Self-nitrogen-doped hierarchical porous carbon spheres as metal-free catalyst for efficient photocatalytic CO2 reduction

It is rather important to develop an efficient, stable and economical photocatalyst for solar-driven CO 2 reduction. Herein, N-doped hierarchical porous carbon spheres (NPCS-F6) with metal-like activity is synthesized using acrylamide and F127 as nitrogen source and soft template, and the effective synergy between nitrogen-containing species and hierarchical pore structure makes the samples exhibit excellent CO 2 reduction properties. XPS characterization illustrates that the introduced pyridine nitrogen and pyrrole nitrogen groups contribute to the increase of CO 2 reduction sites on the surface of the material. N 2 adsorption analysis verifies the existence of hierarchical pore structures in which micropores facilitate the exposure of more adsorption active sites, while mesopores and macropores can be used as transfer channels to connect the internal micropores and reduce the molecular diffusion resistance to improve CO 2 adsorption performance. In addition, the pore structure of the samples prepared with P123 and CTAB as templates is mainly microporous, whereas carbon spheres with hierarchical pores coexisting are obtained with F127 as template. The evaluation of photocatalytic CO 2 reduction activity shows that the CO yield of NPCS-F6 could reach 17.56 μmol·g −1 for 8 h, which is 1.17 times, 1.08 times and 1.07 times of the original NCS-8 (15.05 μmol·g −1 ), NPCS-P6 (P123 as soft template, 16.25 μmol·g −1 ) and NPCS-C1 (CATB as soft template, 16.39 μmol·g −1 ), respectively. Comprehensive characterization results are analyzed to propose a possible mechanism for photocatalytic CO 2 reduction, in which photogenerated electron-hole pairs are generated inside the material under light conditions, and the photogenerated electrons are transferred to the N-6/N-5 active sites through the electron transfer channel N-Q to react with CO 2 molecules adsorbed on the surface to be reduced to CO. Finally, our results provide a favorable and feasible strategy to synthesize metal-free photocatalysts for high-efficiency CO 2 adsorption and targeted reduction of CO 2 to CO.