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 reported^3,4 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 model^5,6 than it does at higher concentrations.