Thermal oxidation of nitric oxide at low concentrations

Abstract

The thermal oxidation of nitric oxide at concentrations of ca. 50 p.p.m. in oxygen + nitrogen mixtures has been investigated by long-path infra-red spectrophotometry from 293 to 372°K. At 293°K, 0.5–175 mm oxygen, and total pressure ca. 630 mm, the reaction is second order with respect to nitric oxide and first order with respect to oxygen. The third-order rate constant, which is about 35 % higher than the accepted value for higher concentrations of nitric oxide, agrees closely with the value previously obtained1 under simulated atmospheric conditions, and is independent of both the total pressure and the pressure of nitrogen. The results are discussed in terms of the mechanism of Trautz.2 With simulated atmospheric conditions at 293–372°K, the apparent activation energy of –0.95 kcal mole–1 is lower than the values of –1.5 and –1.7 kcal mole–1 reported3,4 for higher concentrations in this temperature range. The real activation energy, evaluated using the method of Gershinowitz and Eyring,5 is approximately zero. At very low concentrations, the structure of the activated complex conforms more closely to the transition state model5,6 than it does at higher concentrations.