J. Cruz-Reyes

Preparation of WS2 catalysts by in situ decomposition of tetraalkylammonium thiotungstates

Tungsten tetraalkylammonium thiosalts are used as precursors for the in situ formation of WS2 catalysts in dibenzothiophene (DBT) hydrodesulfurization. The thermal decomposition of alkyl-ammonium thiosalts proceeds directly to WS2 without WS3 formation, as in the case of ammonium thiotungstate (ATT), allowing good control of the catalyst’s stoichiometry. The alkyl-ammonium thiosalts give WS2 particles with different characteristic morphologies. The hydrodesulfurization (HDS) activities of WS2 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 WS2:25C1:7 ,W S 1:7 C0:9 and WS1:3 C0:7 for the in situ catalysts derived from ammonium, methylammonium, and butylammonium precursors, respectively. The improved catalytic properties of WS2 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.

Hydrodesulfurization activity of MoS2 catalysts modified by chemical exfoliation

The surface area of unsupported MoS2 catalysts prepared by thiosalt decomposition is found to increase after undergoing a treatment known as chemical exfoliation. Rate measurements of dibenzothiophene hydrodesulfurization in a batch reactor show that activity decreases for the chemically modified MoS2 catalysts, along with the hydrogenation/hydrodesulfurization ratios (HYD/HDS). These results indicate that both basal and edge planes of the layered sulfides are rearranged by the exfoliation treatment, but that other processes must also be involved. Reference crystalline MoS2 is also discussed in the work.

Influence of preparation conditions on formation of crystalline phases of nickel sulfide

Abstract Nickel sulfide catalysts obtained by homogeneous precipitation were prepared by varying the homogenization time and the sulfiding temperature. The surface area with respect to preparation conditions was studied by the BET method. Structure characterization studies by X-ray diffraction showed the formation of two main crystalline phases (NiS 1.03 and NiS–millerite). Their appearance depends on preparation conditions. Formation of millerite or NiS 1.03 occurs at the constant sulfiding temperature, when time of homogenization of precursors increases.

Structure and catalytic properties of hexagonal molybdenum disulfide nanoplates

Molybdenum disulfide (MoS2) is a compound very useful for its properties; it is used as a lubricant, catalyst in hydrodesulfurization, in hydrogen fuel storage, etc. In this work MoS2 hexagonal nanoplates were synthesized at different temperatures and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). These nanoplates have a size of 25–35 nm as revealed by SEM. With aberration corrected STEM it was possible to measure the interatomic distance of Mo–Mo, which was found to be 2.8 A. The catalytic properties of the nanoplates were measured in the hydrodesulfurization of dibenzothiophene, showing high activity and high direct desulfurization pathway (DDS) selectivity for an unpromoted MoS2 catalyst. Theoretical calculations were performed in 2H-MoS2 as well as 2H-MoS2 with a rotation of 16° and 19° which was applied to the two S planes in the crystalline structure. The results obtained on the rotated 2H-MoS2 yielded indication of a small gap semiconductor of Eg = 0.38 eV when compared to the unrotated 2H-MoS2, which is a semiconductor of Eg = 2.00 eV. This tendency of the rotated crystalline 2H-MoS2 toward metallicity could be responsible for the enhancement of the catalytic properties observed in the material in question, compared to other MoS2-based catalysts.

Hydrodesulfurization catalysts prepared by two methods analyzed by transmission electron microscopy

Abstract Samples of molybdenum sulfide, cobalt sulfide and mixtures in atomic ratios r = Co/(Co + Mo) of 0.0, 0.3, 0.5, 0.7, and 1.0 were prepared by two different methods, homogeneous sulfide precipitation (HSP) and impregnated thiosalt decomposition (ITD). Samples were observed by high-resolution electron microscopy using imaging and diffraction modes. Both preparation methods present the “rag structure” typical of MoS 2 2H with some structural differences between them. The average number of layers (n) in molybdenum disulfide crystals is about the same in both preparation methods, while the average length (L) of the molybdenum disulfide crystallites obtained by HSP is larger than that of those obtained by ITD. The particle size is smaller for ITD samples. The presence of cobalt does not greatly modify the number of layers of the MoS 2 2H stacks in mixed samples. An increase in the intracrystallite disorder is observed.

Phosphorus promoted WS2/Al2O3 catalysts studied by transmission electron microscopy

A series of WS2/Al2O3 catalysts containing a varied amount of phosphorus (0.0,2.5 and 6.0 wt% P2O5), sulfided at two different temperatures (873 and 1073 K), were studied by means of high-resolution transmission electron microscopy (HRTEM). The stacking of (002) layers in the WS2-2H crystallites of the “rag” structure increases with the addition of phosphorus, whereas its length is kept almost constant. At high sulfidation temperatures phosphorus shows a stabilizing effect on the catalysts by retarding the stacking growth of WS2 crystallites. Additionally, an aluminum phosphate crystalline phase is identified in the phosphorus promoted catalysts.

Co Oxidation by Bi 2 MoO 6 -γ(H) Catalyst

We present a study of a Bi-Mo catalyst, obtained by solid state reaction, for the oxidation of CO. Bismuth-molybdate catalysts form several phases (β3-Bi 2 Mo 2 O 9 , α-Bi 2 Mo 3 O 12 and γ-Bi 2 MoO 6 , among others), depending upon the Bi/Mo ratio and the preparation method [1]. The γ-Bi 2 MoO 6 phase is the one that is stable at high temperatures, according to the phase diagram, and is the one we used as catalyst for the conversion of CO [2].

Mixed impregnated thiosalt decomposition catalysts characterized by X-ray diffraction

We used the impregnated thiosalt decomposition method (ITD) to prepare catalysts of molybdenum sulfide promoted with cobalt in atomic ratios (r = Co/(Co + Mo)) ranging from 0.0 to 1.0. Measurements obtained by X-ray diffraction (XRD) show the presence of the MoS2-2H phase in all mixed samples, and segregation of cobalt in two phases: Co9S8, forr ≥ 0.3, and CoS1.035, for 0.3 ≤r ≤ 0.5.

Catalytic properties of WS2 catalysts prepared by in situ decomposition of tetraalkyl-ammonium thiotungstates

Abstract Tungsten disulfide unsupported catalysts obtained by in situ decomposition of tetramethyl- and tetrabutylammonium thiosalts (TMATT and TBATT) presented better hydrodesulfurization performance than catalysts derived from the ammonium thiosalt (ATT). The reaction rate increased with the size of alkyl group in the precursor, however, no correlation of activity with surface area was observed. Auger analysis revealed that the surface concentration of sulfur and carbon varied with the precursor. The improved performance of WS2 catalysts derived from alkylammonium thiosalts in the HDS of DBT is attributed to the formation of tungsten carbide-sulfide species on the surface.

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.Graphical Abstract