Dynamics of carbide formation in iron-supported catalysts of the Fischer–Tropsch process promoted by copper and potassium

The kinetics of the formation of iron carbides during the activation of iron-coated catalyst for Fischer–Tropsch synthesis promoted by copper and potassium, and by carbon monoxide and syngas, is studied. It is established that the presence of copper lowers the initial temperature of hematite reduction to magnetite and leads to the formation of carbide in both CO and СО/Н₂. Potassium slows the rate of magnetite formation, but it accelerates the formation of iron oxide. It is shown that the rate of carbide formation during magnetite reduction for catalysts is half that in the reaction of hematite reduction to magnetite in both CO and СО/Н₂.     

Iron ore sintering

Differential scanning and high temperature reaction calorimetry have been used to characterize a series of natural iron ore and flux samples commonly used during iron ore sintering. Most iron ore samples were shown to contain measurable quantities of goethite, with a characteristic dehydration peak in DSC and TG between 200 and 400°C. At higher temperatures, all samples decomposed to produce magnetite with an accompanying mass loss in the TG profile due to the evolution of oxygen. High temperature reaction calorimetry has been used to measure the heat of solution of the ore in the melt formed during iron ore sintering. The dehydration and calcination of iron ore and flux samples was also examined using high-temperature reaction calorimetry. The results support the DSC/TG findings.     

Formation of protein-metal oxide nanostructures by the sonochemical method: observation of nanofibers and nanoneedles

The sonochemical reaction of iron pentacarbonyl is explored in water and in water with the protein BSA (bovine serum albumen). In water, the reaction is found to produce spherical nanoparticles of magnetite (Fe3O4) with a particle size distribution of <10 to approximately 60 nm. In water with BSA, the reaction produces either nanofibers or nanoneedles, depending on the concentration of BSA. The nanofiber and nanoneedle samples are found to be mixtures of goethite, lepidocrocite, and hematite (alpha-FeOOH, gamma-FeOOH, and alpha-Fe2O3, respectively). The sonochemical reaction of iron pentacarbonyl with BSA in water is thought to proceed through the thermal decomposition mechanism for iron pentacarbonyl with BSA acting as a templating agent.     

高铁CaO-FeOx-SiO2三元体系氧化过程相变热力学分析

通过详细的热力学计算和推导, 对高铁CaO-FeOx-SiO2(CFS)体系(铁含量50%-60%, x=1-1.5)氧化过程中铁氧化物的价态和组分相赋存状态变化规律进行了分析. 结合SEM、EDX及XRD确定物相组成, 图像分析仪对相分布进行定量测量, 化学法分析铁组分变化情况. 研究了氧化过程中, 体系中相变的热力学规律以及磁铁矿相的析出特性, 对不同温度条件下磁铁矿相析出情况进行了详细讨论, 并简要推算了CaO的含量变化对体系相变的影响. 结果表明, 随熔体氧位的增加, 磁铁矿相逐渐形成并饱和以晶体析出, 铁组分会不断向磁铁矿相转移和富集. 体系冷却后主要由磁铁矿、铁橄榄石和钙铁硅酸盐固溶体组成, 氧化过程中, 铁橄榄石相减少, 磁铁矿相增加. 在1423 K以上温度, 控制氧平衡分压lg(pO2/p0)>-7.89时, 体系中的铁组分主要以磁铁矿形式存在, 并在冷却过程析出; 冷却过程中, 磁铁矿是初晶相, 体系中铁离子的摩尔比n(Fe3+)/n(Fe2+)为1/4 时, 磁铁矿初始析出的温度约为1640 K, 随n(Fe3+)/n(Fe2+)比值的增加, 磁铁矿析出温度升高, 在n(Fe3+)/n(Fe2+)为1.8/1 时, 磁铁矿初始析出的温度约为1720 K; 体系中氧化钙含量的增加, 可提高铁在磁铁矿相的富集程度.     

Redox equilibria of iron oxides in aqueous-based magnetite dispersions: effect of pH and redox potential

The effect of pH and redox potential on the redox equilibria of iron oxides in aqueous-based magnetite dispersions was investigated. The ionic activities of each dissolved iron species in equilibrium with magnetite nanoparticles were determined and contoured within the Eh-pH framework of a composite stability diagram. Both standard redox potentials and equilibrium constants for all major iron oxide redox equilibria in magnetite dispersions were found to differ from values reported for noncolloidal systems. The "triple point" position of redox equilibrium among Fe(II) ions, magnetite, and hematite shifted to a higher standard redox potential and an equilibrium constant which was several orders of magnitude higher. The predominant area of magnetite stability was enlarged to cover a wider range of both pH and redox potentials as compared to that of a noncolloidal magnetite system.     

Comparative structural and chemical studies of ferritin cores with gradual removal of their iron contents

Transmission Electron Microscopy (TEM), X-ray Absorption Near Edge Spectroscopy (XANES), Electron Energy-Loss Spectroscopy (EELS), Small-Angle X-ray Scattering (SAXS), and SQUID magnetic studies were performed in a batch of horse spleen ferritins from which iron had been gradually removed, yielding samples containing 2200, 1200, 500, and 200 iron atoms. Taken together, findings obtained demonstrate that the ferritin iron core consists of a polyphasic structure (ferrihydrite, magnetite, hematite) and that the proportion of phases is modified by iron removal. Thus, the relative amount of magnetite in ferritin containing 2200 to 200 iron atoms rose steadily from approximately 20% to approximately 70% whereas the percentage of ferrihydrite fell from approximately 60% to approximately 20%. These results indicate a ferrihydrite-magnetite core-shell structure. It was also found that the magnetite in the ferritin iron core is not a source of free toxic ferrous iron, as previously believed. Therefore, the presence of magnetite in the ferritin cores of patients with Alzheimer's disease is not a cause of their increased brain iron(II) concentration.     

Application of thermomagnetometry to corrosion studies of archaeological iron

Summary Thermomagnetometry has been applied to mineralized archaeological iron samples and samples from accelerated corrosion tests. It has successfully quantified the degree of corrosion, measured by the loss of iron, as well as the amount of magnetite formed and water held in the corrosion and adhered soil layers. Thermomagnetometry, thermogravimetric analysis and differential scanning calorimetry have been applied to the reported corrosion products from archaeological iron. Fourier transform infra-red and Raman spectroscopies and X-ray diffraction analyses were undertaken on the residues and at intermediate heating stages, where the thermal analyses indicated, to identify the reaction products.     

Influence of particle size and mineral phase in the analysis of iron ore slurries by Laser-Induced Breakdown Spectroscopy

Laser-Induced Breakdown Spectroscopy (LIBS) was applied to the analysis of iron ore concentrates. The objective was to determine the influence of particle size and mineral phase on the LIBS signal. The LIBS spectra of hematite and magnetite ore concentrates were qualitatively indistinguishable from each other but magnetite yielded systematically less than hematite. This behavior could be set into an empirical equation to correct the iron peak intensities according to the level of magnetite in the analyzed sample. Similarly, an increase of the LIBS signal was observed as the particle size of the ore samples decreased. Again, an equation could be written down to correct the intensity of either iron or silicon in response to a variation of the average particle size of the ore concentrate. Using these corrections, proper response of the silicon signal against the concentration of silica in the samples was restored. The observed dependence of the strength of the iron signal upon the mineral phase is attributed to oxidation of magnetite into hematite.     

Characterization of rapidly and naturally quenched skeletal iron catalysts for FT synthesis

The slurry phase is a promising system for Fischer-Tropsch (FT) synthesis. Since the liquid medium efficiently removes the heat of reaction so that the steady-state reaction is easily achieved. High catalytic activity is maintained due to removal of waxy products from the catalyst surface by the action of solvent. In addition, CO-rich syngas from coal gasification can be directly used in FT synthesis which may increase the thermal efficiency of the indirect coal liquefaction. One of the important problems to be solved for slurry phase FT is the catalyst attrition and separation from wax residue. Fused iron and Raney iron were found to have high attrition resistance and easy to separate from wax in slurry phase FT synthesis, but their activity is relatively low. Amorphous alloys made by rapid quenching techniques have drawn increasing interest due to their superior mechanical,chemical and magnetic properties compared to the thermodynamically stable crystalline alloys of the same compositions. It is reported that rapidly quenched skeletal Ni catalyst showed higher catalytic activity than Raney Ni in selective hydrogenation of unsaturated organic functional groups.In this paper, Fe50Al50 (by weight) alloys with different quenching rates, rapid quenching (RQ) and natural quenching (NQ) were prepared for FT synthesis. The phase composition of alloys was characterized by XRD. The physical properties, thermal-stability and adsorption properties of skeletal Fe that was prepared by leaching aluminum of the corresponding alloy with aqueous solution of NaOH were also studied by BET, in situ XRD and H2- and CO-TPD. It is found from XRD patterns of the alloys that RQ Fe50Al50 is composed of orthorhombic phase, and NQ Fe50Al50 alloy is mainly composed of monoclinic phase. Meanwhile, diffraction peaks of the RQ alloy are seriously broadened. After leaching aluminum by aqueous solution of NaOH at the same conditions,skeletal Fe from the RQ alloy give the higher specific surface area and larger pore volume. The in -situ XRD shows that skeletal Fe from RQ alloy is composed of elemental iron and magnetite (FeFe2O4) with poor crystalline. The skeletal Fe from NQ alloy is mainly composed of elemental Fe and minor magnetite. The higher content of the magnetite phase in RQ skeletal Fe may arise from the higher activity of the RQ alloy. When the skeletal Fe was heated under Ar flow, the content of magnetite phase increased with temperature and became the main composition at 400℃ for the skeletal Fe from the RQ alloy. For the skeletal iron from the NQ alloy, phase change under heating is less obvious compared with that for the RQ skeletal iron. The H2-TPD profiles of the catalysts showed that two H2 desorption peaks appeared in both NQ and RQ skeletal iron, but the temperatures at maximum desorption rate of RQ skeletal iron were higher than those of the NQ skeletal iron. The CO-TPD experiment showed that NQ skeletal iron had the stronger affinity to CO than RQ skeletal iron. The different properties were explained on the basis of structure.     

Bacteria-mediated precursor-dependent biosynthesis of superparamagnetic iron oxide and iron sulfide nanoparticles

The bacterium Actinobacter sp. has been shown to be capable of extracellularly synthesizing iron based magnetic nanoparticles, namely maghemite (gamma-Fe2O3) and greigite (Fe3S4) under ambient conditions depending on the nature of precursors used. More precisely, the bacterium synthesized maghemite when reacted with ferric chloride and iron sulfide when exposed to the aqueous solution of ferric chloride-ferrous sulfate. Challenging the bacterium with different metal ions resulted in induction of different proteins, which bring about the specific biochemical transformations in each case leading to the observed products. Maghemite and iron sulfide nanoparticles show superparamagnetic characteristics as expected. Compared to the earlier reports of magnetite and greigite synthesis by magnetotactic bacteria and iron reducing bacteria, which take place strictly under anaerobic conditions, the present procedure offers significant advancement since the reaction occurs under aerobic condition. Moreover, reaction end products can be tuned by the choice of precursors used.     

Study on the Precursor Phase Composition of Fused Iron Catalyst for Fischer-Tropsch Synthesis

The effect of the precursor composition of fused iron catalyst on the performance of Fischer-Tropsch synthesis was investigated. XRD, BET and CO2 adsorption experiments were carried out to provide better insight into the relationship therein. The results showed that the selectivity of C5＋ hydrocarbon products was dependent on the mole ratio of Fe^2＋/Fe^3＋, which was represented by a hump-shaped curve. Catalysts with precursors containing Fe3O4 phase favored the magnetite spinal formation during F-T reaction, while Fe（1-x）O-based catalysts were more likely to favor the formation and growth of the iron carbide crystals.     

Iron phosphate mediated magnetite synthesis: a bioinspired approach

The biomineralization of intracellular magnetite in magnetotactic bacteria (MTB) is an area of active investigation. Previous work has provided evidence that magnetite biomineralization begins with the formation of an amorphous phosphate-rich ferric hydroxide precursor phase followed by the eventual formation of magnetite within specialized vesicles (magnetosomes) through redox chemical reactions. Although important progress has been made in elucidating the different steps and possible precursor phases involved in the biomineralization process, many questions still remain. Here, we present a novel in vitro method to form magnetite directly from a mixed valence iron phosphate precursor, without the involvement of other known iron hydroxide precursors such as ferrihydrite. Our results corroborate the idea that phosphate containing phases likely play an iron storage role during magnetite biomineralization. Further, our results help elucidate the influence of phosphate ions on iron chemistry in groundwater and wastewater treatment.

Magnetite was synthesized from a mixed valence iron phosphate precursor through a novel mechanism inspired by biomineralization in magnetotactic bacteria.     

Ferrous Iron Binding Key to Mms6 Magnetite Biomineralisation: A Mechanistic Study to Understand Magnetite Formation Using pH Titration and NMR Spectroscopy

Formation of magnetite nanocrystals by magnetotactic bacteria is controlled by specific proteins which regulate the particles’ nucleation and growth. One such protein is Mms6. This small, amphiphilic protein can self‐assemble and bind ferric ions to aid in magnetite formation. To understand the role of Mms6 during in vitro iron oxide precipitation we have performed in situ pH titrations. We find Mms6 has little effect during ferric salt precipitation, but exerts greatest influence during the incorporation of ferrous ions and conversion of this salt to mixed‐valence iron minerals, suggesting Mms6 has a hitherto unrecorded ferrous iron interacting property which promotes the formation of magnetite in ferrous‐rich solutions. We show ferrous binding to the DEEVE motif within the C‐terminal region of Mms6 by NMR spectroscopy, and model these binding events using molecular simulations. We conclude that Mms6 functions as a magnetite nucleating protein under conditions where ferrous ions predominate.     

钴掺杂的磁性氧化铁纳米粒子的可控合成

通过油酸盐前驱体高温热解法制备出大小均匀的钴掺杂四氧化三铁球形纳米粒子, 其钴/铁摩尔比可以通过调节油酸钴与油酸铁的比例进行调变. 当产物中钴/铁摩尔比从0.024增加到0.156, 所制备的氧化铁纳米粒子的饱和磁矩从39 emu·g^-1逐渐减小到30 emu·g^-1, 而矫顽力从0 Oe升至190 Oe. 在305℃下, 随着反应体系的热解时间由0.5 h 增加到3 h, 所制备出的氧化铁纳米粒子尺寸逐渐由7 nm增加到14 nm. 热解时间较短时, 以高价态的四氧化三铁的晶型为主, 辅之以少量的氧化亚铁; 热解时间增加至2 h, 产物的晶型为四氧化三铁和氧化亚铁的复合物; 而继续增加热解时间至3 h, 除四氧化三铁和氧化亚铁之外, 还出现少量的零价态的CoFe合金, 说明铁(钴)元素经历了由三价到二价, 最后被还原为零价的过程. 随着反应温度的升高, 产物的尺寸逐渐增大, 同时产物中氧化亚铁的含量增多.     

Iron oxyhydroxide colloids stabilized with polysaccharides

 Neutralization of iron salts in aqueous solutions of κ-carrageenan and cellulose sulfate results in iron oxyhydroxide–polysaccharide hybrid colloids with unusual pH stability up to pH 13. It is shown that both polysaccharides form a tight polymer layer surrounding the inorganic particles, which in the case of κ-carrageenan is cross-linked by helical domains forming a self-assembled nanoreactor. The stabilized iron oxyhydroxide particles can undergo further reactions, for example, it is possible by a chemical reaction to produce stabilized magnetite particles. Repetition of the loading/neutralization steps in the reaction results in hybrids with iron contents much higher than the stoichiometric balance of iron and functional groups of the polymer (greater than 100% Fe/SO₄ ⁻). This combination of high iron content with a natural polysaccharide stabilizer makes these colloids interesting for a number of applications, for example, for nutritional purposes or as contrasting agents for tomography. Received: 7 September 1999 Accepted in revised form: 6 December 1999     

Chitosan‐induced synthesis of magnetite nanoparticles via iron ions assembly

Superparamagnetic magnetite nanoparticles were synthesized induced by chitosan hydrogel under ambient conditions via iron ions assembly, and the inducing effect of chitosan hydrogel was discussed. Results of X‐ray diffraction and transmission electron microscopy indicate that the nanoparticles were inverse cubic spinel structure magnetite with diameter about 16 nm, and the superparamagnetic nanoparticles with narrow size distribution dispersed uniformly in chitosan. The magnetization measurements indicated that the nanoparticles showed the typical superparamagnetic behavior. The crystallinity, morphology, and magnetic properties of magnetite nanoparticles were remarkably influenced by the pH values of iron ion solutions. The interaction between magnetite and chitosan was illustrated by FT‐IR and thermogravimetric analysis, which concluded that the magnetite nanoparticles were coated by a chitosan layer via the amino groups of chitosan. The chitosan hydrogel assisted in the synthesis of superparamagnetic magnetite nanoparticles through chelation by amino groups. Copyright © 2008 John Wiley & Sons, Ltd.     

Preparation and characterization of polypyrrole/magnetite nanocomposites synthesized by in situ chemical oxidative polymerization

This work describes the preparation and characterization of polypyrrole (PPy)/iron oxide nanocomposites fabricated from monodispersed iron oxide nanoparticles in the crystalline form of magnetite (Fe₃O₄) and PPy by in situ chemical oxidative polymerization. Two spherical nanoparticles of magnetite, such as 4 and 8 nm, served as cores were first dispersed in an aqueous solution with anionic surfactant sodium bis(2‐ethylhexyl) sulfosuccinate to form micelle/magnetite spherical templates that avoid the aggregation of magnetite nanoparticles during the further preparation of nanocomposites. The PPy/magnetite nanocomposites were then synthesized on the surface of the spherical templates. Structural and morphological analysis showed that the fabricated PPy/magnetite nanocomposites are core (magnetite)‐shell (PPy) structures. Morphology of the PPy/magnetite nanocomposites containing monodispersed 4‐nm magnetite nanoparticles shows a remarkable change from spherical to tube‐like structures as the content of nanoparticles increases from 12 to 24 wt %. Conductivities of these PPy/magnetite nanocomposites show significant enhancements when compared with those of PPy without magnetite nanoparticles, in particular the conductivities of 36 wt % PPy/magnetite nanocomposites with 4‐nm magnetite nanoparticles are about six times in magnitude higher than those of PPy without magnetite nanocomposites. These results suggest that the tube‐like structures of 36 wt % PPy/magnetite nanocomposites may be served as conducting network to enhance the conductivity of nanocomposites. The magnetic properties of 24 and 36 wt % PPy/magnetitenanocomposites show ferromagnetic behavior and supermagnetism, respectively. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1291–1300, 2008     

醇热解法合成超顺磁性氧化铁纳米粒子及其性能

以甲氧基聚乙二醇同时作为溶剂、还原剂及修饰剂,在高温下分解乙酰丙酮铁,制备了纳米Fe3O4粒子,采用透射电子显微镜和X射线衍射分析表征材料的形貌和相组成,傅里叶变换红外光谱仪表征材料的表面修饰物,超导量子干涉仪测试合成的纳米粒子的磁性能,纳米粒度与zeta电势分析仪测试磁性纳米粒子在水中的zeta电势。 结果表明,纳米Fe3O4粒子的大小为（10.1±1.6） nm,粒度均一,单分散性好,在300 K下具有超顺磁性,饱和磁化强度为45 A·m2/kg。 红外结果表明,-COO-共价结合在粒子表面。 zeta电势为-25 mV。 其在水中的稳定性与以三甘醇为反应介质、高温分解法制备的纳米Fe3O4粒子作比较,表现出长时间（60 d以上）的良好分散性。 静电作用及空间位阻效应是其高稳定分散性的原因。     

The effect of polymer adsorption on the wetting properties of partially hydrophobized magnetite

Upon reverse flotation of iron ore, the surface of the iron ore concentrate may become partially hydrophobized due to adsorption of flotation collector, which is facilitated by the calcium ions present in the process water. Hydrophobic areas on the concentrate surface may introduce problems in subsequent pelletization of the concentrate. A possible way to restore the wettability of the surface could be by modifying the surface with a hydrophilic polymer. The effect of hydrophilic polymers of different types, viz. cationic, anionic, and non-ionic, on the wettability of the magnetite surface after adsorption of a surfactant was investigated. Although all the polymers could adsorb on magnetite at pH 8.5, the contact angle measurements revealed that only anionic ammonium polyacrylate could decrease the contact angle of synthetic magnetite after surfactant adsorption to a level close to that of as-synthesized magnetite. Such effect was probably achieved due to shielding of the hydrophobic surfactant chains from the aqueous phase by hydrophilic polyacrylate molecules. The fact that polyacrylate adsorption on magnetite occurred via calcium ions makes polyacrylate suitable for application in calcium-rich process water. The results presented in this work illustrate that ammonium polyacrylate could be successfully used to improve the wettability of magnetite after adsorption of surfactants.     

Isoconversional analysis of thermal dissociation kinetics of hematite in air and inert atmospheres

Dissociation of hematite is an undesirable reaction for iron ore pelletizing process leading to severe deterioration in compressive cold strength and reducibility factors. It was shown that raising temperature in an induration machine would cause hematite’s dissociation, which is either present in the primary ore or formed by oxidation of magnetite in the feed. The oxidation reaction of magnetite is exothermic, which complicates temperature control within the non-isothermal area of preheating. Kinetics of the dissociation reaction is the temperature’s primary function, which controls the extent of the reaction. Pure hematite samples were subjected to several runs of thermal analysis carried out under both air and inert atmosphere, in order to achieve a comprehensive knowledge about the temperature dependencies of dissociation kinetics. Due to the observed uniformity, isoconversional methods were chosen in the present work over isothermal and non-isothermal for calculation of kinetic parameters of the reaction. Respectively, activation energy values of hematite dissociation were found to be 324 and 382 kJ mol^?1 in inert and air atmosphere. The high value of activation energy implies strong dependency of the single-step reaction rate on the temperature. It was also observed that the forward reaction had higher activation energy than the backward reaction; hence, an increase in temperature results in an overall acceleration of the dissociation reaction.