**2. The principles for photochemical method**

gas at some locations could be as high as 70–80% (like Harmatten, Alberta in Canada) that they are considered unusable [1]. A concentration of H2S above 320 ppm in air could lead to pulmonary edema with the possibility of death [2], and H2S must be carefully removed in

Classically, the Claus process is the industrial standard to remove hydrogen sulfide. With this process, gaseous hydrogen sulfide could be decomposed into hydrogen oxide and sulfur (see Eq. (1)) with first thermal step at temperature above 850°C (Eq. (2)) and subsequently catalytic

Although this process is very mature and yields elemental sulfur as a by-product, one big drawback of it is that the energy stored in hydrogen sulfide is partially wasted by the formation of hydrogen oxide. In fact, the energy stored in H2S could be utilized for the generation of hydrogen, a potential energy source in future, or other chemical products like H2O2. Other disadvantages of Claus treatment include additional tail gas treatment and inflexibility to

Various methods that could possibly make better use of hydrogen sulfide have been studied in recent years, like thermal decomposition, electrochemical method, plasmachemical method, and photochemical method [5]. For thermal decomposition, high temperature above 1000 K for significant conversion of H2S is often required. Besides, high pressure and proper catalyst like molybdenum sulfide and other metal sulfide are commonly suggested, too. Interestingly, solar furnace was also suggested as the thermal source from the energy source point of view. Electrochemical method like direct electrolysis is often carried out in basic solutions where H2S is absorbed. Anode poisoning by sulfur is a big challenge. In addition, chemical redox

problem of electrochemical method is the high electricity costs today. Plasma generated from microwave, ozonizer, and glow discharge was also reported to be an active species to induce the decomposition of H2S into H2 and S. In comparison, the plasma method is relatively clean and effective. However, similar to electrochemical method, the big obstacle of the plasma‐

In contrast to others like thermal and electrochemical methods, the photodecomposition of H2S is much less mature. Nevertheless, it is a very attracting method, as it offers us one possible approach to directly harness solar energy and convert them into chemical energy, in a period that we are under the pressure of both exhaustion of fossil fuel and increase in energy demand

2 2 <sup>2</sup> 2 H S O 2 S 2 H O +® + (1)

22 2 2 H S SO 3 S 2 H O + ®+ (3)

2 2 22 2 10 H S 5 O 2 H S SO + 7 S + 8 H O +® + (2)

and Fe3+/Fe2+ are also introduced for indirect electrolysis of H2S. The main

step (Eq. (3)) with activated aluminum(III), titanium (IV) oxide and so on [3].

related human activities.

270 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

adjust to changes [4].

couples such as I3−/I<sup>−</sup>

worldwide.

chemical method is the use of electricity.

As early as 1912, photochemist Giacomo Ciamician has drawn us a picture of the future [6]:

"On the arid lands there will spring up industrial colonies without smoke and without smokestacks; forests of glass tubes will extend over the plains and glass buildings will rise everywhere; inside of these will take place the photochemical processes that hitherto have been the guarded secret of the plants, but that will have been mastered by human industry which will know how to make them bear even more abundant fruit than nature, for nature is not in a hurry and mankind is. And if in a distant future the supply of coal becomes completely exhausted, civilization will not be checked by that, for life and civilization will continue as long as the sun shines! If our black and nervous civilization, based on coal, shall be followed by a quieter civilization based on the utilization of solar energy, that will not be harmful to progress and to human happiness."

However, even after 100 years later of this vision, human civilization is still "made use almost exclusively of fossil solar energy. Would it not be advantageous to make better use of radiant energy?"

In this chapter, we will mainly focus on photocatalysis (photochemical reaction carried out in the presence of catalyst), which has risen during the last half century. Ever since the discovery that TiO2 could split water into hydrogen and oxygen with the assistance of light and electricity in 1972, photocatalysis has aroused great interest of people [7]. Usually, photocatalysis is a chemical process triggered by photogenerated electrons and holes from light-responsive materials. Like photosynthesis happening in nature, the light energy could be converted into chemical energy with photocatalysis. Therefore, some photocatalytic reaction like water splitting for hydrogen and oxygen evolution is called *artificial photosynthesis* and has given high hopes.

Both molecule and inorganic semiconductor systems could be constructed for photocatalysis. Typically, three processes are necessary to complete the photocatalyis (Figure 1) : (1) absorption of photons and subsequent generation of free electrons and holes (see Eq. (4) and the enlarged section on the top right of Figure 1) ; (2)charge transfer and separation of photogenerated carriers (pathway C and D), accompanied with the competitive charge recombination proc‐ esses (pathways A and B); (3) reduction of reaction substrates by electrons; and (4) oxidation of adsorbents by holes (Eq. (5)).

$$\text{Isemiconductor} + hv \to e\_{\text{CB}}^{-} + h\_{\text{VB}}^{\*} \tag{4}$$

$$\mathbf{A} \to \mathbf{A}^- , \mathbf{D} \to \mathbf{D}^\* \tag{5}$$

For molecular systems, these three steps are often occurred on different materials. Taken photocatalytic hydrogen evolution as an example, step 1 is often carried out by one kind molecule (like ruthenium complexes), and step 3 is finished with the help of another molecule (such as recently popular cobalt and nickel complexes), while step 2 occurs both intra and intermolecularly. For semiconductor systems, all three steps could happen on one material (TiO2 for instance), although sometimes cocatalyst (like Pt nanoparticles) is introduced for a higher light-to-chemical energy conversion. Molecular systems could be easily modified and could help us better observe the underlying catalytic mechanism from molecular level; nevertheless, such systems usually lacks long-term stability and we will mainly focus on semiconductor-based photocatalytic systems in this chapter.

**Figure 1.** Schematic photoexcitation in a solid followed by deexcitation events. Adapted with permission from refer‐ ence [8]. Copyright 1995 American Chemical Society.

Various kinds of semiconductors have been developed for photocatalysis. Due to its nontox‐ icity, low cost, and high stability, TiO2 is the most studied semiconductor ever since its big sensation in 1972, and it is still very popular today. However, the crystal symmetry of TiO2 allows only indirect interband transitions, and TiO2 suffers from serious recombination of charge carriers [9]. Most importantly, the wide band gap of TiO2 (3.2 eV for anatase and 3.0 eV for rutile) only makes it response to UV light (with wavelength below 398 nm for anatase and 413 nm for rutile), which only accounts for about 4% of the full solar spectrum [10]. Sensitiza‐ tion and doping are two common methods for modification of TiO2 to increase its responsibility to visible light. Recently, it has been reported that with disorder engineering by hydrogenation, the onset of optical absorption of TiO2 could be shifted to about 1200 nm (corresponding to 1.0 eV), and no obvious loss of photocatalytic activity of TiO2 is observed [11].

In addition to TiO2, many other binary and ternary oxides are also studied, such as d0 metal oxides (SrTiO3, ZrO2, Nb2O5, Ta2O5, Bi2W2O9, etc.), d10 metal oxides (ZnO, In2O3, etc.), and f0 metal oxides (like CeO2). Metal sulfides are another important category of photocatalysts. Among them, CdS has attracted large attentions. The main advantage of CdS is its responsi‐ bility to visible light (with a direct band gap of 2.4 eV), while one big disadvantage is its instability (mainly the oxidation of S2– in the absence of hole scavenger) under light illumina‐ tion. Other sulfides like ZnS, CuInS2, AgIn2S2, and their solid solution have also been well studied for photocatalysis [12]. In particular, carbon materials, like graphene carbon nitride and carbon quantum dots, have lately aroused people's great interests due to their metal-free property and easy preparation [13,14]. Figure 2 shows the band gap and conduction and valence band levels of several typical semiconductors at pH 0. A more comprehensive presentation of the band structure of oxides and sulfide semiconductors was reported by Schoonen and Xu [15]. From the thermodynamic point of view, the conduction and valence band edge of semiconductors is an indication of their reducing and oxidizing ability, respec‐ tively. For instance, oxides often have deep valence band edge and hence strong oxidizing ability.

molecule (like ruthenium complexes), and step 3 is finished with the help of another molecule (such as recently popular cobalt and nickel complexes), while step 2 occurs both intra and intermolecularly. For semiconductor systems, all three steps could happen on one material (TiO2 for instance), although sometimes cocatalyst (like Pt nanoparticles) is introduced for a higher light-to-chemical energy conversion. Molecular systems could be easily modified and could help us better observe the underlying catalytic mechanism from molecular level; nevertheless, such systems usually lacks long-term stability and we will mainly focus on

**Figure 1.** Schematic photoexcitation in a solid followed by deexcitation events. Adapted with permission from refer‐

Various kinds of semiconductors have been developed for photocatalysis. Due to its nontox‐ icity, low cost, and high stability, TiO2 is the most studied semiconductor ever since its big sensation in 1972, and it is still very popular today. However, the crystal symmetry of TiO2 allows only indirect interband transitions, and TiO2 suffers from serious recombination of charge carriers [9]. Most importantly, the wide band gap of TiO2 (3.2 eV for anatase and 3.0 eV for rutile) only makes it response to UV light (with wavelength below 398 nm for anatase and 413 nm for rutile), which only accounts for about 4% of the full solar spectrum [10]. Sensitiza‐ tion and doping are two common methods for modification of TiO2 to increase its responsibility to visible light. Recently, it has been reported that with disorder engineering by hydrogenation, the onset of optical absorption of TiO2 could be shifted to about 1200 nm (corresponding to 1.0

eV), and no obvious loss of photocatalytic activity of TiO2 is observed [11].

In addition to TiO2, many other binary and ternary oxides are also studied, such as d0

oxides (SrTiO3, ZrO2, Nb2O5, Ta2O5, Bi2W2O9, etc.), d10 metal oxides (ZnO, In2O3, etc.), and f0 metal oxides (like CeO2). Metal sulfides are another important category of photocatalysts. Among them, CdS has attracted large attentions. The main advantage of CdS is its responsi‐ bility to visible light (with a direct band gap of 2.4 eV), while one big disadvantage is its instability (mainly the oxidation of S2– in the absence of hole scavenger) under light illumina‐ tion. Other sulfides like ZnS, CuInS2, AgIn2S2, and their solid solution have also been well studied for photocatalysis [12]. In particular, carbon materials, like graphene carbon nitride

metal

semiconductor-based photocatalytic systems in this chapter.

272 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

ence [8]. Copyright 1995 American Chemical Society.

**Figure 2.** Relationship between band structure of semiconductor and redox potentials of water splitting. Reproduced by permission from the Royal Society of Chemistry from reference [12]. All rights reserved.

In all photocatalytic reactions were studied, water splitting is considered to be the Holy Grail of solar energy conversion. Over the last 40 years, scientists have been committed to find ideal photocatalytic systems that could turn water into hydrogen and oxygen by solar light. For a semiconductor qualified for water splitting, the conduction band edge should be more negative than the redox potential of H+ /H2 (0 V vs NHE at pH 0), and the valance band edge should be more positive than the redox potential of O2/H2O (1.23 V vs NHE at pH 0). Never‐ theless, overpotential and large kinetic barriers are also needed to be considered in practice. Several semiconductor systems have been reported for the stoichiometry water splitting for hydrogen and oxygen evolution (with mole ration of 2:1), such as In1-xNixTaO4 (x = 0–0.2) [16], NiO (0.2 wt%)/NaTaO3:La (2%) [17], and the lately reported visible light-responsive carbon dot/C3N4 nanocomposite [14].

As a matter of fact, the photocatalytic decomposition of H2S is similar to that of water splitting. To some extent, the direct decomposition of H2S into H2 and elemental S is much easier than that of H2O from the thermodynamic point of view: the energy needed for H2O decomposition is about 237.2 kJ/mol [18], while that for H2S is only 39.3 kJ/mol [4,10]. The reductive reaction that occurs in the decomposition process of H2S is still hydrogen evolution from protons in most cases (with exception mentioned below), but the oxidative reaction changes from O2 evolution to oxidation of S2−. Therefore, for a semiconductor qualified for H2S decomposition, the conduction band edge should still be more negative than the redox potential of H+ /H2, but the valance band edge only needs to be more positive than the redox potential of H2S/S2− (0.14 V vs NHE at pH 0). This means that for semiconductors that are capable of water splitting are all qualified for H2S decomposition. Besides, for some semiconductor, even if they may be not proper for water splitting due to the less positive valance band edge, they still have the potential for H2S decomposition. One example is silicon. As seen from Figure 2, the valence band edge of silicon is far more negative than the redox potential of H2O/O2, which determines its inability for oxygen evolution. Nevertheless, it could be used in the system of H2S decom‐ position (see below).

Like water splitting could occur in both gas phase (water vapor) and liquid phase, H2S, as an acid gas, could be decomposed in gas phase directly and disposed in liquid phase indirectly after being absorbed by solution. Moreover, here we will have a review of these two cases, respectively.
