Recent advances in the photocatalytic conversion of carbon dioxide to fuels with water and/or hydrogen using solar energy and beyond

Photocatalytic reduction of carbon dioxide to fuels using solar energy is an attractive option for simultaneously capturing this major greenhouse gas and solving the shortage of sustainable energy. Efforts to demonstrate the photocatalytic reduction of CO2 are reviewed herein. Although the photocatalytic results depended on the reaction conditions, such as the incident/absorbing light intensity from the sun or a simulated solar light source, the performance of different systems is compared. When the reactants included CO2 and water, it was necessary to determine whether the products were derived from CO2 and not from impurities that accumulated on/in the catalysts as a result of washing, calcination, or pretreatment in a moist environment. Isotope labeling of 13CO2 was effective for this evaluation using Fourier-transform infrared (FTIR) spectroscopy and mass spectrometry (MS). Comparisons are limited to reports in which the reaction route was verified spectroscopically, the C source was traced isotopically, or sufficient kinetic analyses were performed to verify the photocatalytic events. TiO2 photocatalytically produced methane at the rate of ∼0.1 μmol h−1 gcat−1. In aqueous solutions, formic acid, formaldehyde, and methanol were also produced. When TiO2 was atomically dispersed in zeolites or ordered mesoporous SiO2 and doped with Pt, Cu, N, I, CdSe, or PbS, the methane and CO formation rates were greater, reaching 1–10 μmol h−1 gcat−1. As for semiconductors other than TiO2, CdS, SiC, InNbO4, HNb3O8, Bi2WO6, promoted NaNbO3, and promoted Zn2GeO4 produced methane or methanol at rates of 1–10 μmol h−1 gcat−1, and promoted AIILa4Ti4O15 produced CO at a rate greater than 10 μmol h−1 gcat−1, in addition to the historically known ZnO and GaP (formaldehyde and methanol formation). The photocatalytic reduction of CO2 was also surveyed with hydrogen, because hydrogen can be obtained from water photosplitting by utilizing natural light. CO was formed at a rate of ∼1 μmol h−1 gcat−1 using TiO2, ZrO2, MgO, and Ga2O3, whereas both CO and methanol were formed at a rate of 0.1–1 μmol h−1 gcat−1 using layered-double hydroxides consisting of Zn, Cu, Al, and Ga. When hydrogen is used, in addition to identifying the origin of the carbon, it is critical to confirm that the products are photocatalytically formed, not thermally produced via CO2 hydrogenation. The feasibility of the strategy involving the recycling of a sacrificial electron donor and the direct supply of protons and electrons released from water oxidation catalysts to photocatalysts for the reduction of CO2 to fuels has been demonstrated. However, based on the results obtained to date, it is clear that the practical use of the photocatalytic reduction of CO2 as one possible solution for global warming and the world's energy problems requires the development of more efficient photocatalysts.

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► Efforts to demonstrate photocatalytic reduction of CO2 into fuels are reviewed.
► Formic acid, CO, formaldehyde, methanol, ethanol, methane, and ethane were made.
► Control reaction tests and isotope labeling confirmed the reactions of CO2 + H2O.
► H2 split from water under light also photocatalytically converted CO2 into fuels.
► Layered double hydroxides photocatalytically produced methanol from CO2 + H2.