Abstract: With the continuous progress of global industrialization, fossil fuels have been over-consumed, which results in a large amount of CO₂ discharged into the atmosphere and therefore causes negative effects such as global warming and ecological imbalance. Reducing CO₂ emissions and converting recycled CO₂ to value added chemicals have become important tasks. Around 2010, led by United States of America, followed by Europe and Japan, tens of countries started their national scientific research projects entitled “artificial photosynthesis”, with an investment as much as 100 million USDs. Since 2011, China has also funded similar projects by the National Natural Science Foundation of China and the Ministry of Science and Technology. In September of 2020, the Chinese government even put forward the goal of “carbon emission peak” by 2030 and “carbon neutrality” by 2060.
Green plants or microorganisms make organics from CO₂ and H₂O through “photosynthesis”. The photocatalysts can convert CO₂ and H₂O/H₂ into fuels or chemicals under light irradiation, which simulates the natural “photosynthesis” and is entitled as “artificial photosynthesis”. Nevertheless, the conversion of CO₂ is not limited to be driven by solar energy; alternatively, the electricity generated by non-fossil fuels to drive efficient electrocatalytic or thermocatalytic CO₂ reduction is also a feasible way. At present, the hot spots in basic research are photocatalysis, photothermocatalysis and electrocatalysis for CO₂ reduction. “Green conversion of CO₂” defines the technological approaches with at least one of the features of low energy consumption, low environmental load and high efficiency, which enables them competitive in future industrial applications.
In recent years, the study on CO₂ conversion has made rapid progresses, but some key problems are still to be solved. Photocatalysis provides the mildest way to convert CO₂ into fuels or chemicals, which means the lowest cost in the future application, but currently faces great challenge in efficiency and stability. Not until photothermocatalytic CO₂ reduction was reported in 2014, it has attracted extensive attention. The catalysts realize light-to-thermal conversion and subsequently drive thermocatalysis. Compared with photocatalysis, the efficiency and stability of photothermocatalytic CO₂ reduction are significantly increased, but the consumption of H₂ as feedstock will result in additional cost. Electrocatalytic CO₂ reduction is also a mild process, and its energy utilization efficiency is the highest among three methods. However, due to the reaction environment of aqueous solution, there inevitably occurs the competition of CO₂ reduction and proton reduction; therefore, the improvement of product selectivity is still faced with a bottleneck. Moreover, the separation of liquid or gas phase products somewhat increases production cost. Importantly, the economy of CO₂ conversion should take account of the yield of value-added product. The products of photocatalysis and electrocatalysis mainly concentrated in C1 chemicals (mainly including carbon monoxide, methane, methanol, formic acid salt,etc.), and a few studies reported the production of ethane. However, the photothermocatalysis exhibits superior advantage in this aspect; for instance, the Fischer-Tropsch synthesis with CO₂ and H₂ as feedstocks to produce C2 - C7 products has been reported. Each technological approach has its own advantages but faces different scientific or technical difficulties, which may complement each other and show its strengths in different application fields in the future.
Shortly after China put forward the goal of “carbon emission peak” and “carbon neutrality”, the editorial board ofJournal of Inorganic Materialsorganizes the special issue on “Green Conversion of CO₂”. Although the investigation on CO₂ conversion has made great progress, but still faces various challenges. Looking forward to that more researchers devote to this study to promote it from the basic research to the industrial application, our continuous effort will make our country reverse the disadvantage of terrible carbon emission to the advantage of converting recycled CO₂ to resources effectively.