Enhanced photocatalytic CO2-to-CH4 conversion through K+-ion induced crystallization of carbon nitride

Abstract

The photocatalytic reduction of CO₂ to methane using carbon nitride holds significant promise for sustainable fuel production; however, its performance remains limited due to the poor crystallization of pristine carbon nitride (PCN). In this study, KCl and KBr were used as templates to prepare crystalline carbon nitride (CCN). The results showed that K⁺ ions promoted the formation of CCN by intercalating into the structure, reducing the exposure of terminal amino groups, and significantly improving the photocatalytic CO₂-to-CH₄ conversion efficiency, which increased from less than 5 % in K-free melamine-based PCN (CN-M) to 41 % in CCN-MBr (sample prepared with KBr) and 53 % in CCN-MCl (material synthesized with KCl), respectively. Similar improvements were obtained in dicyandiamide-derived PCN (CN-D) and CCN samples (CCN-DBr and CCN-DCl, prepared with KBr and KCl, respectively). The incorporation of K⁺ into the tri-s-triazine structure played a pivotal role in charge separation, while the reduced exposure of amino groups lowered the band gap of the material. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to identify intermediates during photocatalytic CO₂ reduction. Density functional theory (DFT) calculations showed that the Gibbs free energy for *CHO formation decreased from 0.80 eV (PCN) to 0.53 eV (CCN), favoring methane production over CCN. This study lays the foundation for further research on selective CO₂ reduction to produce hydrocarbon fuels.