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 (Co^II(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 Co^II(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 Co^II(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 Co^II(Ch) on MWCNTs, because two [Co^I(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 Co^II(Ch) adsorbed on MWCNTs as a CO₂ reduction catalyst and [Ru^II(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.