Photoelectrochemical (PEC) hydrogenation of toluene (TL) accompanied with oxidation of water has been reported to produce methylcyclohexane (MCH) as one of the promising hydrogen carriers.¹ In the previous report, the overall PEC MCH production without an external bias voltage has been achieved by using the PEC cell consisting of single crystalline Nb-doped SrTiO₃ (Nb:STO) photoanode and Pt-loaded carbon black (Pt/C)-based membrane-electrode assembly (MEA) cathode. However, the incident-photon-to-current conversion efficiency (IPCE) of the Nb:STO photoanodes used in this prior work was unfortunately quite low: only 3.5% under 350 nm light at 0.5 V_RHE, and thus the further enhancement of quantum efficiencies of this reaction is one of the necessary challenges. Furthermore, past studies focused only on the PEC conversion of pure TL to generate dilute MCH. To establish this PEC MCH production reaction as a practical and industrial artificial photosynthesis system, direct production of MCH up to nearly 100% of concentration is indispensable in order to avoid additional purification process. In this study, the model PEC cell consisting of particulate 1%Al-doped SrTiO₃ (Al:STO) photoanode² and Pt/C MEA cathode was carefully investigated to: 1) improve IPCE of this reaction; and 2) evaluate the feasibility of direct PEC production of the concentrated MCH.

When the pure TL and 0.1 M NaOH electrolyte with pH 13 were applied as an organic and aqueous reactant, respectively, the present PEC cell consisting of Al:STO photoanode and Pt/C MEA cathode successfully performed the spontaneous MCH production without an external bias voltage under irradiation of 300 W Xe lamp. It should be noted that the faradaic efficiencies (FE) for the MCH production from pure TL was confirmed to be almost 100% and that the IPCE of this PEC cell was measured to be 18% at 320 nm light without an external bias voltage, which is significantly greater than that of the reported case with Nb:STO photoanode.¹ This is because that the present photoanodes prepared from Al:STO particles by the particle transfer method³ possess the advantageous structural features for PEC water oxidation.

To evaluate the feasibility of the present PEC cell for direct production of the concentrated MCH, the PEC MCH production from the dilute TL, 0.05—1vol% TL in MCH, with various pH values of aqueous phase were also examined. It was revealed that even in the case of hydrogenating dilute TL, 1vol% in MCH, the PEC cell with the pH 13 NaOH aqueous electrolyte showed a sufficient FE of >94%. This experimental result indicates that the present PEC cell is capable of producing the concentrated MCH with >99%. In addition, when applying further dilute TL, 0.05vol% in MCH, the PEC system with pH 13 NaOH showed an FE of almost 50%, while that with pH 9.9 and 7.5 Na₂SO₄ showed lower FE of approximately 35%. Since the FE is roughly confirmed to be decreased by the competitive HER at the Pt/C, the present experimental observations imply that strong alkaline conditions should contribute to a suppression of the undesirable side reaction. Thus, we can conclude that the utilization of an anion exchange membrane in conjunction with a high pH aqueous phase is beneficial for converting the dilute reactant to the required product in sufficient concentration.

Finally, the concentrated MCH production under intermittent light irradiation was also examined in order to understand the effects of reactant diffusion process on the FE. The FE for concentrated MCH production from 0.05vol% TL in MCH and pH13 NaOH aqueous electrolyte was as high as 83% under this light irradiation condition. This obvious enhancement of FE by the intermittent irradiation indicates the significance of diffusion process of TL, especially in the case of using dilute reactants. Therefore, there is still room to enhance the FE for concentrated MCH production from dilute TL by facilitating the supply of the reactant, TL, to the active sites.

References

1. V. Kalousek, P. Wang, T. Minegishi, T. Hisatomi, K. Nakagawa, S. Oshima, Y. Kobori, J. Kubota and K. Domen, ChemSusChem, 2014, 7, 2690.

2. Y. Ham, T. Minegishi, T. Hisatomi and K. Domen, Chem. Commun., 2016, 52, 5011.

3. T. Minegishi, N. Nishimura, J. Kubota and K. Domen, Chem. Sci., 2013, 4, 1120.