Theoretical study of the combustion kinetics and mechanism of methane on Al(111) surface

Compared with conventional hydrocarbon fuels, hydrocarbon fuels with added energetic particles have higher calorific value and are hold potential applications in high-speed aircraft. In the present work, the ignition and combustion process of CH4 with the addition of aluminum particles are investigated using a density functional theory calculation and kinetic simulation. The geometric configuration and the energy of intermediates involved in the decomposition of methane on the Al(111) surface are analyzed and the dissociation potential energy profiles are drawn to find the optimal reaction path. Three reactions including direct dehydrogenation, O* and OH* assisted dehydrogenation are considered for CHx (x = 1-4) dehydrogenation. Stating from the initial reactants of CH4 and O2, the most preferable path for CO formation is CH4 -> CH3* -> CH2* -> CH* -> C* -> CO* at 1500 K, in which C* is generated from CH* by the OH* assisted dehydrogenation. The favorable pathways for CO2 and H2O formation are CO* -> COH* -> OCOHcis* -> CO2* and O2 -> O* -> OH* -> H2O*. In these reaction paths, the rate-determining step is C* -> CO* with the Gibbs energy barrier of 2.73 eV. The surface reaction of CH4 is seriously affected by the presence of O2 and N2 in the initial atmosphere. Based on DFT energies at 0 K, for the dissociative adsorption of O2 molecule on Al(111) surface, there is no activation energy and releases heat of 9.16 eV. The dissociative adsorption of N2 needs to overcome the energy barrier of 3.50 eV accompanied by the exothermic energy of 2.91 eV. A detailed kinetic mechanism on the methane oxidation in the presence of aluminum particle is developed accounting for surface reactions and gas interactions. Through kinetic simulation of the developed mechanism, the Al surface demonstrates the combustion-enhancing effect on CH4 combustion and this effect decreases with increasing temperature and pressure. Analyzing the gas and surface species concentrations, the dissociative reaction of O2 on Al surface is identified as the key reaction to promote CH4 ignition due to its large heat release.