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Molecules 2020, 25, 3653 4 of 23 Various carbon materials including carbon nanofibers [34], carbon nanotubes [35–38], biochar [39], and carbon felt [40] have also been employed as carriers for CO2 hydrogenation catalysts, taking advantage of their high hydrogen storage capacity, high thermal conductivity, and high specific surface area of carbon carriers. Nanosized materials are used to define nanoscale catalyst structures, in which the composition of catalysts and their surface structures can be adjusted and may bring to darewidespread applications. Despite such achievements, the reactant H2 added in the thermal catalysis process is usually more valuable than the product methane and methanol. Considering the higher cost than that from fossil fuels, the direct hydrogenation reaction of CO2 was rarely used to produce methane or methanol [28]. There are still great challenges in developing catalysts with high catalytic performance and long-term stability, reducing the size of thermal catalytic reactors and decreasing the production costs. In addition, more effective and economical methods to produce H2 are urgently needed [13]. In this case, CO2 conversion to useful fuels are attempted by other methods such as photocatalysis, electrocatalysis, and photoelectrocatalysis. 2.2. Photocatalysis Solar energy is as an ideal energy source to replace traditional fossil fuels because it is an abundant, cheap, clean, and sustainable energy source. Therefore, the use of photocatalysts for solar-driven fuels or chemicals from CO2 is a very attractive approach. Similar to natural photosynthesis, electron–hole pairs are generated when the photocatalysts are exposed to solar light. The photogenerated electrons induce CO2 to undergo a redox reaction that results in hydrocarbon formation. There are three crucial procedures during the photocatalytic conversion of CO2: (1) absorption of sunlight; (2) charge separation and transfer; and (3) catalytic reduction of CO2 and oxidation of H2O [17]. Each procedure during the conversion of CO2 is closely related with the photocatalysts. Until now, the photocatalysts were mainly from semiconductor materials which are abundant on earth and easy to obtain [41]. As for the reaction products, CO, methane, formic acid, and other chemicals containing one or two carbon atoms are usually involved. Until now, many efforts have been made to optimize the structure and composition of photocatalysts or integrate them with other functional units to construct multifunctional catalysts. For example, integration of photocatalysts with metal–organic frameworks (MOFs) has been demonstrated to offer more adsorptive sites for CO2 uptake because of their extreme larger surface area and microporous structure [42–44], resulting in remarkable improvement in CO2 conversion. However, there is a wide gap between the photocatalytic performance of these complexes and the requirements for practical application [45]. The construction of multi-junctions are randomly distributed on the surface of photocatalysts, which improve interfacial electron–hole separation and migration, even though the separation efficiency remains to be raised to a higher level [46]. Meng et al. [16] deposited MnOx nanosheets and Pt nanoparticles on different facets of anatase TiO2 to form surface heterojunction. The results indicated that heterojunction with multiple nodes in the photocatalysts improved the conversion efficiency of CO2. Two dimensional (2D) nanosheets are particularly promising in improving charge separation because the photogenerated electrons and holes will move to the interface with shorter distances. Wei and coauthors [47] synthesized a series of heterostructured CdS/BiVO4 composites by depositing different amounts of CdS on the surface of BiVO4 nanosheets with variable thickness. The results showed that CdS/BiVO4 nanocomposites had higher photocatalytic activity in CO2 reduction than that of pure BiVO4 and CdS. Furthermore, the content of CdS in the composites were responsible for the yield of CO and CH4. Enhancement of photocatalytic activity was attributed to the synergistic effect of forming Z-scheme herterojunction and reduced thickness of BiVO4. According to density functional theory (DFT), theoretical calculations have been made for 2D photocatalysts and other types of catalysts [48–52], such that the characteristics of materials and the role of different components in thePDF Image | Research Progress in Conversion of CO2 to Valuable Fuels
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