Photocatalytic Reduction of CO2 with a Cobalt Chlorin Complex in Water
Authors: Fukuzumi Shunichi 
Published: February 2020

Photoctalytic reduction of CO2 and H2O to CO and H2 has merited increasing attention, because synthesis gas (a fuel gas mixture consisting primarily of CO and H2) can be converted to liquid hydrocarbon fuels by Fischer-Tropsch processes.[1-3] Because the CO2 reduction to CO competes with the proton reduction to H2 as well as CO2 reduction to HCOOH, selective CO production in the photocatalytic reduction of CO2in water has been a challenging issue.
We report herein photoelectrochemical reduction of CO2 to CO with high Faradaic efficiency using a cobalt chlorin complex (CoII(Ch)) adsorbed on multi-walled carbon nanotubes (MWCNTs) as a cathode and a surface-modified BiVO4 photoanode with iron(III) oxide(hydroxide) (FeO(OH)) for oxidation of water in a CO2-saturated aqueous solution (pH 4.6).
The photoelectrochemical reduction of CO2 was performed in a two-compartment cell composed of an FeO(OH)/BiVO4/FTO photoanode and a CoII(Ch)/MECNTs cathode, where the two electrodes were connected with conducting wire as an external circuit and separated by a Nafion membrane (Figure 1). Electrocatalytic reduction of CO2 occurred efficiently using the CoII(Ch)/MECNTs electrode at an applied potential of –1.1 V vs. NHE to yield CO with a Faradaic efficiency of 89% with hydrogen production accounting for the remaining 11% at pH 4.6.[4]
CO was produced with 83% Faradaic efficiency at an applied potential of –1.3 V vs the potential of the photoanode under visible light irradiation in a CO2-saturated aqueous solution (pH 4.6). The amount of O2 produced in the photoelectrochemical oxidation of water is one-half of the amounts of the sum of CO and H2 produced in the electrochemical reduction of CO2 and H2O on the cathode. The difference in the oxidation potential of the FeO(OH)/BiVO4/FTO electrode under dark and that under light illumination was ca. 1.5 V, indicating that the FeO(OH)/BiVO4/FTO photoanode lowered the total bias that enabled simultaneous water oxidation and CO2 reduction. Faradaic efficiency for CO production was much improved by adsorption of CoII(Ch) on MWCNTs, because two [CoI(Ch)]– are located close to each other when the two-electron reduction of CO2to CO occurs.[4,5]
Photocatalytic reduction of CO2 and H2O with triethylamine also occurred efficiently using CoII(Ch) adsorbed on MWCNTs as a CO2 reduction catalyst and [RuII(Me2phen)3]2+ (Me2phen = 4,7-dimethyl-1,10-phenanthroline) as a photocatalyst to yield CO and H2with a ratio of 2.4:1 and the high turnover number of 710.[5]
The present study provides a unique strategy for selective photoelectrocatalytic reduction of CO2 to CO over proton reduction to H2using an earth-abundant metal cathode.

References

[1] Aresta, M.; Dibenedetto, A.; Angelini, A. Chem. Rev. 2014, 114, 1709-1742.
[2] Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazábal, G. O.; Pérez-Ramírez, J. Energy Environ. Sci. 2013, 6, 3112-3135.
[3] Dry, M. E. Catal. Today
2002, 71, 227-241.
[4] Aoi, S.; Mase, K.; Ohkubo, K.; Fukuzumi, S. Chem. Commun. 2015, 51, 10226-10228.
[5] Aoi, S.; Mase, K.; Ohkubo, K.; Fukuzumi, S. Catal. Sci. Technol. 2016, 6, 4077-4080.

Figure 1