Effects of Electron Density Variation of Active Sites in CO<sub>2</sub> Activation and Photoreduction: A Review

Abstract: Photocatalytic reduction of CO2 into value-added chemicals is a feasible approach to harvest solar light energy and storing energy in the form of chemical fuels as well as to mitigate the effects of global climate change and help achieve an artificial carbon cycle. However, the efficiency of CO2 photoreduction is low for commercial purposes. This is mainly due to the difficult adsorption and activation process of CO2 molecules, the unsatisfactory selectivity of target products, and the uncontrolled-subsequent reaction process of the generated carbon products. CO2 photoreduction requires substantial electrons for participation. Hence, these issues are due to the electron density modulation of the active sites of catalysts. Unfortunately, the CO2 photoreduction process involves multi-fundamental steps, which leads to different requirements in electron density modulation. The performance might not be effectively improved by directly enhancing or weakening the total electron density of active sites. In this paper, we summarize recent advances in the influence of electron density variation of the active sites in strengthening the adsorption and activation of CO2 molecules, enhancing the selectivity of target carbon products and modulating the subsequent reaction process of the generated carbon products. This review begins with the effect of different types of active sites in strengthening the adsorption and activation of the CO2 molecules and the related methods for modulating the electron density of active sites. Active sites with high electron densities can significantly enhance the adsorption and activation of CO2. Introducing metal and fabricating the defects on catalyst surfaces are effective strategies for fabricating the electron-rich active sites. After that, we discuss the influence of electron density variation in enhancing the selectivity of target carbon products in detail. In this part, the related effects in the multielectron donation from the catalyst surface, the reactive intermediates, and the competition hydrogen evolution reaction are summarized. Enhancing the electron density of active sites strengthens the former two processes. For multielectron donation, introducing cocatalysts or fabricating heterostructures are the most effective methods for enhancing the electron density of active sites. The adsorption and conversion process of intermediates are mainly affected by the accumulation sites of electrons. The active sites with low coordination are more favorable to achieving the generation of multi-electronic carbon products. In contrast, the hydrogen evolution reaction is significantly inhibited by reducing the electron density of active sites. Moreover, elemental doping is considered one of the most effective strategies. Finally, we describe the method for weakening the electron density of active sites to promote product desorption and inhibit the photooxidation of reactive products.