Green Supercritical CO2 Ion-Exchange Strategy for Cation Engineering in Polyheptazine Imides Towards Efficient Photoreduction CO2 to C2H4.

Photocatalytic reduction of carbon dioxide (CO2) into high-value multicarbon products, such as ethylene (C2H4), remains a significant challenge due to the difficult C-C coupling process. Potassium poly(heptazine imide) (K-PHI) is a promising photocatalyst, yet efficiently exchanging its interlayer cations to tune catalytic selectivity without causing structural degradation is difficult. Herein, an efficient and green supercritical CO2 (SC CO2) assisted ion-exchange strategy was developed to successfully prepare a series of mono-/di-/trivalent cation-doped M-PHI photocatalysts (M = H+, Na+, Sr+, Ca2+, Co2+, Fe3+). Systematic characterizations confirmed that the SC-CO2 treatment successfully achieved in-depth cation substitution without destroying the intrinsic heptazine framework, effectively regulating the interlayer structure and significantly optimizing the photoelectrochemical charge separation. Among the prepared samples, H-PHI exhibited the optimal photocatalytic CO2 reduction performance with an outstanding selectivity toward C2H4 generation. Under simulated sunlight irradiation for 3 h, the yields of CO, CH4, and C2H4 C2H4 C2H4 reached 3564.87, 807.32, and 40.00 μmol·g-1, respectively, significantly outperforming pristine K-PHI and other metal-doped samples. Crucially, isotope-tracing experiments utilizing a SC CO2-DCl treatment detected deuterated CH4 and C2H4 products, providing direct evidence that the hydrogen in the carbon products originates from the introduced protons, thereby elucidating the precise reaction pathway for C-C coupling. This study provides a green and efficient supercritical CO2 ion exchange strategy for the cation engineering of crystalline carbon nitride, and also offers new ideas and methods for designing high-activity photocatalysts for photocatalytic CO2 reduction.