Photoelectrochemical Kolbe Coupling at Bismuth Vanadate Photoelectrodes under Visible-Light Irradiation

Semiconductor photoelectrocatalysis is a promising tool for the development of
synthetic methodologies utilising renewable energy sources. However, development is still
in its infancy and thorough understanding of photoelectrochemical reactions is lacking.
Oxidative decarboxylation, the focus of this thesis, represents an interesting model
reaction as a one electron process that has been extensively studied electrochemically and
has potential application in C-X coupling reactions.
In this work, bismuth vanadate is used as a photoelectrode material due to its
visible-light absorption and prominence in water splitting reactions, as well as some initial
photoelectrosynthesis. Photoelectrodes are prepared by electrostatic spray pyrolysis and
optimised based on photocurrent density generated for the oxidation of phenylacetic acid
solutions. Electrodes prepared by deposition of bismuth 2-ethylhexanoate and vanadyl
acetylacetonate in a mixed ethyl acetate:dimethylsulfoxide solvent system yields
reproducible photocurrent density up to 1.6 mA cm-2 at 1.0 V vs ferrocene.
After optimisation of photoelectrodes, optimisation of reaction conditions
including electrolytic cell, substrate concentrations and applied potential were
investigated. Once an optimised PEC system had been developed, substrate effects of
substituted phenylacetic acids on carboxylate oxidation were investigated, as well as the
effects of supporting electrolytes in low acid concentration media. We suggest that an
intramolecular dissociative electron transfer mechanism dominates for the oxidation of
phenylacetic acid at bismuth vanadate electrodes. Electrolyses of 1 M phenylacetic acid
solutions partially neutralised by triethylamine (0.05 equiv) with optimised photoelectrodes
yields bibenzyl at up to 88% selectivity at mild anodic potentials (<0.5 V vs Fc/Fc+). The
photoelectrochemical system developed in this thesis is a significant improvement on the
>5 V potentials required for similar electrochemical transformations at metallic electrodes.
Electrolysis of phenylacetic acid derivatives shows dimer selectivity (74% - >99%) correlates
well with time-averaged photocurrent density, with no dimer observed for aliphatic acids.
In 50 mM acid solutions containing a supporting electrolyte, a significant drop in
dimer selectivity is observed (~5%). Our studies indicate that an excess of carboxylate is
crucial in determining the selectivity by influencing the adsorption-desorption equilibrium
of the intermediate radical.