Metal Oxide Semiconductors for Photothermal Catalytic CO<sub>2</sub>Hydrogenation Reactions: Recent Progress and Perspectives

Abstract: Owing to the accelerated growth of the human economy and society, the increasing concentration of CO2 in the atmosphere has caused serious ecological and environmental problems because of the greenhouse effect. In response to the challenges posed by climate change, China has made a significant commitment to "peak carbon emissions by 2030 and achieve carbon neutrality by 2060". Ideally, converting CO2 into carbon-based energy and chemicals is supposed to be the best strategy of both worlds, mitigating the greenhouse effect while also addressing the shortage of energy supply. Among the proposed concepts for the above strategy, the scheme of reducing CO2 using renewable green H2 to produce chemicals is preferred, because it can stimulate the potential of clean energy while also reducing CO2 emission. To accelerate this reduction process, many catalytic reactions, including photocatalysis, have been designed and investigated. Owing to its high catalytic efficiency and extensive use of solar energy, photothermal catalytic CO2 hydrogenation in photocatalysis is a desirable method for increasing sun-to-fuel efficiency. There are two main interpretations of photothermal catalytic hydrogenation: (1) only sunlight is used as the energy source to drive the catalyst, which generates heat to promote CO2 conversion. In this case, the reaction still proceeds in the form of thermocatalysis, whereas photocatalysis has a limited effect. (2) Solar and heat energy are coupled to participate in the catalytic reaction, which has a synergistic effect. Therefore, according to the catalytic mode, the rational design and successful synthesis of photothermal catalysts are very important. Metal oxide semiconductors, owing to their unique energy band structure and chemical properties, high stability, and environmental friendliness, are widely used in the research of photothermal catalytic hydrogenation reactions. In this study, we review the research progress on metal oxide materials used in the CO2 hydrogenation reaction by photothermal catalysis. In particular, the most significant results of research in the last five years have been performed mainly from three different catalyst modulation strategies, such as supporting catalysts, applying microstructure engineering, and defect engineering. The mechanisms of these modulation strategies are summarized and presented for further understanding. In addition, this study introduces different types of photothermal hydrogenation reactors, accompanied by the effects of some key parameters on the reactions. Finally, design strategies for metal oxide catalysts are suggested, and an outlook of photothermal abatement technology is presented.