Probing the Nature of the Active Phase of Molybdenum-Supported Catalysts for the Direct Synthesis of Methylmercaptan from Syngas and H2S

Disentangling activity-stability trade-off in the catalytic degradation of malodorous sulfur-containing VOCs driven by active sites' self-dynamic evolution

The catalytic degradation of malodorous sulfur-containing volatile organic compounds (S-VOCs), especially methanethiol (CH3SH), faces an enormous challenge in striking a balance between activity and stability. Herein, we develop the time-tandem and spatial-extended strategy for synthesizing t-MoO3/meso-SiO2 nano-reactor-type catalysts and reveal the migration and transformation behaviors of both carbon and sulfur species at the mesoscopic scale to break the catalytic CH3SH activity and stability trade-off. The dynamic evolution of active centers from initial oxygen sites and acid sites to sulfur vacancies in MoS2 during the reaction process as well as the formation of a new dimethyl disulfide (CH3SSCH3) reaction pathway are identified as the main reason for the catalysts' superior activity and sulfur resistance. H2-TPR, XPS, Raman spectroscopy and other characterizations suggested that the final deactivation is transformed from the conventional sulfide and sulfate poisoning mechanism to MoS2 active phase destroying mechanism. Extending the micropore to mesopore also contributes to the high sulfur-resistance stability due to the greater ability to accommodate deposited elemental sulfur. Furthermore, t-MoO3/meso-SiO2 catalysts have confirmed the excellent performance in the catalytic degradation of dual-component thiols and CH3SH with water vapor present in the realistic environment, also substantiating its wide application potential in the catalytic degradation of S-VOCs.

Influence of Carbonization Conditions on Structural and Surface Properties of K-Doped Mo2C Catalysts for the Synthesis of Methyl Mercaptan from CO/H2/H2S

The cooperative transition of sulfur-containing pollutants of H2S/CO/H2 to the high-value chemical methyl mercaptan (CH3SH) is catalyzed by Mo-based catalysts and has good application prospects. Herein, a series of Al2O3-supported molybdenum carbide catalysts with K doping (denoted herein as K-Mo2C/Al2O3) are fabricated by the impregnation method, with the carbonization process occurring under different atmospheres and different temperatures between 400 and 600 °C. The CH4-K-Mo2C/Al2O3 catalyst carbonized by CH4/H2 at 500 °C displays unprecedented performance in the synthesis of CH3SH from CO/H2S/H2, with 66.1% selectivity and a 0.2990 g·gcat-1·h-1 formation rate of CH3SH at 325 °C. H2 temperature-programmed reduction, temperature-programmed desorption, X-ray diffraction and Raman and BET analyses reveal that the CH4-K-Mo2C/Al2O3 catalyst contains more Mo coordinatively unsaturated surface sites that are responsible for promoting the adsorption of reactants and the desorption of intermediate products, thereby improving the selectivity towards and production of CH3SH. This study systematically investigates the effects of catalyst carbonization and passivation conditions on catalyst activity, conclusively demonstrating that Mo2C-based catalyst systems can be highly selective for producing CH3SH from CO/H2S/H2.

Giant room-temperature nonlinearities in a monolayer Janus topological semiconductor

Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and certain topological properties, such as the Berry curvature and the quantum metric tensor, have attracted considerable interest. Here, we report giant room-temperature nonlinearities in non-centrosymmetric two-dimensional topological materials-the Janus transition metal dichalcogenides in the 1 T' phase, synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently show orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities in 1 T' MoSSe (e.g., > 50 times higher than 2H MoS2 for 18th order harmonic generation; > 20 times higher than 2H MoS2 for terahertz emission). We link this giant nonlinear optical response to topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and heralds the applications of topological materials in optoelectronics down to the monolayer limit.

2D Transition Metal Dichalcogenide with Increased Entropy for Piezoelectric Electronics

Piezoelectricity in 2D transition metal dichalcogenides (TMDs) has attracted considerable interest because of their excellent flexibility and high piezoelectric coefficient compared to conventional piezoelectric bulk materials. However, the ability to regulate the piezoelectric properties is limited because the entropy is constant for certain binary TMDs other than multielement ones. Herein, in order to increase the entropy, a ternary TMDs alloy, Mo_{1- x} W_x S₂ , with different W concentrations, is synthesized. The W concentration in the Mo_{1- x} W_x S₂ alloy can be controlled precisely in the low-supersaturation synthesis and the entropy can be tuned accordingly. The Mo_0.46 W_0.54 S₂ alloy (x = 0.54) has the highest configurational entropy and best piezoelectric properties, such as a piezoelectric coefficient of 4.22 pm V^-1 and a piezoelectric output current of 150 pA at 0.24% strain. More importantly, it can be combined into a larger package to increase the output current to 600 pA to cater to self-powered applications. Combining with excellent mechanical durability, a mechanical sensor based on the Mo_0.46 W_0.54 S₂ alloy is demonstrated for real-time health monitoring.

The nature of K-induced 2H and 1T’-MoS2 species and their phase transition behavior for the synthesis of methanethiol (CH3SH)

The one-step reaction approach from syngas with hydrogen sulfide (CO/H₂/H₂S) over potassium (K) promoted Molybdenum disulfide (MoS₂) materials can provide alternatives for the synthesis of methanethiol (CH₃SH). However, the direct confirmation and determination of the real active nature of K-induced 2H and 1T′-MoS₂ for this reaction and the corresponding phase transformation behavior and origin of K-induced 2H-MoS₂ from/to 1T′-MoS₂ remains unclear. Herein, we proved at the atomic level the precise position of K over 1T′-MoS₂ and 2H-MoS₂ species using the technique of HAADF-STEM. A relationship between K-induced 1T′ and 2H-MoS₂ phases and the catalytic property to synthesize CH₃SH was established, and K-intercalated 1T′-MoS₂ phase was confirmed to have excellent catalytic performances. Moreover, the behavior, origin, and influencing factors of phase transformation of 2H-MoS₂ from/to 1T′-MoS₂ in the existence of K were well proved.

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Converting sulfur-containing pollutants with syngas (CO/H₂/H₂S) is promising

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Thermally induced K-position-dependent phase transformation of MoS₂ was reported

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The precise position of K over MoS₂ was proved at the atomic level

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Relationship between MoS₂ phases and property for synthesizing CH₃SH was established

Synthesis of Higher Alcohols from Syngas over a K-Modified CoMoS Catalyst Supported on Novel Powder and Fiber Commercial Activated Carbons

In the present study, a series of K-modified CoMoS catalysts with compositions of 10% K, 3.6% Co, and 12 wt % Mo supported over novel commercial activated carbons such as powder materials (DAC and OBC-1) and fiber materials (fabric active sorption (TCA) and nonwoven activated material (AHM)) were prepared and characterized by Brunauer-Emmett-Teller (BET), X-ray fluorescence (XRF), scanning electron microscopy (SEM), SEM-energy dispersive X-ray (EDX), and transmission electron microscopy (TEM). The catalytic activities for higher alcohol synthesis from syngas, conducted at T = 300-360 °C, P = 5 MPa, GHSV = 760 L h^-1 (kg cat)^-1, and H₂/CO = 1.0, were investigated. Cat-TCA and Cat-AHM have shown a filamentous morphology with a strip axial arrangement and that a few longitudinal grooves and many irregular particles are distributed on the fiber surfaces. The degree of entanglement of the strip axial arrangement in AHM was found to be more than that in TCA, thus leading to form tangled MoS₂ slabs on AHM and long linear slabs on TCA with long rim sites. The obtained results revealed that the CO conversion increases in the order Cat-TCA < Cat-OBC-1 < Cat-DAC < Cat-AHM. Ethanol, propanol-1, and methanol are the most predominant alcohol products in the collected liquid products, with the byproducts containing mainly butanol-1, isobutanol, amyl alcohol, and isoamyl alcohol. Cat-DAC and Cat-OBC-1 show higher selectivity toward C₃₊, C₄₊, propanol-1, butanol-1, isobutanol, and amyl alcohol-1 than Cat-TCA and Cat-AHM. For powdered activated carbons, microporous catalysts inhibited isomerization because the catalyst that contains the highest micropores (Cat-DAC) produced a considerable amount of linear alcohols compared with Cat-OBC-1.

Role of Zirconia in Oxide-Zeolite Composite for Thiolation of Methanol with Hydrogen Sulfide to Methanethiol

In this paper, the molecular sieve NaZSM-5 was modified with zirconium dioxide (ZrO₂) by a hydrothermal coating process and other methods. By comparing the effects of the crystal phase structure of ZrO₂ and the compositing method on the physicochemical properties and catalytic performance of the obtained composites, the structure–performance relationship of these composite catalysts was revealed. The results indicate that in the hydrothermal system used for the preparation of NaZSM-5, Zr⁴⁺ is more likely to dissolve from m-ZrO₂ than from t-ZrO₂, which can subsequently enter the molecular sieve, causing a greater degree of desiliconization of the framework. The larger specific surface area (360 m²/g) and pore volume (0.52 cm³/g) of the m-ZrO₂/NaZSM-5 composite catalyst increase the exposure of its abundant acidic (0.078 mmol/g) and basic (0.081 mmol/g) active centers compared with other composites. Therefore, this catalyst exhibits a shorter induction period and better catalytic performance. Furthermore, compared with the impregnation method and mechanochemical method, the hydrothermal coating method produces a greater variety of acid–base active centers in the composite catalyst due to the hydrothermal modifying effect.

Synthesis of Methyl Mercaptan on Mesoporous Alumina Prepared with Hydroxysafflor Yellow A as Template: The Synergistic Effect of Potassium and Molybdenum

K-promoted Mo-based catalysts showed great promise for the hydrogenation of CS2 to methyl mercaptan (CH3SH). However, the research on the synergistic effect of K and Mo, and the active site of CS2 hydrogenation to CH3SH were unexplored widely. To solve this problem, the synergistic effect of K and Mo in the K-promoted Mo-based catalysts for CS2 hydrogenation to prepare CH3SH was investigated. The mesoporous alumina was the support and loaded the active components potassium and molybdenum to prepare the catalyst. The results suggested that the active components K and Mo can not only cooperatively regulate the acid-base sites on the catalyst surface, but also stabilize the molybdate species at +5 valence during the reduction process and increase the Mo unsaturated coordination sites. Combined with the results of the catalytic activity evaluation, indicating that the main active site of the catalysts is the weak Lewis acid-base site, and the strong acidic site and strong alkaline site are not conducive to the formation of CH3SH. Moreover, the possible catalytic mechanism of CS2 hydrogenation to CH3SH on the weak Lewis acid-base sites of the catalysts was proposed. The research results of this paper can provide an experimental basis and theoretical guidance for the design of high-performance CH3SH synthesis catalyst and further mechanism research.

Unraveling the Role of Lithium in Enhancing the Hydrogen Evolution Activity of MoS2: Intercalation versus Adsorption

Molybdenum disulfide (MoS₂) is a highly promising catalyst for the hydrogen evolution reaction (HER) to realize large-scale artificial photosynthesis. The metallic 1T'-MoS₂ phase, which is stabilized via the adsorption or intercalation of small molecules or cations such as Li, shows exceptionally high HER activity, comparable to that of noble metals, but the effect of cation adsorption on HER performance has not yet been resolved. Here we investigate in detail the effect of Li adsorption and intercalation on the proton reduction properties of MoS₂. By combining spectroscopy methods (infrared of adsorbed NO, ⁷Li solid-state nuclear magnetic resonance, and X-ray photoemission and absorption) with catalytic activity measurements and theoretical modeling, we infer that the enhanced HER performance of Li _x MoS₂ is predominantly due to the catalytic promotion of edge sites by Li.

Phase-selective synthesis of 1T' MoS2 monolayers and heterophase bilayers

Two-dimensional (2D) MoS₂, which has great potential for optoelectronic and other applications, is thermodynamically stable and hence easily synthesized in its semiconducting 2H phase. In contrast, growth of its metastable 1T and 1T' phases is hampered by their higher formation energy. Here we use theoretical calculations to design a potassium (K)-assisted chemical vapour deposition method for the phase-selective growth of 1T' MoS₂ monolayers and 1T'/2H heterophase bilayers. This is realized by tuning the concentration of K in the growth products to invert the stability of the 1T' and 2H phases. The synthesis of 1T' MoS₂ monolayers with high phase purity allows us to characterize their intrinsic optical and electrical properties, revealing a characteristic in-plane anisotropy. This phase-controlled bottom-up synthesis offers a simple and efficient way of manipulating the relevant device structures, and provides a general approach for producing other metastable-phase 2D materials with unique properties.

Molecular approach to prepare mixed MoW alumina supported hydrotreatment catalysts using H4SiMonW12−nO40 heteropolyacids

Higher catalytic conversions and different selectivity ratios are explained by the formation of the mixed (MoW)S₂ active phase when using mixed MoW heteropolyacid as starting material.

Investigation of the reaction pathway for synthesizing methyl mercaptan (CH3SH) from H2S-containing syngas over K–Mo-type materials

The reaction pathway for synthesizing methyl mercaptan (CH₃SH) using H₂S-containing syngas (CO/H₂S/H₂) as the reactant gas over SBA-15 supported K–Mo-based catalysts prepared by different impregnation sequences was investigated. The issue of the route to produce CH₃SH from CO/H₂S/H₂ has been debated for a long time. In light of designed kinetic experiments together with thermodynamics analyses, the corresponding reaction pathways in synthesizing CH₃SH over K–Mo/SBA-15 were proposed. In the reaction system of CO/H₂S/H₂, COS was demonstrated to be generated firstly via the reaction between CO and H₂S, and then CH₃SH was formed via two reaction pathways, which were both the hydrogenation of COS and CS₂. The resulting CH₃SH was in a state of equilibrium of generation and decomposition. Decomposition of CH₃SH was found to occur via two reaction pathways; one was that CH₃SH first transformed into two intermediates, CH₃SCH₃ and CH₃SSCH₃, which were then further decomposed into CH₄ and H₂S; another was the direct decomposition of CH₃SH into C, H₂S and H₂. Moreover, the catalyst (K–Mo/SBA-15) prepared with co-impregnation exhibits higher catalytic activities than the catalysts (K/Mo/SBA-15 and Mo/K/SBA-15) prepared by the sequence of impregnation. Based on characterization of the oxidized, sulfided and spent catalysts via N₂ adsorption–desorption isotherms, XRD, Raman, XPS and TPR, it was found that two K-containing species, K₂Mo₂O₇ and K₂MoO₄, were oxide precursors, which were then converted into main K-containing MoS₂ species. The CO conversion was closely related to the amount of edge reactive sulfur species that formed the sulfur vacancies over MoS₂ phases.

The reaction pathway for synthesizing methyl mercaptan (CH₃SH) using H₂S-containing syngas (CO/H₂S/H₂) as the reactant gas over SBA-15 supported K–Mo-based catalysts.

On the role of the alkali cations on methanol thiolation

The electronegativity effect of the alkali cations on the formation of methanethiol by reaction of methanol and H₂S was studied with K⁺, Rb⁺, and Cs⁺ supported on γ-Al₂O₃.

Minireview: direct catalytic conversion of sour natural gas (CH4 + H2S + CO2) components to high value chemicals and fuels

Direct sour natural gas catalytic conversion allows to obtain high value products, such as hydrocarbon and organosulfur chemicals, fuels and fertilizers.

Layer-dependent catalysis of MoS2/graphene nanoribbon composites for efficient hydrodesulfurization

Graphene nanoribbons (GNRs) with a defect-rich structure facilitate the preparation of a single-layer MoS₂/GNR composite which shows superior catalytic performance than SL-MoS₂/GS.

The effects of the Mo–Sn contact interface on the oxidation reaction of dimethyl ether to methyl formate at a low reaction temperature

With increasing degrees of Mo–Sn interface contact, the molar ratio of methyl formate (MF) to methanol (MeOH) and formaldehyde (FA) was found to linearly increase simultaneously.