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achieved in a single‐phase system composed of acetonitrile and benzyl alcohol. These works creatively developed the application of MOFs in the field of new energy production.
Chapter 13: Photocatalytic and Photoelectrochemical Reforming of Methane. Solar energy can serve as the input energy for methane activation because of the wide distribution and large reserve. With that, photocatalysis and photoelectrocatalysis are recognized as effective approaches for methane reforming on which more attention has been put in the field of energy preparation. In this chapter, photocatalytic and photoelectrochemical methane reforming are introduced. The differences between these two catalytic processes are investigated in detail. After that, recent research progresses on the methane activation reactions via these two techniques are provided. The related reaction mechanisms are discussed insightfully. At last, promising perspectives on the methane upgrading via solar energy excitation are proposed.
Chapter 14: Photocatalytic and Photoelectrochemical Reforming of Biomass. It is very interesting to combine two renewable resources, e.g. solar energy and biomass, together in one process. Recently photocatalytic and photoelectrochemical reforming of biomass was demonstrated. This chapter discusses the photocatalytic conversion of processed and native lignin, carbohydrates, native lignocellulose, glycerides, and glycerol to hydrogen and value‐added chemicals. Also, the photoelectrochemical (PEC) reforming of biomass to electricity, hydrogen, and biomass‐derived molecules such as glycerol and alcohols, as well as converting 5‐hydroxymethylfurfural to corresponding valuable chemicals, is reviewed.
Chapter 15: Reactors, Fundamentals, and Engineering Aspects for Photocatalytic and Photoelectrochemical Systems. Novel catalyst materials and new reactions have always been the hotspots in the research endeavors. As the places for the ultimate step, the reactors and associated parameters should also be given attention. This chapter provides insightful discussions on the key factors for practical approach of photocatalysis and PEC in solar fuel production. In particular, (i) fundamental rationales and mechanisms, (ii) the design and setup of photoreactors, and (iii) engineering aspects of photocatalytic and PEC systems with potential scalability are provided.
Chapter 16: Prospects of Solar Fuels. By several dream reactions, solar energy can be captured and stored in terms of solar fuels. This chapter first concludes the available processes that have been demonstrated to be feasible, and then attention is paid to explore the feasibility of the processes. Tremendous efforts are required from research, commercialization, and policies to achieve the solar fuel production at a large scale.
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Artificial Photosynthesis and Solar Fuels
Jun Ke
Wuhan Institute of Technology, School of Chemistry and Environmental Engineering, Liufang Campus, No.206, Guanggu 1st road, Donghu New & High Technology Development Zone, Wuhan, 430205, P.R. China
2.1 Introduction of Solar Fuels
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 SOx, NOx, 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].
2.2 Photosynthesis
2.2.1 Natural Photosynthesis
Photosynthesis is a chemical process that occurs in photoautotrophs (organisms that make their own food), in which light energy is converted into sugars and other organic compounds. It consists of a series of chemical reactions that require carbon dioxide and water to begin [5]. The light energy that hits the photoautotrophs is absorbed and drives these chemical reactions to produce carbohydrates and oxygen as a by‐product. The following equation is the basis of photosynthesis:
(2.1)
In past several decades, the rough photosynthesis paths have been reported by scientists based on excessive studies. In the plants, the chloroplast in leaf can absorb the sunlight and trigger the above reaction proceeding in the presence of a series of bioenzymes,