As the amount of Nd³⁺ increases, the oxygen vacancies in Nd−Ce−O composite oxides gradually increase, leading to an amount increasing in the C₂-selective O₂ ⁻ species generated by the oxygen vacancies, and the number of surface moderate basic sites, hence improving the OCM reaction performance.

Abstract

Herein, we synthesised a series of nonstoichiometric Nd−Ce−O composite oxides with a fluorite or rare-earth C-type phase structure using the glycine combustion method for the oxidative coupling of methane (OCM) reaction. As the amount of Nd³⁺ increases, the oxygen vacancies in this series of composite oxides gradually increase, leading to an amount increasing in the C₂-selective oxygen species O₂ ⁻ generated by the oxygen vacancies. In addition, with the increase in Nd³⁺, the number of moderate basic sites on the catalyst surface also improves, another significant factor affecting the OCM reaction performance of the composite oxides. Nd_0.9Ce_0.1O_1.55 possesses optimal low-temperature OCM reaction performance, achieving 8.0 % C₂ yield at 450 °C and 14.1 % C₂ yield at 700 °C. This can be attributed to the nonstoichiometric rare-earth C-type crystalline phase structure of Nd_0.9Ce_0.1O_1.55, which exhibits the highest abundance of oxygen vacancies, selective oxygen species O₂ ⁻, and moderate basic sites.