Physico-chemical Studies on Thermal Decomposition of Alkali Tris(maleato)ferrates(III)

The thermal decomposition of alkali tris(maleato)ferrates(III), M₃ [Fe(C₂ H₂ C₂ O₄ )₃ ] (M =Li, Na, K) has been studied isothermally and non-isothermally employing simultaneous TG-DTG-DTA, XRD, Mössbauer and IR spectroscopic techniques. The anhydrous complexes decompose in the temperature range 215–300°C to yield Fe(II)maleate as an intermediate followed by demixing of the cations forming α-Fe₂ O₃ and alkali metal maleate/oxalate in successive stages. In the final stage of remixing of the cations (430–550°C) a solid state reaction occurs between α-Fe₂ O₃ and alkali metal carbonate leading to the formation of fine particles of respective ferrites. The thermal stabilities of the complexes have been compared with that of alkali tris(oxalato)ferrates(III).     

Physico-chemical Studies on Zinc Hexa(formato)ferrate(III) Decahydrate

Thermal analysis of zinc hexa(formato)ferrate(III) decahydrate, Zn₃ [Fe(HCOO)₆]₂ 10H₂O has been investigated up to 800°C in static air atmosphere employing TG, DSC, XRD, IR, ESR and Mössbauer spectroscopic techniques. After dehydration at 160°C, the anhydrous complex decomposes into α-Fe₂ O₃ and zinc carbonate in successive stages. Subsequently the cations remix to yield fine particles of zinc ferrite, ZnFe₂ O₄ , as a result of solid state reaction between α-Fe₂ O₃ and zinc carbonate at a temperature (600°C) much lower than for ceramic method.     

Preparation of sodium ferrite from the thermolysis of sodium tris(maleato/fumarato)ferrates(III)

Thermal decomposition of sodium tris(maleato)ferrate(III) hexahydrate, Na₃[Fe(C₄H₂O₄)₃]·6H₂O and sodium tris(fumarato)ferrate(III) heptahydrate, Na₃[Fe(C₄H₂O₄)₃]·7H₂O has been studied upto 973 K in static air atmosphere employing TG, DTG, DSC, XRD, Mössbauer and infrared spectroscopic techniques. Dehydration of the maleate complex is complete at 455 K and the anhydrous complex immediately undergoes decomposition till α-Fe₂O₃ and sodium carbonate are formed at 618 K. In the final stage of remixing of cations, a solid state reaction between α-Fe₂O₃ and sodium carbonate leads to the formation of α-NaFeO₂ at a temperature (773 K) much lower than for ceramic method. Almost similar mode of decomposition has been observed for the fumarate complex. A comparison of the thermal stability shows that the fumarate precursor decomposes at a higher temperature than the maleate complex due to the trans geometry of the former.     

Hydrothermal synthesis of nanodispersed α-Fe2O3 with a lamellar shape of crystals

Transformations of oxyhydroxides of iron(III), which are formed in the process of oxidation of iron(II) in Fe(OH)₂ suspensions upon the hydrothermal treatment at 150–220°C in water and in aqueous solutions of potassium hydroxide with concentrations of 1–5 mol/L have been studied. The dependences of the phase composition and dispersity of the arising products and the morphology of α-Fe₂O₃ on the parameters of heat treatment and on the phase composition of FeOOH have been determined. Conditions for obtaining nanodisperse α-Fe₂O₃ with a lamellar shape of crystals have been refined.     

Preparation and properties of dispersed iron (III) oxide on rutile

Samples of 5 and 20 mole% α-Fe₂O₃ supported on rutile TiO₂ were prepared by incipient wetness. A temperature of 390°C was found to be necessary for complete decomposition of the nitrates. Below 400°C there appears to be little evidence for strong interactions between α-Fe₂O₃ and the support. However, ternary phase formation was observed at elevated temperature. Magnetic properties of bulk and supported α-Fe₂O₃ are compared and discussed.     

Mössbauer study of the thermal decomposition of alkali tris(oxalato)ferrates(III)

The thermal decomposition of alkali (Li,Na,K,Cs,NH₄) tris(oxalato)ferrates(III) has been studied at different temperatures up to 700°C using Mössbauer, infrared spectroscopy, and thermogravimetric techniques. The formation of different intermediates has been observed during thermal decomposition. The decomposition in these complexes starts at different temperatures, i.e., at 200°C in the case of lithium, cesium, and ammonium ferrate(III), 250°C in the case of sodium, and 270°C in the case of potassium tris(oxalato)ferrate(III). The intermediates, i.e., Fe¹¹C₂O₄, K₆Fe¹¹₂(ox)₅. and Cs₂Fe¹¹ (ox)₂(H₂O)₂, are formed during thermal decomposition of lithium, potassium, and cesium tris(oxalato)ferrates(III), respectively. In the case of sodium and ammonium tris(oxalato)ferrates(III), the decomposition occurs without reduction to the iron(II) state and leads directly to α-Fe₂O₃.     

Chitosan-mediated synthesis of mesoporous α-Fe2O3 nanoparticles and their applications in catalyzing selective oxidation of cyclohexane

This paper reports the chitosan-mediated synthesis of porous hematite nanoparticles with FeCl₃ as the precursor via a hydrothermal approach at 160 °C. A series of porous chitosan/iron oxide hybrid nanoparticles were obtained via changing the ratio of chitosan to FeCl₃, FeCl₃ concentration and pH value of the reaction solution, and producing porous iron oxide nanoparticles after calcination. The as-prepared samples were characterized by means of X-ray diffraction, transmission electron microscopy, thermal gravimetric analysis, Fourier transform infrared, and N₂ sorption. The particle sizes of these metal oxides were less than 100 nm, and the pore sizes were in the range of 2–16 nm. It was demonstrated that chitosan played a key role in the formation of the porous structures. The resultant α-Fe₂O₃ nanoparticles were used as the support to immobilize Au or Pd nanoparticles, producing Au/α-Fe₂O₃ or Pd/α-Fe₂O₃ nanoparticles. The as-prepared α-Fe₂O₃ nanocatalyst exhibited high selectivity towards cyclohexanone and cyclohexanol for catalyzing cyclohexane oxidation with O₂ at 150°C.     

Sur les carbonates basiques de fer (III): I. Carbonate basique de fer (III) amorphe

The preparation of a non-stoichiometric basic carbonate of iron (III) is described. The amounts of carbon dioxide and water can vary in large limits, depending on the way the samples are dried. The ratio of Fe₂O₃:CO₂ in a fresh product is nearly one, but decomposition takes place already at room temperature and ambient humidity. When heated slowly, the carbon dioxide is given of in two clear steps, an intermediate product being formed at about 200°C. The basic iron (III) carbonate decomposes between 400° and 500°C to an α-Fe₂O₃ with still a small amount of carbon dioxide. The infrared spectra show that in the freshly prepared products the greater part of the CO ions are linked by two oxygen atoms to two iron atoms, and a smaller part probably only by one oxygen to one iron atom. In the intermediate product, part of the CO^2_₃ ions are linked by two oxygen atoms to one iron atom, or a hydrogenocarbonate group may be formed. The X-ray diagrams taken with Mo K_α rays show only two broad lines.     

Catalytic effect investigation of α-Fe2O3 and α-Fe2O3-CMS nanocomposites on the thermal behavior of NC/DEGDN mixture: DSC measurements and kinetic modeling

In this work, the thermal behavior and kinetics of energetic systems containing α-Fe₂O₃ and iron oxide–carbon mesospheres (α-Fe₂O₃-CMS) with nitrocellulose (NC)/diethylene glycol dinitrate (DEGDN)-based composites have been investigated using differential scanning calorimetry DSC and four isoconversional kinetic methods, respectively. The obtained results indicate that NC/DEGDN show only one decomposition peak, corresponding to the decomposition of the nitrate esters. Furthermore, the introduction of α-Fe₂O₃ and α-Fe₂O₃-CMS have lowered the peak temperature by 3.1 °C and 4.7 °C, respectively. Besides, the activation energy of the thermal decomposition of NC/DEGDN was decreased by 11.9 kJ/mol and 27.97 kJ/mol, after the introduction of α-Fe₂O₃ NPs and α-Fe₂O₃ NPs supported on CMS. These results confirm the good catalytic effect of the added catalysts on the thermal decomposition of the NC/DEGDN mixtures. However, the best catalytic effect was provided by the α-Fe₂O₃-CMS. Furthermore, the three considered systems were found to decompose according to different integral models g(α).     

Mössbauer study of the thermal decomposition of some iron(III) dicarboxylates

The thermal decomposition of iron(III) succinate, Fe₂(C₄H₄O₄)₂(OH)₂ and iron(III) adipate pentahydrate, Fe₂(C₆H₈O₄)₃·5 H₂O, has been investigated at different temperatures for different time intervals in static air atmosphere using Mössbauer spectroscopy and nonisothermal techniques (DTG-DTA-TG). The reduction of iron(III) to iron(II) species has been observed at 533 K and 563 K in the case of iron(III) succinate and iron(III) adipate, respectively. At higher temperatures, α-Fe₂O₃ is formed as the final thermolysis product.     

Über die Löslichkeit des Kadmiumferrits in Ameisensäure

The dissolution process of the system CdFe₂O₄/H⁺ has been examined in various concentrations of formic acid. Study of the reaction at 20°, 40°, and 60°C revealed large divergences between expected stoichiometricity in corresponding amounts of ions passing into the solution. The stoichiometry-like process has been achieved only by using 10m-HCOOH and the temperature of 60°C. X-ray identification of preparations after breaking off the dissolution process allowed to find besides cadmium ferrite another iron oxide phases, namely α-Fe₂O₃ and γ-Fe₂O₃.     

Thermal and Magnetic Properties of Maghemite γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> Synthesized by a Precursor Method

Low-dimension ferromagnetic maghemite γ-Fe₂O₃ was synthesized through a precursor route, using basic iron formate Fe(OH)(HCOO)₂ as a precursor. Conditions of formation of γ-Fe₂O₃ and the temperature range of its existence on heating in air were determined. The saturation magnetization of γ-Fe₂O₃ produced through heating the precursor at 350°C 57.5 (T = 4.2 K) and 43.8 emu/g (T = 300 K).     

Selective fabrication of porous iron oxides hollow spheres and nanofibers by electrospinning for photocatalytic water purification

Porous hematite (α-Fe₂O₃) hollow spheres and nanofibers could be obtained via electrospinning and subsequent thermal decomposition in air. The precursor could be fabricated by electrospinning using Fe(NO₃)₃ as the iron source and Polyvinylpyrrolidone (PVP) as a complexing reagent. Upon calcination, pure porous α-Fe₂O₃ hollow spheres and nanofibers could be obtained at 650 °C for 3 h. The novel hollow spheres have an abundantly porous structure as well as large surface areas. Benefitting from the special porous structure, narrow bandgap, and higher surface area, porous α-Fe₂O₃ hollow materials are used as visible-light-responsive photocatalysts. So we have investigated the visible light photodegradation behavior of porous hematite (α-Fe₂O₃) hollow spheres and nanofibers towards organic dyes, as Rhodamine B (RhB). The synergetic effects of higher surface area, pore structures promoted the photocatalytic efficiency for RhB degradation under visible light and contributed to achieving the enhanced and stable photocatalytic activity.     

Influence of temperatures and starting materials used to prepare α-Fe2O3 on its catalytic activity at the thermal decomposition of KClO4

In order to elucidate the influence of preparative history of α-Fe₂O₃ on its reactivity, the catalytic thermal decomposition of KClO₄ by α-Fe₂O₃ was studied by means of DTA and X-ray techniques. The catalysts were prepared by the calcination of three iron salts, Fe(OH)(CH₃COO)₂, FeSO₄ ? 7H₂O and Fe₂(SO₄)₃ ? αH₂O, at temperatures of 500–1200°C in air. The lower the preparation temperature of αFe₂O₃, the larger the specific surface area and reversely the smaller the crystalline size. KClO₄ without α-Fe₂O₃ was found to begin fusion and decomposition simultaneously at about 530°C. The addition of αFe₂O₃ resulted in promotion of the decomposition reaction of KClO₄; a lowering of 30–110°C in the initial decomposition temperature and a solid-phase decomposition before fusion of KClO₄. The influence of preparative history of α-Fe₂O₃ on the decomposition mainly depended on the preparation temperature rather than the starting material. The initial decomposition temperature of KClO₄ increased with an increase of the preparation temperature of α-Fe₂O₃. The effect of α-Fe₂O₃ was discussed on the basis of the charge transfer and the oxygen abstraction models.     

A study of the preparation of solid solutions in the system α-Fe2O3Al2O3

In order to elucidate the formation of precipitated iron catalysts for ammonia synthesis, the formation of solid solutions between α-Fe₂O₃ and Al₂O₃ was studied in the temperature range 500–950°C. The Al₂O₃ content in the solid solutions was found to be below 15 mole%. At temperatures of 800–950°C, solid solutions are formed at an appropriate rate. Specimens with relatively large specific surface areas are obtained at 800°C.     

High Temperature Stable Maghemite Nanoparticles Sandwiched between Hectorite Nanosheets

Maghemite (γ-Fe₂O₃) is a metastable iron oxide phase and usually undergoes fast phase transition to hematite at elevated temperatures (>350 °C). Maghemite nanoparticles were synthesized by the polyol method and then intercalated into a highly swollen (>100 nm separation) nematic phase of hectorite. A composite of maghemite nanoparticles sandwiched between nanosheets of synthetic hectorite was obtained. The confinement of the nanoparticles hampered Ostwald ripening up to 700 °C and consequently the phase transition to hematite is suppressed. Only above 700 °C γ-Fe₂O₃ nanoparticles started to grow and undergo phase transition to α-F₂O₃. The structure and the phase transition of the composite was evaluated using X-ray diffraction, TEM, SEM, physisorption, TGA/DSC, and Mößbauer spectroscopy.     

High temperature reactions of titanium(IV) oxide with some alkali persulfates and their decomposition products

The solid state reactions between TiO₂ and Na₂S₂O₈ or K₂S₂O₈ have been investigated using TG, DTG, DTA, IR, and X-ray diffraction studies in the range of 20 to 1000°C.It has been shown that TiO₂ reacts stoichiometrically (1 : 1) with Na₂S₂O₈ in the range of 160 and 220°C forming the complex sodium monoperoxodisulfato—titanium(IV) as characterized by IR and X-ray analysis. The new complex then decomposes into the reactants above 190°C.An exothermic reaction has been observed between TiO₂ and molten K₂S₂O₇ at mole ratio 1:2 respectively and higher, in the range of 280 and 350°C. The IR and X-ray analyses have shown the formation of a complex namely, potassium tetrasulfato titanium(IV) for which the formula and structure have been proposed. This complex decomposes at higher temperatures into K₂SO₄ and a mixed sulfate of potassium and titanium. The mixed sulfate melts at 620°C and decomposes into K₂SO₄, TiO₂, and the gaseous SO₃.On the other hand, Na₂S₂O₈ decomposes in a special mode producing a polymeric product of Na₁₀S₉O₃₂. Decomposition of this species occurs after melting at 560°C into Na₂SO₄ and sulfur oxides. The decomposition reaction has been proved to be catalysed by TiO₂ itself.     

Preparation of magnesium and calcium ferrites from the thermolysis of M3[Fe(cit)2]2·xH2O precursors

Magnesium and calcium ferrites have been prepared from the thermolysis of M₃[Fe(C₆H₅O₇)₂]₂·xH₂0 (M=Mg, Ca) precursors. Thermal decomposition of the precursors has been studied employing various physico-chemical techniques, i.e., TG-DSC, XRD, IR and Mössbauer spectroscopy. After dehydration the anhydrous precursors undergo an abrupt oxidative pyrolysis to yield α-Fe₂O₃ and a metastable acetone-dicarboxylate intermediate. A subsequent exothermic decomposition leads to the formation of MgO and CaCO₃ from the respective intermediates. Finally ferrite is formed as a result of solid state reaction between MO/MCO₃ and α-Fe₂O₃. Nanosized ferrites of the stoichiometry MgFe₂O₄ and Ca₂Fe₂O₅ have been obtained from magnesium and calcium bis(citrato) ferrates(III). The temperature of ferrite formation is much lower than possible in conventional ceramic method. The results have been compared with the respective oxalate and maleate precursors.     

Festkörperaktivität und Mechanismen in Oxidsystemen. X. Untersuchung der Umsetzung von α-Fe2O3 mit BaCO3 mittels RBS-Spektrometrie

Solid State Reactivity and Mechanisms in Oxide Systems. X Investigation of the Reaction of α-Fe₂O₃ with BaCO₃ by RBS-Spectrometry Plate-like shaped α-Fe₂O₃ single crystals with the hexagonal axis perpendicular to the plane are characterized by Rutherford-Back-Scattering measurements. The preparation of dense BaCO₃ layers of a defined thickness is reported. The reaction at 650°C and 800°C leads to the formation of BaFe₂O₄ at the surface. Supported by model calculations the RBS spectra are interpreted by a sequence of phases which are in accordance with the phase diagram. PbO evaporates from the α-Fe₂O₃ single crystal surface at 500°C without reaction.     

Interactions between the iron and the aluminum minerals during the heating of Venezuelan lateritic bauxites. II. X-ray diffraction study

Four samples of Venezuelan lateritic bauxites were heated to 300, 600 and 1000°C and the thermal reactions were studied by X-ray diffraction (XED) and by chemical extractability of silica and alumina. Gibbsite was converted to boehmite at 300°C, to an amorphous phase at 600°C and partly to corundum at 1000°C, with isomorphic substitution of Fe for some of the Al in the corundum structure. Goethite was converted to protohematite at 600°C and the hematite at 1000°C, with isomorphic substitution for Al for some of the Fe in both α-Fe₂O₃ varieties. Ti contributed by ilmenite is also occluded by the hematites. The occlusion of Ti takes place at 1000°C during the decomposition of the ilmenite and concomitant recrystallization of α-Fe₂O₃.