found to have a rate coefficient k-zb = 6 X 10'" exp (-26,00O/RTI sec-I based on the value of kl, given above. The intermediate can also decompose via (lob) Reaction (lob) is at least partially heterogeneous.
H02N02 -HONO + 0 2 (or other products)