Selective Conversion of CO2 by Single-Site Catalysts

Industrial revolution has led to increased combustion of fossil fuels. Consequently, large amounts of CO2 are emitted to the atmosphere, throwing the carbon cycle out of balance. Currently, the most effective method to reduce the CO2 concentration is direct CO2 capture from the atmosphere and pumping of the captured CO2 deep underground or into the mid-ocean. The transformation of CO2 into high-value chemicals is an attractive yet challenging task. In recent years, there has been much interest in the development of CO2 utilization technologies based on electrochemical CO2 reduction, photochemical CO2 reduction, and thermal CO2 reduction, and CO2 valorization has emerged as a hot research topic. In electrochemical CO2 reduction, the cathodic reaction is the reduction of CO2 to value-added chemicals. The anodic reaction should be the oxygen evolution reaction, and water is the only renewable and scalable source of electrons and protons in this reaction. There is a plethora of research on the use of various metals to catalyze this reaction. Among these, Cu-based materials have been demonstrated to show unique catalytic activity and stability for the electrochemical conversion of CO2 to valuable fuels and chemicals. Moreover, the solar-driven conversion of CO2 into value-added chemical fuels has attracted great attention, and much effort is being devoted to develop novel catalysts for the photoreduction of CO2, especially by mimicking the natural photosynthetic process. The key step in the photocatalytic process is the efficient generation of electron-hole pairs and separation of these charge carriers. The efficient separation of photoinduced charge carriers plays a crucial role in the final catalytic activity. Compared with CO2 reduction via electrocatalysis and photocatalysis, thermal reduction is more attractive because of its potential large-scale application in the industry. Heterogeneous nanomaterials show excellent activity in the electrocatalytic, photocatalytic, and thermal catalytic conversion of CO2. However, nanostructured materials have drawbacks on the investigation of the intrinsic activity of the active sites. In recent years, single-site catalysts have become popular because they allow for maximum utilization of the metal centers, show specific catalytic performance, and facilitate easy elucidation of the catalytic mechanism at the molecular level. Accordingly, numerous single-site catalysts were developed for CO2 reduction to produce value-added chemicals such as CO, CH4, CH3OH, formate, and C2+ products. Value-added chemicals have also been synthesized with the aid of amines and epoxides. This review summarizes recent state-of-the-art single-site catalysts and their application as heterogeneous catalysts for the electroreduction, photoreduction, and thermal reduction of CO2. In the discussion, we will highlight the structure-activity relationships for the catalytic conversion of CO2 with single-site catalysts.