Electrochemical Formation and Activation of Hydrogen Peroxide from Water on Fluorinated Tin Oxide for Baeyer–Villiger Oxidation Reactions

Abstract

The two-electron oxidation of water (2e-WOR) has been studied in the past as a possible method for the alternative preparation of hydrogen peroxide. Often, fluorinated tin oxide (FTO) is used as an anode and FTO itself was found also to be active for 2e-WOR. Because one use of H2O2 is as an oxygen donor for Baeyer–Villiger oxidation of ketones catalyzed by tin compounds and materials, presently we were interested in studying the use of in situ formed H2O2 for these reactions. First, the formation of H2O2 was verified in an acetonitrile/water solvent in a 2e-WOR reaction, which is more efficient than a comparable reaction in water in terms of the H2O2 concentration attained and faradaic efficiency at comparable potentials, that is, ∼3 V vs SHE. Second, initial studies on oxygenation of reactive substrates such as sulfides showed normalized reaction rates (NRRs) for two-electron oxidation reactions that were about 3 times higher than the NRR for H2O2 formation, indicating the formation of an active oxygen-donating or oxidizing species on the electrode surface prior to the formation and release of H2O2 into solution. Third, the Baeyer–Villiger oxygenation of 2-adamantanone at 2.1 V versus SHE in acetonitrile/water showed both the formation of the expected lactone product and hydroxylation at both tertiary and secondary C–H bonds. Hydroxylation is most easily explained by the presence of hydroxyl radical species as supported by the formation of a spin adduct and its identification by electron paramagnetic resonance. However, the potential used, 2.1 V versus SHE, is an underpotential for the formation of a solvated hydroxyl radical in solution, thereby leading to the conclusion that surface-bound hydroxyl species, OH*, are those that are reactive for the apparent one-electron water oxygenation reaction. Fourth, it was shown that although H2O2 can be thermally activated on FTO as a catalyst to a minor degree, electrochemical activation is by far more significant, leading to the use of FTO as an electrochemical catalyst for activation of H2O2 for the Baeyer–Villiger oxygenation and also alkene epoxidation.

Highly Selective Electrochemical Baeyer–Villiger Oxidation through Oxygen Atom Transfer from Water

“…25 The presence of other oxidative species could compete with this mechanism to result in undesired byproducts, as has been reported in recent Baeyer−Villiger attempts that involved H 2 O oxidation. 27,38 We are therefore inspired to propose that the activation of the ketone substrate takes place exclusively on the surface of the electrode.…”

“…23−26 A recent attempt to enable this reaction by electrochemistry involved the intermediates of H 2 O 2 and resulted in poor selectivity (<50%) and low Faradaic efficiencies (<10%). 27 The key competing factor of this process was understood as the production of hydroxyl radicals ( • OH) that would lead to hydroxylation. At the heart of the issue were the relatively poor controls over the oxidation processes, and the overall reaction relied on electrochemically induced chemical reactions away from the surface catalytic site.…”

Hydrogen Peroxide Synthesis via Electrocatalytic Water Oxidation on sp3 and sp2 Carbon Materials Mediated by Carbonates and Bicarbonates

“…Hydrogen peroxide (H 2 O 2 ) is an important chemical, widely used in chemical synthesis, pulp bleaching, water treatment, and the textile industry. − Compared with the traditional anthraquinone method, electrochemical synthesis of H 2 O 2 has the advantages of a simple synthesis process, nontoxic byproducts, in situ synthesis, and direct utilization, especially the electric energy can be provided by photovoltaic devices . There are two types of electrochemical synthesis of H 2 O 2 , namely, cathodic two-electron oxygen reduction reaction (2e – ORR, eq ) and anodic two-electron water oxidation reaction (2e – WOR, eq ).…”

“…9 It is well known that there are different hybrid types of carbon materials. Specifically, the hybrid type of carbon in diamond is mainly sp 3 , graphite is dominated by sp 2 carbon, and carbon fiber paper is generally a mixture of sp 2 and sp 3 carbon. 35 Therefore, it is necessary to explore the mechanism for the difference in selectivity between sp 2 and sp 3 carbon materials for electrocatalytic H 2 O 2 production.…”

“…In this work, two conventional sp 3 and sp 2 carbon materials, namely, BDD and graphite, are selected as anodes to explore the performance and reaction mechanism of electrocatalytic H 2 O 2 production in carbonate-based and bicarbonate-based electrolytes. In order to better reflect the intrinsic properties of the material, we synthesized diamond single crystals with different boron impurity concentrations and chose commercial graphite sheets as electrodes for electrochemical measurements.…”