Boosting the Faradaic Efficiency of Br⁻‐Mediated Photoelectrochemical Epoxidation by Local Acidity on α‐Fe₂O₃

Abstract

The redox mediated photoelectrochemical (PEC) or electrochemical (EC) alkene oxidation process is a promising method to produce high value‐added epoxides. However, due to the competitive reaction of water oxidation and overoxidation of the mediator, the utilization of the electricity is far below the ideal value, where the loss of epoxidation's faradaic efficiency (FE) is ≈50%. In this study, a Br−/HOBr‐mediated method is developed to achieve a near‐quantitative selectivity and ≈100% FE of styrene oxide on α‐Fe2O3, in which low concentration of Br− as mediator and locally generated acidic micro‐environment work together to produce the higher active HOBr species. A variety of styrene derivatives are investigated with satisfied epoxidation performance. Based on the analysis of local pH‐dependent epoxidation FE and products distribution, the study further verified that HOBr serves as the true active mediator to generate the bromohydrin intermediate. It is believed that this strategy can greatly overcome the limitation of epoxidation FE to enable future industrial applications.

Highly Efficient Photoelectrochemical Alkene Epoxidation on a Dye-Sensitized Photoanode

“…It is worth noting that the results shown here significantly surpass those reported in previous studies based on dye-sensitized photoanodes or narrow band gap metal oxide photoanodes for alkene epoxidation (Figure ). ,,, …”

“…Photocurrents (top), selectivity, conversion, and Faradaic efficiency (FE) (maximum values reported in these studies) (bottom) for photoelectrochemical alkene epoxidation with narrow band gap semiconductor photoanodes (BiVO 4 , Fe 2 O 3 , ) and dye-sensitized photoanodes …”

“…These results suggest that the loss in FE is primarily due to the overoxidation of Br − rather than water oxidation at the photoanode or bromine reduction at the cathode. 23,43,44 By contrast, negligible photocurrent was detected in a solution containing 1 and NH 4 PF 6 (Figure S9). Consistent results were obtained for chopped light irradiation over an extended period with a constant bias of 0.6 V vs NHE (Figure 3c), indicating that the bromide mediator is a necessity for PEC epoxidation.…”

“…It is worth noting that the results shown here significantly surpass those reported in previous studies based on dye-sensitized photoanodes or narrow band gap metal oxide photoanodes for alkene epoxidation (Figure 4). 22,23,29,43 After a 1 h PEC reaction, liquid chromatography analysis confirmed nearly complete conversion of 1 to styrene oxide (2) with a Faradaic efficiency (FE) of 65% and a selectivity of 99% (Figure 3e). The headspace gas analysis revealed no production of oxygen, and the FE of hydrogen production at the cathode was close to 100% (Figure S8).…”