Engineered Half-Unit-Cell MoS₂/ZnIn₂S₄ Monolayer Photocatalysts and Adsorbed Hydroxyl Radicals-Assisted Activation of C_α–H Bond for Efficient C_β–O Bond Cleavage in Lignin to Aromatic Monomers

Photocatalysis has high potential in the cleavage of C_β-O bond in lignin into high-value aromatic monomers; however, the inefficient C_α-H bond activation in lignin and a low hydrogen transfer efficiency on the photocatalyst's surfaces have limited its application in photocatalytic lignin conversion. This study indicates that the cleavage of the C_β-O bond can be improved by the generation of the C_α radical intermediate through C_α-H bond activation, and the formation of desirable aromatic products can be significantly improved by the enhanced hydrogen transfer efficiency from photocatalyst surfaces to aromatic monomeric radicals. We elaborately designed the half-unit-cell MoS₂/ZnIn₂S₄ monolayer with a thickness of ∼1.7 nm to promote the hydrogen transfer efficiency on the photocatalyst surfaces. The ultrathin structure can shorten the diffusion distance of charge carriers from the interior to the surfaces and tight interface between MoS₂ and ZnIn₂S₄ to facilitate the migration of photogenerated electrons from ZnIn₂S₄ to MoS₂, therefore improving the selectivity of desirable products. The adsorbed hydroxyl radical (*OH) on the surfaces of MoS₂/ZnIn₂S₄ from water oxidation can significantly reduce the bond dissociation energy (BDE) of C_α-H bond in PP-ol from 2.38 to 1.87 eV, therefore improving the C_α-H bond activation. The isotopic experiments of H₂O/D₂O indicate that the efficiency of *OH generation is an important step in C_α-H bond activation for PP-ol conversion to aromatic monomers. In summary, PP-ol can completely convert to 86.6% phenol and 82.3% acetophenone after 1 h of visible light irradiation by using 3% MoS₂/ZnIn₂S₄ and the assistance of *OH, which shows the highest conversion rate compared to previous works.