An efficient photocatalytic CO2 reduction system improved by emodin as a redox mediator

Solar-driven photoreduction of CO2 to value-added fuels such as CO, CH4, and CH3OH is a promising approach for alleviating the greenhouse effect. Transition-metal-complex molecular catalysts exhibit high catalytic performance for CO2 reduction owing to their tunable structures. In this study, we developed an efficient, inexpensive, and noble-metal-free photocatalytic CO2 reduction system containing an organic photosensitizer proflavine, a cobalt molecular catalyst, and an organic electron transfer relay emodin in DMF/H2O (DMF: N,N-dime-thylformamide) solvent. In the presence of water, a turnover number (TON) of 1333 was achieved for CO2-to-CO conversion under ambient conditions and visible light irradiation, which is a 1.7-fold improvement over the system without emodin. In addition, a high selectivity of 90% was obtained for CO production. Absorption spectroscopy and cyclic voltammetry confirmed the crucial role of emodin in facilitating electron transfer between the photosensitizer and catalyst. We expect that our system will promote the development of an effective homogenous photocatalytic solar-fuel production technology.