Solar-to-Pharmaceutical Raw Material Production: Photoelectrochemical Naphthoquinone Formation Using Stabilized BiVO₄ Photoanodes in Acid Media

In the quest for efficient use of solar energy to produce high-value-added chemicals, we first achieved the photoelectrochemical (PEC) diketonization of naphthalene, using a BiVO₄/WO₃ photoanode, to obtain naphthoquinone, an important pharmaceutical raw material with excellent efficiency by solar energy conversion. In the electrochemical (EC) reaction using F-doped SnO₂ (FTO) substrates and a 0.5 M H₂SO₄ H₂O–acetone (60 vol %) mixed solution containing 5 mM naphthalene, we produced a small amount of naphthoquinone evolution in the dark. However, naphthoquinone (ηNQ)’s Faradic efficiency and its evolution rate at 1.7 VAg/AgCₗ were only 28.5% and 0.48 μmol·cm–²·h–¹, respectively. The PEC reaction using a WO₃ photoanode had very low efficiency for naphthalene diketonization, with low ηNQ and evolution rate values at 1.1 VAg/AgCₗ of 0.3% and 0.039 μmol·cm–²·h–¹, respectively. In contrast, the BiVO₄/WO₃ photoanode strongly enhanced the PEC reaction, and the ηNQ and evolution rates at 1.1 VAg/AgCₗ were boosted up to 37.5% and 4.7 μmol·cm–²·h–¹, respectively. The evolution rate of the PEC reaction in the BiVO₄/WO₃ photoanode was 10 times higher than that of the EC reaction with the FTO substrate regardless of the very low bias voltage. This result suggests that the BiVO₄-based photoanode was very efficient for the selective oxidation of naphthalene even in acid media because of the acetone-mixed electrolyte’s anti-photocorrosion effect and the multilayering of WO₃ and BiVO₄. At a naphthalene concentration of 20 mM, the naphthoquinone evolution rate reached its maximum value of 7.1 μmol·cm–²·h–¹. Although ηNQ tended to decrease with the increase in the electric charge, it reached 100% at a low bias voltage of 0.7 VAg/AgCₗ. An intensity-modulated photocurrent spectroscopy analysis indicated the rate constant of charge transfer at the photoanode surface to the naphthalene molecules was strongly enhanced at a low bias voltage of 0.7–1.1 VAg/AgCₗ, resulting in the high ηNQ value. The acid-resistant BiVO₄/WO₃ photoanode functioned in acetone-mixed electrolytes enabled the realization of a new PEC oxidation reaction driven by solar energy to produce high-value-added pharmaceutical raw materials.