**6. Conclusion**

2+ + 3+ Anode: Fe + 2 Fe *h* ® (28)

<sup>+</sup> Or anode: 3 I + 2 I3 *h* - - ® (30)

<sup>+</sup> Cathode: 2 H + 2 H2 *e*- ® (32)


Overall reaction: H S + 2 H + S 2 2 *hv* ® (33)

<sup>+</sup> +® + (29)

3+ <sup>2</sup> <sup>+</sup> H S 2 Fe 2 Fe 2 H + S (chemical reaction) <sup>2</sup>

286 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

<sup>+</sup> H S I 3 I 2 H + S (chemical reaction) 2 3

Control experiment shows that if there is no existence of Fe2+ or I– in the electrolyte, such experiment is unsuccessful due to the low solubility of H2S 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 Fe2+ and I– could be easily oxidized on it. Nevertheless, due to the

Notably, they further developed this indirect strategy in PEC cell and have made H2O2 and S from H2S in the presence of oxygen [57]. This is quite novel because most study related to H2S decomposition is limited to H2 as the only reduced product now. In addition to the redox

anthraquinone/anthrahydroquinone (AQ/H2AQ) was introduced to the cathode cell for H2O2 production. In fact, AQ is also an important reaction substrate in Hysulf process, one indirect strategy related to the thermal decomposition of H2S. The anode reaction is still the same as Eqs. (30 and 31), but the cathode reaction and the overall reaction change as follows (Eqs.

At zero bias, Pt/p+ n Si photoanode and Pt cathode can simultaneously oxidize I–

reduce AQ to H2AQ, 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.

– in the anode compartment of the cell for S production, another redox couple

<sup>+</sup> Cathode: 2 H + AQ + 2 H AQ <sup>2</sup> *e*- ® (34)

H AQ + O AQ + H O (chemical reaction) 2 2 ® 2 2 (35)

Overall reaction: H S + O + 2 H O + S 2 2 2 2 *hv* ® (36)

 to I3 – and

low stability of the n-type Si anode, further study in this report is unclear.

couple I–

(34–36)):

/I3

In general, H2S 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, H2. The photochemical decomposition of H2S, which emerges with the rise of photocatalysis in the last century, could be one improved method for H2S disposal. Lots of progress in the field of the photochemical decomposition of H2S 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 H2S is indirectly carried out in the form of photocatalytic H2 production from aqueous sulfide solution. Details of the photochemical decomposition of H2S, 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 H2S conversion: photoelectrochem‐ ical cells were built to extract H2 (or H2O2) and S from H2S with the assistance of redox couples.

In 2009, Li et al. reported CdS loaded with PdS and Pt dual cocatalyst can effectively generate H2, with a quantum yield of 93% at 420 nm in the presence of S2–/SO3 2– solution and no deactivation was observed within illumination of 100 h for H2 generation [58]. This is probably the most efficiency system reported relevant to the photocatalytic decomposition of H2S 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 H2S in large scale chemical processing. In present, problems below may be considered in priority:

In gaseous phase systems, the concentration studied for H2S 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 S2– to SO4 2– or H2 generation). This is especially true in solution phase system with S2–/SO3 2– or S2– 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 H2S is not persuasive. Moreover, in solution phase system for H2S decomposition, along with H2 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 H2S. Although systems have been designed for sulfur generation from polysul‐ fide 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 H2S decomposition are mainly metal sulfide loading with noble metal cocatalyst like Pt, RuO2, and so on. Although CdS is consid‐ ered one of the most efficient photocatalyst for H2 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 H2S decomposition, stability and cost could be big problems, too. To conclude, the photochemical decomposition of H2S is still in a relatively early stage. New photocatalytic H2S 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 highlight‐ ed in nanoscience and technology. Moreover, people should keep in mind that oxidation and reduction of H2S is equally important for H2S decomposition if we want to handle H2S in a really green way.
