Electrocatalytic reduction of low concentration CO2.

Utilization of low concentration CO2 contained in the exhaust gases from various industries and thermal power stations without the need for energy-consuming concentration processes should be an important technology for solving global warming and the shortage of fossil resources. Here we report the direct electrocatalytic reduction of low concentration CO2 by a Re(i)-complex catalyst that possesses CO2-capturing ability in the presence of triethanolamine. The reaction rate and faradaic efficiency of CO2 reduction were almost the same when using Ar gas containing 10% CO2 or when using pure CO2 gas, and the selectivity of CO formation was very high (98% at 10% CO2). At a concentration of 1% CO2, the Re(i) complex still behaved as a good electrocatalyst; 94% selectivity of CO formation and 85% faradaic efficiency were achieved, and the rate of CO formation was 67% compared to that when using pure CO2 gas. The electrocatalysis was due to the efficient insertion of CO2 into the Re(i)-O bond in fac-[Re(dmb)(CO)3{OC2H4N(C2H4OH)2}] (dmb = 4,4'-dimethyl-2,2'-bipyridine).

Photoelectrochemical CO2 Reduction Using a Ru(II)-Re(I) Supramolecular Photocatalyst Connected to a Vinyl Polymer on a NiO Electrode.

A Ru(II)-Re(I) supramolecular photocatalyst and a Ru(II) redox photosensitizer were both deposited successfully on a NiO electrode by using methyl phosphonic acid anchoring groups and the electrochemical polymerization of the ligand vinyl groups of the complexes. This new molecular photocathode, poly-RuRe/NiO, adsorbed a larger amount of the metal complexes compared to one using only methyl phosphonic acid anchor groups, and the stability of the complexes on the NiO electrode were much improved. The poly-RuRe/NiO acted as a photocathode for the photocatalytic reduction of CO2 at E = -0.7 V vs Ag/AgCl under visible-light irradiation in an aqueous solution. The poly-RuRe/NiO produced approximately 2.5 times more CO, and its total Faradaic efficiency of the reduction products improved from 57 to 85%.

Hybrid photocathode consisting of a CuGaO2 p-type semiconductor and a Ru(ii)-Re(i) supramolecular photocatalyst: non-biased visible-light-driven CO2 reduction with water oxidation.

A CuGaO2 p-type semiconductor electrode was successfully employed for constructing a new hybrid photocathode with a Ru(ii)-Re(i) supramolecular photocatalyst (RuRe/CuGaO2). The RuRe/CuGaO2 photocathode displayed photoelectrochemical activity for the conversion of CO2 to CO in an aqueous electrolyte solution with a positive onset potential of +0.3 V vs. Ag/AgCl, which is 0.4 V more positive in comparison to a previously reported hybrid photocathode that used a NiO electrode instead of CuGaO2. A photoelectrochemical cell comprising this RuRe/CuGaO2 photocathode and a CoO x /TaON photoanode enabled the visible-light-driven catalytic reduction of CO2 using water as a reductant to give CO and O2 without applying any external bias. This is the first self-driven photoelectrochemical cell constructed with the molecular photocatalyst to achieve the reduction of CO2 by only using visible light as the energy source and water as a reductant.

Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)-Re(I) Complex Photocatalyst and a CoOx/TaON Photoanode.

Photoelectrochemical CO2 reduction activity of a hybrid photocathode, based on a Ru(II)-Re(I) supramolecular metal complex photocatalyst immobilized on a NiO electrode (NiO-RuRe), was confirmed in an aqueous electrolyte solution. Under half-reaction conditions, the NiO-RuRe photocathode generated CO with high selectivity, and its turnover number for CO formation reached 32 based on the amount of immobilized RuRe. A photoelectrochemical cell comprising a NiO-RuRe photocathode and a CoOx/TaON photoanode showed activity for visible-light-driven CO2 reduction using water as a reductant to generate CO and O2, with the assistance of an external electrical (0.3 V) and chemical (0.10 V) bias produced by a pH difference. This is the first example of a molecular and semiconductor photocatalyst hybrid-constructed photoelectrochemical cell for visible-light-driven CO2 reduction using water as a reductant.

Photoelectrochemical CO2 reduction using a Ru(II)-Re(I) multinuclear metal complex on a p-type semiconducting NiO electrode.

A photocathode for CO2 reduction was successfully developed using a hybrid electrode comprising a Ru(II)-Re(I) supramolecular photocatalyst and a NiO electrode. Selective photoexcitation of the Ru photosensitizer unit of the photocatalyst at -1.2 V vs. Ag/AgNO3 selectively afforded CO with high faradaic efficiency.

Efficient Photocatalysts for CO2 Reduction.

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.