The transformation of primary alcohols to carbonyls or acids is a fundamental chemical process of interest from both academic and industrial viewpoints, but typically requires stoichiometric oxidants which are costly and generate potentially toxic waste. As an alternative approach, conversion of biomass-derived alcohols in a photoelectrochemical (PEC) cell is a promising environmentally-friendly route, which takes advantage of renewable carbon feedstocks, renewable solar energy, and renewable electricity from wind, geothermal, or photovoltaic sources. In this talk, we will share our recent progress in designing semiconductor / electrocatalyst composite photoanodes made of earth-abundant elements for photoelectrochemical alcohol oxidation.

Previously, we have studied selective electrocatalytic oxidation of biomass-derived alcohols and 5-hydroxymethylfurfural (HMF) on noble metal catalysts.^1-2 Electroactive homogeneous catalysts such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) represent an alternative to expensive metal catalysts.³ Furthermore, it was recently reported that the electrode potential required to drive TEMPO-mediated alcohol oxidation can be significantly reduced in a PEC reactor, where solar energy is harvested with a bismuth vanadate (BiVO₄) semiconductor electrode.⁴ However, significant electron-hole recombination and efficiency losses due to the competing oxygen evolution reaction (OER) make BiVO₄ a poor electrode material for TEMPO oxidation. While it is common to modify semiconductor electrodes with electrocatalysts to enhance OER, such strategies to selectively promote TEMPO oxidation have not been explored.

We present the first composite photoanode for enhanced TEMPO-mediated alcohol oxidation. BiVO₄ modified with a cobalt-based electrocatalyst layer facilitated TEMPO oxidation at potentials >400 mV lower than unmodified BiVO₄ by enabling facile charge transfer across solid–solid and solid–electrolyte interfaces. The competing OER was completely suppressed, allowing nearly 100% faradaic efficiency for TEMPO oxidation. In this talk, factors that tune the competition between TEMPO oxidation and OER, including photoanode structure, applied potential, and other reaction conditions will be discussed.

References:

(1) Chadderdon, D. J. et al., Green Chem. (2014), 16 3778–3786.

(2) Chadderdon, D. J. et al., ACS Catal. (2015), 5 6926−6936.

(3) Rafiee, M.; Miles, K. C.; Stahl, S. S., JACS (2015), 137 14751−14757.

(4) Cha, H. G.; Choi, K. S., Nature Chem. (2015), 7 328–333.