Re-evaluation of rate constants for the reaction N2H4 (+ M) ⇄ NH2 + NH2 (+ M)

Abstract

Rate constants for the dissociation/recombination reaction N2H4 (+ M) ⇄ NH2 + NH2 (+ M) are determined by a combination of quantum-chemical calculations and statistical unimolecular rate theory. Between 1100 and 2500 K, limiting low-pressure rate constants for hydrazine dissociation in the bath gas Ar of k0 = [Ar] 6.1 × 1020 (T/1000 K)−7.3 exp (-34490 K/T) cm3 mol−1 s−1, limiting high-pressure rate constants of k∞ = 7.6 × 1016 (T/1000 K)−1.0 exp(-33600 K/T) s − 1, and center broadening factors of the falloff curves (between 1100 and 1600 K in Ar) of Fcent = 0.71 exp(- T/1460 K) + 0.29 exp(- T/21 K) + exp(-13400 K/T) were calculated. Using equilibrium constants KC = 1.7 × 103 (T/1000 K)−1.5 exp(-33,460 K/T) mol cm−3, between 300 and 600 K limiting low-pressure rate constants for the reverse recombination of NH2 radicals in the bath gas N2 of krec,0 = [N2] 4.4 × 1020(T/400 K)−6.9 exp (-1630 K/T) cm6 mol−2 s−1, limiting high-pressure rate constants of krec,∞ = 4.4 × 1013 (T/1000 K)0.44 exp(-140 K/T) cm3 mol-1 s−1, and center broadening factors (between 300 and 600 K in N2) of Fcent = exp(- T/1130 K) + exp (-10340 K/T) were obtained. A comparison with experimental results for hydrazine dissociation from the literature suggests incomplete falloff extrapolations toward k∞ and experimental problems in the determination of k0 at temperatures above about 1600 K. Implications of the present re-evaluated rate constants for the modeling of high temperature ammonia oxidation kinetics are discussed, showing an only small influence of their precise values on the overall properties of the process.