Photo(electro)catalysis has attracted much interest in environmental remediation and solar energy conversion. However, those widely studied photo(electro)catalysts (e.g., TiO₂) work via single-electron transfer pathways which generate active radicals for driving photochemical reactions.

For environmental remediation, these active radicals degrade target pollutants as well as other coexisting organics unselectively, causing a low efficiency towards target pollutants. This unselectivity also hinders the development of photo(electro)catalysis for selective organic synthesis.

In a study published in Nature Catalysis, the researchers at Institute of Chemistry of the Chinese Academy of Sciences reported that α-Fe₂O₃ photoanodes act as an efficient oxygen atom transfer (OAT) photocatalyst under mild conditions. They investigated a wide variety of oxygenation reactions, including thioether sulfoxidation, C=C epoxidation, Ph₃P oxygenation and monooxygenation of inorganic ions, which exhibit high selectivity and Faradaic efficiency.

The researchers have developed α-Fe₂O₃ photoanodes that serve as a versatile and efficient OAT catalyst in photoelectrochemical (PEC) cells with water molecules as the oxygen source. This OAT reaction worked via a non-radical pathway that circumvents the unselectivity caused by active radicals in traditional photo(electro)catalysis, providing a new strategy for the selective degradation of environmental pollutants and the production of value-added chemicals.

In this study, the researchers proposed that the adjacent surface-trapped holes (i.e., high-valent iron oxo, Fe^IV=O) on α-Fe₂O₃ surfaces contributed to the OAT reactions.

Rate law analysis of those surface-trapped holes confirmed a second-order kinetics for OAT reactions on α-Fe₂O₃. In contrast, a first-order kinetics appeared for OAT reaction on TiO₂, which underwent a radical pathway and led to a low selectivity and Faradaic efficiency towards those OAT reactions.

Furthermore, the spin-polarized density functional theory + U study showed that the distinct surface electronic structures between α-Fe₂O₃ and TiO₂ contributed to the distinct reaction selectivity.

Photogenerated holes on α-Fe₂O₃ were located at those hybridized Fe 3d and O 2p orbitals, generating high-valent iron oxo (Fe^IV=O) that exhibited a high tendency for OAT reactions. While for TiO₂, the O 2p orbitals contributed to the photogenerated holes and the produced Ti-O· initiated the unselective radical reactions.

This work demonstrates that α-Fe₂O₃ is a versatile and efficient oxygen atom transfer catalyst by using H₂O as the oxygen source. The nonradical reaction mechanism provides a new strategy for improving reaction selectivity in photo(electro)catalysis.