Effect of synthesis conditions on the molybdenum nitride catalytic activity

Properties of the molybdenum nitrides supported on alumina as well as on active carbon has been investigated in the reactions of thiophene and vacuum gas oil (VGO) hydrodesulfurization (HDS) as well as in the reaction of cyclohexene with hydrogen. Supported molybdenum nitride was more active in thiophene hydrogenolysis than sulfided Mo/Al₂O₃. Also in the reaction of VGO HDS alumina supported molybdenum nitride was more active than sulfided counterpart, however Mo₂N supported on active carbon exhibit low activity. In the products of the cyclohexene reaction over supported Mo₂N methyl-cyclopentanes and benzene has been found.     

Hydrodesulfurization of dibenzothiophene on alumina-supported molybdenum nitride

An alumina-supported molybdenum nitrided catalyst was prepared and tested to determine its activity and selectivity during the hydrodesulfurization of dibenzothiophene. The nitrided catalyst was extremely active for the selective C-S bond breakage of dibenzothiophene to produce biphenyl.     

氮化钼深度加氢脱硫催化剂研究进展

介绍了γ-Mo2 N的制备方法 ,包括原料气空速、升温速率、氮化温度和恒温时间对γ-Mo2 N结构及其加氢脱硫活性的影响 ,以及不同的后处理方法和载体对γ-Mo2 N的加氢脱硫活性的影响。阐述了其作为加氢脱硫催化剂的作用机理 ,助催化组分的加入对其催化加氢反应的影响 ,并提出了其实现产业化所应解决的问题     

Structure of molybdenum oxide clusters prepared in zeolite cages

Molybdenum hexacarbonyl entrapped in NaY zeolite was oxidized with molecular oxygen by UV-irradiation at room temperature or by thermal treatment at 343–373 K. Both oxidation procedures resulted in the identical molybdenum(VI) oxide; molybdenum dimer species (Mo-Mo distance: 0.321 nm). The Mo-Mo bonding of the oxide species was degraded on an evacuation at 673 K, while it was considerably stable in the presence of gaseous oxygen.     

Boosting ammonia synthesis over molybdenum nitride through nickel-mediated nitrogen activation and hydrogen transfer

Synergistic optimization of nitrogen dissociation and hydrogen transfer might be an efficient strategy to develop a highly efficient ammonia synthesis catalyst. Herein, the Ni-modified molybdenum nitride is used as the catalyst of ammonia synthesis. The presence of the highly dispersed Ni metal results in enhancement of nitrogen vacancy, causing the acceleration of the exchange and reaction of the lattice N species with the nitrogen species from the gaseous phase. In addition, the presence of Ni species facilitates the hydrogen transfer and spillover, as well as easy hydrogen release. As a result, the optimized molybdenum nitride catalyst with 0.1 wt% Ni shows a 2.5-times higher catalytic activity than that of the catalyst without Ni species at 400°C owing to the coupling of nitrogen activation and hydrogen transfer effects. This general approach could inspire the rational design of other metal catalysts for ammonia synthesis.     

Spectral properties of spherical boron nitride prepared using carbon spheres as template

Spherical boron nitride nanoparticles have been successfully fabricated by temperature-controlled pyrolysis procedure in a N₂ atmosphere, using boron acid and urea as the precursors. The carbon spheres were prepared from glucose (C₆H₁₂O₆) by a hydrothermal method as a template to be used. Comprehensive scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier infrared spectrum (IR) characterizations all confirm that the obtained products are spherical boron nitride. The amount of C₆H₁₂O₆ and reaction time were found to affect the morphology and structure of the as-prepared products. The average diameter of the spherical boron nitride nanoparticles synthesized with the addition of C₆H₁₂O₆ is about 0.3–1 µm. The spherical boron nitride has a high surface area of 176.78 m²g⁻¹ and ~3.5 nm pore size. The as-synthesized nanospheres also exhibit strong photoluminescence (PL) bands at 436, 454, 486, and 616 nm under 312 nm excitation, indicating that they could have potential application in novel optical devices.     

Hydrodesulfurization of model diesel using Pt/Al2O3 catalysts prepared by supercritical deposition

Pt/Al₂O₃ catalysts with Pt loadings ranging from 0.5 to 11 wt.% were synthesized by supercritical carbon dioxide (scCO₂) deposition method. Transmission electron microscopy (TEM) images showed that the synthesized catalysts contained small Pt nanoparticles (1–4 nm in diameter) with a narrow size distribution, no observable agglomeration, and uniformly dispersed on the alumina support. The catalysts were found to be active for hydrodesulfurization of dibenzothiophene (DBT) dissolved in n-hexadecane (n-HD) without sulfiding the metal phase. The reaction proceeded only via the direct hydrogenolysis route in the temperature range 310–400 °C and at atmospheric pressure. The activity increased with increasing the metal loading. Increasing [H₂]₀/[DBT]₀ by either increasing [H₂]₀ or decreasing [DBT]₀, increased the DBT conversion. At a fixed weight hourly space velocity and feed concentration, conversion did not increase with increasing temperature beyond 330 °C. The presence of toluene inhibited the catalyst activity presumably due to competitive adsorption between DBT and toluene. Under the operating conditions, the reaction was far from equilibrium.     

Controlling dispersion and morphology of MoS2 nanospheres by hydrothermal method using SiO2 as template

Monodispersed MoS₂ nanospheres were successfully synthesized by using SiO₂ as hard template. The size and morphology of the MoS₂ nanospheres could be finely controlled by the content of SiO₂ and sulfur precursors. Furthermore, higher surface area of monodispersed MoS₂ nanospheres exhibited high reaction rate for hydrodesulfurization (HDS) of dibenzenethiophene (DBT).     

Kinetic study of the deep hydrodesulfurization of dibenzothiophene over molybdenum carbide supported on a carbon black composite: Existence of two types of active sites

Hydrodesulfurization (HDS) is part of the hydrotreating process, which is an ensemble of several reactions (HDN, HDO, HDS, etc.) taking place simultaneously at the industrial scale. Only sulfur is currently submitted to drastic Europeans specifications and conventional commercial catalysts cannot reach the specifications at low cost. This paper presents the behavior of a potential substitute catalyst tested for the deep HDS of a model molecule such as the dibenzothiophene (DBT). The substitute is molybdenum carbide supported on a mesoporous carbon black composite of surface area 240 m² g⁻¹. A global kinetic study of the deep HDS of DBT (300 ppm S) was performed at 623 K and 5.0 MPa and a global kinetic model was proposed as well as global rate constants were calculated to obtain theoretical plots of concentration versus contact time to compare with the experimental data. The kinetic model and global kinetic orders were confirmed as an acceptable correlation was found between calculated and experimental data. Furthermore, the determination of the global kinetic orders indicated that two types of active sites must be present on the surface in order to explain the observed results.     

Synthesis and properties of oligo- and polymethylsilazanes as a precursor for silicon nitride

The synthesis and properties of polymethylsilazanes were investigated. Controlled ammonolysis of methyl(triisocyanato)silane (MTIS) at the molar ratios NH₃/MTIS=4 and 6 gave polymethylsilazanes3 and4, which were soluble in tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO) but insoluble when isolated as powders. The concentrated polymer solution showed spinnability and film formation. Thermogravimetric analysis of3 and4 showed weight losses of 67 and 16%, respectively. Species4 may be a precursor for silicon/carbon/nitrogen ceramics.     

Effect of NiS addition on nitridation of alumina to aluminum nitride using polymeric carbon nitride as a nitridation reagent

We investigated the effect of adding nickel(II) sulfide (NiS) on nitridation of alumina (Al₂O₃) to aluminum nitride (AlN) using polymeric carbon nitride (PCN), which was synthesized by polymerization of dicyandiamide at 500 °C. The product powders obtained from nitridation of a mixture of δ-Al₂O₃ and NiS powders (mole ratio of 1:0.01) at various reaction temperatures were characterized by powder X-ray diffraction, ²⁷Al magic-angle spinning nuclear magnetic resonance, and Raman spectroscopy. δ-Al₂O₃ began to convert to AlN at 900 °C and completely converted to AlN at 1300 °C. The as-synthesized sample powders contained nitrogen-doped carbon microtubes (N-doped CMTs) with a length of several tens of mm and thickness of ca. 3 µm. The addition of NiS to δ-Al₂O₃ resulted in the enhancement of the amount of N-doped CMTs and nitridation rate, which might be due to the catalytic action of Ni particles on the thermal decomposition of vaporized PCN. The change in Raman spectra with reaction temperatures indicated that the crystallinity of N-doped CMTs was increased by calcining at higher reaction temperatures.     

Synthesis and characterization of mesostructured tungsten nitride by using tungstic acid as the precursor

Mesostructured tungsten nitride was firstly prepared from tungstic acid via the temperature programmed reaction with ammonia. The N₂ adsorption isotherm of as-synthesized tungsten nitride was of type IV with a type H-3 hysteresis loop. BJH pore size distribution was of bimodal distribution (2.5 and 3.5 nm), among which the latter was the main channel of tungsten nitride. The surface area of as-synthesized tungsten nitride was up to 89 m² g⁻¹. XRD pattern showed that the crystal phase of the product was β-W₂N. The effect of synthesis parameters on the surface area of tungsten nitride was investigated extensively. The nitridation mechanism was investigated by in-situ XRD and N₂ adsorption analysis. It was found that H₂WO₄ was initially transformed into WO₃ by eliminating the axial water molecules, and WO₃ retained the layered and porous structure of H₂WO₄. Below 773 K, WO₃ was just partially reduced to W₂₀O₅₈ and W₂₀O₄₀. Above 773 K, β-W₂N phase could be detected. It indicated that during nitridation, WO₃ was gradually reduced and then the homogeneous substitution of oxygen vacancies in the reduced oxides with nitrogen atoms occurred. Based on the experimental results, a reduction-nitridation mechanism was firstly proposed.     

Influence of the V + Mo/Al ratio on vanadium and molybdenum speciation and catalytic properties of V–Mo–ZSM-5 prepared by solid-state reaction

V–Mo–ZSM-5 catalysts with various composition prepared by solid-state ion exchange were investigated with respect to their physico-chemical characteristics using chemical analysis, XRD, BET, DRIFT, UV–vis, ²⁷Al MAS-NMR spectroscopy, H₂ TPR and TPD of NH₃. It was found that all the preparations leads to either metal ions sitting at the bridging oxygen of Si–OH–Al or anchored at Si–OH groups or deposited as oxide. These different solids were tested in the selective catalytic reduction of NO_x by ammonia. The main result is that upon addition of small amount of Mo to V–ZSM-5, catalytic performances were enhanced.     

Methanol synthesis over Pd/SiO2 with narrow Pd size distribution prepared by using MCM-41 as a support precursor

All silicious MCM-41 was investigated as a support or a support precursor for Pd/SiO₂ and prepared catalysts were tested for methanol synthesis from CO and H₂. The methods of Pd loading on the MCM-41 were impregnation, seed impregnation and chemical vapor deposition (CVD). For both impregnations, most Pd existed outside of the pore as large particles, and only a small part of Pd was inserted into the pore of MCM-41 retaining the initial structure. On the contrary, in the catalyst prepared by CVD method, the MCM-41 structure was completely destroyed to become amorphous SiO₂. Yet the average Pd particle size in this catalyst was smaller and its distribution was narrower than those of the catalysts prepared by impregnation methods. In the methanol synthesis from CO hydrogenation the catalyst prepared by CVD showed higher methanol selectivity than other MCM-41-derived catalysts. This result was considered to be due to the more uniform distribution of the Pd particle size.     

Photoelectrochemical water oxidation using hematite modified with metal-incorporated graphitic carbon nitride film as a surface passivation and hole transfer overlayer

Hematite (α-Fe₂O₃) is renowned as a promising photoanode for water oxidation, even though it displays poor photoconversion efficiency. In this study, ∼5 nm-thick graphitic carbon nitride (g-C₃N₄; CN) and metal-incorporated CN (M-CN; M = Ag, Fe, Co) films are uniformly deposited on hematite via a facile one-step evaporation method. Herein, the Co-CN layer leads to the highest photoelectrochemical activity with hematite-based photoanode. The subsequent loading of Co-CN layer with oxygen evolution catalysts (FeNiOOH and CoOOH) further enhances photocurrent density to ∼3.5 mA cm⁻² and oxygen evolution at > 95 % of Faradaic efficiency over 24 h at E = 1.23 V. Detailed analysis based on spectroscopic and electrochemical measurements demonstrate that the primary role of CN layer is improving the charge separation efficiency by passivating the hematite surface. Then the incorporated metals contribute to reducing charge transfer resistance and thereby mediating hole transfer to interfacial water.     

溶剂对柠檬酸络合燃烧法制备的CuO-CeO_2催化剂催化氧化碳烟的影响

分别用无水乙醇和水作溶剂的柠檬酸络合燃烧法制备催化剂Cu_(0.05)Ce_(0.95)-A和Cu_(0.05)Ce_(0.95)-W,使用程序升温氧化法(TPO)评价催化剂对碳烟的氧化活性,通过X射线衍射(XRD)和傅立叶红外光谱(FTIR)对催化剂进行表征,并采用程序升温还原(H_2-TPR)和程序升温脱附(O_2-TPD)测试催化剂的储氧性能和供氧能力。结果表明,Cu_(0.05)Ce_(0.95)-A与Cu_(0.05)Ce_(0.95)-W的晶体结构相同,均形成了Cu-Ce-O固溶体,但仅在Cu_(0.05)Ce_(0.95)-A表面发现高度分散的CuO。Cu_(0.05)Ce_(0.95)-W具有较好的储氧能力,对碳烟的氧化活性较高。     

An evaluation on PAH degradation and characteristics as media of PVA-derivative hydrogels prepared by using a CGA technique

We manufactured PVA-derived hydrogels wth some crosslinkers by using a foam generation technique. Amino acids gels showed remarkably higher swelling ratios, probably because of the highly crosslinked network along with hydrogen bonds. Boric acid and starch would catalyze dehydration while structuring to result in much lower water content and accordingly high gel content, leading to less elastic, hard gels. Bulky materials such as ascorbic acid or starch produced, in general, large pores, and also nicotinamide, highly hydrophobic, was likely to enlarge its pore size, thus leading to reduced swelling. Hydrophilic (or hydrophobic), functional groups which are involved in the reaction or physical linkage, and bulkiness of crosslinkers were found to be more critical to the crosslinking structure and its density than molecular weights that seemed to be closely related to pore sizes. The average sizes of pores were 20 μm for methionine, 10–15 μm for citric acid, 50–70 μm for L-ascorbic acid, 30–40 μm for nicotinamide, and 70–80 μm for starch. Also, amino acid and glucose gels were more elastic than the others. The elasticity of a gel was reasonably correlated with its water content or swelling ratio. On the other hand, L-ascorbic acid among glucose, methionine, citric acid and vitamins, imparted not only the most favorable physical properties and the greatest cell density but also the highest PAH degradation on its derivative gels. The higher biomass ensured the higher degradation rate. The maximum cell density was 0.267 mg/g-hydrogel and degradation rates and efficiencies ranged 0.013–0.007 mM/mg/day and 92-48%, respectively.     

Nanofiltration membranes prepared by direct microemulsion copolymerization using poly(ethylene oxide) macromonomer as a polymerizable surfactant

A novel polymer membrane with nanosized pore structures has been prepared from the direct copolymerization of acrylonitrile (AN) with a polymerizable nonionic surfactant in water‐in‐oil (w/o) or bicontinuous microemulsions. This polymerizable surfactant is ω‐methoxy poly(ethylene oxide)₄₀ undecyl‐α‐methacrylate macromonomer [CH₃O (CH₂CH₂O)₄₀ (CH₂)₁₁ OCO(CH₃)CCH₂, abbreviated: C₁‐PEO‐C₁₁‐MA‐40]. Besides PEO macromonomer, AN, and crosslinker ethyleneglycol dimethacrylate, the microemulsion system contained varying amount of water that formed w/o microemulsions having water droplet structures and bicontinuous microemulsions consisting of interconnected water channel. The polymerized membranes prepared in this study have pore radii ranging from 0.38 to 2.4 nm as evaluated by PEG filtration. The pore size appears to vary linearly with water content in precursor microemulsions. But a sharp change in the gradient of the linear relationship is observed around 25 wt % water content. Membranes made from bicontinuous (>25 wt % water) microemulsion polymerization have a larger and interconnected (open‐cell) nanostructures. In contrast, much smaller closed‐cell (disinterconnected) nanostructures were obtained from w/o (<25 wt % water) microemulsion polymerization and the membrane exhibited a permselectivity toward water in pervaporation separation of high ethanol (>50 wt %) aqueous solutions. The separation factor (α) for 95% ethanol aqueous solution by the membrane derived from the microemulsion containing 10 wt % water is about 20. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2785–2794, 2000     

A study of the oxidative coupling of methane over SrO-La2O3/CaO catalysts by using CO2 as a probe

20%SrO-20%La₂O₃/CaO catalyst (SLC-2), prepared by impregnation, has shown 18% CH₄ conversion and 80% C₂-selectivity for the oxidative coupling of methane (OCM) at 1073–1103 K with CH₄O₂ molar ratio=91 and total flow rate of 100 ml/min. Addition of SrO onto La₂O₃/CaO (LC) catalyst strengthens the surface basicity and leads to an increase in CH₄ conversion and C₂-selectivity. Meanwhile, the reaction temperature required to obtain the highest C₂-yield increases with increasing SrO content. The formation of carbonate on the catalyst surface is the main reason for the deactivation of LC and SLC catalysts. If the amount of CO₂ added into the feed is appropriate and the reaction temperature is high enough, there is no deactivation at all. In such case, the added CO₂ will suppress the formation of CO₂ produced via the OCM reaction, therefore, improves the C₂-selectivity. The FT-IR spectra of CO₂ adspecies recorded at different temperatures show that CO₂ interacts easily with the catalyst surface to form different carbonate adspecies. Unidentate carbonate is the main CO₂ adspecies formed on the catalyst surface. On the LC catalyst surface, the unidentate carbonate was first formed on Ca²⁺ cations at room temperature. If the temperature is higher than 473 K, it will form on La³⁺ cations. On the SLC catalyst surface, if the temperature is lower than 573 K, only the unidentate carbonate formed on Ca²⁺ cations could be observed. When the temperature is higher than 673 K, it will then form on Sr²⁺ cations. This suggests that the unidentate carbonate can migrate on the LC and SLC catalyst surface on one hand, and on the other hand, that the surface composition of SLC catalysts is dynamic in nature. On the basis of both the decomposition temperatures of the carbonate species, and the temperature dependence of the value which is the difference of symmetric and asymmetric stretching frequencies of surface carbonates, the in situ FT-IR technique offered two approaches to measure the surface basicity of the SLC catalyst. The results thus obtained are in good agreement with that of CO₂-TPD. The role of the surface basicity of the SLC catalyst is also discussed.