Abstract
Three types of photocatalytic systems for CO2 reduction, which were recently developed in our group, are reviewed. First, two-component systems containing different rhenium(I) complexes having different roles; i.e., redox photosensitizer and catalyst in the reaction solution are described. The mixed system of a ring-shaped rhenium(I) trinuclear complex and fac-[Re(bpy)(CO)3(MeCN)](+) is currently the most efficient photocatalytic system for CO2 reduction (ΦCO = 0.82 at λex = 436 nm). The second is a series of supramolecular photocatalysts, which have units with different functions in one molecule, i.e., redox photosensitizer, catalyst, and bridging ligand. The highest durability and speed of photocatalysis were achieved by using this system (ΦCO = 0.45, TONCO = 3029, and TOFCO = 35.7 min(-1)). The third is a novel type of artificial Z-Scheme photocatalyst for CO2 reduction, of which photocatalysis is revealed by stepwise excitation of both a semiconductor photocatalyst unit and the supramolecular photocatalyst unit. This system has both strong oxidation and reduction powers.
Highlights
Human beings are facing three serious problems that are related to each other, i.e., shortage of energy sources, shortage of carbon resources, and global warming.[1,2] In the biosphere, CO2 is the main carbon source that is reductively converted to energy-rich carbohydrates by using solar light as the energy source and water as the reductant
On the basis of the investigations described above, we proposed a molecular architecture for constructing an efficient supramolecular photocatalysts for CO2 reduction using a ruthenium(II) photosensitizer and a rhenium(I) catalyst: 1. The electron captured by the photosensitizer unit should be mainly localized in the bridging ligand but not in the peripheral ligands of the photosensitizer unit because the electron should transfer to the catalyst unit
We recently reported on the former type of hybrid, which was a powder-type artificial Z-Scheme photocatalyst; i.e., a supramolecular photocatalyst (19) with a cis,trans-Ru(N^N)(CO)2Cl2 moiety as the catalyst for CO2 reduction and a [Ru(4,4′-Me2bpy)(N^N){4,4′-(H2O3PCH2)2bpy}]2+ photosensitizer having methylphosphonate groups as anchors to metal oxides were adsorbed on tantalum oxynitride powders with silver nanosized particles on the surface (Ag/TaON).[36]
Summary
Human beings are facing three serious problems that are related to each other, i.e., shortage of energy sources, shortage of carbon resources, and global warming.[1,2] In the biosphere, CO2 is the main carbon source that is reductively converted to energy-rich carbohydrates by using solar light as the energy source and water as the reductant. We recently reported on the former type of hybrid, which was a powder-type artificial Z-Scheme photocatalyst; i.e., a supramolecular photocatalyst (19) with a cis,trans-Ru(N^N)(CO)2Cl2 moiety as the catalyst for CO2 reduction and a [Ru(4,4′-Me2bpy)(N^N){4,4′-(H2O3PCH2)2bpy}]2+ photosensitizer having methylphosphonate groups as anchors to metal oxides were adsorbed on tantalum oxynitride powders with silver nanosized particles on the surface (Ag/TaON).[36] This hybrid 19−Ag/TaON was dispersed in methanol and irradiated by visible light (λex > 400 nm) under a CO2 atmosphere to give HCOOH as the main reduction product with CO and H2. In the artificial Z-Scheme photocatalyst, the supramolecular photocatalyst plays crucial roles such as inducing efficient electron transfer from the OERS of the photosensitizer unit to the catalyst unit because the two units are connected to each other and depleting back electron transfer from the OERS of the photosensitizer unit to Ag/TaON because of rapid electron transfer to the catalyst unit
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