Solar-to-Chemical Conversion

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 CO 2significantly 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, CO 2storage and utilization, water splitting, and N 2fixation [3]. Meanwhile, these desirable techniques often have sustainable, clean, and benign metrics, which are beneficial to supporting the future sustainability of human being.

Figure 2.1illustrates a blueprint of sustainable fuel production and consumption, where conventional power plant still consumes fossil fuel and produces CO 2and water [4]. The released CO 2can be captured by using absorption or adsorption techniques and react with H 2O to produce carbon monoxide (CO) and H 2via thermochemical reactions that are triggered by indirect solar heat or solar‐powered electric energy. Subsequently, CO and H 2can be further utilized to transform into hydrocarbon fuels by various thermal catalytic conversions. These findings display a direction of CO 2utilization and fuel productions while the solar energy utilization is still low in this process in spite of reducing CO 2releasing. Inspired by natural photosynthesis, driving transformation of CO 2with H 2O into fuels and O 2under 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 2.1Schematic of solar fuel feedstocks (CO 2, H 2O, and solar energy) and production path on‐site and/or transported to the solar refinery [4].

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, such as chlorophyll II. Meanwhile, CO 2from the air is captured by the leaf through diffusion process and brought to the chloroplast. Finally, the chlorophyll II excited by incident light catalyzes water split and releases hydrogen and oxygen. After that, the oxygen atoms are formed dioxygen and escape from chloroplast to air. Meantime, the hydrogen atoms react with CO 2molecules into hydrocarbons, such as glucose. Until now, the plants fulfill the great photosynthesis process, which creates the suitable ecosystem for advanced living things, in which chemical fuels and O 2can be further consumed to provide energy for aerobic organisms.

By systematically analyzing the process of natural photosynthesis, it can be found that CO 2and water are the initial reactants and chlorophyll II as a catalyst is necessary to accelerate the reaction under sunlight irradiation. Generally speaking, CO 2and water are abundant and cheap in the Earth, which can be obtained easily. Nevertheless, the reaction between CO 2and water cannot proceed in thermodynamic aspect [6]. Therefore, for chemists, it is necessary to develop an efficient catalyst for triggering the photosynthesis reaction, which helps that the O 2and chemical fuels can be produced in a cheap and sustainable way in a factory. Based on this inspiration, simulating natural photosynthesis process is a relentless pursuit for scientists, which is called as artificial photosynthesis [7].

Scientists around the world have been trying to replicate the natural reactions that occur during photosynthesis and have come across the science of artificial photosynthesis. The term artificial photosynthesis is used to refer to any mechanism made to capture light and store energy from the sun in chemical bonds of a solar fuel [8]. In general, as shown in Figure 2.2, the artificial photosynthesis includes three main steps: (i) The first step in artificial photosynthesis is for the reactants coming together. The reactants include sunlight along with water and carbon dioxide that is available in the atmosphere. (ii) These reactants then go through the whole process of photosynthesis artificially. Scientists have been known to use artificial leaves to split water, producing both oxygen and hydrogen. They are now creating artificial leaves using ruthenium and manganese complexes to mimic the natural process of photosynthesis (redox reaction of nicotinamide adenine dinucleotide phosphate (NADPH/NADP +)). (iii) The products of the reactions are created as soon as water is split, producing both oxygen and hydrogen. The hydrogen is then either used directly as a fuel or a reductant for carbon dioxide to produce organic fuels [10].

Figure 2.2Schematic diagrams of (a) natural photosynthesis and (b) artificial photosynthesis based on molecular systems.

Source: Liu et al. [9].

The term “photocatalysis” is often used in papers on catalytic reaction excited by simulated or natural light irradiation, where a catalyst has been used as a reaction center [11]. To simulate the efficient natural photosynthesis reaction, the use of a catalyst is inevitable in the view of physiochemical field. Because biological scientists find that in leave cells the enzyme is the key role in prompting the non‐thermodynamic reaction, i.e. carbon dioxides reacting with water to produce dioxygen and hydrocarbons. Therefore, to accelerate this reaction under certain conditions, a photocatalyst often is used as similar in many industrial fields, such as sulfuric acid products, synthetic ammonia, and syngas. As shown in Figure 2.3, in general, there are many active sites existing on the surface of catalyst, which can act as reaction site and accelerate the transformation of substrates to products under certain reaction conditions [12]. Similarly, the catalytic reaction process is triggered at the active sites on the surface of photocatalyst under light irradiation. It is reported that the electrons and holes can be produced in the photocatalysts, which further participate the redox reaction [13].

Figure 2.3(a) Traditional catalytic processeson the surface of catalysts. (b) Classic photocatalytic paths over photocatalysts.