Investigation of methane oxidation by palladium-based catalyst via ReaxFF Molecular Dynamics simulation

Catalytic oxidations of methane over palladium-based nanoparticles, with and without oxygen coating, are investigated using ReaxFF Molecular Dynamics simulations. The simulation results show the complete dynamic process of the above catalytic reactions at the atomic level and help to reveal the underlying mechanisms both qualitatively and quantitatively. It is found that oxygen molecules are significantly easier to be adsorbed on both bare and oxygen-coated Pd surfaces compared with CH4. The presence of adsorbed O-2 molecules on the surface blocks the active sites for CH4 adsorption on the oxygen-coated Pd surfaces. By comparing the adsorptive dissociation of CH4 over Pd nanoparticles with different levels of oxygen coverage, we find that it is much easier for the adsorptive dissociation of CH4 on oxygen-coated Pd nanoparticles than that on bare Pd nanoparticles at low temperatures. In contrast to the rapid dissociation of CH4 after adsorption, the dissociation of O-2 requires much higher temperature than adsorption. Moreover, the CH4 dissociation rate increases with the rising temperature and is sensitive to the level of oxygen coverage on the surface. In addition, the activation energies for the adsorptive dissociation of CH4 are determined by fixed-temperature simulations from 400 to 1000 K through the changes of CH4 concentration and are found to be 3.27 and 2.28 kcal mol(-1) on 0.3 and 0.7 ML oxygen-coated Pd nanoparticles, respectively, which are consistent with density functional theory calculations and experiments. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.