活性位点电子密度变化对光催化CO_(2)活化和选择转化的影响

Effects of Electron Density Variation of Active Sites in CO_(2) Activation and Photoreduction:A Review

光催化二氧化碳(CO_(2))还原制液体燃料和高值化学品技术不仅能充分利用可再生能源太阳光,实现化学储能;更重要的是,此技术以温室气体CO_(2)为原料,因此可以减缓全球温室效应,构造人工碳循环。然而,光催化CO_(2)还原制液体燃料和高值化学品反应过程中面临诸多挑战:(1)CO_(2)分子吸附和活化过程困难;(2)(高附加值)碳产物选择性低;(3)产物生成后易发生其他副反应导致催化剂失活或产物选择性下降。受到以上三个挑战的制约,目前的反应效率较低,难以满足工业化应用。由于光催化CO_(2)向高值化学品的转化过程为质子耦合光生电子参与的还原反应,因此活性位点的电子密度会显著影响以上挑战的解决。然而,光催化CO_(2)还原过程涉及众多基元步骤,每个基元步骤对于活性位点的电子密度要求并不清晰,这导致无法有针对性设计高效的催化剂来促进光催化CO_(2)分子的有效活化及高选择性转化。本文综述了近期活性位点的电子密度变化对于CO_(2)分子吸附和活化过程、碳产物选择性调控和产物脱附及过氧化的影响规律,并总结了调控活性位点上电子密度的方法,旨在对未来设计高效光催化剂提供参考和理论依据。

Photocatalytic reduction of CO_(2) 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 CO_(2) photoreduction is low for commercial purposes.This is mainly due to the difficult adsorption and activation process of CO_(2) molecules,the unsatisfactory selectivity of target products,and the uncontrolled-subsequent reaction process of the generated carbon products.CO_(2) photoreduction requires substantial electrons for participation.Hence,these issues are due to the electron density modulation of the active sites of catalysts.Unfortunately,the CO_(2) 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 CO_(2) 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 CO_(2) 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 CO_(2).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.