HDS, HDN and HYD activities and temperature‐programmed reduction of unsupported transition metal sulfides

Several transition metal sulfides have been shown to exhibit high catalytic activities in various hydrotreating reactions. The simultaneous catalytic activities for hydrodesulfurization of dibenzothiophene, hydrodenitrogenation of indole and hydrogenation of naphthalene have been studied using high surface area bulk sulfides: NbS₂, MoS₂, ReS₂, RuS₂ and Co₉S₈. The metal–sulfur bond strengths have been assessed by temperature‐programmed reduction of H₂S‐pretreated sulfides, and it is demonstrated that this pretreatment is necessary to obtain reliable TPR data for the sulfides. The catalytic HDS activity is shown to follow the trend in the metal–sulfur bond strengths estimated by this improved method. Furthermore, the metal–sulfur bond strengths determined this way follow the trend estimated previously from theory. The TPR data suggest that a large number of sulfur vacancies exist in active catalysts under industrial reaction conditions. Such vacancies have recently been observed on model catalysts by use of STM. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the chemistry of ammonia over oxide catalysts: Fourier transform infrared study of ammonia,hydrazine and hydroxylamine adsorption over iron–titania catalyst

Topics in Catalysis - The adsorption and transformation of ammonia over Fe2O3/TiO2 catalyst has been investigated by FT-IR spectroscopy in vacuum at increasing temperature. Ammonia is first...     

The effect of metal–solvent properties on the alteration of specific zones on a carbon phase diagram

In this work Fe–Sb and Fe–Ge alloys (up to 10 wt.% Sb, Ge) were used as a solvent-catalyst for diamond synthesis at pressures of 5–6 GPa and temperatures of 1800–1900 K. Carbon solubility, capillary properties and synthesis performance of alloys were investigated. When using alloys with additive content up to 10 wt.%, rapid graphite to diamond transformation was observed. In spite of identical P,T-conditions and identical composition of a solvent–catalyst, different crystal morphology on the top and on the bottom sides of a diamond polycrystalline layer was formed, although their habit type {111} was identical. Thermodynamic and kinetic aspects of these phenomena are discussed.     

Ionic polymer–metal composite mechanoelectric transduction: effect of impedance

Ionic polymer–metal composites (IPMCs) are commonly used as soft actuators due to their electromechanical response. However, the reverse phenomenon, i.e. IPMC's ability to generate charge on application of mechanical strain (mechanoelectric response), is not very well understood. The concept of mechanoelectric transduction and its dependence on complex IPMC architecture comprising of electrode, polymer and composite layer is illustrated with a phenomenological model. The impedance model takes into account the charge transport inside the polymer and layer properties in terms of their impedances. The model lucidly indicates the significance of capacitance in IPMC transduction. The impedance model is used for studying IPMC step and frequency response and the effect of IPMC capacitance on its application as energy harvester.     

Anion-modified zirconia: effect of metal promotion and hydrogen reduction on hydroisomerization of n-hexadecane and Fischer–Tropsch waxes

The effect of metal promoters on the activity and selectivity of tungstated zirconia (8 wt.% W) for n-hexadecane isomerization in a trickle bed continuous reactor is studied by using different metals (Pt, Ni, and Pd) and, in one case, by varying metal loading. Platinum is found to be the best promoter. The effect of hydrogen reduction is investigated using platinum-promoted tungstated zirconia catalysts (Pt/WO₃/ZrO₂, 0.5 wt.% Pt and 6.5 wt.% W). Pretreatment at temperatures between 300 and 400°C for 3 h in hydrogen is found to be slightly beneficial for achieving high yields of isohexadecane. A platinum promoted sulfated zirconia (Pt/SO₄/ZrO₂) is compared with a Pt/WO₃/ZrO₂ catalyst for the hydroisomerization of n-hexadecane in the same reactor at the same n-hexadecane conversion. The former is a good cracking catalyst and the latter is suitable for use as a hydroisomerization catalyst. In a 27-ml microautoclave reactor, studies of the hydroisomerization and hydrocracking of two Fischer–Tropsch (F–T) wax samples are carried out. Severe cracking can be effectively suppressed using a Pt/WO₃/ZrO₂ catalyst so as to obtain branched isomers in the diesel fuel or lube-base oil range.     

The effect of catalyst and heat treatment on the properties of carbon–metal composites

Carbon gel and carbon–nickel–palladium doped gels (C–Ni–Pd) were prepared by carbonising resorcinol–formaldehyde (RF) hydrogel and resorcinol–formaldehyde–nickel–palladium (RF–Ni–Pd) hydrogels at 900 °C in a nitrogen atmosphere. RF and RF–Ni–Pd hydrogels were synthesized through sol–gel polycondensation followed by ambient drying. The aim of this study was the determination of the effect of heat treatment in air at 450 °C on the properties of C–Ni–Pd gels prepared using different Pd salts. In the present work, Ni was added as acetate whereas Pd was added as acetate (CA–Ni–Pd) and as chloride (CB–Ni–Pd). Samples were examined by scanning electron microscopy and X-ray diffraction. Surface area was characterized by N₂ adsorption at ?195.5 °C. Thermogravimetric analysis was carried out in order to determine the thermal characteristics of carbon gel and nickel–palladium composites in air atmosphere. CA–Ni–Pd composite had a higher activity and two-phase reaction compared to the CB–Ni–Pd composite. Further improvement of the electrolyte diffusion into the particles of nickel and palladium was obtained by oxidative thermal treatment. During this process a structural modification of the material took place, consequently leading to changes in the electrochemical properties of the composites.     

Low metal–insulator transition temperature of Ni-doped vanadium oxide films

Elemental doping is the main means to regulate the phase transition of vanadium oxide (VO₂); however, the effects of low valence elemental (<4+) doping on the phase transition of VO₂ are still controversial. In the present work, Ni-doped VO₂ films were prepared on quartz glass by direct current reactive magnetron sputtering and subsequent annealing. With the increase of the Ni doping content, the phase transition temperature of heating (T_H) of the VO₂ films decreased from 73.4 °C to 52.4 °C. The temperature required for the occurrence of phase transition (T_b) was lower than T_MIT. Different from the undoped VO₂ film, the Ni-doped VO₂ films had a T_b of around 30 °C. XRD and Raman results revealed that some rutile VO₂ microcrystals appeared in the vanadium oxide films because of the lattice distortion by incorporated Ni. Hence, rutile VO₂ micro-crystallinities significantly facilitated the phase transition of monoclinic VO₂ to rutile one.     

Hydration of ethene over W–P mixed metal oxide catalysts

W–P mixed metal oxide catalysts are active and selective for the gas-phase hydration of ethene to ethanol. The activity and selectivity of this catalytic reaction depend on the W/P atomic ratio. However, ethene conversion slightly decreases at higher W/(W + P) atomic ratio. The selectivity for ethanol increases with the W/P atomic ratio and reaches the highest value (92%) at W_0.81P_0.19O_x. The W_0.81P_0.19O_x catalyst is less active than the conventional H₃PO₄/SiO₂ catalyst, but the activity is maintained for a long time without the supply of any catalyst components. The reaction temperature does not affect substantially the rate of ethene hydration over the W_0.81P_0.19O_x catalyst. The H₂O/ethene molar ratio of 0.4 is the most appropriate for both reaction rate and selectivity. The active species of W–P mixed metal oxide are amorphous. But there is Keggin structure of W–P oxide species (PW₁₂O₄₀ ³⁻) in the presence of steam. And the species are the active sites for the hydration of ethene, confirmed by in situ Raman spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sol–gel and sonochemically derived transition metal (Co,Ni, Cu,and Zn) chromites as pigments: A comparative study

Cobalt chromite based pigments Co_1−xM_xCr₂O₄ (M=Ni, Cu, and Zn) with different transition metal concentrations (0≤x≤1 with a step of 0.25) have been synthesized applying two aqueous synthesis approaches: sol–gel and sonochemical synthesis routes. The heat treatment of precursor powders was performed between 600 and 800 °C. XRD analysis of the obtained powders revealed that all samples fabricated by sol–gel method have crystallised in a spinel structure, whereas sonochemical synthesis of Ni chromite at lower calcination temperatures showed the formation of mixtures of oxides. In addition, the degree of crystallinity and shaping of sonochemically obtained compounds is lower than sol–gel derived products. The chromites with a higher nickel concentration displayed green colour, while Cu-substituted pigments were nearly black. The spinels with a higher Zn amount were yellowish green.     

Styrene epoxidation over gold supported on different transition metal oxides prepared by homogeneous deposition–precipitation

Epoxidation of styrene by anhydrous tert-butyl hydroperoxide over a number of transition metal oxide (viz. TiO₂, Cr₂O₃, MnO₂, Fe₂O₃, Co₃O₄, NiO, CuO, ZnO, Y₂O₃, ZrO₂, La₂O₃ and U₃O₈) supported nano-size gold catalysts, prepared by the homogeneous deposition–precipitation method, has been investigated. The supported gold catalysts (except Au/MnO₂ and Au/U₃O₈) showed good styrene conversion activity and selectivity for styrene oxide in the epoxidation. The Au loading, Au particle size and performance in the epoxidation of the supported gold catalysts are found to be strongly influenced by the transition metal oxide support used in the catalyst. The Au/TiO₂ and Au/CuO are promising catalysts for the selective epoxidation.     

Rectification behaviour of some metal ion–metal interfaces and concentration cells

The rectification behaviour of three metal ion–metal interfaces and 38 concentration cells was studied. The rectification in AlAl³⁺Al was 35% (–0.4 to +0.80 V d.c.) between 2.0–5.0 V a.c. and for ZnZn²⁺ Al³⁺Al cell was 20% (+0.20 to –0.30 V d.c.). Its negative d.c. potential showed some similarity to a tunnel diode. 20% rectification was obtained when each of Al, Zn, Mg half-cell was coupled with I^–, I₂Pt half-cell and Al half-cell was coupled with Fe³⁺, Fe²⁺Pt half-cell. When the Zn half-cell was associated with Cr³⁺, Cr₂O₇ ^2–Pt half-cell the rectification was 15%, whereas the rectification in all other concentration cells varied from 1 to 12%. The possibility of obtaining much higher percentage of rectification can be explored in a large number of other metal ion–metal interfaces and concentration cells which can be assembled in a similar manner using the table of standard reduction potentials. The characteristics of a concentration cell can be varied by change in concentration of metal ion, redox ratio, variation of pH, temperature, effect of different additives to the cell solution, irradiation of electrode surface etc. Consequently, it will affect the percentage of rectification which may be of some use in commercial applications.     

Lanthanide-based metal–peptide frameworks prepared by ionothermal method: Anion direct effect,DFT calculation and luminescence property

A series of new isomorphic lanthanide-based metal–peptide frameworks, [Ln(PODC)(H₂O)₂]Br (Ln = La(1), Nd(2), Eu(3), Tb(4), H₂PODC = 2, 5-piperazinedione-1, 4-diacetic acid), were synthesized under the ionic liquid medium. Crystal structure measure results exhibit that they are new three-dimensional frameworks, where bromine anions of ionic liquid are enveloped in the pore. Interestingly, it is found that bromine anions can effectively counteract the effects of lanthanide ions (e.g. lanthanide contraction). Via the first-principles DFT calculation, it is obtained that bromine anions of ion liquid influence the electronic properties of PODC^2 − ligand and thereby directly control the structure. Luminescence analysis demonstrates that compounds 3 and 4 have good fluorescence properties, where the emission peaks can be ascribed to the transitions of ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) for 3 and ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) for 4.     

Separation of propylene and propane with a microporous metal–organic framework via equilibrium-kinetic synergetic effect

Adsorption separation of olefin and paraffin can greatly lower the energy consumption associated with the currently utilized distillation technique but remains a great challenge. Herein, we report the efficient separation of propylene (C₃H₆) and propane (C₃H₈) in a phosphate anion-functionalized metal–organic framework (MOF) ZnAtzPO₄ by synergetic effect of equilibrium and kinetics. The material features periodically expanded and contracted apertures decorated with electronegative groups, offering eligible pore shape and pore chemistry to effectively trap C₃H₆ under moderate isosteric heat of adsorption (27.5 kJ mol⁻¹) while obstruct the diffusion of C₃H₈. It simultaneously combines excellent thermodynamic selectivity (uptake ratio of 1.71) and kinetic selectivity (~31) for C₃H₆/C₃H₈ separation, meanwhile can be easily regenerated. Breakthrough experiment for C₃H₆/C₃H₈ gas mixture was conducted and confirmed the outstanding separation capability of ZnAtzPO₄. The equilibrium and kinetics cooperative C₃H₆/C₃H₈ adsorption separation was for the first time found in anion-functionalized MOFs, and further confirmed by computational studies.     

Syntheses,structures and properties of a series of 3s–5d–4f mixed metal substituted sandwich-type arsenotungstates

A class of 3s–5d–4f mixed metal substituted sandwich-type arsenotungstates [H₂N(CH₃)₂]₈H₃[LnNa(H₂O)₄(OH)WO(H₂O) (B-α-AsW₉ O₃₃)₂]·8H₂O [Ln = La^III (1), Ce^III (2), Pr^III (3)] have been isolated from an aqueous solution reaction system (pH = 4) of Na₂WO₄·2H₂O, C₂H₇N·HCl, NaAsO₂ and Ln(NO₃)₃·6H₂O and structurally characterized by elemental analyses, IR spectra, UV spectra and single-crystal X-ray diffraction. It is the most prominent in 1–3 that the [LnNa(H₂O)₄(OH) WO(H₂O)(B-α-AsW₉O₃₃)₂]^11 − polyanion consists of two trivacant Keggin [B-α-AsW₉O₃₃]^9 − moieties linked by one [WO(H₂O)]^4 + group and a dimeric [LnNa(H₂O)₄(OH)]^3 + group resulting in the special 3s–5d–4f mixed metal substituted sandwich-type assembly. Interestingly, lanthanide and sodium ions simultaneously occupy the two positions located at the central belt of 1–3 with the site occupancy of 50% for each position. Moreover, the electrochemical and electrocatalytic properties of only 1 and 2 have been measured by cyclic voltammetry (CV) in 0.5 mol·L^− 1 Na₂SO₄ + H₂SO₄ aqueous solution. 1 and 2 illustrate electrocatalytic activities for the hydrogen peroxide reduction.     

The effect of phase transition of crednerite/delafossite CuMn1−xCrxO2 on optical,thermal, and power factor properties

This study aims to investigate the effect of the phase transition of CuMn_1−xCr_xO₂ compound on the Jahn–Teller effect which in turn affects the optical, thermal, and thermoelectric power factor properties. The CuMn_1−xCr_xO₂ samples were synthesized by a solid-state reaction method. The ab initio computation was applied to evaluate the electronic and optical properties in order to confirm the experiment data. The appearance of the phase transition from crednerite CuMnO₂ to delafossite CuCrO₂ was confirmed by X-ray diffraction (XRD) and the ab initio computation through displaying the mixed crednerite/delafossite phase; and, the existence of the Jahn–Teller effect was confirmed by the X-ray photoelectron spectroscopy (XPS) technique exhibiting the occurrence of mixed-state Mn³⁺/Mn⁴⁺ ions. The results obtained from XRD, XPS, and the ab initio computation implied the decrease of the Jahn–Teller behavior with increased x content under the influence of the phase transition from the crednerite phase to the delafossite phase of CuMn_1−xCr_xO₂. Surprisingly, the Jahn–Teller distortion reduction caused an increase in the energy gap of the optical property, electrical resistivity, and activation energy in thermally activated band conduction. The effect suffered the specific heat behavior by being separated into two groups of crednerite and delafossite, and enhanced the small polaron behavior by increasing the activation energy of thermally activated band conduction. The phase transition reduced the results of thermal conductivity, thermopower, and thermoelectric power factor properties. In other words, the effect of the phase transition from the crednerite CuMnO₂ phase to the delafossite CuCrO₂ phase on CuMn_1−xCr_xO₂ compound reduced the Jahn–Teller effect with increased Cr content which in turn caused changes in the optical, thermal, and thermoelectric power factor properties. The effect of the phase transition is advantageous for the improvement of material properties.     

The role of solute conformation,solvent–solute and solute–solute interactions in crystal nucleation

The induction time for nucleation can differ based on the solutions used to conduct a crystallization, which can in turn impact the efficiency and economics of a crystallization process, the crystal size distribution, the morphology and ultimately functionality of the final product. Establishing a link between the nucleation pathway/solution structure and nucleation induction time is essential to achieve improved comprehension of the process of crystal nucleation from solution. In this study, the role of solute conformation, solvent–solute interaction, and solute–solute interaction in nucleation was examined using tolbutamide as a model compound in toluene and toluene–alcohol solutions. Through a combination of induction time experiments, attenuated total reflection Fourier transformed infrared spectroscopy, nuclear magnetic resonance spectroscopy, molecular dynamics simulations, and quantum chemical calculations, it was found that not only solvent–solute interactions but also solute–solute interactions and structural similarities between molecular self-assemblies in the solution and synthons in the crystal structure, can significantly influence the nucleation induction time.     

The influence of precursors and additives on the hydrothermal synthesis of VO2: A route for tuning the metal–insulator transition temperature

Thermochromic materials have attracted the attention of scientific and technological researchers due to their ability to change color depending on the temperature. Vanadium dioxide (VO₂) is capable of considerable polymorphs and has aroused interest mainly because its metal–insulator transition (MIT) presents a thermochromic characteristic at a relatively low temperature. This work aimed to obtain vanadium oxide nanostructures using hydrothermal synthesis to tune the MIT temperature. Ammonium metavanadate or vanadium pentoxide was used as a precursor of vanadium, oxalic acid as a reducing agent, and sodium molybdate as an additive. The starting materials were homogenized and inserted in a hydrothermal reactor at 180 °C. After 24 h of synthesis, part of the resulting product was heat-treated at 400 °C for 3 h. The powders obtained were characterized by their structure, morphology, and thermal properties. The results showed a fiber/rod-shaped VO₂ (M) morphology. Distinct strategies were used to obtain the crystalline phase of interest (VO₂(M)), and the presence of a reversible change occurring at ~68 °C was evaluated according to the parameters from the VO₂ phase transition. The addition of sodium molybdate favored a 22% reduction in the MIT temperature when the precursor used was vanadium pentoxide, indicating possible doping in the structure increased the effects of smaller crystallite size and the presence of crystalline phases. This work opens new perspectives for applications of the vanadium oxides obtained, such as in thermal sensors and/or intelligent materials.     

Turbidimetric and intrinsic viscosity study of EVA copolymer–solvent systems

Both Hansen solubility parameter and Flory–Huggins interaction parameter of two EVA [Poly(ethylene-co-vinyl acetate)] copolymers with different vinyl acetate content have been obtained by means of intrinsic viscosity measurements. To calculate this last parameter it was also necessary to determine the theta solvent at different temperatures of the two EVA copolymers with turbidimetric measurements. The results indicate that the vinyl acetate content is a variable which influences the composition of the theta solvent and Flory–Huggins parameter (the higher the vinyl acetate content, the lower the Flory–Huggins parameter), although its influence over the Hansen solubility parameter is almost negligible.     

Epoxidation of cyclooctene over mesoporous Ga,Ga–Nb,and Ga–Mo oxide catalysts

We demonstrate the epoxidation of cyclooctene to epoxycyclooctane over mesoporous Ga and mixed Ga–Nb and Ga–Mo oxides synthesized via self-assembly hydrothermal-assisted approach. These mesoporous catalysts displayed high epoxide selectivities at moderate cyclooctene conversions. Mesoporous Ga oxides with average particle sizes between 2 and 3 μm exhibited higher cyclooctene conversions, while larger particle sizes between ~ 4.5–6.5 μm showed lower conversions. The incorporation of Nb led to an increase in the acidity of the resultant Ga–Nb mixed oxides. These mesoporous Ga–Nb mixed oxides displayed 80–100% selectivity for the epoxide at conversion levels of cyclooctene in the 17 to 30% range. Finally, a mesoporous Ga–Mo oxide with molar composition of 35% Mo displayed the highest cyclooctene conversion of all mesoporous samples at 60 °C. The conversion of this sample was 41% with 100% selectivity.