Effect of different valence metals doping on methane activation over La₂O₃(001) surface

La₂O₃ as a catalyst is used for oxidative coupling of methane (OCM) reactions due to its excellent stability and high C₂ selectivity, but poor activity on methane dissociation limits its wide application. Different valence metals are doped on the La₂O₃(001) surface to improve the methane conversion activity, and the activation of methane on metal-doped La₂O₃(001) surfaces has been investigated via the density functional theory (DFT) calculations. The relationship between the valence states of doped metals and the methane conversion activities shows that doping low valence metals (Li, Na, K, Mg, Ca, Sr and Ba) and equivalent metals (Al, Ga, In) can significantly improve the conversion activity of methane. Among them, the activation energy of methane on the Li-La₂O₃(001) surface is the lowest, which is only 13.0 kJ/mol. However, doping of high valence metals (Zr, Nb, Re and W) cannot improve the CH₄ dissociation activity. Furthermore, the relationships between surface oxygen vacancy formation energies, acid-base properties and the activation energies of CH₄ have also been investigated. The results show that with the increase of metal valence state, the oxygen vacancy formation energy increases, while the dissociation activity of CH₄ decreases. The introduction of alkali and alkaline earth metals increases the alkalinity of La₂O₃(001) surface, and the alkalinity of La₂O₃(001) doped with the alkali metal is stronger than that with the alkaline earth metal, exhibiting higher dissociation activity of CH₄. Our research may provide a guide for improving methane conversion activity on La₂O₃ catalysts.