Mechanism of selective and complete oxidation in La2O3-catalyzed oxidative coupling of methane

Abstract

Periodic density functional theory (DFT) calculations were performed to study the evolution of surface oxygen species on a La₂O₃ catalyst during the oxidative coupling of methane (OCM) reaction, and to establish the catalytic mechanism of the selective and complete oxidation of CH₄. The lattice oxygen (O²⁻) site on the stoichiometric La₂O₃ surface activates CH₄via heterolytic C–H bond cleavage to yield CH₃⁻ and H⁺ fragments, which bind to surface Lewis acid (La³⁺) and Brønsted base (O²⁻) sites, respectively. In the presence of the H⁺ fragment, the CH₃⁻ fragment binds quite strongly to the above La₂O₃ surface, but O₂ adsorption facilitates its desorption as a gaseous CH₃ radical at relatively low reaction temperatures, while molecular O₂ adsorbed at the La³⁺ site becomes a superoxo radical (O₂˙⁻) species. This O₂˙⁻ species reacts with a second CH₄ molecule via direct H abstraction to produce another gaseous CH₃ radical, accompanied by its transformation into a hydrogenperoxo (HO₂⁻) species, which transfers an O atom to a neighboring O²⁻ site and converts the latter into a peroxo (O₂²⁻) site. Whereas CH₄ activation at the O²⁻ site is essentially an acid–base reaction, that at the O₂²⁻ site is clearly a redox reaction, which occurs via an O insertion mechanism to directly form gaseous CH₃OH as further confirmed by our ab initio molecular dynamics (AIMD) calculations. CH₃OH is further oxidized by an additional O₂²⁻ site to yield CO₂ following a similar mechanism, whereas CO may form from dehydrogenation of the CH₂O intermediate at the same site. Thus, the O₂²⁻ site is proposed to be responsible for complete oxidation of CH₄ in the OCM reaction, whereas the O²⁻ and O₂˙⁻ sites are responsible for the formation of gaseous CH₃ radicals and thus C₂ products. Our proposed catalytic mechanism is based on first principles DFT calculations, which gives a comprehensive view of CH₄ interaction with the different oxygen species on the La₂O₃ catalyst surface, and provides critical insights into the possible evolution of surface oxygen species and the detailed surface reaction network.