Characterization of Porous Nanocomposites Formed by Cobalt Ferrites Dispersed in Sol-Gel Silica Matrix

Composites of cobalt ferrite particles dispersed in a silica matrix (CoFe₂O₄/SiO₂) were prepared by the sol-gel process using tetraethylorthosilicate (TEOS) as a precursor of silica and metallic nitrates as precursors of ferrite. Samples of SiO₂ and CoFe₂O₄/SiO₂ were prepared in monolithic shape, dried at 110^∘C, treated at various temperatures and their characteristics were compared. After the thermal treatment, the surface area of the silica matrix decreased, above 700^∘C it densified, and above 1100^∘C it crystallized. The same heat treatment in the composite led to the crystallization of CoFe₂O₄ particles in the SiO₂ matrix and the increase in particle size, with the consequent increase in magnetization. The presence of particles in the matrix reinforced its structure, avoiding large changes in surface area and porosity and in the structure of the matrix after high temperature thermal treatment.     

Nanocomposites NiFe2O4/SiO2 and CoFe2O4/SiO2-Preparation by Sol-Gel Method and Physical Properties

This paper aims to characterise the systems NiFe₂O₄/SiO₂ and CoFe₂O₄/SiO₂ prepared by the sol-gel method. After heat treatment, the various samples have been studied by means of X-ray diffraction, Mössbauer spectroscopy, magnetic measurements and transmission electron microscopy (HR TEM).X-ray diffraction and Mössbauer spectra confirmed the presence of the spinel phase. HR TEM observations revealed the nanocrystals with the size in the range of 2–25 nm. Magnetic measurements showed a superparamagnetic behaviour of the samples heated at lower temperature (800°C) and ferrimagnetic character for the samples heated at higher temperature (900, 1000°C).The final phase composition of the heated samples depends on the preparation conditions. The samples, treated up to 300°C in vacuum and then subsequently heated at 800°C or 900°C, do not contain hematite (the most stable phase at higher temperatures). On the contrary, the samples heated at 1000°C or 1250°C display certain content of hematite.     

Physico-Chemical and Catalytic Study of the Co/SiO2 Catalysts

This paper is focused on the physico-chemical and catalytic properties of Co/SiO₂ catalysts. Silica-supported cobalt catalysts were prepared by sol-gel and impregnation methods and characterized by BET measurements, temperature programmed reduction (TPR_H2), X-ray diffraction (XRD), and thermogravimetry-mass spectroscopy (TG-DTA-MS). The sol-gel method of preparation leads to metal/support catalyst precursor with a homogenous distribution of metal ions into bulk silica network or on its surface. After drying the catalysts were calcined at 500, 700, and 900°C. The reducibility of the supported metal oxide phases in hydrogen was determined by TPR measurements. The influence of high temperature—atmosphere treatment on the phase composition of Co/SiO₂ catalysts was investigated by XRD and TG-DTA-MS methods. At least five crystallographic cobalt phases may exist on silica: metallic Co, CoO, Co₃O₄, and two different forms of Co₂SiO₄ cobalt silicate. Those catalysts in which cobalt was chemically bonded with silica show worse reducibility as a result of strongly bonded Co-O-Si species formed during high-temperature oxidation. The TPR measurements show that a gradual increase in the oxidation temperature (500–900°C) leads to a decrease in low-temperature hydrogen reduction effects (<600°C). The decrease of cobalt oxide reduction degree is caused by cobalt silicate formation during the oxidation at high temperature (T 1000°C). The catalysts were tested by the reforming of methane by carbon dioxide and methanation of CO₂ reactions.     

Cobalt-doped silica membranes for gas separation

Cobalt-doped silica membranes were synthesized using tetraethyl orthosilicate-derived sol mixed with cobalt nitrate hexahydrate. The cobalt-doped silica structural characterization showed the formation of crystalline Co₃O₄ and silanol groups upon calcination. The metal oxide phase was sequentially reduced at high temperature in rich hydrogen atmosphere resulting in the production of high quality membranes. The cobalt concentration was almost constant throughout the film depth, though the silica to cobalt ratio changed from 33:1 at the surface to 7:1 at the interface with the alumina layer. It is possible that cobalt has more affinity to alumina, thus forming CoOAl₂O₃. The He/N₂ selectivities reached 350 and 570 at 160 °C for dry and 100 °C wet gas testing, respectively. Subsequent exposure to water vapour, the membranes was regenerated under dry gas condition and He/N₂ selectivities significantly improved to 1100. The permeation of gases generally followed a temperature dependency flux or activated transport, with best helium permeation and activation energy results of 9.5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and 15 kJ mol⁻¹. Exposure of the membranes to water vapour led to a reduction in the permeation of nitrogen, attributed to water adsorption and structural changes of the silica matrix. However, the overall integrity of the cobalt-doped silica membrane was retained, given an indication that cobalt was able to counteract to some extent the effect of water on the silica matrix. These results show the potential for metal doping to create membranes suited for industrial gas separation.     

CoFe2O4/SiO2 nanocomposites by thermal decomposition of some complex combinations embedded in hybrid silica gels

The study reports the preparation of CoFe₂O₄/SiO₂ nanocomposites by a new modified sol–gel method starting from cobalt nitrate, iron nitrate, and diols: 1,2-ethanediol (EG), 1,3-propanediol (1,3PG), and tetraethylorthosilicate (TEOS), for final compositions of 30 %CoFe₂O₄/70 %SiO₂ and 50 %CoFe₂O₄/50 %SiO₂. The method is based on the formation of a Co(II), Fe(III)—carboxylate precursors mixture, during the redox reaction between the NO ₃ ^? ion and the diol (~140 °C) within the silica gels. The thermal decomposition of these complex combinations takes place at ~300 °C leading to the corresponding amorphous metal oxides within the pores of the hybrid gels. Depending on the subsequent thermal treatment, CoFe₂O₄ can be obtained as single phase or in a mixture with Co₂SiO₄. The CoFe₂O₄ crystallites sizes are in the nanometer range (3–10 nm). The obtained nanocomposites have a hard magnet behavior, as a result of the high anisotropy of CoFe₂O₄ having large hysteresis cycles.     

Mössbauer and related studies on the formation of some mixed oxide catalysts of iron and cobalt

A series of mixed oxides and ferrites of iron and cobalt has been prepared by taking iron and cobalt in the atomic ratio 10.50, 11.33 and 13.00, respectively. These samples were prepared by calcination of the stoichiometric amount of their respective nitrate salts for 6 h in air at 500±10°C. Characterization of the samples has been carried out using Mössbauer spectroscopy. Percentage formation of -Fe₂O₃ and CoFe₂O₄ has been determined using the same technique. These results have been supplemented by X-ray diffraction studies. The particle size has been calculated using Scanning Electron Microscopy. the decomposition of 0.5% w/v hydrogen peroxide at 40°C over the catalyst has also been studied.     

Comparative study of the synthesis of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> and NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> in silica through the polymerized complex route of the sol–gel method

In this work the synthesis of CoFe₂O₄-SiO₂ and NiFe₂O₄-SiO₂ nanocomposites was studied via the sol–gel method, using the polymerized complex route. The polymerized precursors obtained by the reaction of citric acid, ethylene glycol, tetraethylorthosilicate, ferric nitrate, and cobalt nitrate or nickel chloride were characterized by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. NMR and IR spectra of the precursors, without and with metallic ions, show the formation of polymeric chains with ester and ether groups and complexes of metal-polymeric precursor. The nanocomposites were obtained by the thermal decomposition of the organic fraction and characterized by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). XRD patterns show the formation of CoFe₂O₄ and NiFe₂O₄ in an amorphous silica matrix above 400 °C in both cases. When the calcination temperature was 800 °C the particle size of the crystalline phases, calculated using the Scherrer equation, reached ∼35 nm for the two oxides. VSM plots show the ferrimagnetic behavior that is expected for this type of magnetic material; the magnetization at 12.5 KOe of the CoFe₂O₄-SiO₂ and NiFe₂O₄-SiO₂ compounds was 29.5 and 17.4 emu/g, respectively, for samples treated at 800 °C.     

An easy synthesis of nanostructured magnetite-loaded functionalized carbon spheres and cobalt ferrite

An easy method in a solvothermal system has been developed to synthesize nanostructured magnetite (Fe₃O₄)-loaded functionalized carbon spheres (CSs) and cobalt ferrite (CoFe₂O₄). Surface-tunable CSs loaded with iron oxide (Fe₃O₄) nanoparticles were prepared using an acetylferrocene Schiff base (OPF), whereas spinel cobalt ferrite (CoFe₂O₄) was synthesized via metal complexes of a ferrocenyl Schiff base with phenol moiety (Co-OPF). The formed composite powder was investigated using X-ray powder diffraction, Raman spectrometry, Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and vibrating sample magnetometry. It was found that most of the iron oxide nanoparticles were evenly distributed upon the surface of the CSs. Furthermore, the surface of the iron oxide-loaded CSs has large numbers of functional groups. Good saturation magnetization was achieved for the formed magnetic nanoparticles.     

Cobalt/silica gel system as a chemisorbent for oxygen

A study has been made of oxygen chemisorption and the phase composition of cobalt/silica gel samples containing from 1.1 to 7.7% Co by weight. Reduction in hydrogen leads to the formation of metallic cobalt with a crystal size of about 50 Å. The quantity of chemisorbed O₂ at 20°C increases in proportion to the increase of [Co]. The uptake of O₂ is essentially completed at 300°C with the formation of Co₃O₄. The high adsorption capacity for O₂ is preserved unchanged in repetitive reduction-oxidation cycles. The residual content of O₂ in the gas after passage through the layer of the cobalt/silica gel chemisorbent is 3·10^–6% by volume.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiya Akademii Nauk, Seriya, Khimicheskaya, No. 1, pp. 40–43, January, 1992.     

Thermal behavior of Ni,Co and Fe succinates embedded in silica matrix

This paper presents the thermal behavior of Co, Ni and Fe succinates obtained by sol-gel synthesis using Co(II), Ni(II) and Fe(III) nitrates, 1,4-butanediol and tetraethyl orthosilicate as reactants. The thermal analysis revealed the formation of succinates at 413–453 K and their decomposition to ferrites at 503–623 K. The rate constants for the decomposition of succinates to ferrites, calculated using the isotherms at 473, 523, 573 and 623 K, were used to determine the activation energy of each ferrite (NiFe₂O₄, Ni_0.3Co_0.7Fe₂O₄, Ni_0.7Co_0.3Fe₂O₄ and CoFe₂O₄) embedded in the silica matrix. By increasing the Ni content in the mixed Ni–Co ferrites, the activation energy decreases from 13.530 to 1.944 kJ mol^?1. The formation and decomposition of succinate precursors and the formation of silica matrix were confirmed by FT-IR spectroscopy, while the formation of CoFe₂O₄ and NiFe₂O₄ single-phases embedded in the silica matrix was confirmed by X-ray diffraction analysis. The nanocrystallites size decreases from 31.7 (CoFe₂O₄) to 18.5 nm (NiFe₂O₄). The optical band gap of mixed Co–Ni ferrites was significantly higher than that corresponding to CoFe₂O₄. The photocatalytic activity of the samples was evaluated against Rhodamine B under visible light. All the samples have photocatalytic activities, the best performance being obtained in the case of Ni_0.7Co_0.3Fe₂O₄.

     

Formation of uniform particles of cobalt compounds and cobalt

Depending on experimental conditions, precipitation from cobalt (II) sulfate solutions in the presence of urea yields finely dispersed cobalt compounds of different chemical compositions and morphologies. The needle-type particles, generated in closed systems, were identified as cobalt (II) basic carbonate. In systems open to air, spherical particles of cobalt (II) basic cyanato carbonate are formed. The latter transform to spherical Co₃O₄ particles on calcining at 300°C, and then can be reduced to metallic cobalt powder by reacting with hydrogen at 300°C. In the presence of sodium dodecylsulfate, unique cone-type particles containing dodecylsulfate ions are produced.Supported in part by the Air Force Contract F49620-85-C-0142.     

Physicochemical and catalytic investigations of grafted poly(tetrafluoroethylene) complexed with some transition metal nitrates

Radiation-induced graft polymerization of methacrylic acid into poly(tetrafluoroethylene) by the mutual irradiation technique has been studied. The obtained hydrophilic solids were treated with solutions containing calculated amounts of iron, cobalt, nickel, or copper nitrates. The amounts of these transition metal nitrates were fixed at 16 wt%, expressed as metal oxide. IR, XRD, TG, and catalysis of CO oxidation reaction by O₂ have been carried out on the various prepared solids. The results obtained revealed that most of iron and copper species were contributed in complex formation of the polymeric material via carboxylic groups while some of the cobalt and nickel were involved in the complex formation and the rest remained as separate phases. The thermal treatment either in air at 400°C or in vacuo at 240°C led to degradation of the treated grafted polymeric material with subsequent formation of poly(methacrylic acid) together with Co₃O₄ (poorly crystalline phase) and NiO (having moderate crystallinity) and minute amounts of Fe₂O₃ and CuO (undetected by XRD). The PTFE-g-PMAAc treated with the different transition metal nitrates and subjected to heating under vacuum at 240°C exhibited different catalytic activities that vary in the order: Co > Ni > Cu ? Fe. The catalytic activity was mainly dependent on the produced amount of free oxide and their degree of division. © 1993 John Wiley & Sons, Inc.     

Thermal behavior of Co<Subscript>x</Subscript>Fe<Subscript>3−x</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> nanocomposites obtained by a modified sol–gel method

The thermal behavior of Co_xFe_3?xO₄/SiO₂ nanocomposites obtained by direct synthesis starting from nonahydrate ferric nitrate and hexahydrate cobalt nitrate in different ratios with and without the addition of 1,4-butanediol was studied. For the synthesis of Co_xFe_3?xO₄ (x = 0.5–2.5) dispersed in the silica matrix a wide Co/Fe molar ratio was used. The decomposition processes, formation of crystalline phases, gases evolvement and mass changes during gels annealing at different temperatures were assessed by thermal analysis. The absence of succinate precursor and a low mass loss were observed in the case of the gel obtained in the absence of 1,4-butanediol. In case of gels obtained using a stoichiometric ratio of Co/Fe, no clear delimitation between Co and Fe succinates was observed, while for samples with a Fe or Co excess, the formation of the two succinates was observed. The evolution of the crystalline phase after annealing (673, 973 and 1273 K) investigated by X-ray diffraction analysis and Fourier transformed infrared spectrometry revealed that in samples with Fe excess, stoichiometric Fe/Co ratio or low Co excess, the cobalt ferrite (CoFe₂O₄) was obtained as a single phase, while in samples with higher cobalt excess, olivine (Co₂SiO₄) as a main phase, cobalt oxide and CoFe₂O₄ as secondary phases were obtained after annealing at 1273 K. The SEM images confirmed the nanoparticles embedding in the silica matrix, while the TEM and X-ray diffraction data showed that the obtained nanoparticles’ size was below 10 nm in most samples.     

Structure and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> ferrites synthesized by sol–gel and microwave calcination

CoFe₂O₄ ferrites were synthesized sol–gel with cobalt chloride, ferric chloride and citric acid as the main raw material. X-ray diffraction, vibrating sample magnetometer and simultaneous thermal analysis were applied to character the structure and magnetic properties of traditional and microwave calcined samples. The samples with pH 5 and molar ratio of citric acid to metal nitrate 1–1.2 showed the optimal structure and magnetic properties. Microwave calcination reduces the synthesis time from 2 h for conventional calcination to 15–30 min. The saturation magnetization (σ _s ) for sample microwave-calcined at 550 °C for 30 min reaches to 75.89 emu/g, much higher than that of conventional-calcined samples.     

Prenucleation formations in control over synthesis of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocrystalline powders

Nanocrystalline cobalt ferrite powders were synthesized by hydrothermal treatment of co-precipitated hydroxides in the conditions of an external heating of the autoclave and under microwave heating of the reaction medium. In the microwave-heating mode, the prenucleation clusters formed under ultrasonic treatment of a suspended mixture of cobalt and iron hydroxides is transformed into CoFe₂O₄ nanocrystals during the first minute of synthesis at a temperature satisfying the equilibrium-existence conditions of cobalt ferrite. In the case of a slow external heating of the autoclave, there is no effect of this kind, which is attributed to the disintegration of the prenucleation clusters before the dehydration of the hydroxides to give crystalline cobalt ferrite becomes thermodynamically favorable. The main factor determining the increase in the formation rate of crystallites of CoFe₂O₄ nanopowders and the decrease in their size is the generation of prenucleation centers in the starting mixture of cobalt and iron hydroxides.     

Synthesis and characterization of Co (III) salen complex immobilized on cobalt ferrite‐silica nanoparticle and their application in the synthesis of spirooxindoles

The preparation, characterization and catalytic application of Co (III) salen complex loaded on cobalt ferrite‐silica nanoparticle [CoFe₂O₄@SiO₂@ Co (III) salen complex] are described. Co (III) salen complex loaded on ferrite cobalt‐silica nanoparticles is characterized by transmission electron microscopy, scanning electron microscopy coupled with energy‐dispersive X‐ray, vibrating‐sample magnetometer and Fourier transform‐infrared analyses. The thermal stability of the material is also determined by thermal gravimetric analysis. An average crystallite size is determined from the full‐width at half‐maximum of the strongest reflection by using Scherrer's approximation by powder X‐ray diffractometry. The efficiency of CoFe₂O₄@SiO₂@Co (III) salen complex is investigated in the synthesis of spirooxindoles of malononitrile, various isatins with 1,3‐dicarbonyles. The nanocatalyst demonstrated excellent catalytic activity that gave the corresponding coupling products in good to excellent yields. Moreover, the recoverability and reusability of CoFe₂O₄@SiO₂@Co (III) salen complex is investigated where nanocatalyst could be recovered and reused at least five times without any appreciable decrease in activity and selectivity, which confirmed its high efficiency and high stability under the reaction conditions and during recycling stages.     

Solution Preparation of Li(Co,Fe)O2 Coatings for Molten Carbonate Fuel Cell Components

An aqueous solution process has been used for dip coating onto substrates of 316L stainless steel. Coatings of LiCoO₂, Li(Co_0.5Fe_0.5)O₂ and LiFeO₂ were applied and heat treated to 650°C for 3 hrs. Thermal analysis, X-ray diffraction analysis, and SEM analysis were carried out to characterize the microstructure of the coatings. Results showed that the coatings transformed from a gel to a porous, crystalline layer between 270 and 350°C. Microhardness measurements at low load (50 g) were used as an indication of the surface coverage. Samples subjected to 10 thermal cycles at 10°C/min to 650°C and back to ambient, to simulate use in a molten carbonate fuel cell, showed no decrease in microhardness.     

A new selective and high affinity polymeric complexing agent for transition metal ions

Summary The synthesis and characteristics of a new chelating glycinohydroxamate-containing polymer resin is described. The functionality of the polymer is 1.76 mmolg^–1. The hydrogen capacity, water regain and adsorption capacities for iron(III), cadmium(II), cobalt(II), copper(II), nickel(II) and zinc(II) were measured at various pH values; uptake of the metal ions increased with pH and was quantitative above pH 3 for most of the metal ions. All cations studied showed high exchange rates towards the resin. The half saturation times for iron(III), cadmium(II), copper(II) and zinc(II) were all less than 1 min. The coordination behaviour of the resin was studied with the help of e.p.r., i.r., u.v. and potentiometry. The pK _a of the resin is 10.70 and the log value of the stability constants for iron(III), copper(II), lead(II), zinc(II), cobalt(II), manganese(II), cadmium(II) and nickel(II) were measured as 21.81, 19.50, 19.20, 18.59, 18.51, 18.46, 18.37 and 18.36, respectively, at 25 ° C and I = 0.2M KCl.     

Structural-Chemical Transformations of Two-Dimensional Iron-Oxygen Nanostructures in the Course of Transport Reduction

Transport reduction was shown to allow finely controlling the Fe^{+ 3}/Fe^{+ 2} ratio in iron-oxygen two-dimensional nanostructures (nanolayers, thickness 3-15 Å) on silica. It was found by Müossbauer spectroscopy that isolated surface iron-oxygen groups >-Si-O-Fe(OH)₂ and (>-Si-O-)₂FeOH are not reduced at 400-600°C, which is explained by their covalent bonding with silica. The transport reduction of samples with applied nanolayers (one and four) at T 600°C was shown to form bulk phases [iron(II) silicate and metallic iron] on the silica surface. The features of structural-chemical transformations on transport reduction of iron-oxygen nanolayers on supports are primarily associated with the specifics of phase formation in nanostructures.     

Gravimetric determination of beryllium with cupferron

Summary Beryllium has been determined with cupferron by precipitating the metal at p_H 5.8. The complex can be weighed directly as Be (C₆H₅O₂N₂)₂ containing 3.18% of the metal after drying at 110°C. The metal has also been separated with cupferron masking iron (III), aluminium, cerium, thorium, copper, nickel, cobalt, zinc and cadmium with EDTA The beryllium content of beryl has been determined in the same way.

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Zusammenfassung Zur Bestimmung von Beryllium wird mit Kupferron bei pn 5,8 gefällt und der Komplex Be (C₆H₅O₂N₂)₂ (mit 3,18% Be) nach Trocknen bei 110°C direkt ausgewogen. Eisen(III), Aluminium, Cer, Thorium, Kupfer, Nickel, Kobalt, Zink und Cadmium können mit ÄDTA maskiert werden. Auf diese Weise wurde auch der Be-Gehalt von Beryll bestimmt.