Lewis Acid Sites in (110) Facet-Exposed BiOBr Promote C–H Activation and Selective Photocatalytic Toluene Oxidation

Abstract

Solar photocatalytic technology has exhibited significant potential for upgrading the value-added chemicals industry through efficient C–H bond activation, such as selective photocatalytic toluene oxidation. However, improvement in the C–H bond activation of toluene is still a challenge. Herein, (110) facet-exposed BiOBr (EC-BiOBr) synthesized via a facile crystal facet control strategy exhibited an increasing exposure of Lewis acid sites, as confirmed by in situ Fourier-transform infrared spectroscopy (FT-IR) using ammonia as a probe molecule. In situ FT-IR results substantiated the improved absorption capacity of EC-BiOBr for toluene. Density functional theory (DFT) calculations indicated that the Lewis acid–base pairs formed by Bi sites and O sites can adsorb toluene directionally, precisely matching the orbit spaces of the conduction band (Bi 6p state) and valence band (O 2p and Br 4p states). Benefiting from the oriented adsorption of toluene, the electron in the C–H bond could transfer to a photogenerated hole precisely, thus achieving C–H bond activation. Compared to (001) facet-exposed BiOBr (H–BiOBr), an 11-fold improvement in the toluene conversion rate (from 233 to 2460 μmol g–1 h–1) was observed in the EC-BiOBr group, and the benzaldehyde formation rate increased from 233 to 1623 μmol g–1 h–1. Active species identification and DFT calculations revealed that the superoxide radicals were involved as the primary reactive species in the subsequent oxidation of benzyl radicals generated from the C–H bond activation of toluene. This work highlights the importance of the surface acid sites regulated by the crystal facet control strategy, which is conducive to the rational design of photocatalysts with high performance in C–H bond activation.