Kinetic Studies of Ammonia Oxidation in Shock Waves. I. The Reaction Mechanism for the Induction Period

The oxidation of ammonia has been studied at temperatures between 1550 and 2300°K by using the shock-tube technique combined with ultraviolet absorption spectroscopy. The measurements of the induction periods for the appearance of OH have shown that they are strongly dependent on the concentration of oxygen and that the activation energy required is 42.5±1.2 kcal./mol. The intermediates of the reaction, NH, NO and NH2, have been detected spectroscopically, and the time relations between their appearance and their accumulation have been determined. As a result, the following mechanism has been postulated as the main steps governing the induction period of the high-temperature oxidation of ammonia: NH2+O2 → NH+HO2 (I) NH+O2 → NO+OH (II) OH+NH3 → NH2+H2O (III) HO2 → OH+O (IV) O+NH3 → OH+NH2 (V) where Reaction I is considered to be rate controlling.