Читать книгу Solar-to-Chemical Conversion - Группа авторов - Страница 13

In the Earth, all chemicals, in particular energy system, including fuels and their combustion products, are enclosed in a substance cycle. It is well known that fossil fuels including coal, petroleum, and natural gas mainly stem from the evolution of ancient animals and plants under the stratum for tens of thousands of years. When these fossil fuels are combusted, massive chemical energies can be converted into thermal energies, which can then be used to make liquid water vaporize for driving the electric generators to produce various available energy forms, such as electric power. Furthermore, during the combustion of fossil fuels, the intrinsic chemical reactions are oxidation of hydrocarbons, accompanying with outputs of thermal energy. Finally, the hydrocarbons and their derivatives are converted into carbon dioxide and water along with SO_x, NO_x, and cokes under imperfect combustion. It is known that when massive CO₂ and water are released into the air, the natural plants and alga can utilize CO₂ to produce hydrocarbons as own constituents and release dioxygen in the presence of sunlight and water, being defined as photosynthesis. Subsequently, these plants and alga enter food chains and finally become fossil fuels. Until now, carbon as energy carrier realizes the global cyclic process.

However, with the rapid development of industrial activities, energy consumption demands sharply increase, which greatly destroys the balance of global substance cycle [1]. Subsequently, the releasing amount of CO₂ significantly raises accompanying with other pollutants, resulting in a series of global pollutions, such as global warming and ozone depletion [2]. To overcome the coming energy crisis and environmental issues, chemists attempt to make the substance cycle rebalance by means of various promising solar‐driven techniques, such as photocatalysis, CO₂ storage and utilization, water splitting, and N₂ fixation [3]. Meanwhile, these desirable techniques often have sustainable, clean, and benign metrics, which are beneficial to supporting the future sustainability of human being.

Figure 2.1 illustrates a blueprint of sustainable fuel production and consumption, where conventional power plant still consumes fossil fuel and produces CO₂ and water [4]. The released CO₂ can be captured by using absorption or adsorption techniques and react with H₂O to produce carbon monoxide (CO) and H₂ via thermochemical reactions that are triggered by indirect solar heat or solar‐powered electric energy. Subsequently, CO and H₂ can be further utilized to transform into hydrocarbon fuels by various thermal catalytic conversions. These findings display a direction of CO₂ utilization and fuel productions while the solar energy utilization is still low in this process in spite of reducing CO₂ releasing. Inspired by natural photosynthesis, driving transformation of CO₂ with H₂O into fuels and O₂ under benign conditions is more desirable, where direct sunlight or solar‐source electricity is the main energy source, as present in Figure 2.1. Nevertheless, it is demonstrated that the reaction is non‐thermodynamic and extremely low rate under spontaneous condition. Therefore, to achieve the considerable efficiency of natural photosynthesis and commercialization, catalysts have to be introduced to accelerate the reaction rate, as similar as the chlorophyll, which is named by artificial photosynthesis.

Figure 2.1 Schematic of solar fuel feedstocks (CO₂, H₂O, and solar energy) and production path on‐site and/or transported to the solar refinery [4].