BiVO₄ Photoanodes for TEMPO‐Mediated Benzyl Alcohol Oxidation in Organic Media

Abstract

The photoelectrochemical production of fuels, exemplified by light‐driven water splitting to hydrogen and oxygen, offers a sustainable option to offset dependence on fossil fuels. A low‐cost, efficient, and stable photoelectrochemical approach to solar fuels remains elusive but using similar materials and photoelectrodes for chemical production or biomass conversion offers an appealing alternative. This work reports a facile method for fabricating pristine (undoped) BiVO4 photoanodes to carry out TEMPO‐mediated benzyl alcohol oxidation to benzaldehyde in organic media (TEMPO=2,2,6,6‐tetramethylpiperidinyl‐N‐oxyl). The best performing BiVO4 photoanode studied here gave a faradaic efficiency (FE) of 85±5% for benzaldehyde formation in the presence of TEMPO and pyridine during a 2.5‐hour reaction. Compared with direct electrocatalytic conversion under the same conditions, light capture and conversion by the BiVO4 surface decreased the required applied bias by 46 %. To our knowledge, this is the first report of visible light assisted, TEMPO‐mediated benzyl alcohol oxidation using pristine BiVO4 photoanodes in organic media.

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

“…The fabrication of a dense WO 3 coating film limits exposure of the FTO surface to the electrolyte and provides a suitable substrate for the direct deposition of BiVO 4 layers. ,− Figure C,D shows top-down views of BiVO 4 two-layer films deposited on top of FTO|WO 3 (1L), where BiVO 4 appears as an aggregated network of particles with morphological features typical of monoclinic BiVO 4 . Previously prepared two-layer coatings of BiVO 4 without the WO 3 underlayer have displayed less aggregated, more open surface morphology . The cross-sectional images in Figure E,F indicate the deposition of uniform WO 3 and BiVO 4 films to give a defined sequential layering within the heterojunction.…”

“…While purged with N 2 , the reactor was not completely sealed from atmosphere and a FE of 45% was found for the production of benzaldehyde. This result was unexpectedly low given our previous findings . Control experiments with an air-saturated solution revealed that the FE was contingent on excluding the presence of oxygen in the reactor.…”

Photoelectrochemical approaches for the conversion of lignin at room temperature

“…117 Recent studies have examined the utility of coupling bismuth vanadate, a 2.4 eV band gap n-type semiconductor, with N -oxyl mediators to achieve specific organic oxidation reactions such as the conversion of 5-hydroxymethyfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) 118,119 or the formation of 1-tetralone from tetralin mediated by N -hydroxysuccinimide (NHS). 120 Specifically in the case of benzyl alcohol oxidation, BiVO 4 -based photoanodes, both with NHS 120 and TEMPO, 13 have demonstrated the production of benzaldehyde with faradaic efficiencies of ≥85%. This reaction has also been the focus of dye-sensitized photoanode based systems relying on TEMPO to mediate the production of benzaldehyde both with TEMPO being dissolved in the electrolyte 121 and with surface bound chromophore–TEMPO dyad or triad type surface adsorbates.…”

“…With this motivation, Sherman, Yoo, and Leem research groups have developed photoelectrocatalytic processes for oxidative chemical transformations in primary aliphatic and/or benzyl alcohols by combining a DSPEC and aminoxyl radical mediators (ARMs). 13–19…”

“…19 This photoelectrocatalytic oxidation process leads to enhanced selectivity and effective chemical transformations of aliphatic and/or benzylic alcohols under ambient conditions. 13,14,18,19 Our recent work on developing DSPEC processes incorporating ARMs for achieving benzyl alcohol oxidation and lignin degradation will be discussed later. The photoelectrocatalytic mechanisms and the key mechanistic pathways of chemical transformation for solar or visible light driven lignin degradation will also be considered.…”