Nitrate Radical Facilitates Indirect Benzyl Alcohol Oxidation on Bismuth(III) Vanadate Photoelectrodes

Abstract

Bismuth(III) vanadate (BiVO 4) films show activity for direct benzyl alcohol (PhCH 2 OH) oxidation to benzaldehyde (PhCHO) in acetonitrile solvent. Introducing tetrabutylammonium nitrate (Bu 4 NNO 3) drastically reduces the overpotential required to generate the PhCHO product while maintaining a high faradaic efficiency (FE) > 90 %. BiVO 4 corrosion accompanies PhCH 2 OH oxidation. However, the presence of nitrate ions (NO 3 À) results in significantly less bismuth-and vanadium-ion leaching (determined by ICP-MS analysis), as well as reduced surface roughening (determined by SEM imaging). In this reaction, it is proposed that rate-determining NO 3 À oxidation generates a highly reactive nitrate radical (NO 3 •) that reacts with PhCH 2 OH by hydrogen-atom abstraction (HAT). NO 3 À is stoichiometrically consumed by the irreversible formation of electrochemically inert HNO 3 , characterized by an EC i mechanism, rather than a catalytic EC' mechanism. In the presence of PhCH 2 OH, NO 3 À oxidation on BiVO 4 becomes more facile; every order of magnitude increase in PhCH 2 OH concentration shifts the NO 3 À / NO 3 • equilibrium potential negatively by 200 mV. The shift results from the introduction of a consumption pathway for the nitrate radical intermediate via a coupled chemical step with benzyl alcohol. This report is the first example of photoelectrochemical NO 3 • generation to initiate indirect PhCH 2 OH oxidation.

Base-Assisted Nitrate Mediation as the Mechanism of Electrochemical Benzyl Alcohol Oxidation

“…Since NO 3 – oxidation remains irreversible in all solution conditions, another mechanism must be at play. The electrochemical system detailed here closely matches our recent findings of nitrate-assisted (but not mediated) reactivity on BiVO 4 photoelectrodes . In that report, the irreversibility of the NO 3 – /NO 3 • couple is thought to result from stoichiometric nitric acid formation; HNO 3 is a weak electrolyte in acetonitrile (p K a, MeCN ∼ 9) .…”

“…The irreversible NO 3 – oxidation involved in indirect PhCH 2 OH oxidation mirrors the photoelectrochemical example on BiVO 4 films but differs from the mediation behavior observed photochemically on CdS nanowires . The presence of lithium cations was shown not to influence the voltammetry (Figure a, red), confirming that the improvements afforded by LiNO 3 are due to nitrate anion.…”

“…Recently, indirect alcohol oxidation through nitrate radical (NO 3 • ) formed from nitrate anion (NO 3 – ) has been revisited with photocatalytic and photoelectrochemical demonstrations on CdS nanowires and BiVO 4 photoanodes . On CdS nanowires, NO 3 – mediation is observed; on BiVO 4 photoelectrodes, NO 3 – reacts stoichiometrically.…”

Research Progress in Organic Synthesis by Means of Photoelectrocatalysis

“…The high overpotential can hinder the electrocatalysis, so optimizing the reactor design and changing the coordination sites of organic compounds can reduce overpotential. Nitro radicals can reduce the overpotential required for BiVO 4 owing to the direct conversion of benzyl alcohol into acetonitrile while maintaining a high Faradaic efficiency (FE) (>90 %) [49] …”

A Highly Efficient Bismuth Nitrate/Keto-ABNO Catalyst System for Aerobic Oxidation of Alcohols to Carbonyl Compounds under Mild Conditions

“…Bismuth, as a non-toxic, cheap, and readily available metal, has been applied as a suitable ecofriendly reagent in cosmetic and medicinal chemistry. Moreover, due to the non-toxicity and inexpensive characteristics of bismuth and its compounds, they are also used as efficient catalysts for many organic reactions [23][24][25][26][27][28]. Recently, our group employed bismuth(III) salts as a highly efficient co-catalyst for the direct oxidative esterification of alcohols [29].…”