Carbon-rich potassium poly(heptazine imide) for stoichiometric photocatalytic water reduction to hydrogen and glycerol oxidation to high-value products via a direct photocarrier redox pathway

Abstract

Carbon-rich potassium poly(heptazine imide) (CKPHI) was synthesized via a direct ionothermal method using a supramolecular complex comprising 2,4,6-triaminopyrimidine (TAP), melamine (MA), and cyanuric acid. Increasing the TAP-to-MA ratio in the precursor promotes substitution of nitrogen atoms in the π-conjugated aromatic framework with carbon atoms and partial conversion of terminal amine groups into cyano and CH moieties, thereby increasing the carbon-to‑nitrogen (C/N) ratio in the resulting CKPHIs. This structural tuning significantly influences light absorption, charge separation, and surface charge transfer behavior, leading to optimized photocatalytic performance of potassium poly(heptazine imide) for simultaneous water reduction and glycerol oxidation. Under AM 1.5G solar illumination (100 mW cm⁻²) in 10 vol% aqueous glycerol solution, Pt-loaded CKPHI achieves high yields of hydrogen (1648 μmol g⁻¹ h⁻¹), glyceraldehyde (1260 μmol g⁻¹ h⁻¹), dihydroxyacetone (390 μmol g⁻¹ h⁻¹), and cyclic diglycerol (39 μmol g⁻¹ h⁻¹), with glyceraldehyde selectivity reaching 75 %. Moreover, Pt/CKPHI enables stoichiometric hydrogen evolution and selective glycerol oxidation under anaerobic conditions via a direct photocarrier redox pathway, effectively suppressing undesirable CC bond cleavage and overoxidation to carbon dioxide by reactive oxygen species. This work highlights the critical role of structural engineering in ionic carbon nitrides for improving charge dynamics and achieving efficient charge balance in photocatalytic hydrogen generation coupled with biomass valorization.