The transformation of primary alcohols to higher valuable carbonyls or acids has attached enormous attention from both academic and industrial viewpoints, but typically requires stoichiometric oxidants, which are costly and generate potentially toxic waste. Previously, we have studied selective electrocatalytic oxidation of biomass-derived alcohols and 5-hydroxymethylfurfural (HMF) on noble metal catalysts.[1-3] Electroactive homogeneous catalysts such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) represent an alternative to expensive metal catalysts.[4] Conversion of biorenewable feedstock in a photoelectrochemical cell (PEC) [5] is potentially one of the most environmentally-friendly chemical production routes available, which takes advantage of renewable carbon sources from the earth, renewable photons from the sun, and renewable electrons from wind, geothermal, or photovoltaic sources. However, there is a need to develop new composite materials which can simultaneously facilitate light absorption, charge separation, charge transport, and electrochemical reactions. We have investigated the use of composite photoanodes for efficient TEMPO-mediated oxidations. Bismuth vanadate (BiVO₄) semiconductor films modified with an earth-abundant cobalt phosphate (Co-Pi) layer facilitated TEMPO oxidation at overpotentials almost 500 mV lower than unmodified BiVO₄, while simultaneously suppressing the undesired oxygen evolution reaction. The photoanode materials were characterized using XRD, XPS, SEM, EDS, UV/vis and photoelectrochemical measurements. The composite photoanodes achieved complete oxidation of HMF to 2,5-furandicarboxylic acid (FDCA) with 88% yield, whereas an unmodified BiVO₄ only reached partial oxidation intermediates (FDCA yield <1%) under the same conditions. Transient photocurrents revealed the role of Co-Pi for promoted TEMPO oxidation is two-fold: (1) to efficiently remove photogenerated charges from the semiconductor and (2) promote charge transfer across the catalyst-electrolyte interface for TEMPO oxidations.

References:

(1) Zhang, Z.Y. et al., Appl. Catal. B, (2012), 119, 40-48.

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

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

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

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