A comprehensive review on oxygen defect rich catalysts for low-temperature CO2 hydrogenation to methane and alcohols

The thermo-catalytic hydrogenation of greenhouse gas CO 2 into more valuable products, primarily fuels such as methane and alcohols, is a promising pathway of carbon utilization. Since CO 2 is known for its inertness with high activation energy, which makes it difficult to activate for the hydrogenation reaction, thus demanding higher temperatures for its conversion to useful products. Widespread efforts have been made to optimize catalysts for better catalytic performance at less severe operating conditions. This has garnered research community interest in utilizing the oxygen defect-rich metal catalysts, where missing oxygen atoms, or oxygen vacancies (V O s), contribute to the adsorption and geometry distortion of CO 2 , easing its reaction with H 2 . This review presents an overview of the potential of oxygen defect-rich catalysts for low-temperature CO 2 hydrogenation, particularly focusing on those with supports such as CeO 2 , ZrO 2 , and TiO 2 . The fundamentals of V O s, including their types, impact, formation, and characterization techniques, are discussed, followed by an examination of their role in improving catalytic performance and steering reaction pathways towards methane and alcohols. Emphasis is placed on relevant optimization parameters, including catalyst features (metal loading and dispersion, type of metal, structure, etc.) , presence and density of V O s and hydrogenation promoters, and reaction conditions (temperature, pressure, H 2 :CO 2 feed ratio, flow rate). Recent advances are summarized, and lastly, current challenges and prospects are discussed.