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

Abstract

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 CH_x (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* → OCOH_cis* → 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.