Precursor-tuned defect engineering in carbon nitride for sustainable and selective photooxidation of benzyl alcohol

Abstract

This study reports nitrogen-deficient CN (Nv-CN) derivatives, synthesized from various nitrogen-rich precursors, as highly efficient photocatalysts for the selective aerobic oxidation of benzyl alcohol to benzaldehyde under visible-light irradiation. Comprehensive structural and photophysical analyses confirmed that defect-engineering significantly alters both the framework and electronic structure of CN. Among the derivatives, Nv-CN(C) prepared from cyanamide exhibited the highest density of N-vacancies, mid-gap states, and polar surface functionalities, which collectively enhanced visible-light harvesting, charge separation, and the adsorption and electron transfer to molecular oxygen, leading to efficient generation and stabilization of reactive oxygen species. Compared to its pristine analogue, Nv-CN(C) achieved near-complete benzyl alcohol conversion (96%) with > 99% selectivity toward benzaldehyde. Mechanistic investigations employing ROS scavengers identified a cooperative pathway involving superoxide radicals (center dot O-2(-) ), singlet oxygen (O-1(2)), and photogenerated holes as the dominant oxidative species, while hydroxyl radicals (center dot OH) played only a minor role. According to the C-13 CP-MAS NMR analysis, the defective regions of CN(C) are mainly located at the outer nitrogen atoms of the heptazine rings, which play an important role as active sites. Substrate scope studies confirmed the broad applicability of Nv-CN(C) to both electron-rich and electron-deficient benzyl alcohol derivatives, maintaining high selectivity across diverse substrates.