Electron-rich pyrimidine rings enabling crystalline carbon nitride for high-efficiency photocatalytic hydrogen evolution coupled with benzyl alcohol selective oxidation

Abstract

Photocatalytic water splitting over polymeric carbon nitride (PCN) has been seriously limited by the poor charge carrier transfer ability and sluggish four-electron water oxidation kinetics. Herein, crystalline carbon nitride (CCN-Pr) with electron-rich pyrimidine rings introduced in the molecular structure is synthesized by a two-step self-assembly and molten-salt annealing strategy for photocatalytic hydrogen evolution coupled with benzyl alcohol selective oxidation to benzaldehyde, instead of the kinetically sluggish water oxidation reaction. Owing to the synergistically tuned band and electronic structures, the obtained CCN-Pr exhibits excellent photocatalytic performances, with the highest hydrogen and benzaldehyde production rates reaching 149.39 μmol h⁻¹ and 154.62 μmol h⁻¹, respectively. The apparent quantum yield for hydrogen evolution is determined to be 20.27% at 420 nm, encouragingly standing at the highest level reported for simultaneous hydrogen and benzaldehyde production over PCN-based photocatalysts. It is well evidenced that the introduced electron-rich pyrimidine rings could finely tune the band structures for extended optical absorption and matched redox potentials for water reduction and benzyl alcohol oxidation. Theoretical calculation and experimental results reveal that the electronic structure engineered by pyrimidine rings alters the charge density distribution for promoted charge transport, and creates abundant reactive sites to accelerate the surface oxidation reaction kinetics. This work provides a reliable strategy to design efficient photocatalysts with band and electronic structures engineered by the tunable molecular structures, and also paves an alternative way to promote the economic benefits and the technology upgrading of solar energy conversion and utilization.