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 CO 2 methanation reaction. However, the intrinsic activity and reaction mechanism remain unclear. In this work, we found that in the CO 2 methanation reaction, the active phase of the lanthanum oxide support is La 2 O 2 CO 3 transformed by the reaction between La 2 O 3 and CO 2 . After the induction period, Ni/La 2 O 3 catalysts show a performance almost identical to that of Ni/La 2 O 2 CO 3, with superior CO 2 methanation performance compared to those of Ni/Al 2 O 3 and Ni/SiO 2 catalysts. Furthermore, Ni/La 2 O 3 catalysts show negligible changes in the activity of the CO 2 methanation reaction during an 80 h long-term stability test at 673 K, achieving a CH 4 production rate of 444 mmol CH 4 ·g Ni –1 ·s –1 with a CO 2 conversion rate of 77% and a CH 4 selectivity of 97%. Transmission electron microscopy (TEM) results indicate that the particle size of Ni in Ni/La 2 O 3 remained stable after the induction period. The O 2 temperature-programmed desorption (O 2 -TPD) and electron spin resonance (ESR) results reveal that oxygen vacancies are formed at the metal–oxide interfaces of Ni/La 2 O 3 and Ni/La 2 O 2 CO 3 catalysts (Ni–O v –La). The interfacial oxygen vacancies enhance H 2 adsorption with an additional H 2 desorption peak at temperatures higher than those on metallic Ni surfaces. Rich medium basic sites on the La 2 O 2 CO 3 surfaces promote CO 2 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 CO 2 hydrogenation, one involving the hydrogenation of CO intermediates on Ni and the other involving the hydrogenation of surface carbonates to formate intermediates on La 2 O 2 CO 3 supports with H spillover from Ni atoms, leading to CH 4 formation. This work provides deep insights into the oxygen vacancy formation on transition-metal-modified La 2 O 3 and La 2 O 2 CO 3 surfaces and sheds light on the design of lanthanum oxide-based catalysts.