Nickel Nanoparticles Supported on Lanthanum Oxycarbonate with Interfacial Oxygen Vacancies as Catalysts for CO2 Hydrogenation to Methane

Lanthanum oxide used as a promoter or a support for nickel-based catalysts can effectively enhance the CO2 methanation reaction. However, the intrinsic activity and reaction mechanism remain unclear. In this work, we found that in the CO2 methanation reaction, the active phase of the lanthanum oxide support is La2O2CO3 transformed by the reaction between La2O3 and CO2. After the induction period, Ni/La2O3 catalysts show a performance almost identical to that of Ni/La2O2CO3, with superior CO2 methanation performance compared to those of Ni/Al2O3 and Ni/SiO2 catalysts. Furthermore, Ni/La2O3 catalysts show negligible changes in the activity of the CO2 methanation reaction during an 80 h long-term stability test at 673 K, achieving a CH4 production rate of 444 mmol CH4·gNi–1·s–1 with a CO2 conversion rate of 77% and a CH4 selectivity of 97%. Transmission electron microscopy (TEM) results indicate that the particle size of Ni in Ni/La2O3 remained stable after the induction period. The O2 temperature-programmed desorption (O2-TPD) and electron spin resonance (ESR) results reveal that oxygen vacancies are formed at the metal–oxide interfaces of Ni/La2O3 and Ni/La2O2CO3 catalysts (Ni–Ov–La). The interfacial oxygen vacancies enhance H2 adsorption with an additional H2 desorption peak at temperatures higher than those on metallic Ni surfaces. Rich medium basic sites on the La2O2CO3 surfaces promote CO2 adsorption and activation as surface carbonates. The temperature-programmed surface reaction (TPSR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results reveal two distinct reaction pathways for CO2 hydrogenation, one involving the hydrogenation of CO intermediates on Ni and the other involving the hydrogenation of surface carbonates to formate intermediates on La2O2CO3 supports with H spillover from Ni atoms, leading to CH4 formation. This work provides deep insights into the oxygen vacancy formation on transition-metal-modified La2O3 and La2O2CO3 surfaces and sheds light on the design of lanthanum oxide-based catalysts.