Ammonium Thiotungstate Research Articles

A New Route for the Synthesis of Ammonium Thiotungstate, a Catalyst Precursor

Ammonium thiotungstate (ATT), (NH4)2WS4, is prepared by a new method using (NH4)2S, and compared with ATT obtained by conventional sulfidation with H2S(g). The XRD spectra of both samples are very similar to the simulated spectrum of ATT. UV–Vis and FT-IR spectra of both samples have characteristic bands for ATT. SEM micrographs show a similar morphology between the samples. TGA-DTA curves of both samples are the same, corresponding to the decomposition of ATT. Analytical results thus show that the ATT obtained by this new, safer, method is the same as the ATT obtained by sulfidation with poisonous H2S(g). The HDS activity of the WS2 catalyst derived from ATT prepared by the new method described here is equivalent to that of ATT prepared by the conventional method.

Naphthalene hydrogenation over nickel–tungsten sulfide catalysts synthesized in situ from DMSO–hydrocarbon medium emulsions

A method for synthesizing unsupported nickel–tungsten sulfide hydrodearomatization catalysts by breaking SPAN-80 surfactant-stabilized nonaqueous emulsions of solutions of different precursors in dimethyl sulfoxide (DMSO) in situ in a hydrocarbon medium has been first studied using ammonium thiotungstate (NH4)2WS4 and 1-butyl-1-methylpiperidinium nickel thiotungstate [BMPip]2Ni[WS4]2 as precursors and nickel nitrate hexahydrate as a nickel source. The synthesized nickel–tungsten catalysts have been characterized by TEM and XPS. The catalytic activity of the in situ synthesized Ni–W particles in naphthalene hydrogenation has been studied at temperatures of 350–400°C and a hydrogen pressure of 5.0 MPa.

Control over structural-dimensional characteristics of tungsten disulfide particles in aerosol-assisted chemical vapor deposition

Solutions of ammonium thiotungstate in dimethylformamide were used to synthesize spherical tungsten disulfide particles with average radius of 500–100 nm by the method of aerosol-assisted chemical vapor deposition. Nanoparticles with composition close to stoichiometric tungsten disulfide are formed at pyrolysis temperatures not lower than 800°C. It was found that the average particle radius linearly decreases as the reagent concentration in solution becomes lower, and the nebulizer power has no effect within the range under study on the size characteristics and structure of the particles obtained. It was demonstrated that the particles have a layered structure that is formed in all probability by S–W–S packets, which must provide high antifriction properties of the material in its use as a high-temperature solid lubricant. The results obtained indicate that the size of tungsten disulfide particles can be controlled in a wide range in the course of the aerosol-assisted chemical vapor deposition. This may be of interest for developing a technology for creating high-temperature wear-resistant antifriction coatings.

Ultralow Density, Monolithic WS2, MoS2, and MoS2/Graphene Aerogels.

We describe the synthesis and characterization of monolithic, ultralow density WS2 and MoS2 aerogels, as well as a high surface area MoS2/graphene hybrid aerogel. The monolithic WS2 and MoS2 aerogels are prepared via thermal decomposition of freeze-dried ammonium thio-molybdate (ATM) and ammonium thio-tungstate (ATT) solutions, respectively. The densities of the pure dichalcogenide aerogels represent 0.4% and 0.5% of full density MoS2 and WS2, respectively, and can be tailored by simply changing the initial ATM or ATT concentrations. Similar processing in the presence of the graphene aerogel results in a hybrid structure with MoS2 sheets conformally coating the graphene scaffold. This layered motif produces a ∼50 wt % MoS2 aerogel with BET surface area of ∼700 m(2)/g and an electrical conductivity of 112 S/m. The MoS2/graphene aerogel shows promising results as a hydrogen evolution reaction catalyst with low onset potential (∼100 mV) and high current density (100 mA/cm(2) at 260 mV).

Transformation of Mo and W thiosalts into unsupported sulfide catalysts: A temperature dependent in-situ spectroscopic investigation

The thermal decomposition of ammonium thiomolybdate (ATM), ammonium thiotungstate (ATT), tetramethylammonium thiomolybdate (TMATM) and tetramethylammonium thiotungstate (TMATT) are investigated as a function of temperature by using thermogravimetric analysis (TGA), temperature-programmed decomposition with mass spectroscopy (TPS–MS), in-situ Fourier transform infrared (FTIR) and Raman spectroscopies, and X-ray photoelectron spectroscopy (XPS). The results allow for correlations to be made between the changes in the bulk and surface structures of the materials, and the evolution of gas-phase decomposition products. The major difference between the ammonium and tetramethylammonium precursors is the complexity of the thermal decomposition profile, which is found to follow two steps over a wide temperature range for the former, but one step over a relatively narrow range for the latter materials. Raman and FTIR spectra reveal the decomposition of the ammonium and tetramethylammonium groups, along with the decomposition of the initial sulfide structures to metal disulfides. For both sets of precursors, XPS results show that the surface of the resulting materials at various temperatures of treatment does not track directly with the state of the bulk material. While the ATM, TMATM, and TMATT-derived material surfaces are reduced to the 4+ state at the highest temperature, indicating disulfides, the ATT-derived materials still retained a significant amount of W6+ state consistent with the starting precursor.

Tribology of Aqueous Thiomolybdate and Thiotungstate Additives in Low-Pressure Contacts

The wear reducing properties of aqueous solutions of ammonium thiomolybdate and ammonium thiotungstate were evaluated using a frontal cylinder-on-cylinder test tribometer on an AISI 304 steel–steel contact. The antiwear characteristics of the lubricating solutions of thiomolybdate and thiotungstate were evaluated at 24.5, 30, and 40 N keeping fixed the entrainment speed, sliding distance, temperature, and additive concentration. Although there was a reduction in wear for the two salts compared to fluid reference, it is worth mentioning that the thiotungstate additive was slightly more effective under tested lubricating contact conditions than thiomolybdate additive. Energy-dispersive X-ray spectroscopy (EDS) analysis of a wear track lubricated with thiomolybdate showed Mo and S elements and for the track lubricated with thiotungstate W and S were detected. Raman spectra of the wear track showed peaks at 373, 404 and 208, 253, 280, 353 cm−1 corresponding to 2H-MoS2 and FeS, respectively. Ammonium thiomolybdate releases sulfur atoms that also react with the freshly exposed surface, forming FeS, which takes part in reducing wear.

Unsupported Ni/Mo(W)S2 Catalysts from Hexamethylenediammonium Thiometallates Precursors: In Situ Activation During the HDS of DBT

Unsupported Ni/MoS2 and Ni/WS2 HDS cata- lysts were prepared by in situ activation of hexamethyl- enediammonium thiometallates promoted with Ni. The method involved an aqueous solution reaction of ammo- nium thiomolybdate (ATM) or ammonium thiotungstate (ATT) with Ni(NO3)2� 6H2O and hexamethylenediamine. Ni-promoted precursors were then in situ activated during the hydrodesulfurization (HDS) of dibenzothiophene (DBT) producing Ni/MoS2 and Ni/WS2 catalysts. Solids were analyzed after the in situ activation by scanning electron microscopy, transmission electron microscopy, X- ray diffraction, and for textural properties by using the BET and BJH methods. Catalysts with relatively high surface area and type IV N2 adsorption-desorption isotherms were obtained. The use of the hexamethylene- diammonium precursor led to a significant nickel promo- tion of MoS2 and WS2 catalysts. For Ni/MoS2, the use of this carbon-containing precursor was found to be more beneficial for the final HDS catalytic activity than using the classical ammonium tetrathiomolybdate (ATM) without carbon. For Ni/WS2, compared to tetraalkylammonium thiosalts, the lower amount of carbon in excess formed during the decomposition of the hexamethylenediammo- nium precursor coupled with a lower crystallization rate of WS2 favors a correct nickel accommodation on the WS2 edges.

Trimetallic NiMoW unsupported catalysts for HDS

Abstract Trimetallic unsupported NiMoW sulfide catalysts were prepared of by decomposition-sulfidation of bimetallic Mo-W thiosalts precursors impregnated with Ni. The catalytic activity of the catalysts was determined, both in situ and ex situ , in the dibenzothiophene (DBT) hydrodesulfuration (HDS) reaction. The sample with a relative composition R1 = Mo/[Mo − W] = 0.85 and R2 = Ni/[Mo + W] = 0.5 presented a substantial increment ( k = 23 × 10 −7 mol/s g) with selectivity to hydrogenation by a factor of 1.24 over hydrodesulfuration. Scanning electron microscopy (SEM) micrographs showed changes in morphology as a function of R1 suggesting that distinct crystalline phases were formed. Also, X-ray diffraction (XRD) measurements of precursors and catalysts revealed that the bimetallic thiosalt structure presents the same planes of the rhombohedral structure as ammonium thiomolybdate (ATM) or ammonium thiotungstate (ATT) indicating that redistribution of Mo and W occurred preferably in specific orientations as the (0 0 2), (0 2 0) and (3 1 0). The ammonium thiomolybdate-thiotungstate (ATMW) bimetallic thiosalt with R1 = 0.85 showed very different structure from the original Mo or W single precursors, among all the samples. Extended Huckel calculations revealed that promotion of (Mo-W)S 2 precursors with Ni forming trimetallic NiMoWS catalysts after sulfidation, increases the availability of electrons over the Fermi level increasing the metallic character of the catalysts.

Friction reduction by water-soluble ammonium thiometallates

Lubricant bases for metalworking applications make extensive use of water-soluble additives to reduce friction and wear. In order to do such task, these additives must form a lubricating film that separates the contact surfaces thus imparting good surface finishes to the worked parts. This paper presents a study on the tribological performance of aqueous solutions of ammonium thiomolybdate and ammonium thiotungstate. Tests were carried out on a pin-on-disc tribometer for a steel–aluminum contact while keeping load, entrainment speed, sliding distance, temperature, and concentration of the additive constant to study the lubrication effect of these two salts. Chemical analysis of the wear track indicates the presence of an in-contact-formed solid film enriched with MoS2 and WS that reduces friction markedly.

The catalytic activity of Ni/W bimetallic sulfide nanostructured catalysts in the hydrodesulfurization of dibenzothiophene

Two types of catalysts containing NiW bimetallic sulfide nanostructures were prepared by a chemical method employing ammonium thiotungstate and nickel nitrate as metal-sulfide precursors followed by sulfidation in H2S/H2 at 400 °C. The nanostructures were grown with excess of Ni, at atomic ratio R = 0.75, 0.85 (R = Ni/Ni + W). High resolution electron microscopy (HRTEM) micrographs revealed the formation of two types of nanostructures, nickel sulfide nanoparticles and long nanorods of tungsten suboxide, both coated by WS2 layers. The Ni/W catalyst containing mostly nanorods presented twice the catalytic activity (pseudo-zero order constant rate k = 12 × 10−7 mol/s.g) of the Ni/W catalyst containing nanoparticles (k = 6.3 × 10−7 mol/s.g) with a low selectivity for tetrahydrodibenzothiophene (THDBT) and high selectivity to cyclohexylbenzene (CHB, 50 mol%). In turn the Ni/W catalyst containing nanoparticles presented a catalytic activity comparable to a Ni/Mo catalyst without inorganic fullerene (IF) nanostructures (k = 7.2 × 10−7 mol/s.g) but with higher selectivity for hydrogenation to THDBT, (14 mol%) than the sample with nanorods.

Nickel–tungsten bimetallic sulfide nanostructures of fullerene type

Bimetallic NiW sulfide nanostructures of the inorganic fullerene-like (IF-like) type were prepared by a chemical method employing ammonium thiotungstate and nickel nitrate as metal-sulfide precursors followed by sulfidation in H2S/H2 at 400 °C. The nanostructures were grown with a Ni excess, at an atomic ratio R = 0.85 (R = Ni/Ni + W). The x-ray diffraction patterns showed poorly crystalline WS2, WO2, NiS, and Ni9S8 phases. High-resolution electron microscopy micrographs revealed the formation of two fullerene-like nanostructures, nickel sulfide nanoparticles and long nanotubes filled with tungsten suboxide, both coated by several WS2 layers. The surface area of 18 m2/g measured by nitrogen adsorption (BET surface-area) revealed that these materials contained micropororosity.

Synthesis of tetraalkylammonium thiometallate precursors and their concurrent in situ activation during hydrodesulfurization of dibenzothiophene

The synthesis of tetraalkylammonium thiomolybdates and thiotungstates, (NR 4) 2MS 4 {R: heptyl or cetyltrimethyl, M: Mo or W} using an a queous solution method is reported. This method significantly improves the yield of tetraalkylammonium compounds compared to previous studies. The one-step rapid substitution of [NH 4] + ions from ammonium thiomolybdate (ATM) and/or ammonium thiotungstate (ATT) with [(Heptyl) 4N] + and/or [(CTriM)N] + ions, respectively, is described. Tetraheptylammonium thiomolybdate, tetraheptylammonium thiotungstate, cetyltrimethylammonium thiomolybdate and cetyltrimethylammonium thiotungstate were synthesized and characterized using Fourier transform infrared (FTIR) and UV-Vis spectroscopies. Thermal analyses (TGA-DTA) were done to study the fragmentation and decomposition behavior of their molecular structures. These tetraalkylammonium thiometallate precursors were in situ-activated concurrently during the hydrodesulfurization (HDS) of dibenzothiophene (DBT), producing MoS 2 and WS 2 catalysts, respectively. These catalysts analyzed by scanning electron microscopy showed large voids and S/M around 2 (M: Mo, W). High surface area (200–400 m 2/g) and type IV adsorption–desorption nitrogen isotherms were obtained. The nature of the alkyl group affects both the surface area and HDS selectivity. In this respect, a high selectivity for direct C–S bond cleavage is observed for MoS 2 and WS 2 formed from THepATM and THepATT precursors, respectively. The X-ray diffraction study showed that the catalysts are poorly crystalline with a very weak (0 0 2) intensity except for the WS 2 catalyst formed from THepATT.

Activation of tetraalkylammonium thiotungstates for the preparation of Ni-promoted WS 2 catalysts

A comparative study of the synthesis of unsupported Ni/WS 2 catalysts by ex situ and in situ decomposition of thiosalts is herein reported. Ammonium thiotungstate (ATT), tetramethylammonium thiotungstate (TMATT), tetrapropylammonium thiotungstate (TPATT) and tetrabutylammonium thiotungstate (TBATT) and their nickel-promoted counterparts were used as precursors. Ex situ activation was performed under a H 2S/H 2 flow whereas the in situ activation consists of the decomposition of the thiosalt in the presence of a hydrocarbon solvent during the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The catalytic activity depends on both the method of activation and the nature of the precursor. The tetraalkylammonium thiotungstate precursors have been characterized by thermogravimetric analysis and the final catalysts by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and BET specific surface area measurements. Compared to ATT, the in situ activation of tetraalkylammonium thiotungstates has a negative impact on the HDS activity of the as-formed NiW catalysts while a moderate gain in activity was observed for ex situ activated catalysts. The presence of carbon in the tetraalkylammonium precursors accelerates the crystallization rate of WS 2-based catalysts leading to well-organized WS 2 structure but with low specific surface areas. The accelerating rate of crystallization also limits the accommodation of Ni on the WS 2 edges and then hinders the synergetic effect. This situation differs strikingly from MoS 2-based catalysts for which no accelerating effect of carbon on the crystallization rate was observed.

Comparative study of WS2 and Co(Ni)/WS2 HDS catalysts prepared by ex situ/in situ activation of ammonium thiotungstate

A comparative study of the syntheses of unsupported WS2 and M/WS2 (M = Co, Ni) catalysts by ex situ/in situ decomposition of ammonium thiotungstate (ATT) is herein reported. Ex situ activation was performed under a H2S (15% volume)/H2 flow, whereas in situ activation consists in the direct decomposition of ATT or Co(Ni)/ATT precursors in the presence of a hydrocarbon solvent during the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Precursors were characterized by thermogravimetric analysis and final catalysts by X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area (BET). Catalysts activated using the in situ mode of activation present higher specific surface areas with the noticeable exception of the Ni/ATT precursor. Activity measurements showed that the in situ activated WS2 and Ni/WS2 catalysts exhibit higher activity than the ex situ activated catalysts.

Synthesis of tetraalkylammonium thiometallates in aqueous solution

An aqueous solution method for the preparation of tetraalkylammonium thiometallates (R 4N) 2MS 4 (R=pentyl or hexyl and, M=Mo or W) is reported. The one-step rapid substitution of [NH 4] + ions from ammonium thiomolybdate (ATM) and ammonium thiotungstate (ATT) with [(pentyl) 4N] + and [(hexyl) 4N] + ions during reactions with (pentyl) 4NBr and (hexyl) 4NBr, respectively, is described. One application for these tetraalkylammonium thiomolybdates and thiotungstates is as precursors for MoS 2 and WS 2 catalysts, which are used in industrial hydrodesulfurization and hydrodenitrogenation processes. The synthesized thiometallates were characterized using the spectroscopic techniques Fourier transform infra red (FTIR), ultra violet (UV) and, nuclear magnetic resonance ( 13C NMR) for determining their chemical structures. Thermal analyzes (TGA-DTA) were done to study the fragmentation and decomposition behavior of their molecular structures.

Hydrogenation of cyclohexanone on nickel-tungsten sulfide catalysts

Unsupported Ni-W sulfide catalysts were tested for cyclohexanone hydrogenation under atmospheric pressure conditions and showed a high selectivity towards cyclohexene. They were prepared employing ammonium thiotungstate complexes and nickel nitrate as metal-sulfide precursors. The data of catalytic activity show a synergetic effect as a function of stoichiometric composition, R=Ni/(Ni+W). A maximum synergetic effect occurred at R=0.7. The texture and surface composition characterization of the catalysts were carried out by BET surface area measurements, AES and XPS. The surface of catalysts was found to be nickel deficient, compared to the nominal values, as determined by AES and XPS. The electronic state of Ni changed as the catalyst stoichiometry varied, but the electronic state of W did not change.

Preparation of WS 2 catalysts by in situ decomposition of tetraalkylammonium thiotungstates

Tungsten tetraalkylammonium thiosalts are used as precursors for the in situ formation of WS 2 catalysts in dibenzothiophene (DBT) hydrodesulfurization. The thermal decomposition of alkyl-ammonium thiosalts proceeds directly to WS 2 without WS 3 formation, as in the case of ammonium thiotungstate (ATT), allowing good control of the catalyst’s stoichiometry. The alkyl-ammonium thiosalts give WS 2 particles with different characteristic morphologies. The hydrodesulfurization (HDS) activities of WS 2 catalysts derived from alkylthiosalts are higher than those of catalysts derived from the ammonium thiosalt. The reaction rate increases with the size of the cation in the precursor. No correlation of catalytic activities with surface areas is found. The S/W and C/W surface ratios determined by Auger electron spectroscopy decrease with increasing cation size. Surface composition is WS 2.25C 1.7, WS 1.7 C 0.9 and WS 1.3 C 0.7 for the in situ catalysts derived from ammonium, methylammonium, and butylammonium precursors, respectively. The improved catalytic properties of WS 2 catalysts derived from alkylammonium thiosalts in the HDS of DBT are attributed to the formation of carbon-containing tungsten sulfide phases on the catalyst’s surface.

Evolution of crystalline phases in nickel–tungsten sulfide catalysts

Unsupported Ni–W sulfide catalysts were prepared employing ammonium thiotungstate complexes and nickel nitrate as metal-sulfide precursors. Structural characterization was carried out by XRD and HRTEM measurements. XRD patterns show crystallinity enhancement of WS 2 particles as the W content in the mixed catalysts was decreased. HRTEM demonstrates that WS 2 layers start to coat NiS particles when the amount of Ni was increased.

The crystal structure of ammonium thiotungstate (NH4)2WS4

The crystal structure of ammonium thiotungstate (NH₄)₂WS₄