Synergistic enhancement of photocatalytic NADH regeneration over mesoporous carbon nitride by potassium and sulfur doping

High-efficiency regeneration of reduced nicotinamide adenine dinucleotide (NADH) is essential for practical applications of cofactor-dependent redox enzymatic processes. Herein, we report a synergistic K/S co-doping strategy to substantially enhance photocatalytic NADH regeneration performance of mesoporous carbon nitride (MPCN). Systematic investigation of single-doped catalysts revealed that K⁺ intercalation improved interlayer charge transport and induced cyano-terminated defects, greatly upgrading the final NADH yield; while sulfur substitution narrowed the bandgap from 2.54 to 2.02 eV and extended visible-light absorption from 488 to 613 nm, significantly increasing the photocatalytic activity. After optimizing K and S sources, dosages, and dopant ratio, MPCN-KS₂ achieved a remarkable photocatalytic activity of (27. 89 ± 0.27) mM/(g·min) and NADH yield of (87.31 ± 0.62) % within 20 min, outperforming single-doped MPCN and pristine MPCN. Comprehensive characterizations demonstrated that KCl and thioacetamide synergistically modulated electronic structure and crystallinity: K⁺ intercalation facilitated interlayer charge transport while S-derived impurity states enhanced light harvesting. The conduction-band negative shift induced by K doping effectively compensated for the thermodynamic driving force loss caused by S doping, achieving balanced enhancement of charge-carrier generation, separation, and utilization. This work provides insightful design principles for multi-element cooperative modification of carbon nitride photocatalysts toward efficient artificial photosynthesis.