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Molecules 2020, 25, 3653 15 of 23 Researchers not only improved the noble metal mediator, but also further studied the coenzyme NADH. NADH is necessary in most biocatalytic reductions of CO2 to provide electrons for dehydrogenase. NADH is expensive and easily forms enzymatically inactive dimers, NAD2 , resulting in reduction of enzyme activity [143]. It is therefore necessary to develop economical methods for NADH cofactor regeneration. To achieve this goal, chemical, photochemical, and electrochemical regeneration of NADH has been developed over the last few decades [122,144–146]. Electrochemically mediated electron injection into the enzyme not only bypasses the requirement of NADH but also simplifies product separation [147]. Amao and Shuto [148] have demonstrated the use of viologen-modified FDH immobilized on an ITO electrode to electrochemically convert CO2 to formic acid. This study showed evidence of electrons directly transferring to FDH active sites from electrodes and represents a new strategy for CO2 reduction in the absence of NADH. However, finding alternative compounds to replace NADH is also an attractive research direction. Hence, researchers have tried to use cheaper electron mediators, such as methyl viologen (MV2+), instead of NADH. Miyatani et al. [149] developed a system using MV which combined the synthesis of formic acid from CO2 (bicarbonate ion) with FDH and MV2+, and photoreduction with ZnTMPyP as photosensor and TEOA as electron donor. Formic acid was successfully formed from bicarbonate ions and FDH in the absence of NADH. Moreover, investigations have demonstrated that the oxidation of formate to carbon dioxide does not occur readily with MV2+. Therefore, the overall yield of formate would be preserved without loss from reoxidation [150]. Ishibashi et al. [151] used methyl viologen (MV2+) instead of NADH. A photoelectrochemical system was composed of TiO2 nanoparticles as photocatalyst, MV2+ as electron carrier, and FDH as biocatalyst, in which CO2 was successfully reduced to formic acid without sacrificing reagent, external bias, and NADH. 3. Conclusions and Outline Thermal catalysis, photocatalysis, electrocatalysis, photoelectrocatalysis, and enzyme catalysis can effectively alleviate the greenhouse gas CO2 . The catalyst is the key component of different CO2 reducing systems. A suitable catalyst can not only reduce energy consumption but also facilitate generation and transfer of electrons. On one side, from the perspective of energy consumption, we wish to complete the conversion of CO2 while consuming as little energy source as possible to save energy and the environment. Therefore, photocatalysis and photoelectrocatalysis that adopt clean and sustainable solar energy to drive the conversion are of interest, in which porphyrin-based macrocycles or their combination with other components presented promising properties of light-capturing and charge transferring. On the other side, the selectivity of products has challenged the development of CO2 conversion. Specific product generation from CO2 will greatly reduce the cost of subsequent product separation and purification. This has inspired researchers to pay particular attention to biocatalysts because of their high specificity and efficiency in catalyzing biochemical reactions. Among the aforementioned methods, enzyme coupled to photocatalysis and enzyme coupled to photoelectrocatalysis has integrated the two sides successfully, showing great potential in solar energy utilization and specific conversion of CO2. They are worthy of more investigation to make biocatalysis compatible with photocatalysis or photoelectrocatalysis. In general, the conversion of CO2 to valuable fuels or chemicals appears to have a bright future, and continuous efforts are needed to improve the catalytic efficiency, conversion rate, and product selectivity. Author Contributions: Conceptualization, S.W. and L.X.; software, L.X. and Y.X.; validation, S.W.; resources, S.W.; date curation, F.L. and Y.L.; writing—original draft preparation, L.X. and Y.X.; writing—review and editing, S.W.; visualization, S.W. and F.L.; supervision, S.W.; project administration, S.W.; funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (21773310) and Key R. & D. Program of Shandong Province (2019GGX103047). Conflicts of Interest: The authors declare no conflicts of interest.PDF Image | Research Progress in Conversion of CO2 to Valuable Fuels
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