Highly stable 2D/2D ZnCo2O4 NSL/pg-C3N4 composite with defective sites for enhancing selective photocatalytic CO2 reduction in a batch/continuous flow reactor

Well-designed two-dimensional (2D) Zinc cobalt oxide (ZnCo2O4) nano-slabs (NSL) were integrated with porous graphitic carbon nitride (pg-C3N4) using a simple chemical and self-assembly method to form a 2D ZnCo2O4/pg-C3N4 heterojunction. The photocatalytic performance of the composite was tested for the reduction of CO2 to useful chemicals and fuels in a batch and continuous photoreactor systems. Porous and unique nanotextures synthesized using a template-free method were beneficial in obtaining higher light-harvesting ability and rapid charge separation efficiency. Using various sacrificial reagents, the activity of the 2D ZnCo2O4/pg-C3N4 for the generation of CO during photocatalytic CO2 reduction was investigated, and their performance was optimized. The optimized 10 wt% ZnCo2O4/pg-C3N4 composite performed 4.22 and 15.67 times better for CO2 reduction with H2O to CO in a flow reactor than ZnCo2O4 and pg-C3N4, respectively. This enhanced photocatalytic efficiency was due to the S-scheme heterojunction with the enhanced charge separation within the structured composite. Using a flow reactor, the highest CO yield of 3.76 µmol g−1 h−1 was obtained, which was increased to 616 µmol g−1 h−1 in a batch-type process during CO2 reduction with H2O. Using a methanol/water mixture during CO2 reduction, the greatest CO yield rate of 13675 µmol g−1 h−1 was attained, which was 22.2 times higher than when water was used in a batch photoreactor system. The accumulation of reactants and products inside the reactor and increased contact between the intermediate products and the effective charge carrier separation were the causes of this enhanced photoactivity. The promising QY was achieved in a batch process; however, stable photoactivity was observed in a continuous-flow reactor. This work provides the importance of a structured composite photocatalyst in a batch and continuous flow photoreactor to control the product yield, selectivity and stability for photocatalytic CO2 reduction application.