Effects of Reaction Variables on Fischer–Tropsch Synthesis with Co-Precipitated K/FeCuAlO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Catalysts

Abstract  

The effects of reduction temperature and reaction temperature, pressure and space velocity on iron-based K/FeCuAlO _x Fischer–Tropsch catalysts prepared by co-precipitation were investigated. The catalyst reduced at 150 °C deactivated quickly due to an abundance of unreduced iron species. With increasing reduction temperature, the iron oxide’s phase transformed from hematite (α-Fe₂O₃) to magnetite (Fe₃O₄) and finally to metallic iron (α-Fe). The induction period to reach steady-state catalytic activity was reduced at increased reduction temperatures due to in situ reduction by syngas during reaction. CO conversion increased with increasing reaction temperature, and selectivity to C₅+ decreased with increasing reaction pressure and space velocity. At reaction temperatures up to of 300 °C, CO₂ formation by the water–gas shift reaction was linearly correlated with the extent of CO conversion, and CO₂ formation was slightly suppressed at ≥350 °C by a reverse water–gas shift reaction.     

Effect of planetary ball milling on phase transformation of a silica-rich iron ore

The phase transformation of a silica-rich iron ore during planetary milling has been studied. A mixture of silica-rich low grade iron ore and iron powder (4.75 wt.%) was subjected to high energy ball milling and the resulting products were characterized at various milling times. The major constituents of the silica-rich iron ore are hematite (54.2%) and quartz (44.6%). Upon milling, these phases have undergone phase transformation to form fayalite (Fe₂SiO₄) as the major phase along with unreacted quartz as the minor phase. X-ray and Mössbauer spectroscopy studies confirmed the formation of fayalite phase. It was observed that the optimal constitution of the product, in which fayalite is the major phase, could be attained within 15 h of milling.     

Characterization of Li–Zn–Fe crystalline phases in low temperature ceramic glaze

Ceramic glaze containing Li₂O and ZnO was prepared at a low firing temperature of 1100 °C. Addition of 0–30 wt.% iron oxide content developed brown color with a metallic sparkling effect from crystallization after soaking at 980–1080 °C. Using XRD, SEM/EDS and Raman microscopy the crystalline phases were determined as lithium zinc ferrite (Li_xZn_1?2xFe_2+xO₄ where x = 0.05–0.20), hematite (α-Fe₂O₃) and anorthite (CaAl₂Si₂O₈). The most preferable metallic sparkling effect was caused by the lithium zinc ferrite phase obtained from the glaze containing 10 wt.% of iron oxide. Thermal analysis by STA after heat treatment indicated that crystallization temperature of lithium zinc ferrite and the effective soaking temperature depended on the iron oxide content in the glaze. The influence of excessive iron oxide content on the crystallization behavior of lithium zinc ferrite, anorthite and hematite phases is discussed.     

Electrochemical synthesis and the physicochemical properties of a nanodispersed system based on iron and aluminum oxides

An Al₂O₃–Fe₂O₃ dispersed oxide system has been obtained electrochemically in aqueous solutions containing chloride ions with the subsequent thermal treatment. The phase and elemental composition and structural characteristics of the samples have been studied using X-ray phase analysis, Mössbauer spectroscopy, and scanning electron microscopy. The effect of the electrolysis mode and thermal treatment on the characteristics of the samples which were synthesized is shown. It is found that a high-temperature treatment (1100°C) facilitates the formation of a complex oxide material whose phase composition is a combination of the α-Al₂O₃ (corundum) and α-Fe₂O₃ (hematite) crystalline phases and whose particles have nanometer sizes.     

Effect of reducing agents on microstructure and catalytic performance of precipitated iron-manganese catalyst for Fischer–Tropsch synthesis

Effects of reducing agents on the textural properties and bulk/surface phase compositions of a precipitated iron-manganese catalyst were investigated by N₂-physisorption, X-ray photoelectron spectroscopy (XRD), Mössbauer effect spectroscopy (MES), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (LRS). Fischer–Tropsch synthesis (FTS) was performed in a slurry-phase continuously stirred tank reactor. The characterization results indicated that the hematite in the fresh catalyst was converted mainly to magnetite in H₂ atmosphere without the formation of intermediate metallic iron. Large amounts of Fe₃O₄ and small amounts of ε′-Fe_2.2C and χ-Fe_2.5C were formed after syngas pretreatment. In contrast, CO activation led to the formation of large amounts of χ-Fe_2.5C and carbonaceous species on the surface of magnetite. In the FTS reaction, the CO-activated catalyst presented the highest initial activity compared to the H₂ and syngas-reduced catalysts, and remained unchanged in the activity following the transformation of iron carbides to Fe₃O₄. Furthermore, the FTS activity of the H₂-reduced catalyst increased gradually accompanied with the conversion of magnetite to iron carbides. All of the results suggested that the formation of iron carbides (especially for χ-Fe_2.5C) on the surface layers provides probably the active sites for FTS, whereas the Fe₃O₄ formed plays a negligible role in the FTS activity.     

Re-formation of metastable ε-Fe2O3 in post-annealing of Fe2O3/SiO2 nanostructure: Synthesis,computational particle shape analysis in micrographs and magnetic properties

Several Fe₂O₃/SiO₂ nanostructures were synthesized by the combination of the microemulsion and a sol-gel methods. Based on X-ray powder diffraction (XRD) and magnetic measurements (giant coercivity ~2.13 T) we identified ε-Fe₂O₃ (hard magnet) as the dominant crystalline phase. TEM analysis showed a wide size distribution of iron oxide nanoparticles (from 4 to 50 nm) with various morphologies (spherical, ellipsoidal and rod-like). We quantitatively described (computational analysis, MATLAB code) morphological properties of nanoparticles using the ellipticity of the shapes. The as-synthesized hard magnetic material was subjected to a post-annealing treatment at different temperatures (200, 500, 750, 1000 and 1100 °C) in order to investigate stability, formation and transformation of the ε-Fe₂O₃ polymorph. We found decreasing coercivity in the thermally treated samples up to the temperature of 750 °C (H_c=1245 Oe), followed by an observation of a surprising jump in coercivity H_C~1.5 T after post-annealing at 1000 °C. We conclude that the re-formation of the ε-Fe₂O₃ structure during post-annealing at 1000 °C is the origin of the observed phenomena. The phase transformation ε-Fe₂O₃→α-Fe₂O₃ and crystallization of amorphous silica in quartz and cristobalite were observed in the sample treated at 1100 °C.     

In situ high-temperature phase transformation studies on pyrite

Oil shales and coal have considerable amount of pyrite which undergoes various thermal transformations during their processing or combustion. Reactions and changes in pyrite chemistry vary considerably under different environmental conditions. In this paper, we report an in situ high-temperature X-ray diffraction study of phase transformations in pyrite under variable environmental conditions (atmospheric pressure (1 atm.), low air pressure (<0.001 atm.), inert and carbon dioxide atmosphere). We observe that while heating of pyrite in air promotes the formation of hematite (α-Fe₂O₃), magnetite (Fe₃O₄) is a major product in low pressure environment. On the other hand, in the inert environments (nitrogen and argon) pyrrhotite, a non-stoichiometric iron sulphide, is the most dominant product. However, in carbon dioxide (CO₂) environment, pyrrhotite is an intermediate low temperature product which further transforms into magnetite and hematite, attributed to the dissociation of the CO₂ into O₂ and CO providing conducive conditions for the oxidation. We also propose the possible reaction pathways including self-dissociation of CO₂.     

Investigation of the high-temperature behaviour of coal ash in reducing and oxidizing atmospheres

The high-temperature behaviour of ashes from a suite of coals exhibiting a wide range of mineralogies has been investigated. Phase analysis of ash samples quenched from various temperatures under either a reducing (60% CO/40% CO₂) or an oxidizing (air) atmosphere was performed by Mössbauer spectroscopy, scanning electron microscopy (SEM)/automatic image analysis (AIA), and X-ray diffraction (XRD). It was found that significant partial melting of the ashes occurred at temperatures as low as 200–400 °C below the initial deformation temperature (IDT) defined by the ASTM ash cone fusion test. Melting was greatly accelerated under reducing conditions, for which the percentage of melted ash increased rapidly between 900 and 1100 °C, saturating at temperatures above ≈ 1200 °C. The observation of such phases as wustite (FeO), fayalite (Fe₂SiO₄), hercynite (FeAl₂O₄), and ferrous glass in samples quenched from 900 to 1200 °C indicates that ash melting in a reducing atmosphere is usually controlled by the iron-rich corner of the FeO-Al₂O₃-SiO₂ phase diagram. Ashes rich in CaS are an exception to this rule, for large quantities of iron sulphide are formed and the melting behaviour is controlled in part by the FeO-FeS phase diagram. Under oxidizing conditions, potassium appears to be the most important low-temperature fluxing element, as the percentage of glass in samples quenched from temperatures below 1100 to 1200 °C was proportional to the amount of the potassium-bearing mineral illite contained in the coal. Above 1200 °C in air, calcium and, to a lesser extent, iron became effective as fluxing elements; melting accelerated between 1200 and 1400 °C, and was near completion between 1400 and 1500 °C for most ashes. To retard ash melting, it is generally concluded that aluminium is the most desirable constituent of ash, whereas the most undesirable constituents are iron, calcium, and potassium.     

Sol-gel auto-combustion synthesis of Ba–Sr hexaferrite ceramic powders

We report the mechanism involved in sol-gel auto-combustion synthesis of Ba–Sr-hexaferrite (Ba_1-xSr_xFe₁₂O₁₉; x = 0, 0.25, 0.5, 0.75 and 1, BSFO) ceramic powders through the analysis of the phases evolved during annealing of the as-synthesized powders, along with their structure and morphological studies. The XRD patterns of the as-synthesized samples indicate the formation of barium/strontium monoferrite ((Ba/Sr)Fe₂O₄) and maghemite (γ-Fe₂O₃) phases along with a minute amount of hematite (α-Fe₂O₃) phase. Annealing of these samples facilitates formation of BSFO phase through the solid state reaction between BaFe₂O₄ and γ-Fe₂O₃ phase. Interestingly, after annealing the samples with x = 0, 0.5 and 1, at 1000 °C for 2 h, we observed that phase pure Ba–Sr hexaferrite structure forms, but for samples with x = 0.25 and 0.75, high amount of hematite (α-Fe₂O₃) phase is observed, especially for x = 0.75. The reason associated with this could be the large difference between the ionic radii of Ba²⁺ and Sr²⁺ ions occupying the oxygen site. Furthermore, our study on annealing dependent phase evolution confirms that, this difference in ionic radii forbids the formation of a single phase Ba–Sr hexaferrite. The growth of clear hexagonal-shaped plate-like particles with varied particle sizes was observed for all the samples. The particle size variation may be due to the influence of the ionic radii difference on the sinterability of the samples. Our study provides a better understanding of synthesis mechanism of Ba–Sr hexaferrite samples.     

Hydrothermal evolution, optical and electrochemical properties of hierarchical porous hematite nanoarchitectures

Hollow or porous hematite (α-Fe₂O₃) nanoarchitectures have emerged as promising crystals in the advanced materials research. In this contribution, hierarchical mesoporous α-Fe₂O₃ nanoarchitectures with a pod-like shape were synthesized via a room-temperature coprecipitation of FeCl₃ and NaOH solutions, followed by a mild hydrothermal treatment (120°C to 210°C, 12.0 h). A formation mechanism based on the hydrothermal evolution was proposed. β-FeOOH fibrils were assembled by the reaction-limited aggregation first, subsequent and in situ conversion led to compact pod-like α-Fe₂O₃ nanoarchitectures, and finally high-temperature, long-time hydrothermal treatment caused loose pod-like α-Fe₂O₃ nanoarchitectures via the Ostwald ripening. The as-synthesized α-Fe₂O₃ nanoarchitectures exhibit good absorbance within visible regions and also exhibit an improved performance for Li-ion storage with good rate performance, which can be attributed to the porous nature of Fe₂O₃ nanoarchitectures. This provides a facile, environmentally benign, and low-cost synthesis strategy for α-Fe₂O₃ crystal growth, indicating the as-prepared α-Fe₂O₃ nanoarchitectures as potential advanced functional materials for energy storage, gas sensors, photoelectrochemical water splitting, and water treatment.     

Facile synthesis of morphology and size-controlled α-Fe2O3 and Fe3O4 nano-and microstructures by hydrothermal/solvothermal process: The roles of reaction medium and urea dose

This paper reports a systematic study of the influences on the synthesis of α-Fe₂O₃ and Fe₃O₄ via a hydro/solvothermal process at 200 °C. Both the reaction medium and urea dose have been investigated. The products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM). Results showed that the reaction mediums, such as water and ethylene glycol, played important roles in forming different types of iron oxides. Pure crystalline α-Fe₂O₃ was formed via the hydrothermal process, and Fe₃O₄ was obtained through a solvothermal route with ethylene glycol as reaction medium. Increasing urea dose tuned the particle sizes of α-Fe₂O₃ and Fe₃O₄ from a few hundreds to several tens of nanometers. With addition of urea, the morphology of α-Fe₂O₃ evolved from olive-like to rhomb-like, and Fe₃O₄ evolved from hollow sphere, to pinecone-like, and finally into cracked nanostructures. The variations of the surface area of products were mainly dependent on the microstructure and intrinsic features of the iron oxide particles. Results of the mechanistic studies indicated that the generation of CO₂ and NH₃ via in situ thermal decomposition of urea was crucial for the formation of α-Fe₂O₃ and Fe₃O₄ nano-and microstructures. The as-synthesized α-Fe₂O₃ and Fe₃O₄ were used as catalysts for methylene blue degradation in the presence of H₂O₂, and α-Fe₂O₃ showed a higher degradation efficiency. Our findings demonstrated a promising strategy for the developments of rationally designed iron oxides.     

Influence of annealing treatment on magnetic properties of Fe2O3/SiO2 and formation of ε-Fe2O3 phase

Magnetic properties of Fe₂O₃/SiO₂ samples were studied after being produced by sol-gel synthesis and formation of ε-Fe₂O₃ polymorph. Samples were thermally treated, using different annealing temperatures and annealing times. The size and morphological characteristics of the iron oxide nanoparticles were examined using a TEM microscope. We used the “ellipticity of shapes”, which is a measure of how much the shape of a nanoparticle differs from a perfect ellipse, in order to quantitatively describe morphological properties of nanoparticles. Coercivity measurements were used to identify and monitor the formation of the epsilon-iron oxide phase during the thermal treatments (annealing). Coercivity values were in the range from 1.2 to 15.4 kOe, which is in accordance with previous experience regarding the existence of ε-Fe₂O₃. We have determined the optimal formation conditions for the ε-Fe₂O₃ polymorph (t=1050 °C for 7 h, H_C=15.4 kOe), as well as the narrow temperature interval (1050–1060 °C) in which the polymorph abruptly vanished (H_C=2300 Oe), on the basis of results of the magnetic properties. The threshold temperature for the ε-Fe₂O₃ phase transformation was measured as 1060 °C. We found that different annealing temperatures and annealing times significantly affected magnetic properties of the examined samples.     

EQUILIBRIUM STUDIES IN THE SYSTEM FeO-AI2O3,-SiO2*

Equilibrium relationships on the liquidus surface in the system Fe0-Al₂O₃SiO₂ have been established by a modified quenching procedure. The crystal phases which separate from melts heated in iron crucibles are fayalite (2FeO·SO₂), hercynite (FeO·Al₂O₃), tridymite and cristobalite (SiO₂), mullite (3Al₂O₃·2SiO₂), corundum (Al₂O₃), and wastite (approximately FeO). A considerable portion of this system is liquid at 1400°C. Diagrams show the isotherms and the index of refraction of the glasses formed. Two quintuple points have been established in this investigation. One point is at the composition, SiO₂ 42% by weight, Al₂O₃13%, and FeO 45%, and is a eutectic involving the phases fayalite, hercynite, and tridymite at 970°C. ± 200C. The preferred composition of the second quintuple point is 48% of SO₂, 23% of Al₂O₃, and 29% of FeO; it is a peritectic, and the crystal phases are mullite, hercynite, and tridyrnite. Crystallization from this melt without a change in the weight composition calls for the resorption of mullite at a temperature of 1100°C. ±20°C.     

Hematite nanoflakes as anode electrode materials for rechargeable lithium-ion batteries

Hematite (α-Fe₂O₃) nanoflakes and nanocubes were synthesized by liquid-solid-solution method and their properties as anode electrode materials for rechargeable Li⁺-ion batteries were measured. When changing the water to ethanol volume ratio in the synthesis system, the nanocrystals can be changed from α-Fe₂O₃ to α-FeOOH, with shapes being tuned from nanoflakes to nanocubes, non-uniform particles and nanowires. When assembled as the anode electrode materials in rechargeable Li⁺-ion batteries, the hematite nanoflakes showed one more plateau in the first discharge progress of the voltage-composition curves than hematite nanocrystals with other shapes in the literature. X-ray diffraction, high-resolution transmission electron microscope and electrochemical data showed that this extra plateau came from the formation of Li₂Fe₃O₄ nanoclusters and amorphous Li₂O. This experiment showed that like sizes, shapes of nanocrystals may also affect the detailed electrochemical progress.     

Iron oxide synthesis using a continuous hydrothermal and solvothermal system

Iron oxide synthesis via a continuous hydrothermal and solvothermal reaction were studied. In the hydrothermal synthesis, fine α-Fe₂O₃ (hematite) particles were obtained at 250–420 °C and 30 MPa. The α-Fe₂O₃ crystals were grown in sub-critical water via a dissolution and precipitation process. The growth of α-Fe₂O₃ crystals in supercritical water was suppressed due to the rather low solvent power of water. Crystalline Fe₃O₄ was obtained as the temperature was raised above the supercritical state in the solvothermal preparation. Isopropanol (IPA) was oxidized in acetone which provided a reducing atmosphere. Acetone molecule adsorption onto the Fe₃O₄ particle surface inhibited crystallite growth.     

Effect of niobium valence on the mechanochemical activation of niobium oxides–hematite magnetic ceramic nanoparticles

Three types of nanostructured systems: xNbO·(1?x)α-Fe₂O₃, xNbO₂·(1?x)α-Fe₂O₃, and xNb₂O₅·(1?x)α-Fe₂O₃ were synthesized by ball milling at different molar concentrations (x=0.1, 0.3, 0.5, and 0.7). The effect of Nb valence and milling time on mechanochemical activation of these systems were studied by X-ray diffraction and the Mössbauer spectroscopy measurements. In general, Nb-substituted hematite was obtained at lower molar concentrations for all Nb oxides. For the NbO–Fe₂O₃ system the favorable substitution of Fe²⁺ for Nb²⁺ in the octahedral sites in the NbO lattice was observed after 12 h milling for x=0.7. In the case of the NbO₂–Fe₂O₃ and Nb₂O₅–Fe₂O₃ systems a formation of orthorhombic FeNbO₄ compound was observed, in which Fe³⁺ cations were detected. For the highest concentration of NbO₂ (x=0.7) iron was completely incorporated into the FeNbO₄ phase after 12 h milling. The molar concentrations of x=0.3 and 0.5 were the most favorable for the formation of ternary FeNbO₄ compound in the Nb₂O₅–Fe₂O₃ system. Influence of ball milling on thermal behavior of the powders was investigated by simultaneous DSC–TG measurements up to 800 °C.     

Crystallization and magnetic property of iron oxide nanoparticles precipitated in silica glass matrix

Crystallization and magnetic property of Fe₂O₃ nanoparticle precipitated in SiO₂ matrix was investigated. Fe₂O₃/SiO₂ nanocomposite thin film was obtained by annealing of the amorphous Fe-Si-O thin film deposited by RF-magnetron sputtering of (α-Fe₂O₃)_1−x/(SiO₂)_x composite targets. The Fe₂O₃ crystallite size increased with decreasing SiO₂ area ratio, x of the target and increasing annealing temperature. ?-Fe₂O₃ with the crystallite size of 20-30 nm was obtained after annealing the film deposited in SiO₂ area ratio, x = 0.33-0.42 at 900 °C. Lower SiO₂ area ratio (x) than 0.25 and higher annealing temperature resulted in precipitation of α-Fe₂O₃ with the larger crystallite size than 40 nm. In the case of SiO₂ area ratio, x ≥ 0.50, the annealed film was amorphous and showed higher magnetization and smaller coercivity due to the precipitation of very small crystalline γ-Fe₂O₃. The ?-Fe₂O₃/SiO₂ composite thin film showed ferromagnetic hysteresis with coercive force of 0.14 T.     

Formation and gasification of carbon deposits on NiSiO2 catalysts

The deposition of carbon from CH₄ and CO on $\text{Ni}\text{SiO}{}_2$ catalysts was studied in pulse-flow experiments as well as volumetrically with a low-field magnetic permeameter. It was found that carbon, deposited from CH₄ according to: CH₄ → C + 4H, gave rise to the formation of nickel carbide, Ni₃C, only at the surface of the nickel particles (T< 300 °C). However, carbon, deposited from CO according to: 2CO → C + CO₂, led to the formation of a bulk nickel carbide as well as dissolution of carbon interstitially. The reactivity of the carbon thus deposited was studied with both H₂ and H₂O. The rate of reaction with hydrogen appeared to be a function of temperature: the rate passed through a maximum at 200 °C and dropped steeply above 300 °C. The only product of the reaction was CH₄. The reaction with H₂O produced besides CH₄, CO₂ and (at low carbon surface coverages) H₂.     

Mössbauer analysis of iron phases in brown coal ash and fireside deposits

The iron phases present in an electrostatic precipitator ash, an uncooled ash deposit and a cooled superheater ash deposit from Hazelwood Power Station, Australia, burning Morwell brown coal has been examined using Mössbauer spectroscopy. The principal iron phase in the precipitator ash and the uncooled ash deposit from a hot gas offtake was calcium aluminoferrite (Ca₂Fe_{2 ? x}Al_xO₅). Minor amounts of hematite (α-Fe₂O₃) and magnetite (Fe₃O₄) were also detected in the precipitator ash. The cooled superheater ash deposit contained a (Mg, Fe, Al) oxide spinel as the primary iron phase; small quantities of hematite were also detected in this deposit close to the heat exchanger interface. The formation of these iron phases has been rationalized on the basis of the average composition of coal delivered to the power station and supplementary ash chemistry data obtained from other techniques. The evidence suggests that the calcium aluminoferrite in the precipitator ash is derived from inorganic constituents (distributed throughout the coal organic matrix) and the hematite and magnetite are of mineral origin (discrete particles).     

Bauxite ‘red mud’ in the ceramic industry. Part 1: thermal behaviour

Samples of red mud, by-products of alumina production from bauxite, are studied in the 120–1400°C interval. An extensive characterization was performed by thermal and X-ray diffraction analyses. The identification of gaseous species released upon heating was carried out by coupling the thermal analizer with a gas-chromatographic/mass spectrometer. Density evolution was also determined as a function of the heat treatment. Results indicate primary H₂O release from aluminium hydroxides, followed by carbonate decomposition with CO₂ evolution below 900°C. Alkaline oxides, mainly CaO and Na₂O, lead to the formation of Ca₃Al₂O₆ and NaAlSiO₄ between 900 and 1100°C. At the highest temperatures, reduction of Fe³⁺ to Fe²⁺, involving O₂ release, promotes the formation of Fe₂TiO₄, with the disappearance of the rutile-TiO₂ phase. The various solid state reactions, ascertained at different stages of the heating process, and possible mass balances are discussed with reference to the state diagrams of principal red mud components.