Heterojunction WO₃–BiVO₄ Photoanodes for TEMPO-Mediated Benzyl Alcohol Dehydrogenation in Organic Media

Photoelectrosynthetic approaches to fine chemical production offer a means of directly integrating renewable energy into processes that rely on fossilized carbon-based energy. This work investigates the use of heterojunction semiconductor photoanodes comprised of a tungsten oxide (WO3) base layer and bismuth vanadate (BiVO4) photoactive layer to drive the production of benzaldehyde via benzyl alcohol dehydrogenation. Illumination of the WO3|BiVO4 photoanode drives interfacial oxidation of solution-dissolved TEMPO which mediates oxidation of benzyl alcohol to benzaldehyde. Detailed photoelectrochemical and impedimetric studies show the importance of TEMPO as a mediator and the influence of other solution components for achieving overall dehydrogenation. Optimized reaction conditions result in a Faradaic efficiency of 66% (19 ± 2% product yield) for benzaldehyde and a 43% Faradaic efficiency (10 ± 4% product yield) for cathodic hydrogen production with added bias. As opposed to using oxygen as a terminal acceptor, this study focuses on the photoelectrochemical production of two value-added products and reveals some of the compound challenges presented when considering a complete cell reaction as opposed to isolated half-cell chemistry.