Mechanism of the catalytic oxidation of methane on Pt(111) surfaces in moist environment: A density functional theory study

Density functional theory studies were carried out to gain mechanistic understanding of the catalytic oxidation of methane on Pt surfaces, especially in moist environment. Transition-state theory was used to estimate the energy barriers for each of the elementary reactions involved in the catalytic oxidation process. The optimal adsorption geometry and the corresponding chemisorption energy were determined for all the species involved in each elementary step to elucidate the energetics of the pathways for methane adsorption and oxidation on Pt surfaces. As a first step, the scope of this study is limited to a Pt(1 1 1) surface. The elementary steps involve the dissociative chemisorption of methane on the Pt(1 1 1) surface, dehydrogenation reactions of adsorbed CH, species, and oxidation reactions of adsorbed reactive intermediates by adsorbed O and OH species. Microscopic reaction pathways and corresponding transition-state structures were identified. The results indicated that the primary reaction pathway is CH4* -> CH3* -> CH2* -> CH* -> HCO* -> HCOO* -> CO2*. In moist environment, the primary reaction pathway is CH4* -> CH3* -> CH2* -> CH* + OH* -> CHOH* -> CO* + OH* -> COOH* -> CO2*. The rate-determining step for the reaction pathway from methane to adsorbed carbon dioxide is the dissociative chemisorption of methane due to its relatively high energy barrier. The presence of water in reactants can promote the catalytic oxidation process.