Photocatalytic reduction of CO2 using metal complexes

•Review on photocatalytic systems for CO2 reduction using metal complexes.•Mixed systems, supramolecular photocatalysts, and hybrid systems with inorganic materials.•Systematical classification of the systems based on roles of each component.•Detailed features and abilities of the photocatalytic systems.

Developing photocatalytic systems for CO2 reduction will provide useful and energy-rich compounds and would be one of the most important focuses in the field of “artificial photosynthesis” and “solar fuels”. Such studies have been conducted in the past three decades from the perspective of basic science and for solving the shortage of fossil resources, which include both energy and carbon sources. More recently, focus has been placed on the mitigation of global warming through the reduction of atmospheric CO2. This review summarizes the enormous body of reported literature in this field, particularly studies that describe photocatalytic systems that use transition metal complexes as key players, i.e., as catalysts (Cat) and/or photosensitizers (PS). In addition, we briefly describe the evaluation of various photocatalytic systems, especially the performance of reductants (D) and solvents. Furthermore, we analyze the types of photocatalytic systems and classify each component in these systems according to their role: (1) PS, (2) Cat for CO2 reduction catalysts, and (3) D. Briefly, we summarize the important features of each component and provide typical examples. The next section discusses the photocatalytic abilities of each of the three categories of photocatalytic systems: multicomponent systems comprising PS and Cat, supramolecular photocatalysts comprising a multinuclear complex, and hybrid systems constructed with metal-complex photocatalysts and inorganic materials, such as semiconductors or electrodes.