Preparation of MoS2 and WS2 catalysts by in situ decomposition of ammonium thiosalts

The in situ decomposition of ammonium thiometallates during the hydrodesulfurization (HDS) of dibenzothiophene (DBT), to obtain molybdenum disulfide and tungsten disulfide catalysts, was investigated. It was found that very efficient catalysts for the HDS of DBT were obtained by in situ decomposition. Mechanical uniaxial pressing of the precursors (ammonium thiometallates) affected both textural and catalytic properties of the catalysts. Surface areas of molybdenum and tungsten disulfides increased as a function of uniaxial pressing, while catalytic activities went through a maximum. For MoS₂, the hydrogenation selectivity was much higher for in situ catalysts than for ex situ ones. For WS₂ catalysts, the hydrogenation selectivity was less sensitive to the condition of decomposition (ex situ/in situ). The surface S/M (M = Mo, W) atomic ratio from the Auger signal decreased as a function of uniaxial pressing, while the C/M ratio remained almost constant at 1.6. The best catalyst showed an experimental S/Mo ratio that is slightly higher than the stoichiometric value. The effect of in situ decomposition and mechanical deformation of thiometallate precursors is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ferromagnetism in freestanding MoS2 nanosheets

Freestanding MoS₂ nanosheets with different sizes were prepared through a simple exfoliated method by tuning the ultrasonic time in the organic solvent. Magnetic measurement results reveal the clear room-temperature ferromagnetism for all the MoS₂ nanosheets, in contrast to the pristine MoS₂ in its bulk form which shows diamagnetism only. Furthermore, results indicate that the saturation magnetizations of the nanosheets increase as the size decreases. Combining the X-ray photoelectron spectroscopy, transmission electron microscopy, and electron spin resonance results, it is suggested that the observed magnetization is related to the presence of edge spins on the edges of the nanosheets. These MoS₂ nanosheets may find applications in nanodevices and spintronics by controlling the edge structures.     

Exfoliated MoS2 catalysts in coal liquefaction

A technique is described for the application of MoS₂ to coal particles. The method was used to determine the importance of several parameters to the performance of this direct coal liquefaction catalyst. Improved performance was obtained when MoS₂ was intercalated with lithium, then exfoliated in a mixture of tetrahydrofuran and water in the presence of coal. Performance was compared by subjecting dried coal/catalyst mixtures to uniform microautoclave liquefaction tests. Measurements of coal conversion and hydrogen consumption show that a combination of reduction of the MoS₂ stacking and improved coal/catalyst dispersion is beneficial.     

Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices

Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS₂ monolayer using the first principles approach. The results are compared with those of graphene/MoS₂ superlattice. The distortions of the geometry of germanene, silicene, and MoS₂ layers due to the formation of the superlattices are all relatively small, resulting from the relatively weak interactions between the stacking layers. Our results show that both the germanene/MoS₂ and silicene/MoS₂ superlattices are manifestly metallic, with the linear bands around the Dirac points of the pristine germanene and silicene seem to be preserved. However, small band gaps are opened up at the Dirac points for both the superlattices due to the symmetry breaking in the germanene and silicene layers caused by the introduction of the MoS₂ sheets. Moreover, charge transfer happened mainly within the germanene (or silicene) and the MoS₂ layers (intra-layer transfer), as well as some part of the intermediate regions between the germanene (or silicene) and the MoS₂ layers (inter-layer transfer), suggesting more than just the van der Waals interactions between the stacking sheets in the superlattices.     

Hydrothermal synthesis of orthorhombic LiMnO2 nano-particles

Orthorhombic LiMnO₂ nano-particles were successfully prepared under mild hydrothermal conditions from KMnO₄ and MnCl₂ sources. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the LiMnO₂ particles. The influence of the molar ratio of Li/Mn and of the holding temperature was studied. The synthesized nano-particles were found to be highly crystallized with a diameter of 50-100 nm.     

High activity Ni/MoS2 catalysts obtained from alkylthiometalate mixtures for the hydrodesulfurization of dibenzothiophene

An efficient method for improving the catalytic properties of unsupported Ni/MoS₂ catalysts is mixing thiometalate precursors applying the appropriate precursors and thermal conditions. High active catalysts for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) are prepared by the controlled decomposition of physical mixtures of Ni(diethylentriamine)₂MoS₄ (NDTA-TM) and [(Propyl)₄N)]₂MoS₄ (TPA-TM). The catalysts with a higher content of NDTA-TM are very active with a high selectivity for the direct desulfurization pathway (DDS) due to the synergistic effect of nickel. In addition the presence of a large amount of carbon may produce single-slabs of nickel promoted carbon containing molybdenum sulfides. The activity enhancement is attributed to an increased number of NiMoS active sites originated by the chemical interaction between the precursors NDTA-TM and TPA-TM during the mixing procedure. Furthermore, the carbon content in the final products is related to the enhancement of the activity and the preference of the DDS pathway. The controlled decomposition of mixtures of NDTA-TM + TPA-TM yields catalysts which are about twofold more active than an industrial NiMo/Al₂O₃ catalyst. This improvement may be attributed to an intense interaction of the precursors during the synthesis causing a re-dispersion of nickel atoms from NDTA-TM over the surface of carbon containing molybdenum sulfide provided by the precursor TPA-TM, increasing the amount of active sites. The catalysts from mixtures of (NH₄)₂MoS₄ (A-TM) and NDTA-TM behave similarly to the pure precursors.     

Deactivation of MoS2 catalysts during the HDS of thiophene

Catalytic properties of unsupported MoS₂ catalysts in the thiophene hydrodesulfurization reaction were determined in the temperature range 623–653 K. The catalysts were prepared by ex situ decomposition of ammonium thiomolybdate (ATM) crystals in a mixture of 15% H₂S in H₂ at 673 K. Activity of catalysts decreased very rapidly before reaching a steady state after 15 h on‐stream. The thiophene conversion went down from 10–12 to 3–4% in that time. The surface area of the catalysts also decreased during the catalytic reaction from 40–50 to 8–10 m²/g. Selectivity for hydrodesulfurization, hydrogenation and isomerization reactions was affected distinctly by the deactivation process. By increasing the reaction time, double‐bond isomerization increased, hydrogenation of butenes decreased and hydrodesulfurization remained constant. Results indicate that the main cause of activity decay was surface area loss that was due to sintering of MoS₂ crystallites. Selectivity variation indicates that different active sites are involved for the three reactions. A deactivation model involving diminution of active sites located in edge and rim sites of small MoS₂ particles is proposed to explain the variation of product distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrothermal synthesis of binary Ru-Ti oxides with excellent performances for supercapacitors

Hydrous, crystalline, binary (Ru-Ti)O₂·nH₂O with compositions equal to the ratios of metallic ions in the precursor solutions are successfully synthesized by a mild hydrothermal process. The maximum utilization of RuO₂·nH₂O (ca. 793 F/g) occurs at the composition of 60 M% TiO₂·nH₂O although phase separation is clearly found for this TiO₂-enriched binary oxides. The nano-structured architecture with a high BET surface area (ca. 253 m²/g) of the hydrothermal-derived (Ru-Ti)O₂·nH₂O with annealing at 200 °C favors the physical adsorption of water and maintains a high water content which is novel and never found before. Due to this novel nanostructure, the annealed (Ru-Ti)O₂·nH₂O synthesized by means of the hydrothermal process exhibits excellent performances (i.e., high utilization of RuO₂, high power property, and long cycle life) for supercapacitors.     

Hydrothermal synthesis and electrochemical behavior of orthorhombic LiMnO2

The most highlighted point of this work to emphasize is that it is the first trial to use Mn₃O₄ oxide as a precursor to synthesize orthorhombic LiMnO₂ by the hydrothermal method. A well-ordered orthorhombic LiMnO₂ phase was formed by the hydrothermal treatment of Mn₃O₄ with excess LiOH aqueous solution at 170 °C. According to TEM observation, the as-synthesized powder was single crystalline particle oxide. Comparing with other orthorhombic LiMnO₂ prepared by low temperature synthetic route and by high temperature calcination, the orthorhombic LiMnO₂ prepared by the hydrothermal route showed enhanced battery performance as a lithium battery cathode material. We believe that the new hydrothermal synthesis is expected as an excellent alternative of powder preparation method of high capacity cathode material to be used for Li-ion secondary battery.     

Modifications of Unpromoted and Cobalt-Promoted MoS2 During Thermal Treatment by Dimethylsulfide

Previous results have shown that the active surface in stabilized active sulfide catalysts is carbided. This fact led us to reconsider the influence of organosulfide agents in the activation of hydrotreatment catalysts. Structural and morphological consequences of dimethylsulfide treatment of (Co)/MoS₂-based solids were studied. Results suggest that the electronic promotion of Mo by Co substantially influences the carbon replacement of sulfur atoms at the edges of molybdenum sulfide layers.     

Catalytic Activity of Exfoliated MoS2 in Hydrodesulfurization, Hydrodenitrogenation and Hydrogenation Reactions

The activity of exfoliated MoS₂ in the hydrodesulfurization (HDS) of dibenzothiophene, the hydrodenitrogenation (HDN) of carbazole and the hydrogenation of naphthalene has been determined. The catalytic activity was compared to MoS₂ prepared by the decomposition of molybdenum naphthenate (MoNaph). Exfoliated MoS₂ was found to give better overall HDS activity compared to MoNaph derived MoS₂ catalyst, whereas MoNaph derived MoS₂ was found to give higher hydrogenation and HDN activity. These results are discussed in terms of the morphology of the two catalysts. The relative activity of the two catalysts in the hydrotreating reactions is shown to be different to that obtained during Cold Lake bitumen hydrocracking.     

Hydrothermal assisted synthesis and hot-corrosion resistance of nano lanthanum zirconate particles

The nano La₂Zr₂O₇ (LZ) particles with pyrochlore microstructure were successfully synthesized from a mixture of La(NO₃)₃, Zr(NO₃)₄ and C₁₉H₄₂BrN (CTAB) using hydrothermal assisted (HTA) synthesis which consists of two steps: hydrothermal treatment and calcination. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR) and X-ray diffraction (XRD) spectroscopy were employed to study morphologies and phase compositions. The results suggest that HTA process led to very rapid synthesis of nano LZ compared to the conventional solid reaction process. The particles produced by HTA synthesis have cubic shape and the distribution of its grain size is from 10 nm to 30 nm. The present work demonstrates that the nano La₂Zr₂O₇ produced via HTA synthesis which have better hot-corrosion resistance is an ideal material for thermal barrier coatings.     

Hydrothermal Syntheses and Catalytic Properties of Dispersed Molybdenum Sulfides

Reactions of ammonium thiomolybdate under mild hydrothermal conditions were studied, and the range of conditions leading to the dispersed MoS₂ product has been determined. The morphology and the catalytic properties of the hydrothermal MoS₂ preparations have been compared with those of reference MoS₂ produced from the thermal decomposition of thiosalts. It has been shown that the length of the slabs and their stacking number of the crystallites of hydrothermal MoS₂ preparations differ considerably from those in the reference sulfides. The morphological differences strongly influence the hydrogenation to hydrodesulfurization activity ratio in these systems.     

Hydrothermal synthesis of S-doped TiO2 nanoparticles and their photocatalytic ability for degradation of methyl orange

A simple synthesis route to nanocrystalline S-doped TiO₂ photocatalysts by a hydrothermal method at 180 °C was developed and the photocatalytic activity of the obtained powders for the degradation of methyl orange was studied. The products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The phase composition (anatase/rutile ratio) and the photocatalytic activity of the final materials were found to be markedly influenced by the amount of the incorporated sulphur. On increasing the S-dopant amount, the anatase/rutile ratio and the photocatalytic activity of the as-prepared powders increased.     

Hydrothermal synthesis of vanadium pentoxide nanostructures and their morphology control

Different morphologies of vanadium pentoxide (V₂O₅) from 1D to 3D, including nanospheres, nanowires, urchin-like and flower-like nanostructures, have been synthetized by a simple hydrothermal method. Some parameters, such as the reaction temperature, the volume of polyvinyl pyrrolidone (PVP) and possible formation mechanisms of different V₂O₅ nanostructures were discussed. The results demonstrate that PVP and the reaction temperature play a critical role on the morphology of vanadium pentoxide.     

Hydrothermal synthesis of LiNi0.9Co0.1O2 cathode materials

Ultrafine powders of LiNi_0.9Co_0.1O₂ were prepared under mild hydrothermal conditions. The product was characterized by XRD, TEM and EDS tests, which indicated that the obtained products were pure and well-crystallized LiNi_0.9Co_0.1O₂. The ICP-AES results indicated the products were lithium-deficient compounds. The addition of KOH hardly effected the crystallinity of the product but gave larger crystals.     

Exploring the Effects of the Interaction of Carbon and MoS2 Catalyst on CO2 Hydrogenation to Methanol

Hydrogenation of CO₂ to form methanol utilizing green hydrogen is a promising route to realizing carbon neutrality. However, the development of catalyst with high activity and selectivity to methanol from the CO₂ hydrogenation is still a challenge due to the chemical inertness of CO₂ and its characteristics of multi-path conversion. Herein, a series of highly active carbon-confining molybdenum sulfide (MoS₂@C) catalysts were prepared by the in-situ pyrolysis method. In comparison with the bulk MoS₂ and MoS₂/C, the stronger interaction between MoS₂ and the carbon layer was clearly generated. Under the optimized reaction conditions, MoS₂@C showed better catalytic performance and long-term stability. The MoS₂@C catalyst could sustain around 32.4% conversion of CO₂ with 94.8% selectivity of MeOH for at least 150 h.     

Alcohol Synthesis from Syngas over K2CO3/CoS/MoS2 on Activated Carbon

Supported K₂CO₃/Co–MoS₂ on activated carbon was prepared by a co-impregnation technique and has been characterized by X-ray diffraction (XRD) and BET. Active ingredients ranged from 39 to 66% and included molysulfide and cobalt sulfide. XRD analysis indicates that cobalt and molybdenum sulfides are found in the Co₃S₄ and Co₉S₈ phases. These catalysts were performance tested in a fixed-bed reactor under higher alcohol synthesis conditions, 2000–2400 psig and 270–330°C. Active chemicals on the carbon extrudates decreased the surface area dramatically, as measured by BET. Surprisingly, at the high level of active chemicals, alcohol productivity and selectivity were decreased. An increase in the reaction temperature led to a decrease in the selectivity of methanol and an increase in selectivity of hydrocarbons. Total alcohol productivity was also increased as gas hourly space velocity (GHSV) was increased. Co₉S₈ may play a role in the catalyst aging process. In prolonged reaction periods (140 h), sulfur is lost from the surface, possibly as H₂S. The quantity of Co₉S₈ on the surface appears to increase as the catalyst ages.     

Prediction of a highly sensitive molecule sensor for SOx detection based on TiO2/MoS2 nanocomposites: a DFT study

First principles calculations within density functional theory have been carried out to investigate the adsorptions of SO_x (x?=?1, 2) molecules on TiO₂/MoS₂ nanocomposites in order to fully discover the gas sensing capabilities of TiO₂/MoS₂ composite systems. The van der Waals interactions were included to obtain the most stable geometrical structures of TiO₂/MoS₂ nanocomposites with adsorbed SO_x molecules. SO_x molecules preferentially interact with the doped nitrogen and fivefold coordinated titanium sites of the TiO₂ anatase nanoparticles because of their higher activities in comparison with the other sites. The results presented include structural parameters such as bond lengths and bond angles and energetics of the systems such as adsorption energies. The variation of electronic structures are discussed in view of the density of states and molecular orbitals of the SO_x molecules adsorbed on the nanocomposites. The results show that the adsorption of the SO_x molecule on the N-doped TiO₂/MoS₂ nanocomposite is energetically more favorable than the adsorption on the undoped one, implying that the nitrogen doping helps to strengthen the interaction of SO_x molecules with TiO₂/MoS₂ nanocomposites. These calculated results thus provide a theoretical basis for the potential applications of TiO₂/MoS₂ nanocomposites in the removal and sensing of harmful SO_x molecules.     

Hydrothermal synthesis of crystalline α-/β-MnO2 nanorods via γ-MnOOH nanorod precursors

The crystalline α-MnO₂ and β-MnO₂ nanorods have been successfully prepared via a facile hydrothermal method from γ-MnOOH nanorods precursor, respectively. The samples were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), field emission scanning electron microscope (FESEM) and Fourier transformed infrared spectra (FTIR). The morphology and structure of γ-MnOOH nanorods precursors have a great influence on the crystal structure of the obtained products. The α-MnO₂ nanorods are prepared from the 100°C γ-MnOOH precursor, while the β-MnO₂ nanorods are obtained from the 150°C γ-MnOOH precursor, respectively. Besides, the catalytic activity of the prepared α-MnO₂ and β-MnO₂ nanorods for the H₂O₂ decomposition has been investigated comparatively, and the latter shows better catalytic activity.