**1. Introduction**

Hydrogen sulfide (H2S) is an extremely toxic and corrosive gas with an odor of rotten eggs. Usually, H2S 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 H2S 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 H2S. As a matter of fact, the H2S content of sour natural

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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 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].

$$2\,\mathrm{H}\_{2}\mathrm{S} + \mathrm{O}\_{2} \to 2\,\mathrm{S} + 2\,\mathrm{H}\_{2}\mathrm{O}\tag{1}$$

$$210\,\mathrm{H\_2S} + 5\,\mathrm{O\_2} \to 2\,\mathrm{H\_2S} + \mathrm{SO\_2} + 7\,\mathrm{S} + 8\,\mathrm{H\_2O}\tag{2}$$

$$2\,\mathrm{H}\_{2}\mathrm{S} + \mathrm{SO}\_{2} \rightarrow 3\,\mathrm{S} + 2\,\mathrm{H}\_{2}\mathrm{O}\tag{3}$$

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 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 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 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 couples such as I3−/I<sup>−</sup> and Fe3+/Fe2+ are also introduced for indirect electrolysis of H2S. 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 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‐ chemical method is the use of electricity.

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 worldwide.
