Low Temperature Catalytic Decomposition of Hydrogen Sulfide into Hydrogen and Diatomic Gaseous Sulfur

Obtaining elemental sulfur for Martian sulfur concrete

A potential candidate material for the construction of Mars habitats is concrete made from the Martian regolith and sulfur extracted from the regolith itself. Sulfur concrete, which has excellent mechanical properties, can be prepared at a low temperature (<150 °) and without water (unlike Portland-cement concrete). The surface of Mars has a much higher concentration of sulfur than those of the Earth, the Moon, or the asteroids. Sulfur on Mars, however, exists not as elemental sulfur—which is needed in concrete production—but as sulfates (usually hydrated) and sulfides. This paper surveys thermochemical and electrochemical methods that might be used to produce elemental sulfur from its compounds contained in the minerals on Mars. Possible methods include chemical or electrochemical oxidation or decomposition of sulfides, which include sulfides that exist naturally on Mars as well as sulfides that are produced via chemical or electrochemical reduction of sulfates. Some of the methods to obtain elemental sulfur—such as chemical or electrochemical oxidation or decomposition of metal sulfides or hydrogen sulfide—have already been demonstrated. The methods of producing elemental sulfur from sulfur-containing minerals on Mars will have the added benefit of generating byproducts (e.g. water, hydrogen, oxygen, and metals) that are useful for explorations of the Red Planet. In the future, chemical processes for the production of elemental sulfur may also have important industrial applications on Earth.

Exploring the Reaction Mechanism of H2S Decomposition with MS3 (M = Mo, W) Clusters

H₂S is abundantly available in nature, and it is a common byproduct in industries. Molybdenum sulfides have been proved to be active in the catalytic decomposition of hydrogen sulfide (H₂S) to produce hydrogen. In this study, density functional theory (DFT) calculations are carried out to explore the reaction mechanisms of H₂S with MS₃ (M = Mo, W) clusters. The reaction mechanism of H₂S with MoS₃ is roughly the same as that of the reaction with WS₃, and the free-energy profile of the reaction with MoS₃ is slightly higher than that of the reaction with WS₃. The overall driving forces (-ΔG) are positive, and the overall reaction barriers (ΔG _b) are rather small, indicating that such H₂ productions are product-favored. MS₃ (M = Mo, W) clusters have clawlike structures, which have electrophilic metal sites to receive the approaching H₂S molecule. After several hydrogen-atom transfer (HAT) processes, the final MS₄·H₂ (IM-4) complexes are formed, which could desorb H₂ at a relatively low temperature. The singlet product MS₄ clusters contain the singlet S₂ moiety, similar to the adsorbed singlet S₂ on the surface of sulfide catalysts. The theoretical results are compared with the experiments of heterogeneous catalytic decomposition of H₂S by MoS₂ catalysts. Our work may provide some insights into the optimal design of the relevant catalysts.

Syn-gas from waste: the reduction of CO2 with H2S

In this paper, we demonstrate that syngas (H₂/CO) can be produced from oil waste (H₂S/CO₂) forming SO₂ and S as secondary products at 600–800 °C in a flow reactor set-up.

A vapor-phase-assisted growth route for large-scale uniform deposition of MoS2 monolayer films

In this work a vapor-phase-assisted approach for the synthesis of monolayer MoS₂ is demonstrated, based on the sulfurization of thin MoO_3-x precursor films in an H₂S atmosphere. We discuss the co-existence of various possible growth mechanisms, involving solid-gas and vapor-gas reactions. Different sequences were applied in order to control the growth mechanism and to obtain monolayer films. These variations include the sample temperature and a time delay for the injection of H₂S into the reaction chamber. The optimized combination allows for tuning the process route towards the potentially more favorable vapor-gas reactions, leading to an improved material distribution on the substrate surface. Raman and photoluminescence (PL) spectroscopy confirm the formation of ultrathin MoS₂ films on SiO₂/Si substrates with a narrow thickness distribution in the monolayer range on length scales of a few millimeters. Best results are achieved in a temperature range of 950-1000 °C showing improved uniformity in terms of Raman and PL line shapes. The obtained films exhibit a PL yield similar to mechanically exfoliated monolayer flakes, demonstrating the high optical quality of the prepared layers.

First-principles investigation of H2S adsorption and dissociation on titanium carbide surfaces

The adsorption and dissociation reactions of H₂S on TiC(001) are investigated using first-principles density functional theory calculations. The geometric and electronic structures of the adsorbed S-based species (including H₂S, SH and S) on TiC(001) are analyzed in detail. It is found that the H₂S is bound weakly, while SH and atomic S are bound strongly on the TiC(001) surface. The transition state calculations show that the formation of SH from H₂S (H₂S → SH + H) is very easy, while the presence of a co-adsorbed H will inhibit the further dissociation of SH (SH + H → S + H + H). In contrast, the hydrogenation of the adsorbed SH is rather easy (SH + H → H₂S). Therefore, the dissociative SH can be removed via the hydrogenation reaction. It is concluded that it is difficult for H₂S to dissociate completely to form atomic S and poison the TiC surface. The results will further provide understanding of the mechanism of the sulfur tolerance of the TiC anode of proton exchange membrane fuel cells (PEMFCs).