Electrochemical and quantum chemical studies of the reactions of transition metals M (M = Co, Fe and Ni) with LiF and Li2O

It has been a puzzle that transition metals can unexpectedly react with lithium-based matrixes of LiF and Li₂O in the potential versus Li/Li⁺ range from 0.01 to 3.5 V at room temperature. The electrochemical and theoretical investigations on the reactions of transition metals M (M = Co, Fe and Ni) with LiF and Li₂O were carried out. The electrochemical reactivity of metal cobalt with LiF and Li₂O has been examined by the discharge and charge, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. Density functional theoretical calculation results suggested that the stable compounds MLiF and MLi₂O could be formed by the insertion of transition metal (M) in lithium-based matrixes with exothermic as an intermediate. The theoretical calculations provide an understanding in chemical reaction of M with LiF and Li₂O. The small molecular or clusters reaction may play an important role in the electrochemical reaction of metal with transition Li₂O or LiF, which could be used to explain for the unexpectedly reaction of transition metal with LiF and Li₂O.     

Dispersion and Solid State Ion Exchange of VCl3, CrCl3?6H2O, MnCl2?4H2O and CoCl2?6H2O onto the Surface of NaY Zeolite Using Microwave Irradiation

Transition metal/Y zeolites have been prepared by solid state ion exchange with microwave irradiation of mechanical mixtures of VCl₃, CrCl₃ ⋅ 6H₂O, MnCl₂ ⋅ 4H₂O or CoCl₂ ⋅ 6H₂O with NaY zeolite at 900 W in 10–20 min. The prepared transition metal/Y zeolites were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), simultaneous thermal analysis (STA) and surface area measurement (BET). Concentration of dispersed transition metal on zeolite was measured and results revealed that transition metal was dispersed and ion exchanged onto the surface of NaY zeolite.     

New photocatalyst for the degradation of organic dyes based on [Co2(1,4-BDC)(NCP)2]n·4nH2O

One metal–organic framework, [Co₂(1,4-BDC)(NCP)₂]_n·4nH₂O (1) (1,4-H₂BDC denotes benzene-1,4-dicarboxylic acid, HNCP denotes 2-(4-carboxyphenyl)imidazo(4,5-f)(1,10)phenanthroline), has been hydrothermally synthesized and characterized via single crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis and diffuse-reflectance UV–vis spectrum study. Compound 1 shows a 2D layer structure, which is further extended into a 3D supramolecular network structure through π–π stacking interactions. The photocatalytic study on compound 1 indicates that it is an active catalyst for the degradation of orange G, rhodamine B, methylene blue and methyl violet.     

MoS2 morphology and promoter segregation in commercial Type 2 Ni–Mo/Al2O3 and Co–Mo/Al2O3 hydroprocessing catalysts

A series of commercial liquid-phase-sulfided Type 2 Ni–Mo/Al₂O₃ and Co–Mo/Al₂O₃ catalysts used in different trickle-phase reactions has been characterized by (scanning) transmission electron microscopy combined with energy-dispersive X-ray analysis ((S)TEM-EDX) and X-ray diffraction (XRD) analysis. In contrast to what is reported for H₂S/H₂ sulfided Type 2 model catalysts, MoS₂ stacking is not prominent in liquid-phase sulfided Type 2 commercial catalysts. The presence or absence of MoS₂ stacking has been shown to be highly dependent on the sulfidation procedure applied. Although Type 2 commercial Ni–Mo catalysts have high MoS₂ dispersion, there is a significant Ni sulfide segregation. This is not the case for their Co-containing counterparts.     

Model study of pyrite demineralization by hydrogen peroxide oxidation at 30 °C in the presence of metal ions (Ni2+, Co2+ and Sn2+)

Dissolution of pyrite involving oxidation by hydrogen peroxide (H₂O₂) in the presence of metal ions (Ni²⁺, Co²⁺ and Sn²⁺) has been investigated. Before oxidation, pure and well crystalline structure of the acid washed pyrite sample, used in the present investigation, was confirmed by X-ray diffraction and chemical analysis. Oxidation of pyrite was examined by the determination of soluble sulfur. The rate of oxidation of pyrite with H₂O₂ is best represented by determining the rates of total soluble sulfur production. Each experiment was carried out for short (1–4 h) and extended (24 h) time periods. Pyrite is oxidized by H₂O₂ (1:1) up to the extent of 31.3% at short time period, which however remained the same even at extended time period. Increased amount of soluble sulfur has been observed when pyrite was oxidized by H₂O₂ (1:1) in the presence of Ni²⁺ or Co²⁺ or Sn²⁺ ion at short time period. The effectiveness of these metal ions in relation to pyrite oxidation at short time period decreases in the order Co²⁺>Sn²⁺>Ni²⁺, while at extended time period the order is Co²⁺>Ni²⁺>Sn²⁺. With Co²⁺ ion, the highest pyrite oxidation is obtained both at short (34.0%) and extended (35.0%) time period, while it is the lowest 31.3% with Ni²⁺ ion at short time and 25.3% with Sn²⁺ ion at extended time period. The effect of chloride ion on the rate of oxidation of pyrite is not pronounced in the metal ion containing systems. Substantial depletion in the concentration of externally added metal ions is in good agreement with the level of oxidation and infers certain adsorption or precipitation of metal ions on pyrite surface. The results of this study throw a new light of the influence of metal ions in the dissolution of pyrite in oxidation systems and has considerable applications in fields of demineralization, desulfurization and environmental science.     

Fischer–Tropsch synthesis over Co/TiO2 catalyst: Effect of catalyst activation by CO compared to H2

Co/TiO₂ catalyst activation for Fischer–Tropsch (FT) reaction by CO in comparison to H₂ has been performed. The catalyst, prepared by incipient wetness impregnation, has been characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses after separate reduction using CO and H₂ respectively. Temperature programmed reduction (TPR) analyses were also conducted to study the reduction behaviour of the catalyst in presence of H₂ and CO respectively. CO improved catalyst reduction and produced a more stable and active catalyst with higher selectivity and yield for C_5 + hydrocarbons at extended time-on-stream.     

One novel polynuclear complex [Cu2(bpc)2(N3)4Ca(H2O)2Na2(H2O)2]n having three different transition and non-transition metal centres

The complex [Cu₂(bpc)₂(N₃)₄Ca(H₂O)₂Na₂(H₂O)₂]_n (1) (bpc²⁻=2,2^′-bipyridyl-3,3^′-dicarboxylate) has been prepared and characterised by elemental analyses, spectroscopic and single crystal X-ray diffraction study. Single crystal X-ray analysis revealed a novel polynuclear complex containing three different metal centres such as an alkali metal (sodium), an alkaline earth metal (calcium) and a transition metal atom (copper) connected together by azide and carboxylato groups.     

Ethene hydroformylation on Co/SiO2 catalysts

The results from ethene hydroformylation at 173°C showed that a Co(acac)₃/SiO₂ catalyst prepared from Co(acac)₃ precursor by gas‐phase deposition was three times as active as a catalyst prepared by impregnation from cobalt nitrate, but oxo‐selectivities were similar. The high propanal selectivities on the Co(acac)₃/SiO₂ seem to be related to the presence of highly dispersed active sites favouring CO insertion. As dispersion is decreased from 23 to 8% due to increasing metal content (from 5 to 16 wt%), oxo‐selectivity decreased from 39 to 25%. The activity of Co(acac)₃/SiO₂ remained unchanged during 68 h on stream. The gas‐phase deposition technique described here is a promising method for the preparation of active, selective and stable heterogeneous hydroformylation catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation of thin film alloys with electrodeposition from heterotrinuclear M1–M2–M1 complexes (M1: NiII, CuII; M2: CuII, CoII, ZnII, CdII)

M₁ ^II–M₂ ^II–M₁ ^II type linear complexes (where M₁ = Ni²⁺, Cu²⁺ and M₂ = Cu²⁺, Zn²⁺, Co²⁺ and Cd²⁺ were prepared and dissolved in dimethyl sulfoxide. They were then electrodeposited on mild steel surfaces by the use of rotating disc electrodes. The deposition potentials were determined from cyclic voltammetric i-E scans. The metal films deposited on mild steel surfaces were characterized with atomic absorption spectrometer (AAS), X-ray fluorescence (XRF) and ion chromatography (IC) methods. Although the stoichiometric quantity of M₂ metal cation was half of the M₁ metal cation in the complex, the amount of M₂ metal deposited upon the surface was markedly greater. The amount of M₂ ion deposited on the surface was found to increase with the hardness of the ion. The X-ray diffraction (XRD) patterns showed that the excessively deposited metal on the surface was in metallic form as well as alloy. The size of the deposited film particles was investigated by the use of atomic force microscope (AFM) technique and the particles were observed to be bigger than nanoparticle size.     

Synthesis and characterisation of La(Co1/2Ti1/2)O3

Single phase, dense La(Co_1/2Ti_1/2)O₃ (LCT) ceramics have been fabricated using conventional solid state synthesis. Samples were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and their dielectric properties were studied at radio and microwave frequencies. X-ray and electron diffraction conclusively revealed that LCT contained in-phase and antiphase rotations of the O-octahedra, consistent with and a⁻a⁻c⁺ tilt system in the Glazer classification. However, XRD indicated that the Co and Ti ions were disordered on the B-site whereas TEM and Raman spectroscopy exhibited reflections and modes which suggested that partial ordering may be present. Moreover, some Raman bands could only be explained by assuming that at least some of the octahedra exhibited a Jahn–Teller distortion. Dielectric measurements indicated that LCT is insulating with low dielectric loss, 0.0024 at 1 MHz and frequency independent relative permittivity, ε_r=25. A quality factor, Q×f_o=38,000 was obtained at microwave frequencies along with a temperature coefficient of the resonant frequency, TCF=−42 MK⁻¹.     

Effect of CO2 in the feed stream on the deactivation of Co/γ-Al2O3 Fischer–Tropsch catalyst

The influence of CO₂ on the deactivation of Co/γ-Al₂O₃ Fischer–Tropsch (FT) catalyst in CO hydrogenation has been investigated. The presence of CO₂ in the feed stream reveals a negative effect on catalyst stability and in the formation of heavy hydrocarbons. The CO₂ acts as a mild oxidizing agent on cobalt metal during Fischer–Tropsch synthesis. During FT synthesis on Co/γ-Al₂O₃ of 70 h, the CO conversion and C₅₊ selectivity in the presence of CO₂ decreased more significantly than in the absence of CO₂. CO₂ is found to be responsible for the partial oxidation of surface cobalt metal at FT synthesis environment with the co-existence of generated water.     

Improved electrochemical performance of layered LiNi0.4Co0.2Mn0.4O2 via Li2ZrO3 coating

A Li₂ZrO₃ coating technique was successfully applied to layered lithium transition metal mixed oxide LiNi_0.4Co_0.2Mn_0.4O₂ to enhance its electrochemical performance using Zr(NO₃)₄·5H₂O and CH₃COOLi·2H₂O as coating reagents. The existence of Li₂ZrO₃ coating layer was identified by X-ray diffraction (XRD), Environmental scanning electron microscopy (ESEM), Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The Li₂ZrO₃ shell decreased the exposed area of electrode core to electrolyte, and thus suppressed the dissolution of transition metal and side reaction between them. In addition, coating with Li₂ZrO₃ improved the ion transportation characteristics. As a result, an enhanced electrochemical performance in terms of discharge capacity, rate performance, and capacity retention were obtained, especially at higher temperature of 50 °C. Analysis of electrochemical impedance spectra (EIS) showed that the coated material exhibited lower charge transfer resistance (R_ct) with less variation during cycling, which indicated the electrode/electrolyte interface of coated material was more favorable and stable for electrochemical reaction.     

Effect of citric acid content on synthesis of LiNi1/3Mn1/3Co1/3O2 and its electrochemical characteristics

LiNi_1/3Mn_1/3Co_1/3O₂ has been synthesized using different citric acid (chelating agent) contents to study the effect on morphology and electrochemical characteristics of the powdered compound synthesized. The citric acid content is expressed as R′ which is the ratio of citric acid to metal ions. The processing with large value of R′ yields the powder having particle size of about 200 nm. X-ray diffraction (XRD) analysis shows the powder has single phase layered rhombohedral structure. First cycle coulombic efficiency of the powder prepared with R′ = 3, is ～93% in the voltage range of 4.6–2.5 V.     

Low-temperature Oxidation of Carbon Monoxide on Co/ZrO2

A 10%Co/ZrO₂ catalyst prepared by impregnation was tested for its activity for the oxidation of CO to CO₂ in excess oxygen. Activity tests showed that conversion could be obtained at temperatures as low as 20 °C. Time-on-stream studies showed no loss of activity in these experiments, indicating that this catalyst is stable in the experimental oxidizing conditions. The activation energy for the CO to CO₂ oxidation reaction was calculated as E_a = 54 kJ/mol over this catalyst. Characterization of the material by thermogravimetric analysis, temperature-programmed techniques, X-ray photoelectron spectroscopy, and laser Raman spectroscopy indicate that Co₃O₄ is present on monoclinic ZrO₂ after the calcination of the catalyst.     

Electronic structure and magnetic properties of graphene/Co composite

The results of measurements of XPS spectra and magnetic properties of powder graphene/Co composite prepared by hydrogen reduction of CoCl₂·6H₂O deposited on few-layers graphene matrix are presented. XPS Co 2p measurements show two sets of 2p_3/2,1/2-lines belonging to oxidized and metallic Co-atoms. This means that metal in composite is partly oxidized. This conclusion is confirmed by magnetic measurements which show the presence of the main (from the metal) and shifted (from the oxide) hysteresis loops. The presence of oxide layer on the metal surface prevents the metal from the full oxidation and (as shown by magnetic measurements) provides the preservation of ferromagnetic properties after long exposure in ambient air which enables the use of graphene/metal composites in spintronics devices.     

Strong dependence on pressure of the performance of a Co/SiO2 catalyst in Fischer–Tropsch slurry reactor synthesis

A 10 wt% Co/SiO₂ catalyst was prepared by the incipient wet-impregnation method and tested in Fischer–Tropsch synthesis in a slurry reactor under conditions approaching industrial practice. The catalyst precursor was investigated by X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and photoelectron spectroscopy (XPS). The XPS and XRD techniques revealed the presence of a crystalline Co₃O₄ spinel-type phase, while-in addition-TEM and XPS analyses pointed to the formation of another amorphous Co₃O₄ spinel phase, both species interacting weakly with the silica substrate. The influence of total pressure on the conversion, selectivity and stability of the catalyst was studied. Upon increasing the overall pressure from 20 to 40 bar, not only activity increased but also the catalyst are not deactivating. These results are explained in terms of an increase of gases solubility in the solvent, this increment of CO concentration in the liquid phase favours carbonyl species formation and the cobalt particles segregation that implies an increase in the metal surface area.     

Encapsulation mechanism of N2 molecules into the central hollow of carbon nitride multiwalled nanofibers

Nitrogen molecules have been encapsulated into the central hollows of vertically aligned carbon nitride (CN) multiwalled nanofibers by dc plasma-enhanced chemical vapor deposition with C₂H₂, NH₃, and N₂ gases on a Ni/TiN/Si(1 0 0) substrate at 650 °C. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectra showed the existence of nitrogen molecules in CN nanofibers. Elemental mapping images with electron energy loss spectroscopy of the CN nanofiber and catalyst metal, and optical emission spectroscopy spectra of the plasma showed the distribution of nitrogen atoms and molecules in the CN nanofiber, catalyst metal, and gaseous precursor, respectively. These studies showed that atomic nitrogen diffused into the catalytic metal particle because of the concentration gradient and then saturated at the bottom of the particle. Saturated nitrogen atom participated in the formation of the CN nanofiber wall but most of nitrogen was trapped in the central hollow of the nanofiber as molecules.     

Wetting and interfacial phenomena in relation to joining of alumina via Co/Nb/Co interlayers

When designing multilayer metallic interlayers for transient-liquid-phase (TLP) bonding of ceramics, the phenomena that occur at the interface between the liquid metal alloy formed at the joining temperature and the solid ceramic to be joined have a major effect on the success or failure of the joining process. To assess the behavior of liquids that could develop when using Co/Nb/Co trilayers to bond Al₂O₃ ceramics, the wettability and interfacial behavior of pure Co and Co–Nb alloys on sapphire and polycrystalline Al₂O₃ were studied. Contact angles were measured at 1500 °C using the sessile-drop technique, and the microstructures of the resulting metal/ceramic interfaces were characterized by scanning electron microscopy and energy-dispersive spectroscopy. Observations were assessed and evaluated in the context of predictions of thermodynamic properties. Additions of Nb to Co reduced the contact angle and thus improved the adhesion between the ceramic and metal. Nb additions also enhanced dissolution of Al₂O₃.     

Investigations of the divalent metal ions Zn2+, Co2+, Ni2+ doped ultra-wide temperature stable BBNN system

Multi-component BaTiO₃–Bi_0.5Na_0.5TiO₃–Nb₂O₅ (BBNN) system doped with divalent metal ions (Zn²⁺, Co²⁺, Ni²⁺) was prepared by the conventional solid–state method.The X-ray diffraction patterns revealed all samples exhibited perovskite (P4mm) single phase. The dielectric properties and micro-mechanisms were discussed and validated. Novel theories, based on the characteristics of the different kinds of dielectric polarization, are proposed to explain the dielectric anomalies in the dielectric system. The relationships between microstructure and the dielectric properties were investigated systematically for the first time. The samples doped with 15 mol% Zn²⁺/Co²⁺presented good dielectric properties of an ultra-broad temperature stable range (from −50 and >300), high dielectric constant (ε~1925 for Zn²⁺/ ε~1341 for Co²⁺) and low dielectric loss (tan δ<0.02) were obtained. These features made the ceramics system have high practical values for miniaturization and harsh environments applications.     

Magnetic interactions and hysteresis loops study of Co/CoFe2O4 nanoparticles

Co/CoFe₂O₄ nanoparticles with the mean size of 8.8, 10.8 and 16.9 nm were prepared by thermal decomposition of metal salts in the presence of citric acid. The X-ray diffraction patterns and Rietveld refinements confirmed coexistence of Co-ferrite and metallic cobalt phases in the nano-powders. Scanning electron microscope images showed an increase in particles aggregates mean diameter with increasing the annealing temperature. Magnetic hysteresis loops showed a demagnetization jump at low fields, which was attributed to different reversal fields of ferrite and the cobalt phases. Field-dependent behavior of maximum magnetization (M_max), remanence (M_r), squareness (S) and coercivity (H_c) were studied through minor loops measurements. The calculated S value of the loops showed a maximum, between anisotropy and coercive fields. A sharp increase in H_c of larger particles was observed with increasing the applied field when compared to smaller particles. Henkel plots showed that the samples are interacting. Negative deviation of Henkel plots from linear behavior and negative δm plots revealed the dominant role of dipole–dipole interactions in the nano-aggregates.