A shock-tube study of ammonia pyrolysis

Abstract

The pyrolysis of NH3 was studied behind reflected shock waves at temperatures 2500–3000 K, using mass spectrometric analysis of dynamically sampled gas. The initial mixtures contained 0.14% to 6% NH3, with Kr as diluent, at total gas concentrations of about 2 × 10−6 mol cm−3. Concentration profiles of NH3, NH2, NH, and N2 were measured. It was found that the apparent rate coefficient for overall removal of NH3 is increased by increasing the initial NH3 concentration, but is decreased by addition of H2. Addition of H2 also suppressed NH, but left the NH2 concentration relatively unchanged. A close correlation was found between NH2 concentration and N2 formation rate, indicating that NH2 participates in the reaction which produces N2.The experimental results are consistent with a chain mechanism in which NH3 is removed by unimolecular decomposition and by attack by H, NH, and NH2. Computer analysis yields a rate constant of 1.2 × 1016 exp (−91 000 cal mol−1/RT) cm3 mol−1 s−1 for the unimolecular process.