Selective Photocatalytic Reduction of CO2-to-CO in Water using a Polymeric Carbon Nitride Quantum Dot/Fe-Porphyrin Hybrid Assembly

Visible light-driven conversion of CO2 into more value-added products is a promising technology not only for diminution of CO2 emission but also for solar energy storage in the form of chemical energy. However, photocatalytic materials that can efficiently and selectively reduce CO2-to-CO in a fully aqueous solution typically involve precious metals that limit their suitability for large scale applications. Herein, a novel photocatalytic assembly is reported, consisting of polymeric carbon nitride quantum dots (CNQDs) as the visible light absorber and a Fe-porphyrin complex (Fe-p-TMA) as the catalyst for CO2-to-CO conversion. Both components were carefully selected to allow for excellent solubility in water as well as improved electronic communication through complementary electrostatic and pi-pi interactions. This CNQD center dot[Fe-p-TMA] hybrid assembly, at the optimized molar ratio, can produce CO with a turnover number (TON) exceeding 10(5) and selectivity similar to 96% after 10 hours of visible light irradiation (400-700 nm). It is postulated that the enhanced CO2-to-CO transformation performance is due to the convenience of a more direct charge transfer (CT) pathway between the CNQDs and [Fe-p-TMA] motif.