Comparison of molybdenum carbide and tungsten carbide for the hydrodesulfurization of dibenzothiophene

The molybdenum and tungsten carbides (Mo₂C and W₂C) were synthesized, characterized and tested in hydrodesulfurization (HDS) of dibenzothiophene (DBT). The phase purity of these catalysts was established by X-ray diffraction (XRD), and the surface properties were determined by N₂ BET specific surface area (Sg) measurements, CO chemisorption and high-resolution transmission electron microscopy (HRTEM). The activities of catalysts were determined during the HDS of DBT at a temperature of 613 K and under a 6 MPa total pressure. Both molybdenum and tungsten carbides were active in HDS of DBT. The reactivity studies showed that molybdenum carbide was more active than tungsten carbide related to weight. However, W₂C was shown to possess stronger hydrogenating properties.     

Catalytic performances of platinum doped molybdenum carbide for simultaneous hydrodenitrogenation and hydrodesulfurization

The hydrotreating activity of molybdenum carbide doped with platinum (0.3 wt.%) was studied and compared to that of a pure, non-modified Mo₂C. 4,6-Dimethyldibenzothiophene (4,6-DMDBT, 300 ppm of S) and carbazole (100 ppm of N) were designated as model compounds and subjected to hydrodesufurization (HDS) and hydrodenitrogenation (HDN) processes either separately or simultaneously. In both cases the molybdenum carbide doped with platinum (Mo₂C-Pt) turns out to be more active than Mo₂C. The increase of the hydrotreating activity, owing to the presence of platinum in molybdenum carbide can be related to the raise of hydrogenation activity of the modified catalyst. The platinum modified molybdenum carbide was stable (displays a long lifetime) under HDN and HDS reaction conditions. The predominant reaction products are bicyclohexyl (BCH) for the HDN process or 3,3′-dimethylbiphenyl (3,3′-DMBPh) and methylcyclohexyltoluene (MCHT) for the HDS process, respectively.     

Synthesis of nanophase tungsten carbide by electrical discharge machining

Tungsten carbide nanopowders were synthesized successfully by electric discharge machining followed by annealing under a nitrogen atmosphere. The tungsten workpieces were initially melted and evaporated on the working surface during the electric discharge machining process, and then the tungsten powders were reacted with the carbon electrode and the working medium of kerosene to form the nanocrystalline WC_1−x powders. The powders produced were characterized by XRD, SEM, and TEM. When annealing the powders under an N₂ atmosphere, the cubic phases of WC_1−x gradually changed to hexagonal W₂C and then were transformed fully to nanocrystalline hexagonal WC at 1200 °C, with the nanocrystalline tungsten carbide encapsulated in a carbon shell. On the other hand, under an H₂ atmosphere, the WC_1−x phase changed via a W₂C phase to reduced powders of pure tungsten at 1000 °C or were reduced directly from WC_1−x to elemental W.     

Use of unsupported and silica supported molybdenum carbide to treat chloroarene gas streams

The gas phase catalytic hydrodechlorination (HDC) of mono- and di-chlorobenzenes (423 K ≤ T ≤ 593 K) over unsupported and silica supported Mo carbide (Mo₂C) is presented as a viable means of detoxifying Cl-containing gas streams for the recovery/reuse of valuable chemical feedstock. The action of Mo₂C/SiO₂ is compared with MoO₃/SiO₂ and Ni/SiO₂ (an established HDC catalyst). The pre- and post-HDC catalyst samples have been characterized in terms of BET area, TG-MS, TPR, TEM, SEM, H₂ chemisorption/TPD and XRD analysis. Molybdenum carbide was prepared via a two step temperature programmed synthesis where MoO₃ was first subjected to a nitridation in NH₃ followed by carbidization in a CH₄/H₂ mixture to yield a face-centred cubic (-Mo₂C) structure characterized by a platelet morphology. Pseudo-first order kinetic analysis was used to obtain chlorobenzene HDC rate constants and the associated temperature dependences yielded apparent activation energies that decreased in the order MoO₃/SiO₂ (80 ± 5 kJ mol⁻¹) ≈ MoO₃ (78 ± 8 kJ mol⁻¹) > Ni/SiO₂ (62 ± 3 kJ mol⁻¹) ≈ -Mo₂C (56 ± 6 kJ mol⁻¹) ≈ -Mo₂C/SiO₂ (53 ± 3 kJ mol⁻¹). HDC activity was lower for the dechlorination of the dichlorobenzene reactants where steric hindrance influenced chloro-isomer reactivity. Supporting -Mo₂C on silica served to elevate HDC performance, but under identical reaction conditions, Ni/SiO₂ consistently delivered a higher initial HDC activity. Nevertheless, the decline in HDC performance with time-on-stream for Ni/SiO₂ was such that activity converged with that of -Mo₂C/SiO₂ after three reaction cycles. A temporal loss of HDC activity (less extreme for the carbides) was observed for each catalyst that was studied and is linked to a disruption to supply of surface active hydrogen as a result of prolonged Cl/catalyst interaction.     

Acid and base characteristics of molybdenum carbide catalysts

The acid and base properties of a high surface area Mo₂C catalyst were characterized using the temperature programmed desorption of CO₂ and NH₃, the decomposition of isopropyl alcohol (IPA) as a test reaction and monitoring changes in the associated rates and product selectivities on the addition of acid and base site poisons. The Mo₂C catalyst was prepared using the temperature programmed reaction method and passivated prior to exposure to air. Prior to carrying out the temperature programmed desorption experiments and reaction rate measurements, the Mo₂C catalyst was reduced in H₂ at 400 °C. Results obtained for the reduced Mo₂C catalyst were compared with those for MgO, HZSM-5 and 1% Pt/SiO₂ catalysts. The study provided evidence for the presence of both acid and base sites on Mo₂C. The base and acid sites on the Mo₂C catalyst were weaker than those on the MgO and HZSM-5 catalysts, respectively. The base and acid sites were likely created as a consequence of charge transfer from molybdenum to carbon.     

Activity of carbided molybdena–alumina for CO2 hydrogenation

The relationship between the catalytic activity of carbided molybdena–alumina and the methane desorption from carbidic carbon through temperature-programmed surface reaction (TPSR) were studied. The effects of passivation and hydrogen treatment on the catalytic activities of molybdenum carbides for CO₂ hydrogenation were determined. When the 973 K-carbided catalyst was reduced at 773 K with hydrogen, the catalyst exhibited the highest activity for the reaction, the activity decreasing with increasing H₂ pretreatment temperature. Passivation of this catalyst decreased the reaction rate by 20%. TPSR results were correlated with the activity to reveal that molybdenum carbide with slightly deficient carbidic carbon (Mo₂C_0.962C_1.0) serves as an active site for CO₂ hydrogenation.     

Group VI transition metal carbides as alternatives in the hydrodechlorination of chlorofluorocarbons

The synthesis of transition metal carbides of tungsten and molybdenum has been carried out via temperature programmed reactions (TPRs) of metal oxides or passivated nitrides. Their specific chloropentafluoroethane conversion rates were at best one order of magnitude less than that of a reference Pd based catalyst. The intrinsic rates range from 4.7 to 14.7 nmol m⁻² s⁻¹ and decrease as follows: Mo₂C>WC>W₂C≈WC_1−x>MoC_1−x. The group VI carbide samples catalyse hydrodehalogenation and dehydrofluorination. WC appears to be as selective towards pentafluoroethane (HFC-125) as the Pd based catalyst. Then the selectivity decreases in the following sequence: W₂C>Mo₂C>WC_1−x>MoC_1−x. All the carbide catalysts deactivate at the early stages of the reaction. Based on the XPS results and the product distribution of the reaction, the deactivation has been mainly attributed to a site blocking phenomenon due to a strong deposit of polymeric carbon and of hydrofluorocarbon polymers. Polymerisation of detected unsaturated compounds take place on acidic sites probably generated by fluoride and/or chloride in the course of the reaction.     

Effect of phosphorus addition on the hydrotreating activity of NiMo/Al2O3 carbide catalyst

A series of phosphorus promoted γ-Al₂O₃ supported NiMo carbide catalysts with 0–4.5 wt.% P, 13 wt.% Mo and 2.5 wt.% Ni were synthesized and characterized by elemental analysis, pulsed CO chemisorption, BET surface area measurement, X-ray diffraction, near-edge X-ray absorption fine structure, DRIFT spectroscopy of CO adsorption and H₂ temperature programmed reduction. X-ray diffraction patterns and CO uptake showed the P addition to NiMo/γ-Al₂O₃ carbide, increased the dispersion of β-Mo₂C particles. DRIFT spectra of adsorbed CO revealed that P addition to NiMo/γ-Al₂O₃ carbide catalyst not only increases the dispersion of Ni-Mo carbide phase, but also changes the nature of surface active sites. The hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) activities of these P promoted NiMo/γ-Al₂O₃ carbide catalysts were performed in trickle bed reactor using light gas oil (LGO) derived from Athabasca bitumen and model feed containing quinoline and dibenzothiophene at industrial conditions. The P added NiMo/γ-Al₂O₃ carbide catalysts showed enhanced HDN activity compared to the NiMo/γ-Al₂O₃ catalysts with both the feed stocks. The P had almost no influence on the HDS activity of NiMo/γ-Al₂O₃ carbide with LGO and dibenzothiophene. P addition to NiMo/γ-Al₂O₃ carbide accelerated CN bond breaking and thus increased the HDN activity.     

Hydrodesulfurization over supported monometallic, bimetallic and promoted carbide and nitride catalysts

The preparation of alumina-supported β-Mo₂C, MoC_1−x (x≈0.5), γ-Mo₂N, Co–Mo₂C, Ni₂Mo₃N, Co₃Mo₃N and Co₃Mo₃C catalysts is described and their hydrodesulfurization (HDS) catalytic properties are compared to conventional sulfide catalysts having similar metal loadings. Alumina-supported β-Mo₂C and γ-Mo₂N catalysts (Mo₂C/Al₂O₃ and Mo₂N/Al₂O₃, respectively) are significantly more active than sulfided MoO₃/Al₂O₃ catalysts, and X-ray diffraction, pulsed chemisorption and flow reactor studies of the Mo₂C/Al₂O₃ catalysts indicate that they exhibit strong resistance to deep sulfidation. A model is presented for the active surface of Mo₂C/Al₂O₃ and Mo₂N/Al₂O₃ catalysts in which a thin layer of sulfided Mo exposing a high density of sites forms at the surface of the alumina-supported β-Mo₂C and γ-Mo₂N particles under HDS conditions. Cobalt promoted catalysts, Co–Mo₂C/Al₂O₃, have been found to be substantially more active than conventional sulfided Co–MoO₃/Al₂O₃ catalysts, while requiring less Co to achieve optimal HDS activity than is observed for the sulfide catalysts. Alumina-supported bimetallic nitride and carbide catalysts (Ni₂Mo₃N/Al₂O₃, Co₃Mo₃N/Al₂O₃, Co₃Mo₃C/Al₂O₃), while significantly more active for thiophene HDS than unpromoted Mo nitride and carbide catalysts, are less active than conventional sulfided Ni–Mo and Co–Mo catalysts prepared from the same oxidic precursors.     

Electro-oxidation of hydrogen on Mo---W carbide alloy catalysts in acid electrolyte

The rates of anodic oxidation of H₂ in acid electrolyte on WC and on the isostructural ternary alloy Mo_{1 - x}W_xC, with Mo occupying 70–80% of the metal sites, have been compared experimentally. The ternary catalyzes the reaction as effectively as WC, by the same mechanism, and is equally tolerant of CO entrained in the H₂ feed. Evidence is presented which indicates that proton discharge is rate limiting on these carbides, and that their performance is limited by scarcity of active sites and hence open to improvement by appropriate preparative technique.     

纳米碳纤维/碳毡负载碳化钨和碳氮化钨的肼分解性能

以纳米碳纤维/碳毡(CFF)为载体,分别采用碳热氢气还原法和碳热氨气还原法制备了负载型碳化钨、碳氮化钨整体催化剂,XRD结果表明催化剂的活性相分别为W₂C和WC_xN_y, TEM表征发现其粒子尺寸分别为2～40 nm和2～20 nm。采用1 N肼分解推力器评价了上述催化剂的反应性能,W₂C/CFF和WC_xN_y/CFF表现出了相比于Ir/CFF更好的肼分解综合性能:除了初活性略低以外,启动加速性、稳态燃压以及比活性均高于贵金属Ir催化剂。此外,W₂C/CFF和WC_xN_y/CFF催化剂的稳定性明显优于Ir/CFF,这主要是由于CFF载体在贵金属Ir的作用下发生了更显著的甲烷化反应。     

Synthesis and characterization of cobalt–molybdenum bimetallic carbides catalysts

Co₃Mo₃C, Co₆Mo₆C and MCM41-supported Co₃Mo₃C catalyst are prepared by a simple one-step thermal decomposition method without the conventional temperature-programmed carburization. The resultant carbides are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscope (HRTEM) and BET surface area measurements. The as-prepared Co₃Mo₃C/MCM41 catalyst exhibits good performance in both probe reactions of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), which proves the one-step decomposition method to be an effective route for the preparation of bimetallic carbide catalyst.     

Hydrotreating of heavy vacuum gas oil (HVGO) on molybdenum and tungsten nitrides catalytic phases

A series of supported and unsupported Mo₂N and W₂N phases were synthesized by means of the treatment under ammonia atmosphere at 700°C of Mo and W oxides. The X-ray diffraction and electron microscopy techniques verified the formation of the Mo₂N and W₂N ceramic phases, while the N₂ adsorption (BET) was used to determine the surface areas, between 46–133 m²/g for Mo₂N (unsupported) and 81–101 m²/g for W₂N (unsupported). The supported phases had surface areas between 109–113 and 109–122 m²/g, for Mo₂N/Al₂O₃ and W₂N/Al₂O₃, respectively. The catalytic hydrotreating of a heavy vacuum gas oil (HVGO) derived from Maya crude (i.e. 2.21 wt.% S, 0.184 wt.% N₂) was performed on both, supported and unsupported Mo nitrides and W nitrides, which promoted the HDN reaction preferentially, up to 26.6% on Mo₂N/Al₂O₃ and up to 22.3% on W₂N/Al₂O₃, against 3.26% on the reference catalyst, i.e. CoMo/Al₂O₃ at 350°C and 80 kg/cm². Also, the rates for HDN increased with the crystallite size in the unsupported W₂N series. Also, the pore volume and mean pore diameters of the Mo₂N/Al₂O₃ and W₂N/Al₂O₃ series improve substantially with respect to the pure ceramic phases.     

Tetralin hydrogenation catalyzed by Mo2C/Al2O3 and WC/Al2O3 in the presence of H2S

Supported molybdenum and tungsten carbides were synthesized by temperature-programmed reactions. These materials were characterized by XRD, EDS analysis, HRTEM and CO chemisorption. Hydrogenation of tetralin was carried out at a total pressure of 4 MPa (3.06 MPa of H₂), at 573 K, without or with sulfur (200 ppm of sulfur as DMDS). The resulting activities were compared with those of MoS₂/Al₂O₃ and Pt (1% (w/w) metal loading) supported on Al₂O₃ or SiO₂. In the absence of sulfur, WC/Al₂O₃ showed an initial activity similar to that of Pt/SiO₂, higher than that of MoS₂/Al₂O₃ but lower than that of Pt/Al₂O₃. In the presence of H₂S, WC/Al₂O₃ showed a steady-state activity similar to that of Pt/Al₂O₃ (which suffered a marked deactivation). Post-reaction characterization did not show any sulfur poisoning of the supported carbides. Therefore the supported carbides are sulfur-tolerant and promising catalysts for the hydrogenation of aromatics in diesel fuels in the presence of small amounts of S-containing compounds.     

自牺牲模板法制备氮掺杂碳化钼/碳析氢电催化剂

采用g-C₃N₄为自牺牲模板和氮源,葡萄糖为碳源,钼酸铵为钼源,制备具有二维纳米结构的氮掺杂碳化钼修饰碳纳米片(N-Mo₂C/C),并评价其电催化析氢性能。利用X射线衍射仪(XRD)、场发射扫描电镜(FESEM)、透射电镜(TEM)、拉曼(Raman)等测试手段对N-Mo₂C/C的组成、形貌及结构进行分析。结果表明,氮掺杂的Mo₂C纳米颗粒均匀分散在二维碳纳米片上,粒径主要分布在3~5 nm。利用电化学工作站测试 N-Mo₂C/C的电催化析氢性能,在1 mol/L KOH溶液中,电流密度为10 mA/cm²时其对应的过电势为185 mV,Tafel斜率为69 mV/dec,经20 h循环可维持稳定的析氢电势。     

Study of unsupported and active carbon supported β-Mo2C prepared from MoCl5 precursor

We have studied the preparation of unsupported and lignite supported β-Mo₂C using MoCl₅ as precursor. The catalysts were characterized by XRD and TEM techniques and tested in the reaction of propene hydrogenation at room temperature and pressure. Total carburization was observed at 923 K and although incomplete, carburization also occurred at 473 or 723 K. The high temperature supported carbide exhibited a remarkable lifetime in the test reaction (72 h), whereas, the massive carbide rapidly deactivated (<1 h). The low temperature carburized catalysts also rapidly deactivated (10 min). However, in a remarkable way, processing of the used catalysts by H₂ at 723 or 873 K, not only restored the initial activity, but increased their stability with time on stream (3 and >24 h, respectively). The correlation between the degree of carburization, dispersion and catalyst stability is discussed.     

Thermal stability of tungsten carbide in 7 mol.% calcia–zirconia solid solution matrix heat treated in argon

Zirconia polycrystals stabilised with 7 mol.% CaO containing 10 vol.% WC particles (Ca-PSZ/WC) were obtained by using zirconia nanopowder and WC micropowder. Cold isostatically pressed samples were pressureless sintered in argon at 1350–1950 °C. The influence of the sintering temperature and the incorporation of WC particles on the phase composition and mechanical properties of the composites were studied. Decomposition of WC due to the reaction with the zirconia matrix was found. W₂C and metallic tungsten were detected as decomposition products when heat treated below 1750 °C. At higher temperatures, ZrC is formed. The mechanism of WC decomposition was discussed. The zirconia polycrystals modified with in situ formed W and W₂C inclusions showed a bending strength of 417 ± 67 MPa, a fracture toughness of 5.2 ± 0.3 MPa m^0.5 and a hardness of 14.6 ± 0.3 GPa.     

TPD study and carbazole hydrodenitrogenation activity of nitrided molybdena–alumina

The relationship between the activity and surface molybdenum species of nitrided 12.5% MoO₃/Al₂O₃ was studied in the hydrodenitrogenation (HDN) of carbazole at 573 K and 10.1 MPa total pressure. The surface molybdenum species were determined by the desorption of nitrogen gas during TPD. The surface area of NH₃-cooled Mo/Al₂O₃ nitrided at 773 and 1173 K was decreased by 8% and 61% from 245 m² g⁻¹ of the fresh MoO₃/Al₂O₃, respectively. The NH₃-cooled Mo/Al₂O₃ catalysts had slightly higher surface area than the He-cooled catalysts. The HDN rate increased with increasing nitriding temperature in the HDN of carbazole on the nitride catalysts. The NH₃-cooled Mo/Al₂O₃ catalysts nitrided at 1173 K were the most active in carbazole HDN and the He-cooled catalyst nitrided at 773 K was the least.     

Hydrodenitrogenation of carbazole over a series of bulk NixMoP catalysts

The hydrodenitrogenation (HDN) of carbazole over bulk Ni_xMoP (0 ≤ x ≤ 1.1) catalysts is reported at 583 K and 3.0 MPa H₂. X-ray diffraction (XRD) revealed the presence of NiMoP and MoP in the Ni_xMoP catalysts. The Ni_xMoP had higher CO uptake, higher acidity and lower TOFs for the HDN of carbazole than MoP. However, the selectivity to bicylohexane (BCHX) was greater on the Ni_xMoP catalysts compared to MoP, and the Ni_0.07MoP had the highest BCHX selectivity and highest TOF among the Ni_xMoP catalysts. The improved selectivity is attributed to the enhanced CO uptake and acidity that resulted in increased hydrogenation of carbazole to terahydrocarbazole which in turn readily undergoes CN bond cleavage on acid sites to produce BCHX.     

High surface area silicon carbide doped with zirconium for use as catalyst support. Preparation, characterization and catalytic application

Zirconium doped SiC with a surface area from 88 to 200 m² g⁻¹ was synthesized using the shape memory concept method followed by calcination in air at a temperature of ≤480°C. The material obtained was composed of β-SiC and small ZrO₂ particles dispersed throughout the material matrix and a significant amount of an amorphous phase containing Si, Zr and O. Molybdenum oxycarbide, the active isomerization phase, supported on such a material displayed a similar behavior to that obtained on pure SiC for the n-heptane isomerization reaction. A comparison made with the molybdenum oxycarbide catalyst supported on pure ZrO₂ showed that the Zr doped SiC was not simply made of silicon carbide coated with a layer of ZrO₂ on the surface but probably an amorphous phase containing Si, Zr and O which displays a similar behavior as pure SiC.