Photocatalytic degradation of H₂S aqueous media using sulfide nanostructured solid-solution solar-energy-materials to produce hydrogen fuel

Highlights

• Synthesis of alloy photocatalyst nanoparticles through a facile hydrothermal route.

• Simultaneous photo-degradation of H₂S solutions and production of hydrogen fuel.

• HS⁻ plays a crucial role in photo-redox processes, serves as e/h scavenger species.

• Mechanism of pH influence on the photocatalyst performance in H₂S media.

Abstract

H₂S is a corrosive, flammable and noxious gas, which can be neutralized by dissolving in alkaline media and employed as H₂-source by utilizing inside semiconductor-assisted/photochemical reactors. Herein, through a facile hydrothermal route, a ternary nanostructured solid-solution of iron, zinc and sulfur was synthesized in the absence and presence of Ag-dopant, and applied as efficient photocatalyst of hydrogen fuel production from H₂S media. The effect of pH on the photocatalyst performance was scrutinized and the maximum activity was attained at pH = 11, where HS⁻ concentration is high. BET, diffuse reflectance and photoluminescence studies indicated that the ternary solid-solution photocatalyst, in comparison to its solid-solvent (ZnS), has a greater surface area, stronger photon absorption and less charge recombination, which justify its superiority. Moreover, the effect of silver-dopant on the photocatalyst performance was examined. The investigations revealed that although silver could boost the absorption of photons and increase the surface area, it could not appreciably enhance the photocatalyst performance due to its weak influence on retarding the charge-recombination process. Finally, the phenomenon was discussed in detail from mechanistic viewpoint.

Introduction

Hydrogen sulfide (H₂S) is a toxic, corrosive (Eq. (1)) and flammable (Eq. (2)) gas [1], [2], [3], [4], [5]; its emission to the environment is harmful and could be lethal at concentrations above 500 ppm [6]. This hazardous material has the stench of rotten eggs, and can be easily produced by acidification of sulfide media (Eq. (3)) [7], [8].

$${\text{H}}_2\text{S}+\text{Fe}\to \text{FeS}+{\text{H}}_2$$

$${\text{H}}_2\text{S}+\frac{3}{2}{\text{O}}_2\to {\text{SO}}_2+{\text{H}}_2\text{O}$$

$${\text{S}}^{2-}{\displaystyle \stackrel{{\text{H}}^{+}}{\leftrightarrow }}{\text{HS}}^{-}{\displaystyle \stackrel{{\text{H}}^{+}}{\leftrightarrow }}{\text{H}}_2\text{S}$$

This noxious gas is biologically generated via microbial activities of sulfur reducing bacteria (SRB), by degradation of sulfur-containing compounds in sewage environments [9]. Hydrogen sulfide is also recognized as by-product of sour gas and oil, which is the main cause of severe corrosion and hydrogen damage/embrittlement in petroleum and gas industries [10], [11]. Therefore, the deactivation of this noxious/devastating gas is not only important from environmental but also crucial from corrosion as well as industrial standpoints. Beside these aspects, the elimination of H₂S is highly interesting from energy/fuel perspective; here, by photocatalytic decomposition of H₂S solutions, without needing external hole-scavenger additives, hydrogen fuel can be effectually produced [12], [13] upon appropriate semiconductor photocatalyst/solar-energy materials [14]. The method of photocatalytic utilization of H₂S media is deemed to be economical and efficient [15], [16], [17]. This is because, a vast amount of H₂S gas is annually generated during various industrial/natural processes, including hydrodesulfurization of crude oil, acid leaching of sulfide ores, sewage treatment, geothermal activities, etc. [18], [19], [20]. Therefore, using low-cost, efficient, solar-energy materials, this hazardous feed could be economically/effectually converted into hydrogen clean fuel.

Concerning photocatalytic production of hydrogen fuel, it is worth noting that zinc sulfide (ZnS) is a non-toxic, photostable, semiconductor material with a proper (high) conduction-band energy-level [21], [22]; this is a reason why ZnS is usually selected as a good photocatalyst base for photochemical reduction of protons and production of hydrogen solar fuel [23], [24]. This compound (E_g ∼ 3.5 eV [25]) however is only active in the UV region–which forms a small portion of sunlight (∼5% [26]). To absorb photons in the visible region (∼43% of sunlight [26]), therefore, it is crucial to decrease the bandgap of the semiconductor/solar-energy material without noticeable lowering its conduction-band energy-level [27]. To this end, one of the effective methods is the application of dopant to synthesize a solid solution or alloy semiconductor material [24], [27]. For ZnS semiconductor, the use of cadmium [28], indium [29], and copper [30] alloying components has been reported in the literature; albeit the toxicity as well as the abundancy of dopants should also be taken into account.

In this paper, we focused on Fe dopant and synthesized the ternary $\text{F}{\text{e}}_{0.2}^{\left(\text{III}\right)}\text{Z}{\text{n}}_{0.7}^{\left(\text{II}\right)}\text{S}$ solid-solution (IZS) through a facile hydrothermal route [24], and applied it for the first time as an effective photocatalyst of hydrogen fuel production from H₂S aqueous media. We synthesized $\text{F}{\text{e}}_{0.2}^{\left(\text{III}\right)}\text{Z}{\text{n}}_{0.7}^{\left(\text{II}\right)}\text{S}$ because this composition exhibited the greatest extent of photon absorption [31]. Concerning the selection of iron dopant, it is also worth noting that Fe is an eco-friendly abundant element which can combine with sulfur and produce FeS. This compound is relatively stable (K_sp = 8 × 10⁻¹⁹) and being industrially applied as a high temperature (550–900 °C) catalyst of H₂S decomposition to produce hydrogen gas [32]:

$$\text{FeS}+{\text{H}}_2\text{S}{\displaystyle \stackrel{\Delta }{\to }}{\text{FeS}}_2+{\text{H}}_2$$

In the present work, instead of oxide semiconductor materials, we employed sulfide one. The choice was based on the ability of sulfide materials to form a chemical bond with proton species (Eq. (3)), which in turn could result in the facilitation of electron transport from the photoexcited semiconductor solar-energy-material to proton species on the photocatalyst surface [14].

The effect of pH of medium on the photocatalyst performance was another interesting issue, which was scrutinized in this article. Due to the ability of silver atoms to exhibit surface Plasmon resonance and enhance the photocatalyst performance [27], [4], we also examined the effect of silver on the photocatalyst activity.