Photoelectrocatalytic C–H halogenation over an oxygen vacancy-rich TiO2 photoanode

Abstract

Photoelectrochemical cells are emerging as powerful tools for organic synthesis. However, they have rarely been explored for C–H halogenation to produce organic halides of industrial and medicinal importance. Here we report a photoelectrocatalytic strategy for C–H halogenation using an oxygen-vacancy-rich TiO2 photoanode with NaX (X=Cl−, Br−, I−). Under illumination, the photogenerated holes in TiO2 oxidize the halide ions to corresponding radicals or X2, which then react with the substrates to yield organic halides. The PEC C–H halogenation strategy exhibits broad substrate scope, including arenes, heteroarenes, nonpolar cycloalkanes, and aliphatic hydrocarbons. Experimental and theoretical data reveal that the oxygen vacancy on TiO2 facilitates the photo-induced carriers separation efficiency and more importantly, promotes halide ions adsorption with intermediary strength and hence increases the activity. Moreover, we designed a self-powered PEC system and directly utilised seawater as both the electrolyte and chloride ions source, attaining chlorocyclohexane productivity of 412 µmol h−1 coupled with H2 productivity of 9.2 mL h−1, thus achieving a promising way to use solar for upcycling halogen in ocean resource into valuable organic halides.

Photocatalytic Chlorination of Methane Using Alkali Chloride Solution

“…According to previous reports, the charge transfer resistance in an electrode/electrolyte can be determined by the arc radius in electrochemical impedance spectra (EIS). 63,64 A smaller radius implies a smaller resistance. Ab EIS Nyquist plot (Figure 5b) displays that the arc radius of Ag 2 O/NTO is obviously smaller than that of NTO.…”

Chloride Oxidation by One- or Two-Photon Excitation of N-Phenylphenothiazine

“…Chlorine atoms have long been known as a powerful tool for C–H activation yet historically have seen little use in practical applications due to the harsh reaction conditions necessary to generate Cl • . Over the last decade, a number of groundbreaking studies have demonstrated facile chloride oxidation using photoredox catalysts, which have enabled new approaches to functionalize inert C–H bonds. − Additionally, environmentally motivated applications rely on chloride oxidation as an important fundamental process for HCl splitting to store solar energy − or for electric gradient generation for sea water desalination. − However, the vast majority of these examples rely on noble metal photocatalysts to generate chlorine atoms because the one-electron reduction potential, (Cl • /– ), requires an incredibly potent photooxidant. − Indeed, chloride ions are so redox inert that they are frequently used as counterions for common photocatalysts such as ruthenium polypyridyl compounds. , Synthesizing ruthenium or iridium coordination compounds that are strong photooxidants requires skillful ligand engineering ,− and such catalysts often suffer from stability issues. − Moreover, photocatalysts based on noble metals are impractical at scale given the low natural abundance of these rare elements. Herein, we report a rare example of light-induced chloride oxidation catalyzed by an inexpensive, commercially available organic photocatalyst through unconventional one- and two-photon pathways.…”

Interfacial Bi–S bonds modulate band alignment for efficient solar water oxidation

“…4d were measured to probe the intrinsic photoelectric properties of CIS, CIS/BWO, and CIS-BWO films. 15,42 The CIS/BWO photoanode exhibits an obvious SPV signal under light irradiation, indicating that photogenerated electrons migrate to the FTO substrate and the photogenerated carrier separation is effective. Meanwhile, the CIS-BWO photoanode exhibits a relatively weak SPV signal compared with the CIS/BWO photoanode due to the type II band alignment, which allows electrons to migrate to the photoanode surface, while the photogenerated holes are rarely detected on the surface.…”