Performance of Zn-Al co-doped La₂O₃ catalysts in the oxidative coupling of methane

Zn-doped and Zn-Al co-doped La₂O₃ catalysts were prepared by citric acid sol-gel method and characterized by a series of in situ technologies, to investigate the structure-activity relationship of La₂O₃-based catalysts in the oxidative coupling of methane (OCM). The in situ XRD results reveal a thermal expansion of the La₂O₃ crystal along the c-axis at high temperature. The H₂-TPR results show two types of oxygen species on the La₂O₃-based catalysts, viz., the strong-binding oxygen species and weak-binding oxygen species; in addition, the XPS results indicate that the strong-binding oxygen species is probably attributed to anion radical O⁻. The doping with Zn can significantly increase the number of oxygen vacancies in the Zn-doped La₂O₃ catalysts, which can promote the activation of oxygen and generate more strong-binding oxygen species; as a result, the Zn-doped La₂O₃ catalyst shows better performance in OCM in comparison with the unmodified La₂O₃ catalyst. Moreover, the co-doping with Al can promote the dispersion of Zn in La₂O₃ and further raise the number of strong-binding oxygen species in the Zn-Al co-doped La₂O₃ catalysts, which is beneficial to enhance the selectivity to C₂₊ hydrocarbons in the OCM reaction