Photochemical Decomposition of Hydrogen Sulfide

Hydrogen sulfide is an extremely toxic gas which is generated from both nature factors and human factors. A proper method for the efficient decomposition of hydrogen is of great importance. Using traditional Claus process, hydrogen sulfide could be decom‐ posed into hydrogen oxide and sulfur. One drawback of this process is that the energy stored in hydrogen sulfide is partially wasted by the formation of hydrogen oxide. In fact, the energy could be utilized for the generation of hydrogen, a potential energy source in future, or other chemical products. Various methods that could possibly make better use of hydrogen sulfide have been studied in recent years, like thermal decomposition, plas‐ ma method, electrochemical method, and photochemical method. In particular, there have been high hopes in photochemical method due to the possible direct solar energy conversion into chemical energy. Unlike traditional photocatalytic water splitting, hydro‐ gen sulfide decomposition is more accessible from the thermodynamic point of view. Photocatalytic hydrogen sulfide decomposition could occur in both gas phase and solu‐ tion phase and various systems have been reported. Besides, the photoelectrochemical decomposition of hydrogen sulfide is also highlighted. In this chapter, we will simply in‐ troduce the current situation for photochemical decomposition of hydrogen sulfide.


Introduction
Hydrogen sulfide (H 2 S) is an extremely toxic and corrosive gas with an odor of rotten eggs. Usually, H 2 S could be released due to nature factors like microbial metabolism in the absence of oxygen and volcanic eruptions. However, in modern society, the main source of H 2 S should be more attributed to human activities like the refinery of crude oil (desulfurization) and the sweetening of natural gas. For example, due to the exhaustion of high-quality natural gas reservoirs and our continued growth in energy demand, people has to turn to some sour nature gas reservoirs with a large amount of H 2 S. As a matter of fact, the H 2 S content of sour natural 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 H 2 S above 320 ppm in air could lead to pulmonary edema with the possibility of death [2], and H 2 S must be carefully removed in related human activities.
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 step (Eq. (3)) with activated aluminum(III), titanium (IV) oxide and so on [3].
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 H 2 S could be utilized for the generation of hydrogen, a potential energy source in future, or other chemical products like H 2 O 2 . Other disadvantages of Claus treatment include additional tail gas treatment and inflexibility to adjust to changes [4].
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 H 2 S 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 H 2 S is absorbed. Anode poisoning by sulfur is a big challenge. In addition, chemical redox couples such as I 3− /I − and Fe 3+ /Fe 2+ are also introduced for indirect electrolysis of H 2 S. The main 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 H 2 S into H 2 and S. In comparison, the plasma method is relatively clean and effective. However, similar to electrochemical method, the big obstacle of the plasmachemical method is the use of electricity.
In contrast to others like thermal and electrochemical methods, the photodecomposition of H 2 S 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 worldwide.

The principles for photochemical method
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 TiO 2 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 ( 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 (TiO 2 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 reference [8]. Copyright 1995 American Chemical Society.
Various kinds of semiconductors have been developed for photocatalysis. Due to its nontoxicity, low cost, and high stability, TiO 2 is the most studied semiconductor ever since its big sensation in 1972, and it is still very popular today. However, the crystal symmetry of TiO 2 allows only indirect interband transitions, and TiO 2 suffers from serious recombination of charge carriers [9]. Most importantly, the wide band gap of TiO 2 (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]. Sensitization and doping are two common methods for modification of TiO 2 to increase its responsibility to visible light. Recently, it has been reported that with disorder engineering by hydrogenation, the onset of optical absorption of TiO 2 could be shifted to about 1200 nm (corresponding to 1.0 eV), and no obvious loss of photocatalytic activity of TiO 2 is observed [11].
In addition to TiO 2 , many other binary and ternary oxides are also studied, such as d 0 metal oxides (SrTiO 3 , ZrO 2 , Nb 2 O 5 , Ta 2 O 5 , Bi 2 W 2 O 9 , etc.), d 10 metal oxides (ZnO, In 2 O 3 , etc.), and f 0 metal oxides (like CeO 2 ). Metal sulfides are another important category of photocatalysts. Among them, CdS has attracted large attentions. The main advantage of CdS is its responsibility to visible light (with a direct band gap of 2.4 eV), while one big disadvantage is its instability (mainly the oxidation of S 2-in the absence of hole scavenger) under light illumination. Other sulfides like ZnS, CuInS 2 , AgIn 2 S 2 , 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, respectively. For instance, oxides often have deep valence band edge and hence strong oxidizing ability.

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 + /H 2 (0 V vs NHE at pH 0), and the valance band edge should be more positive than the redox potential of O 2 /H 2 O (1.23 V vs NHE at pH 0). Nevertheless, 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 In 1-x Ni x TaO 4 (x = 0-0.2) [16], NiO (0.2 wt%)/NaTaO 3 :La (2%) [17], and the lately reported visible light-responsive carbon dot/C 3 N 4 nanocomposite [14].
As a matter of fact, the photocatalytic decomposition of H 2 S is similar to that of water splitting. To some extent, the direct decomposition of H 2 S into H 2 and elemental S is much easier than that of H 2 O from the thermodynamic point of view: the energy needed for H 2 O decomposition is about 237.2 kJ/mol [18], while that for H 2 S is only 39.3 kJ/mol [4,10]. The reductive reaction that occurs in the decomposition process of H 2 S is still hydrogen evolution from protons in most cases (with exception mentioned below), but the oxidative reaction changes from O 2 evolution to oxidation of S 2− . Therefore, for a semiconductor qualified for H 2 S decomposition, the conduction band edge should still be more negative than the redox potential of H + /H 2 , but the valance band edge only needs to be more positive than the redox potential of H 2 S/S 2− (0.14 V vs NHE at pH 0). This means that for semiconductors that are capable of water splitting are all qualified for H 2 S 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 H 2 S 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 H 2 O/O 2 , which determines its inability for oxygen evolution. Nevertheless, it could be used in the system of H 2 S decomposition (see below).
Like water splitting could occur in both gas phase (water vapor) and liquid phase, H 2 S, 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.

Photocatalytic hydrogen sulfide decomposition by gas phase reaction
Jardim et al. studied the gas phase destruction of H 2 S with a low concentration range of hundreds of ppm using TiO 2 as the catalyst and black light lamp as the light source [19]. In the existence of oxygen and water vapor, H 2 S could be effectively decomposed (about 99% efficiency) and the main product is determined as SO 4 2- . The deactivation of TiO 2 would happen with a H 2 S concentration larger than 600 ppm, and it was mainly caused by the adsorption of by-product on its surface. No elemental S was detected by the color change of TiO 2 from white to yellow, and hydrogen evolution was not considered in this study. Notably, if oxygen is absent in the system, H 2 S could be barely removed.
In a similar experiment, with the assistance of in situ FT-IR, Anderson et al. confirmed that no other gaseous products like SO 2 or SO, and SO 2− adsorbed on TiO 2 may be one intermediate during the "eight electron transfer" process [20]. Furthermore, Sano et al. have found that the photodeposition of Ag on TiO 2 would promote the adsorption of H 2 S on the sample, possibly due to the partially oxidized silver surface, and the deposited Ag could act as a cocatalyst for removal of H 2 S. Both factors made Ag-deposited TiO 2 more efficient for H 2 S degradation [21].
In addition, Sánchez et al. have tried glass "Raschig" ring, poly(ethylene terephthalate) (PET), and cellulose acetate (CA) as the supports to load TiO 2 for photocatalytic treatment of H 2 S gas [22]. Glass rings supported TiO 2 (which has underwent fire treatment) outperforms PET and CA supported TiO 2 . For PET and CA supports with low temperature treatment, PET supports displayed the higher photocatalytic activity, and TiO 2 caused the degradation of CA supports under illumination. Different from reports before, although SO 4 2-is one main product of H 2 S removal, SO 2 was detected from these systems.
The interaction of H 2 S with the semiconductor surfaces has also been investigated. Two adsorption modes of H 2 S with high defect density rutile TiO 2 (110) surfaces were suggested: dissociative adsorption with both H and S atom attached to the Ti atom at low H 2 S concentration and molecular adsorption at high H 2 S concentration [23]. Moreover, the preadsorption of H 2 S would significantly block O 2 adsorption on TiO 2 surfaces even in the presence of large Ti 3+ cations. Using Langmuir isotherm, Sopyan further discovered that H 2 S adsorbed more strongly on rutile (0.7 molecules / nm 2 ) rather than anatase (0.4 molecules/nm 2 ). This is in sharp contrast with other molecules like acetaldehyde and ammonia [24]. Consequently, photocatalytic activity of anatase film is only 1.5 times higher than that of rutile for degradation of H 2 S. In all the above systems, H 2 S in gas phase is studied within low concentration (tens to hundreds of ppm) and people mainly concerns with the oxidation product of H 2 S. Little attention is paid to the reductive reaction of H 2 S. Nevertheless, the reduction of H 2 S (which is often the conversion of H 2 S into H 2 ) is more attractive from an energy point of view.
Early in 1990s, Naman has combined thermal and photocalytic decomposition of H 2 S together and studied the influence of light influx on the thermal decomposition of H 2 S by V x S y on different substrates (TiO 2 , Al 2 O 3 , and ZnO) [25]. Under light irradiation, the conversion of H 2 S to H 2 was increased by 27.6%, 44.6%, and 16.5% at 500°C, respectively. The Arrhenius activation energy for H 2 S decomposition has also calculated to be 50% of that in darkness. The author tentatively attributes this photoactivation effect on thermal decomposition to the photoexcitation of semiconductors (including V x S y ) and the subsequent generated charge carriers.
In 2008, Li et al. have compared the activity of five typical semiconductors TiO 2 , CdS, ZnS, ZnO, and ZnIn 2 S 4 for the direct decomposition of H 2 S in gaseous phase [26]. With illumination of Xe lamp and Pt loading (0.2 wt%), the efficiency of the decomposition of 5% H 2 S in argon decreases as a sequence of ZnS > TiO 2 > ZnIn 2 S 4 > ZnS > CdS under the gas flow rate of 6 ± 0.5 mL/min. Various noble metal loadings on ZnS have been compared, and it turns out that Ir is superior than others (Pd, Pt, Ru, Rh, and Au), which improves the hydrogen evolution efficiency from 1.2 to 4.5 μmol/h. Doping ZnS was also carried out, and transition metal Cu 2+ doping (0.5% mol) could greatly promote the decomposition process and improve efficiency of the hydrogen evolution by about 20 times in contrast to blank ZnS. In addition, the absorption edge of ZnS shift from 400 to 450 nm after Cu doping, and this contributes to a photocatalytic H 2 production rate of 17 μmol/h under visible light irradiation (λ > 420 nm). Similarly, one limitation of this research is that only the reduction product, H 2 , is detected in the system and the oxidative products are ignored.
Although systematic experimental studies of the photocatalytic decomposition of H 2 S in gaseous phase are scarce, thermodynamic analysis of solar-based photocatalytic H 2 S decomposition has recently been reported, which may be instructive for further studies on experiments [27]. Analysis indicates that energy efficiency of this process is not significantly affected by the intensity of solar irradiation. Exergy efficiency (the second law efficiency) will decrease with the increase of solar intensity, while the hydrogen yield will increase. Although the exergy efficiency value of current catalyst is calculated to be less than 1%, the author envisioned that an exergy efficiency of 10% could be achieved in the near future, and a maximum exergy efficiency of 27% may be obtained for a chemical conversion ratio of 0.6 if close to optimum cases of the quantum efficiency and the catalyst band gap can be obtained.

Photocatalytic hydrogen sulfide decomposition in solution 4.1. H 2 S decomposition in aqueous solution
In comparison with solid gas phase photocatalysis, more often H 2 S is first absorbed in solution. Under these circumstances, H 2 S mainly participate in the photocatalytic reaction in the form of S 2-or HS − , depending on the pH of the system. (The dissociative constants for the first and the second dissociation of H 2 S at 298 K are 1.02 × 10 -7 and 1.3 × 10 -12 , respectively.) Hydrogen (in most cases) is generated in these systems as a result of the proton reduction. H 2 S has a high solubility in pure water (ca. 0.1 M at 298 K); however, due to the limited availability of S 2-and HS − , the hydrogen evolution efficiency is low [28]. More often, H 2 S is absorbed by alkaline solution like NaOH and KOH solution, and sulfide solution is often used to replace the gaseous H 2 S for photocatalytic H 2 evolution.
In 1976, Wrighton et al. reported that when using Na 2 S as the sacrificial reagents in the presence of NaOH, the photocorrosion of CdS or CdSe photoelectrodes could be effectively inhibited [29]. The added S 2-in solution is oxidized, judging from the color change of the solution from transparent to yellow. H 2 was evolved at the Pt counter electrode. Later, Nozik proved that when the Schottky-type n-CdS/Pt photochemical diodes was suspended in the solution containing 1 M Na 2 S and 1 M NaOH, hydrogen evolution could be observed with the illumination of simulated sunlight [30].
Early exhaustive studies of such work were conducted by Grätzel et al. [31]. When loaded with RuO 2 (0.1 wt%), CdS shows a high H 2 evolution rate of 0.128 mL g -1 h -1 in the presence of 0.1 M Na 2 S (pH 3). With the S 2-ions present in the photocatalytic solution, a H 2 S to H 2 conversion efficiency of 90% was calculated. Also, the concomitantly formed oxidation product S would not interfere with the water reduction (hydrogen evolution). The reaction mechanisms are shown in Eqs. (6)(7)(8): CdS hv e h -+ + ® + During the photocatalytic process, although oxygen reduction can compete with hydrogen reduction, the existence of O 2 has little effect on the system's efficiency. Furthermore, the study shows that a basic solution and a higher RuO 2 loading (but no more than 0.5 wt%) could obviously improve the efficiency, while increase of the concentration of S 2-in solution and Pt loading on CdS seems has no significant influence on hydrogen evolution. Under optimal conditions, a quantum yield of 0.35 is obtained. In a similar CdS involved photocatalytic system with S 2-as the electron donor, Reber et al. pointed out that in contrast to an acidic environment, only disulfide instead of elemental sulfur is formed in an alkaline medium [32].
Photocatalytic hydrogen evolution with in situ H 2 S absorption in alkaline solution has been carried out with various kinds of semiconductor photocatalysts, too (

H 2 S decomposition through S 2-/SO 3 2-solution
One challenge often encounters with alkaline sulfide solution for photocatalytic hydrogen evolution is the interference of by-product. Disulfide and polysulfide ions usually form in alkaline sulfide solution by reaction between S 2-and elemental S immediately after the photooxidation (see Eqs. (10)(11)(12)). These ions are yellow and can act as an optical filter, which reduces the absorption of photocatalyst. In addition, polysulfide would compete with protons for reduction. Therefore, with the accumulation of disulfide, the hydrogen evolution efficiency of related systems is slowed down. A common solution for this is the addition of SO 3 2-into the system. The additional sulfite could react with sulfur and avoid the generation of polysulfide; meanwhile, colorless thiosulfate is formed, which is thermodynamically less easily reduced than protons: Then the net oxidative reaction that occurs in such a photocatalytic system is and the whole photocatalytic hydrogen evolution reaction corresponds to Photocatalytic hydrogen evolution systems based on S 2-/SO 3 2-solution is widely reported, and some typical reports are given in Table 2 [10]. As a matter of fact, the S 2-/SO 3 2-solution is one of the most famous sacrificial donors for photocatalytic hydrogen evolution under basic environment; this is especially true for metal sulfide photocatalysts. CdS, ZnS, CuInS 2 ZnInS 2 , and their solid solution are all well studied for photocatalytic hydrogen evolution with such system. Metal sulfide often suffers from instability in photocatalytic processes as a result of the self-oxidation of sulfide with other sacrificial donors, but this could be effectively inhibited in the presence of sulfide in solution. This may be one important reason for the wide use of S 2-/ SO 3 2-solution in photocatalysis. In contrast, metal oxide is less popular in such system, probably due to their small response in the visible light region.
Using S 2-/SO 3 2-solution, Kudo et al. have developed a series of visible light-responsive ZnS-CuInS 2 -AgInS 2 solid solution photocatalysts for hydrogen evolution under irradiation from the solar simulator [34]. With increasing the proportion of CuInS 2 and AgInS 2 in the solid solution, the absorption spectrum of the photocatalyst could be extended to near-infrared region; however, hydrogen evolution was only observed with light absorption of wavelength less than 650 nm. When loaded with 0.75 wt% Ru, the initial hydrogen rate of 8.2 L m -2 h -1 and a quantum yield of 7.4% (at both 480 and 520 nm) could be observed with irradiation of solar simulator. Furthermore, they have demonstrated that with this photocatalytic system, a solar hydrogen evolution rate of about 2 L m -2 h -1 could be obtained for a reactor of 1 m 2 in November in Tokyo [12]. In addition, CdS loaded with RuO 2 (0.25 wt%) is also evaluated for its potential for commercial application. When the CdS-RuO 2 concentration is 2.0 mg mL -1 in 500 mL solution of 0.1 M Na 2 S and 0.1 M Na 2 SO 3 (with a surface area of 112 cm 2 ), a hydrogen generation rate of 28 mL h -1 could be achieved under solar light irradiation [35].

Thiosulfate cycle for H 2 S decomposition
To make more efficient use of solution with mixed sulfide and sulfite for photocatalytic hydrogen evolution, Grätzel et al. further propose the concept "thiosulfate cycle" [36]. Under light illumination, S 2 O 3 2-could be disproportionated into S 2-and SO 3 2-with the assistance of TiO 2 (see specific reaction in Eqs. (16)(17)(18)(19) and overall reaction in Eq. (20) With such a cycle, no sulfur or thiosulfate would accumulate in such system. Figure 3 shows such a possible two-compartment system composed of CdS and TiO 2 . Ideally, for the generation of 1 mol of H 2 , 2 mol and 4/3 mol photons are needed to be absorbed by CdS and TiO 2 , respectively. However, whether the efficiency of the two half cycles could match each other effectively is one important question unveiled to us, and there is no further clear report of this system till now.

H 2 S decomposition in ethanolamine solution
In addition to hydroxide alkaline solution, some other additives are introduced to promote the absorption of H 2 S in solution. For example, ethanolamine solution is frequently used in gas sweetening industry. Naman and Grätzel have dissolved H 2 S in aqueous solution of alkanolamines (including monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA)) and studied the photocatalytic efficiency of such system with vanadium sulfide as the photocatalyst [37]. Taking monoethanolamine for instance, one monoethanolamine was able to dissolve one molecule of H 2 S (see Eqs. (22 and 23)). However, one big disadvantage of this method is that ethanolamines themselves could be decomposed under light illumination and the amount of ammonia detected from the photocatalytic system could even be higher than that of H 2 : 2HOCH 2 CH 2 NH 2 + H 2 S ⇌ (HOCH 2 CH 2 NH 3 ) 2 S (22) (HOCH 2 CH 2 NH 3 ) 2 S + H 2 S ⇌ (HOCH 2 CH 2 NH 3 ) 2 (SH) 2 (23) Furthermore, Li et al. used anhydrous ethanolamine solution to absorb H 2 S [38]. Different from early report that system containing aqueous MEA outperforms that contains DEA and TEA, nonaqueous DEA solution is best for H 2 S decomposition with CdS-based photocatalyst. Such a system is also better than system with NaOH-Na 2 S solution from both the point of lifetime and rate for photocatalytic hydrogen evolution. In addition, the reduction of polysulfide in H 2 S-DEM system is effectively depressed and could hardly compete with the proton reduction, which commonly occurs in NaOH-Na 2 S system.

Extraction of elemental sulfur
Although numerous kinds of catalysts have been reported for the decomposition of H 2 S through the above-mentioned method and hydrogen indeed evolves from solution, one problem is that S 2-often transforms into polysulfide, thiosulfate, or sulfite. How to deal with these by-products is another big challenge for us. Elemental S is more favored as the byproduct; nevertheless, it could not be recovered from such photocatalytic system. To obtain pure sulfur, people have developed several ideas.
One simple method is to take advantage of the limited acid stability of complex sulfur species. Both polysulfide and thiosulfate would produce S when the pH value of the system decreases to a certain extent. That is, if the outlet reaction solution after photocatalysis (containing polysulfide or thiosulfate) encounters the inlet acidic gas H 2 S, elemental S could possibly be precipitated from the system with a proper drop of pH: In this regard, Linkous et al. have designed a circulating photoreactor for H 2 S decomposition ( Figure 4) [39]. The feasibility of this system was conducted. In the photoreactor, hydroxide would be generated along with H 2 evolution, and the pH of the solution would increase. Nevertheless, when this solution flows into the scrubber tower, pH would decrease due to the input H 2 S gas. For the fresh reaction solution constituted of both S 2-and SO 3 2-, S 2 O 3 2-would be generated after photocatalysis, and pH must be lowered to 4.2 (by neutralization with H 2 S) for sulfur release from S 2 O 3 2-. Then S could be collected as precipitates and the remaining solution (enriched with HSand HSO 3 -) would be sent back to the photoreactor for another round of photocatalysis, with a low pH (≤ 4.2). For fresh reaction solution only constituted of S 2-, polysulfide would be generated and the pH of the solution need to be lower than 10 for precipitation of sulfur. Normally, the photocatalytic systems using S 2-or SO 3 2-as the electron donor are more efficient for hydrogen evolution under basic conditions (pH ≥10); in some cases, the system could not even work under a relatively acidic environment (like pH 4). This urges us to reconsider the effect of SO 3 2-under such circumstances: as described above, SO 3 2-are widely used in S 2-involved hydrogen evolution system to avoid the generation of polysufide (which competes not only with catalyst from light absorption but also with protons for reduction by electrons), but the acidity necessary for the release of S from the obtained thiosulfate would greatly reduce the photocatalytic activity of the catalysts. In their study, Linkous pointed out that if the depth of reaction solution in photoreactor is less than 1 cm, in order to reduce the light absorption of polysufide, S 2-alone as the electron donor for photocatalytic hydrogen evolution is probably more suitable for the cyclic sulfur release in a CdS/Pt involved system. Additionally, another problem of this design is that if the commonly studied suspension system is used for photoreaction, photocatalyst could not be easily separated with the solution. Therefore, catalyst may need to be immobilized for circulating.

Photoelectrochemical decomposition of hydrogen sulfide
In addition to the photocatalytic decomposition of H 2 S alone, sometimes photochemical method is combined with electrochemical method for the decomposition of H 2 S, that is, the photoelectron-chemical (PEC cell) decomposition of H 2 S. Actually, the colloidal semiconductor photocatalyst system mentioned above could also be seen as some kind of short-circuit PEC cell, in which both anodic and cathodic reaction occurs on the surface of semiconductors at the same time (similar to the "photochemical diodes" developed by Nozik [30]). In this section, traditional PEC cells (with separated anode and cathode connected by wires) would be mainly focused. These cells could not only decompose hydrogen sulfide but also generate electricity. In addition, voltage bias could be applied to the cells if the drive force of light is not enough for hydrogen sulfide decomposition.
In 1987, Kainthla and Bockris reported a PEC cell for the decomposition of H 2 S based on CdSe anode and Pt cathode [55]. CdSe film was directly grown on Ti substrate. Using polysulfide (prepared by H 2 S dissolution in NaOH and subsequent addition of sulfur) as the electrolyte, an open circuit voltage of 0.62 V and short circuit current of 8.82 mA cm -2 could be achieved. H 2 bubbles could be observed to leave the Pt cathode when photocurrent flows through the cell and a Faraday efficiency of 0.97 is calculated. With the gradual accumulation of polysufide during the reaction, elemental sulfur would precipitates from the solution when polysulfide reaches its solubility limit. Stability of the cell is also tested and short circuit drops less than 10% with continuous illumination of 2 weeks. The total cell conversion efficiency (ε) given as the ratio of the recoverable energy to the input energy is calculated based on Eq. (26) ( 26) where 0.171I is the chemical energy storage for H 2 S decomposition into H 2 and S, IV cell is the electrical energy generated from the cell, and W light is the intensity of light. Maximum light to chemical energy storage, light to electrical energy, and total cell conversion efficiency occurs at cell voltage of 0, 0.3, and 0.275 V, with the corresponding efficiency to be 1.5%, 1.8%, and 2.85%, respectively. In this regard, the PEC cell can be operated in a manner that electrical energy or chemical energy can be selectively collected.
It is noteworthy that for eliminating the competition of polysulfide with proton for reduction in this cell, which is also a big problem in suspension systems, anode and cathode are placed in two compartments, and Nafion membrane is used to prevent the contact of polysulfide with cathode. If there is no Nafion membrane, only polysulfide is reduced into sulfide, and no H 2 could be detected from the system. Under this circumstance, no net chemical reaction happens in the cell, and light energy could only be converted into electrical energy.
Another advantage for PEC cells is that some strategies for the electrochemical decomposition of H 2 S could be extended to PEC cell. One strategy is the indirect decomposition of H 2 S with the assistance of redox couple like I -/I 3 -(or I -/IO 3 -) and Fe 3+ /Fe 2+ , in which the electrical energy or solar energy is first stored in the redox intermediate species, and then the intermediate could drive the following chemical reactions. Although indirect strategy may consume more additional energy for H 2 S decomposition from a thermodynamical point of view, it is kinetically more favored and is beneficial for the extraction of elemental sulfur from the system. Lately, Li and Wang et al. have adopted this strategy in PEC cells for H 2 S decomposition and achieved good results. PEC cell with p-type Si deposited with protective TiO 2 /Ti n+ doping layer and H 2 evolution cocatalyst Pt (Pt/TiO 2 /Ti/n+p-Si) as the photocathode and Pt plates as anode was reported for the decomposition of H 2 S [56]. In a two-compartment cell separated by Nafion membrane, freshly prepared 0.2 M of FeSO 4 (or KI) in 0.5 M of H 2 SO 4 solution and 0.5 M of H 2 SO 4 was used as the anodic and cathodic electrolyte, respectively. After H 2 S bubbling into the anode compartment, S and H 2 could be separately produced from the anode and the cathode under light illumination at an applied potential of 0.2 V vs RHE. In this system, the chemical redox couple is significant for the conversion of H 2 S into H 2 and S (Eqs. (27)(28)(29)(30)(31)(32)(33)): Overall reaction: H S + 2 H + S hv ® Control experiment shows that if there is no existence of Fe 2+ or Iin the electrolyte, such experiment is unsuccessful due to the low solubility of H 2 S in acidic solution. Besides, n-type Si coated with 3,4-ethylenedioxythiophene (PEDOT) as the anode was also tested in this system, and it turns out that Fe 2+ and Icould be easily oxidized on it. Nevertheless, due to the low stability of the n-type Si anode, further study in this report is unclear.
Notably, they further developed this indirect strategy in PEC cell and have made H 2 O 2 and S from H 2 S in the presence of oxygen [57]. This is quite novel because most study related to H 2 S decomposition is limited to H 2 as the only reduced product now. In addition to the redox couple I -/I 3 -in the anode compartment of the cell for S production, another redox couple anthraquinone/anthrahydroquinone (AQ/H 2 AQ) was introduced to the cathode cell for H 2 O 2 production. In fact, AQ is also an important reaction substrate in Hysulf process, one indirect strategy related to the thermal decomposition of H 2 S. The anode reaction is still the same as Eqs. (30 and 31), but the cathode reaction and the overall reaction change as follows (Eqs. (34)(35)(36) At zero bias, Pt/p+ n Si photoanode and Pt cathode can simultaneously oxidize Ito I 3 -and reduce AQ to H 2 AQ, respectively. Solar to chemical conversion efficiency was estimated to be 1.1%. If Pt cathode is replaced with carbon plate, a higher photocurrent could be observed.

Conclusion
In general, H 2 S is a highly polluted gas that must be carefully handled and removed. The traditional Claus process suffers from high-energy consumption and waste of potential energy, H 2 . The photochemical decomposition of H 2 S, which emerges with the rise of photocatalysis in the last century, could be one improved method for H 2 S disposal. Lots of progress in the field of the photochemical decomposition of H 2 S has been made in both gaseous phase and liquid phase. The mechanism of such reaction has been studied, and the efficiency of these systems has been calculated. Most often, the photochemical decomposition of H 2 S is indirectly carried out in the form of photocatalytic H 2 production from aqueous sulfide solution. Details of the photochemical decomposition of H 2 S, such as extraction of elemental sulfur from reaction system and the cyclic operation, were also of preliminary consideration. In addition, photochemistry was combined with electrochemistry for H 2 S conversion: photoelectrochemical cells were built to extract H 2 (or H 2 O 2 ) and S from H 2 S with the assistance of redox couples.
In 2009, Li et al. reported CdS loaded with PdS and Pt dual cocatalyst can effectively generate H 2 , with a quantum yield of 93% at 420 nm in the presence of S 2-/SO 3 2-solution and no deactivation was observed within illumination of 100 h for H 2 generation [58]. This is probably the most efficiency system reported relevant to the photocatalytic decomposition of H 2 S till now. However, a lot of scientific problems are still unsolved, and there is a long, long way to go for the real application of the photocatalytic decomposition of H 2 S in large scale chemical processing. In present, problems below may be considered in priority: In gaseous phase systems, the concentration studied for H 2 S decomposition is often low (with a volume concentration on ppm level); they are not practical in real industrial process. Also, people tend to focus on half of the reaction (oxidation of S 2-to SO 4 2-or H 2 generation). This is especially true in solution phase system with S 2-/SO 3 2-or S 2-as the electron donor: most reports only consider how to improve the efficiency of hydrogen evolution. Without the thorough consideration of both oxidizing and reducing reactions, the photochemical decomposition of H 2 S is not persuasive. Moreover, in solution phase system for H 2 S decomposition, along with H 2 evolution, the simultaneously generated polysulfide or thiosulfate is also a pollutant to environment; subsequent processing of such reaction solution should be cared for meaningful utilization of H 2 S. Although systems have been designed for sulfur generation from polysulfide or thiosulfate solution, successful trials are limited and the subsequent separation of sulfur from solution is also a challenge.
Current catalysts with high efficiency of photochemical H 2 S decomposition are mainly metal sulfide loading with noble metal cocatalyst like Pt, RuO 2 , and so on. Although CdS is considered one of the most efficient photocatalyst for H 2 generation under visible light, the high toxicity of CdS should be taken seriously. New materials are needed to be exploited, and carbon materials may be alternative photocatalysts in consideration of cost, stability, and toxicity. Besides, noble metal poisoning by sulfide is another problem could happen sometimes and new earth abundant (low cost) cocatalyst resistive to sulfide poisoning is necessary. Transition metals like Fe, Co, and Ni and their compounds could be promising from the current available data. Similar in PEC cells for H 2 S decomposition, stability and cost could be big problems, too.
To conclude, the photochemical decomposition of H 2 S is still in a relatively early stage. New photocatalytic H 2 S decomposition systems with low cost, high quantum efficiency, and long stability should be further developed, especially those responsive to the visible light region, which account for 43% in the full solar spectra. (Taking similar photocatalytic water spitting as a reference, a quantum yield of 30% at 600 nm is the starting point for practical application, which corresponds to about 5% solar energy conversion.) This may be fulfilled with optimized structure design, including chemical composition, electron and band structure, crystal structure and crystallinity, surface state, morphology, and so on, which is currently highlighted in nanoscience and technology. Moreover, people should keep in mind that oxidation and reduction of H 2 S is equally important for H 2 S decomposition if we want to handle H 2 S in a really green way.