Probing hydrogen-nitrogen chemistry: A theoretical study of important reactions in NxHy, HCN and HNCO oxidation

As an indirect storage medium of hydrogen, ammonia (NH3) has drawn significant attention from academia and industry. Understanding nitrogen combustion chemistry is a major challenge in applying ammonia for converting chemical energy to thermal energy. Diazene (N2H2), diazenyl radical (NNH), amidogen radical (NH2), hydrogen cyanide (HCN) and isocyanic acid (HNCO) are the crucial intermediate species in the combustion of NH3 or its mixtures with other hydrocarbons. In light of that, this study provides advanced theoretical treatment of 14 important reactions in the oxidation of these intermediates, including isomerization, dissociation and abstraction reactions. The rate constants of all these reactions, and the temperature-dependent thermochemistry of the species involved in the reactions, were calculated utilizing high level quantum chemical methods. Rovibrational properties of the reactants, products and stationary points were determined at the M06-2X/6-311++C (d,p) level of theory. Coupled cluster (CCSD(T)) methods were employed, with two large basis sets (cc-pVTZ and cc-pVQZ), and complete basis set of extrapolation techniques to compute the energies of the resulting geometries. All calculated results were compared with experimental and theoretical results in the literature. Finally, the implications of this work for combustion modeling were investigated, and the simulated species' profiles of HCN and HNCO demonstrated the influence of the updated rate coefficients on kinetic model predictions. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.