**4.1. H2S decomposition in aqueous solution**

In comparison with solid gas phase photocatalysis, more often H2S is first absorbed in solution. Under these circumstances, H2S mainly participate in the photocatalytic reaction in the form of S2– or HS<sup>−</sup> , depending on the pH of the system. (The dissociative constants for the first and the second dissociation of H2S 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. H2S has a high solubility in pure water (ca. 0.1 M at 298 K); however, due to the limited availability of S2– and HS<sup>−</sup> , the hydrogen evolution efficiency is low [28]. More often, H2S is absorbed by alkaline solution like NaOH and KOH solution, and sulfide solution is often used to replace the gaseous H2S for photocatalytic H2 evolution.

In 1976, Wrighton et al. reported that when using Na2S as the sacrificial reagents in the presence of NaOH, the photocorrosion of CdS or CdSe photoelectrodes could be effectively inhibited [29]. The added S2– in solution is oxidized, judging from the color change of the solution from transparent to yellow. H2 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 Na2S and 1 M NaOH, hydrogen evolution could be observed with the illumi‐ nation of simulated sunlight [30].

Early exhaustive studies of such work were conducted by Grätzel et al. [31]. When loaded with RuO2 (0.1 wt%), CdS shows a high H2 evolution rate of 0.128 mL g–1 h–1 in the presence of 0.1 M Na2S (pH 3). With the S2– ions present in the photocatalytic solution, a H2S to H2 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. (6–8):

$$\text{CdS} + hv \rightarrow e^- + h^\* \tag{6}$$

$$2\ 2\ \mathrm{H}^{+} + 2\ e^{-} \to \mathrm{H}\_{2} \tag{7}$$

$$\text{H}\_2\text{S} + 2\text{h}^+ \rightarrow 2\text{H}^+ + \text{S} \tag{8}$$

Moreover, the overall reaction corresponds to H2S splitting into H2 and S with the assistance of two photons (Eq. (9)):

$$\text{H}\_2\text{S} + 2h\text{v} \rightarrow \text{H}\_2 + \text{S} \tag{9}$$

During the photocatalytic process, although oxygen reduction can compete with hydrogen reduction, the existence of O2 has little effect on the system's efficiency. Furthermore, the study shows that a basic solution and a higher RuO2 loading (but no more than 0.5 wt%) could obviously improve the efficiency, while increase of the concentration of S2– 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 S2– 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].

**4. Photocatalytic hydrogen sulfide decomposition in solution**

In comparison with solid gas phase photocatalysis, more often H2S is first absorbed in solution. Under these circumstances, H2S mainly participate in the photocatalytic reaction in the form

the second dissociation of H2S 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. H2S has a high solubility in pure water (ca. 0.1 M at 298 K); however, due to the limited availability of S2– and

In 1976, Wrighton et al. reported that when using Na2S as the sacrificial reagents in the presence of NaOH, the photocorrosion of CdS or CdSe photoelectrodes could be effectively inhibited [29]. The added S2– in solution is oxidized, judging from the color change of the solution from transparent to yellow. H2 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 Na2S and 1 M NaOH, hydrogen evolution could be observed with the illumi‐

Early exhaustive studies of such work were conducted by Grätzel et al. [31]. When loaded with RuO2 (0.1 wt%), CdS shows a high H2 evolution rate of 0.128 mL g–1 h–1 in the presence of 0.1 M Na2S (pH 3). With the S2– ions present in the photocatalytic solution, a H2S to H2 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

Moreover, the overall reaction corresponds to H2S splitting into H2 and S with the assistance

CdS *hv e h* - + +®+ (6)

<sup>2</sup> 2 H 2 H *e* + - + ® (7)

+ + H S 2 2H S <sup>2</sup> +® + *h* (8)

HS 2 H S 2 2 + ®+ *hv* (9)

, the hydrogen evolution efficiency is low [28]. More often, H2S is absorbed by alkaline solution like NaOH and KOH solution, and sulfide solution is often used to replace the gaseous

, depending on the pH of the system. (The dissociative constants for the first and

**4.1. H2S decomposition in aqueous solution**

276 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

H2S for photocatalytic H2 evolution.

nation of simulated sunlight [30].

shown in Eqs. (6–8):

of two photons (Eq. (9)):

of S2– or HS<sup>−</sup>

HS<sup>−</sup>

Photocatalytic hydrogen evolution with in situ H2S absorption in alkaline solution has been carried out with various kinds of semiconductor photocatalysts, too (Table 1). The mechanism of such systems is similar to that contains sulfide solution. One recent example with relatively high efficiency for hydrogen evolution was reported by Kale et al. with nanostructure Bi2S3, which has a direct band gap of 1.3–1.7 eV [33]. Both nanorod and hierarchical nanoflower Bi2S3 were synthesized by hydrothermal method. With continuous H2S bubbling into KOH solution, a hydrogen evolution efficiency of 8.88 and 7.08 mmol g–1 h–1 was observed for nanoflower and nanorod, respectively, under solar irradiation (from 11:30 a.m. to 2:30 p.m.).



**Table 1.** Photocatalytic systems directly using H2S gas dissolved in alkaline solution for hydrogen evolution.

#### **4.2. H2S decomposition through S2–/SO3 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 S2– and elemental S immediately after the photooxidation (see Eqs. (10–12)). 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 SO3 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:

$$\text{S}^{2-} + \text{2}h^\* \rightarrow \text{S} \tag{10}$$

$$\mathbf{S}^{2^{-}} + \mathbf{S} \to \mathbf{S}\_{2}^{2-} \tag{11}$$

$$\rm{S}\_{2}^{2-} + 2\,\rm{S} \to \rm{S}\_{4}^{2-} \tag{12}$$

$$\text{S}\text{O}\_3^{2-} + \text{S} \rightarrow \text{S}\_2\text{O}\_3^{2-} \tag{13}$$

Then the net oxidative reaction that occurs in such a photocatalytic system is

**photocatalyst light source aqueous reaction**

278 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

> 420 nm

450-W Xe, > 420 nm

450-W Xe, > 420 nm

450-W Xe, > 420 nm

450-W Xe, > 420 nm

450-W Xe, > 420 nm

450-W Xe, > 420 nm

500-W Hg, > 420 nm

> 400 nm

FeGaO3

CuGa2O4

CuGaO2

CuGa1.4Fe0.6O4

CuGa0.065In0.935O2

Nb2Zr6O17−xNx

than protons:

CdS in HY zeolite 250-W Hg,

**4.2. H2S decomposition through S2–/SO3**

CdS-TiO2

**solution**

H2S + NaOH / Na2SO3

**Table 1.** Photocatalytic systems directly using H2S gas dissolved in alkaline solution for hydrogen evolution.

of related systems is slowed down. A common solution for this is the addition of SO3

2 2

**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 S2– and elemental S immediately after the photooxidation (see Eqs. (10–12)). 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

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

**Cocatal./H2 activity (µmol·h-1 g-1)**

H2S + KOH NiOx/4730 7.5 (550 nm) [49]

H2S + KOH 3212 5.3 (550 nm) [50]

H2S + KOH RuO2/9548 15.0 (550 nm) [50]

H2S + KOH 7316 11.4 (550 nm) [51]

H2S + KOH RuO2/8656 13.6 (550 nm) [51]

H2S + KOH 8566 13.5 (550 nm) [52]

H2S + NaOH Pt/9800 41 (> 420 nm) [53]

<sup>2</sup> S2 S *h* - + + ® (10)

<sup>2</sup> S SS - - + ® (11)

24000 - [54]

2– into the

**Quantum yiled (%) Ref.**

$$\text{S}\text{O}\_3^{2-} + \text{S}^{2-} + 2h^\* \rightarrow \text{S}\_2\text{O}\_3^{2-} \tag{14}$$

and the whole photocatalytic hydrogen evolution reaction corresponds to

$$\text{S}^{2-} + \text{SO}\_3^{2-} + 2\text{ H}\_2\text{O} + 2\text{ }hv \rightarrow \text{S}\_2\text{O}\_3^{2-} + 2\text{ OH}^- + \text{H}\_2\tag{15}$$

Photocatalytic hydrogen evolution systems based on S2–/SO3 2– solution is widely reported, and some typical reports are given in Table 2 [10]. As a matter of fact, the S2–/SO3 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, CuInS2 ZnInS2, 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 S2–/ SO3 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 S2–/SO3 2– solution, Kudo et al. have developed a series of visible light-responsive ZnS– CuInS2–AgInS2 solid solution photocatalysts for hydrogen evolution under irradiation from the solar simulator [34]. With increasing the proportion of CuInS2 and AgInS2 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 m2 in November in Tokyo [12]. In addition, CdS loaded with RuO2 (0.25 wt%) is also evaluated for its potential for commercial application. When the CdS-RuO2 concentration is 2.0 mg mL–1 in 500 mL solution of 0.1 M Na2S and 0.1 M Na2SO3 (with a surface area of 112 cm2 ), a hydrogen generation rate of 28 mL h–1 could be achieved under solar light irradiation [35].



Adapted with permission from reference [10]. Copyright 2010 American Chemical Society.

**Table 2.** Photocatalysts for hydrogen evolution using S2-/SO3 2- related solution as the sacrificial donor under visiblelight irradiation.

## **4.3. Thiosulfate cycle for H2S decomposition**

**photocatalyst light source Cocatalyst / H2**

280 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

CdS 300-W Xe, > 420 nm Pt-PdS/29 233 93 (420 nm) CdS/ZnS 350-W Xe, > 430 nm 900 10.2 (420 nm) CdS/TiO2 350-W Hg, > 420 nm Pt/6400 -

CdS/ZnO 300-W Xe Pt/3870 3.2 (300-600 nm)

CdSe 700-W Hg, > 400 nm 436 13.4 (> 400 nm) In2S3 300-W Xe, > 400 nm Pd/960.2 2.1 (430 nm) CuInS2 500-W Xe, > 420 nm Pt/84 - ZnIn2S4 300-W Xe, > 430 nm Pt/562 18.4 (420 nm) ZnIn2S4:Cu 300-W Xe, > 430 nm Pt/757.5 14.2 (420 nm) AgGaS2 500-W Hg, > 420 nm Pt/2960 12.4 (> 420 nm) CuGa3S5 300-W Xe, > 420 nm NiS/∼2800 1.3 (420-520 nm) AgIn5S8 300-W Xe, > 420 nm Pt/200 5.3 (411.2 nm) Ag2ZnSnS4 300-W Xe, > 420 nm Ru/1607 3 (500 nm) Cu2ZnGeS4 300-W Xe, > 420 nm Ru/1233 - CuGa2In3S8 300-W Xe, > 420 nm Rh/10667 15 (560 nm) AgGa2In3S8 300-W Xe, > 420 nm Rh/3433 15 (490 nm) AgInZn7S9 300-W Xe, > 420 nm Pt/3164.7 20 (420 nm) ZnS-In2S3-CuS 300-W Xe, > 400 nm 360 000 22.6 (420 nm)

CdS/LaMnO3 300-W Xe, > 420 nm 375 c-CdS/Pt/hex-CdS 500-W Hg-Xe, > 420 nm 13 360 - CdS/Na2Ti2O4(OH)2 300-W Xe, > 420 nm Pt/2680 43.4 (420 nm) CdS/Zr0.25Ti0.75PO4 300-W Xe, > 430 nm Pt/2300 27.2 (420 nm) CdS/AgGaS2 450-W Hg, > 420 nm Pt/4730 19.7 (> 420 nm) CdS:Ag 900-W Xe Pt/33480 ∼25 (450 nm) CdS-ZnS:Ag 900-W Xe Pt/40957.5 37 (450 nm) CdS:In/Cu 300-W W-H, > 420 nm Pt/2456 26.5 (420 nm) CdS:Mn 500-W Xe, > 420 nm RuOx/1935 7 (> 420 nm) Cd0.1Zn0.9S:Ni 350-W Xe, > 420 nm Pt/585.5 15.9 (420 nm) (Zn0.95Cu0.05)0.67Cd0.33S 300-W Xe, > 420 nm Pt/3633.3 31.8 (420 nm) ZnS:C 500-W Hg, > 420 nm Pt/∼90 - ZnS:Ni 300-W Xe, > 420 nm 160 1.3 (420 nm) ZnS:Pb/Cl 300-W Xe, > 420 nm 93 -

**activity (µmol·h-1 g-1)**

**Quantum yiled (%)**

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, S2O3 2– could be disproportionated into S2– and SO3 2– with the assistance of TiO2 (see specific reaction in Eqs. (16–19) and overall reaction in Eq. (20)). Oxidation products like SO4 2– and S2O6 2– are excluded from the system, and the 1:2 stoichiometric ratio of S2– and SO3 2– is maintained during the whole irradiation time:

$$\text{TiO}\_2 + 2\text{ }hv \to 2\text{ }h^\* + 2\text{ }e^- \tag{16}$$

$$2\,\mathrm{S}\_2\mathrm{O}\_3^{2-} + 2\,h^\* \to \mathrm{S}\_4\mathrm{O}\_6^{2-} \tag{17}$$

$$\mathrm{S\_4O\_6^{2-}} + 3\,\mathrm{OH^-} \rightarrow \mathrm{SO\_3^{2-}} + \frac{3}{2}\,\mathrm{S\_2O\_3^{2-}} + \frac{3}{2}\,\mathrm{H\_2O} \tag{18}$$

$$\text{S}\_2\text{O}\_3^{2-} + 2\text{ e}^- \rightarrow \text{S}^{2-} + \text{SO}\_3^{2-} \tag{19}$$

$$\frac{3}{2}\text{ S}\_2\text{O}\_3^{2-} + 3\text{ OH}^- + 2\text{ hv} \rightarrow 2\text{ SO}\_3^{2-} + \text{S}^{2-} + \frac{3}{2}\text{ H}\_2\text{O} \tag{20}$$

Therefore, if a system contains both photocatalytic hydrogen generation (Eq. (15)) and sulfite generation (Eq. (20)) compartments and one coordinates to the other well, three molecules of H2 would be produced with the oxidation of one mol of S2– into SO3 2– through the thiosulfate cycle (Eq. (21)):

$$\text{S}^{2-} + 3\text{ H}\_2\text{O} + 10\,\text{h}\nu \rightarrow \text{SO}\_3^{2-} + 3\,\text{H}\_2\tag{21}$$

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 TiO2. Ideally, for the genera‐ tion of 1 mol of H2, 2 mol and 4/3 mol photons are needed to be absorbed by CdS and TiO2, 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.

**Figure 3.** Schematic illustration of H2S decomposition by two photosystems, linked through the S2O3 2–/S2–/SO3 2– redox system.

## **4.4. H2S decomposition in ethanolamine solution**

In addition to hydroxide alkaline solution, some other additives are introduced to promote the absorption of H2S in solution. For example, ethanolamine solution is frequently used in gas sweetening industry. Naman and Grätzel have dissolved H2S in aqueous solution of alkanolamines (including monoethanolamine (MEA), diethanolamine (DEA), and triethanol‐ amine (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 H2S (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 H2:

$$\text{H}\_2\text{HOCH}\_2\text{CH}\_2\text{NH}\_2 + \text{H}\_2\text{S} \tag{22}$$

$$\text{(HOCH}\_2\text{CH}\_2\text{NH}\_3\text{)}\_2\text{S} + \text{H}\_2\text{S} \Rightarrow \text{(HOCH}\_2\text{CH}\_2\text{NH}\_3\text{)}\_2\text{(SH)}\_2\tag{23}$$

Furthermore, Li et al. used anhydrous ethanolamine solution to absorb H2S [38]. Different from early report that system containing aqueous MEA outperforms that contains DEA and TEA, nonaqueous DEA solution is best for H2S decomposition with CdS-based photocatalyst. Such a system is also better than system with NaOH-Na2S solution from both the point of lifetime and rate for photocatalytic hydrogen evolution. In addition, the reduction of polysulfide in H2S-DEM system is effectively depressed and could hardly compete with the proton reduction, which commonly occurs in NaOH-Na2S system.

## **4.5. Extraction of elemental sulfur**

H2 would be produced with the oxidation of one mol of S2– into SO3

282 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

2 2

**Figure 3.** Schematic illustration of H2S decomposition by two photosystems, linked through the S2O3

In addition to hydroxide alkaline solution, some other additives are introduced to promote the absorption of H2S in solution. For example, ethanolamine solution is frequently used in gas sweetening industry. Naman and Grätzel have dissolved H2S in aqueous solution of alkanolamines (including monoethanolamine (MEA), diethanolamine (DEA), and triethanol‐ amine (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 H2S (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

2HOCH2CH2NH2 + H2S⇌ (HOCH2CH2NH3)2

S+H2S ⇌ (HOCH2CH2NH3)2

**4.4. H2S decomposition in ethanolamine solution**

(HOCH2CH2NH3)2

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 TiO2. Ideally, for the genera‐ tion of 1 mol of H2, 2 mol and 4/3 mol photons are needed to be absorbed by CdS and TiO2, 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

<sup>2</sup> 3 2 S 3 H O 10 SO + 3 H *hv* - - + +® (21)

cycle (Eq. (21)):

system till now.

system.

than that of H2:

2– through the thiosulfate

2–/S2–/SO3

S (22)

<sup>2</sup> (23)

(SH)

2– redox

Although numerous kinds of catalysts have been reported for the decomposition of H2S through the above-mentioned method and hydrogen indeed evolves from solution, one problem is that S2– 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 H2S, elemental S could possibly be precipitated from the system with a proper drop of pH:

$$\rm{S\_2O\_3^{2-}} + \rm{H\_2S} \rightarrow \rm{S+HS^-} + \rm{HSO\_3^-} \tag{24}$$

$$\rm S\_2^{2-} + H\_2S \to S + 2HS^- \tag{25}$$

In this regard, Linkous et al. have designed a circulating photoreactor for H2S decomposition (Figure 4) [39]. The feasibility of this system was conducted. In the photoreactor, hydroxide would be generated along with H2 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 H2S gas. For the fresh reaction solution constituted of both S2– and SO3 2–, S2O3 2– would be generated after photocatalysis, and pH must be lowered to 4.2 (by neutralization with H2S) for sulfur release from S2O3 2–. Then S could be collected as precipitates and the remaining solution (enriched with HS– and HSO3 – ) 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 S2–, 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 S2– or SO3 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 SO3 2– under such circumstances: as described above, SO3 2– are widely used in S2– 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, S2– alone as the electron donor for photo‐ catalytic 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.

**Figure 4.** Generalized scheme for light-driven H2S decomposition using an immobilized photocatalyst. Reprinted from reference [39], Copyright (1995), with permission from Elsevier.
