Abstract
Photocatalytic reduction of carbon dioxide (CO(2)) into high-value multicarbon products, such as ethylene (C(2)H(4)), 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 CO(2) (SC CO(2)) assisted ion-exchange strategy was developed to successfully prepare a series of mono-/di-/trivalent cation-doped M-PHI photocatalysts (M = H(+), Na(+), Sr(+), Ca(2+), Co(2+), Fe(3+)). Systematic characterizations confirmed that the SC-CO(2) 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 CO(2) reduction performance with an outstanding selectivity toward C(2)H(4) generation. Under simulated sunlight irradiation for 3 h, the yields of CO, CH(4), and C(2)H(4) C(2)H(4) C(2)H(4) 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 CO(2)-DCl treatment detected deuterated CH(4) and C(2)H(4) 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 CO(2) 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 CO(2) reduction.