Deep reduction behavior of iron oxide and its effect on direct CO oxidation

Reduction of metal oxide oxygen carrier has been attractive for direct CO oxidation and CO₂ separation. To investigate the reduction behaviors of iron oxide prepared by supporting Fe₂O₃ on γ-Al₂O₃ and its effect on CO oxidation, fluidized-bed combustion experiments, thermogravimetric analyzer (TGA) experiments, and density functional theory (DFT) calculations were carried out. Gas yield (γCO₂) increases significantly with the increase of temperature from 693 K to 1203 K, while carbon deposition decreases with the increase of temperature from 743 K to 1203 K, where temperature is a very important factor for CO oxidation by iron oxide. Further, it were quantitatively detected that the interaction between CO and Fe₂O₃, breakage of O-Fe bonds and formation of new C-O bonds, and effect of reduction degree were quantitatively detected. Based on adsorptions under different temperatures and reducing processes from Fe³⁺ into Fe²⁺, Fe⁺ and then into Fe, it was found that Fe²⁺ → Fe⁺ was the reaction-controlling step and the high oxidation state of iron is active for CO oxidation, where efficient partial reduction of Fe₂O₃ into FeO rather than complete reduction into iron may be more energy-saving for CO oxidation.     

Controlled manufacturing of nanoparticles by the laser pyrolysis: Application to cementite iron carbide

The laser pyrolysis is an attractive technique for the synthesis of different nanostructures from gas-phase precursors. The characteristics of this synthesized method are here exemplified by the production of almost pure cementite Fe₃C nanomaterials, obtained by the pyrolysis of methyl methacrylate and iron pentacarbonyl (vapors). Those nanopowders exhibited core (Fe₃C)-shell (MMA polymer-based) morphologies and mean particle diameters of about 8-9 nm. Preliminary magnetic measurements indicate rather high values for the saturation magnetization. By irradiating the same reactive mixture with a lower intensity radiation, the chemical content of nanopowders shifts towards mixtures of iron and maghemite/magnetite iron oxides.     

Structural and optical properties of porous iron oxide

The structural and optical properties of the novel porous iron oxide fabricated by wood template have been investigated. The obtained porous iron oxide was characterized to be α- Fe₂O₃ by Fourier transform infrared and Raman spectroscopy. X-ray absorption fine structure measurement revealed that the bond length of Fe-O₁ of the porous iron oxide has good agreement with that reported for the α- Fe₂O₃ crystal structure while the bond lengths for Fe-O₂ and Fe-Fe deviate slightly from those of the α- Fe₂O₃ crystal structure. Photoluminescence from the porous iron oxide exhibited broad emission bands around 760 and 890 nm, which are believed to be due to the unique nanoscale structure of the porous iron oxide.     

Particle size dependence of relaxivity for silica-coated iron oxide nanoparticles

We investigate the particle size dependence of the relaxivity of hydrogen protons in an aqueous solution of iron oxide (Fe₃O₄) nanoparticles coated in silica for biocompatibility. The T₁ and T₂ relaxation times for various concentrations of silica-coated nanoparticles were determined by a magnetic resonance scanner. We find that the relaxivity increased linearly with increasing particle size. The T₂ relaxivity (R₂) is more than 50 times larger than the T₁ relaxivity (R₁) for the nanoparticle contrast agent, which reflects the fact that the T₂ relaxation is mainly influenced by outer sphere processes. The high R₂/R₁ ratio demonstrates that silica-coated iron oxide nanoparticles may serve as a T₂ contrast agents in magnetic resonance imaging with high efficacy.     

Fabrication and analysis of buried iron silicide microstructures using a focused low energy electron beam

We fabricated iron and iron silicide microstructures on an Si(1 0 0) clean surface via electron beam induced process of Fe(CO)₅ multilayer and subsequent annealing. The fabricated microstructures were in situ analyzed by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). We successfully analyzed the coverage and chemical states of the artificial deposited iron structure area-selectively by AES. The artificial iron structure was fabricated after heating to above 350 K to desorb residual Fe(CO)₅ species. The artificial structure was observed even after 1190 K annealing by SEM, but AES measurements showed it to be covered by Si atoms. We concluded that the buried iron silicide microstructure was formed by the present process.     

Assessment of cardiac iron by MRI susceptometry and R2* in patients with thalassemia

A magnetic resonance imaging cardiac magnetic susceptometry (MRI-CS) technique for assessing cardiac tissue iron concentration based on phase mapping was developed. Normal control subjects (n=9) and thalassemia patients (n=13) receiving long-term blood transfusion therapy underwent MRI-CS and MRI measurements of the cardiac relaxation rate R2*. Using MRI-CS, subepicardium and subendocardium iron concentrations were quantified exploiting the hemosiderin/ferritin iron specific magnetic susceptibility. The average of subepicardium and subendocardium iron concentrations and R2* of the septum were found to be strongly correlated (r=0.96, P<.0001), and linear regression analysis yielded CIC (μg Fe/g_{wet tissue})=(6.4±0.4)·R2* _septum (s⁻¹) − (120±40). The results demonstrated that septal R2* indeed measures cardiac iron level.     

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ₂ = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ₁, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ₁ and the intensity I₂, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.     

Inhibition of hydrogen–oxygen flames by iron pentacarbonyl at atmospheric pressure

The chemistry of inhibition of laminar premixed hydrogen–oxygen flames by iron pentacarbonyl at atmospheric pressure was studied experimentally and by numerical simulation. Flame speed and chemical structure were analyzed. Flame burning velocities and inhibition effectiveness were measured and simulated for various equivalence ratios. The concentration profiles of a number of Fe-containing products of Fe(CO)₅ combustion, including Fe, FeO₂, FeOH, and Fe(OH)₂, were first measured using probing molecular beam mass spectrometry in an atmospheric-pressure H₂/O₂/N₂ flame. A comparison of the experimental and modeling results shows that they are in satisfactory agreement with each other, indicating that the reaction mechanism proposed previously for flame inhibition by iron pentacarbonyl is adequate for predicting the chemical structure of flames. The key recombination stages of active species catalyzed by Fe-containing species for flames of various stoichiometries can be determined by calculations of the production rates of H and O atoms and OH radicals as well as by analysis of the kinetic model.     

Influence of poly(aminoquinone) on corrosion inhibition of iron in acid media

The inhibitor performance of chemically synthesized water soluble poly(aminoquinone) (PAQ) on iron corrosion in 0.5 M sulphuric acid was studied in relation to inhibitor concentration using potentiodynamic polarization and electrochemical impedance spectroscopy measurements. On comparing the inhibition performance of PAQ with that of the monomer o-phenylenediamine (OPD), the OPD gave an efficiency of 80% for 1000 ppm while it was 90% for 100 ppm of PAQ. PAQ was found to be a mixed inhibitor. Besides, PAQ was able to improve the passivation tendency of iron in 0.5 M H₂SO₄ markedly.     

Iron/iron oxides core-shell nanoparticles by laser pyrolysis: Structural characterization and enhanced particle dispersion

Well-dispersed nanoparticles with iron/iron carbide core and iron oxide shell structures may constitute an excellent magnetic material for different applications as magnetic nanofluids, contrast agents in magnetic resonance imaging, sensors and catalysts. Based on the ability of the CO₂ laser pyrolysis technique to synthesize nanoparticles of the Fe/Fe₂O₃ core-shell type, we further improve the powder dispersion by first collecting the nanoparticles in a toluene bubbler, positioned downstream and prior to the collection filter. Structural characterisation of the samples by electron microscopy and X-ray diffraction was performed. Conditions in which clusters contain a reduced number of nanoparticles (around 50) are evidenced. Mean core-shell particle sizes of 15 nm were estimated. Finally, preliminary results on the morphology of iron/iron oxide core-shell nanoparticles as hydrocarbon-based magnetic nanofluids are presented.     

Studies of iron and iron oxide layers by electron spectroscopes

Thin iron oxide layers prepared “in situ” in the ultra high vacuum on polycrystalline iron substrate were investigated by electron spectroscopy methods—X-ray photoelectron spectroscopy (XPS) and elastic peak electron spectroscopy (EPES), using spectrometer ADES-400. The texture and the average grain size of the iron substrate foil have been examined by glancing angle X-ray diffraction (XRD). Qualitative and quantitative estimation of investigated oxide layers was made using (i) the relative sensitivity factor XPS method, (ii) comparison of binding energy shifts of Fe 2p photoelectron line and (iii) non-linear fitting procedure of Fe 2p photoelectron lines.Both, sputter-clean polycrystalline iron substrate and finally grown Fe_2.2O₃ layer, were investigated by the EPES method to measure the electron transport parameters used for quantitative electron spectroscopy, such as the electron inelastic mean free path (IMFP) values. The IMFPs were measured in the electron kinetic energy range 200-1000 eV with the Cu standard. The surface excitation parameters using Chen and Werner et al. approaches were evaluated and applied for correcting these IMFPs. The discrepancies between the evaluated parameters obtained using the above quantitative and qualitative approaches for characterising the iron oxide layers were discussed.     

Measurement and correction of stimulated echo contamination in T2-based iron quantification

The purpose of this study was to characterize the effects of stimulated echo contamination on MR-based iron measurement derived from quantitative T₂ images and develop a method for retrospective correction. Two multiple spin-echo (MSE) pulse sequences were implemented with different amounts of stimulated echo contamination. Agarose-based phantoms were constructed that simulate the relaxation and susceptibility properties of tissue with different concentrations of dispersed (ferritin-like) and aggregated (hemosiderin-like) iron. Additionally, myocardial iron was assessed in nine human subjects with transfusion iron overload. These data were used to determine the influence of stimulated echoes on iron measurements made by an MR-based iron quantification model that can separately measure dispersed and aggregated iron. The study found that stimulated echo contamination caused an underestimation of dispersed (ferritin-like) iron and an overestimation of aggregated (hemosiderin-like) iron when applying this model. The relationship between the measurements made with and without stimulated echo appears to be linear. The findings suggest that while it is important to use MSE sequences with minimal stimulated echo in T₂-based iron quantification, it appears that data acquired with sub-optimal sequences can be retrospectively corrected using the methodology described here.     

Solid state synthesis and room temperature magnetic properties of iron phosphide nanoparticles

Room temperature magnetic properties have been achieved for nano-crystalline iron phosphide synthesized from the direct solid state reaction of iron chloride and tri-octylphosphine (TOP). The magnetization continuously increased with higher magnetic fields, indicating a super-paramagnetic behavior. It is observed that room temperature magnetism is possible for the material showing antiferromagnetic nature at low temperatures. In the present synthesis, TOP acted as a source of phosphorus as well as a surfactant. X-ray diffraction (XRD) studies revealed that the black powder is a mixture of FeP and Fe₂P. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed elongated as well spherical particles. Energy dispersion X-ray analysis (EDAX) confirmed a non-stoichiometric iron phosphide. Presence of TOP was confirmed by infra-red (IR) spectroscopy, and thermo-gravimetric analysis (TGA) indicated about 6% wt. loss due to presence of organics.     

Magnetic properties of iron loaded MCM-48 molecular sieves

Mesoporous molecular sieves of MCM-48 type were loaded with iron by the wet impregnation method, using Fe(III) nitrate or Fe(II) sulfate aqueous solutions as Fe sources, to obtain a magnetic porous composite. The iron loaded materials were characterized by XRD, N₂ adsorption and DRUV-vis and compared with the Si-MCM-48 host. Their magnetic properties were studied by measuring the hysteresis loops up to 1.5 T at different temperatures (5-300 K) and by magnetization vs. temperature curves following the conventional zero field cooling (ZFC) and field cooling (FC) protocols. Materials with high structure regularity and surface area are obtained, which exhibit a mixed paramagnetic and superparamagnetic behavior, arising in isolated iron ions inserted in the host framework, and in small iron oxide clusters or nanoparticles forming inside the pores, respectively. Larger hematite particles (8-13 nm) grown on the external surface provide a quite small ferromagnetic contribution to the hysteresis loop.     

Compositional and structural characterization of nanoporous films produced by iron anodizing in ethylene glycol solution

We report on the composition and morphology of as-grown anodic oxide films onto the iron surface in an ethylene glycol solution containing some NH₄F and H₂O by anodizing under direct current bias. Decrease in the content of NH₄F and the temperature of electrolyte allow us to form either nanochannel or nanotubular films over a larger potential window, ca. from 30 to 100 V. By this way, the films in thickness of up to10 μm have been formed. Mössbauer spectra recorded at room to cryogenic temperatures under conversion electron and transmission modes revealed the formation of lepidocrocite (γ-FeOOH) film containing some Fe(OH)₂ and/or FeF₂·4H₂O. An increase in anodizing voltage results in fabrication of more porous and less Fe(II) compounds containing films.     

The iron oxidation state in Mg-Al-Fe mixed oxides derived from layered double hydroxides: An XPS study

Mixed oxides with large surface area and high thermal stability can be obtained by thermal treatment of the layered double hydroxides (LDH). Mg-Al-Fe mixed oxide samples with varying Mg/Al ratio and 5 mol.% of Fe were prepared in this way and the iron oxidation state (Fe_ox) in these compounds was studied by X-ray photoelectron spectroscopy (XPS), using a calibration based on the relation of Fe_ox to the splitting between the O 1s and Fe 2p_3/2 centroids. The XPS results confirm Fe³⁺ as a dominant oxidation state in the studied mixed oxides. A vacuum-induced reduction of iron in the Fe₂O₃ and Mg-Al-Fe oxide samples has been observed and an influence of the Mg:Al ratio on this effect in mixed oxides has been detected. The role of the local variations of the electron density distribution in the close neighbourhood of the surface oxygen atoms in the mixed oxides in the reduction processes is discussed.     

Hybrid system of iron porphyrin on aminosilanized c-Si

Adsorption of iron porphyrin (Fe^IIITPPS₄) Fe(III)meso-tetra(4-sulfonatophenyl) porphine on aminosilanized surface of crystalline Si (c-Si) was investigated. Formation of nanometric structures of Fe^IIITPPS₄ on c-Si, the surface of which has been modified by various silanization procedures, was studied. Aqueous, ethanol and acetone solutions of 3-aminopropyltrietoxysilane (APTES) were prepared for deposition on c-Si by spinning or immersion techniques. Smooth homogeneous APTES films of thickness 100–500 nm were produced by multiple spin coating procedure. Thin APTES films of thickness 2.5 nm were fabricated by dipping technique followed by washing procedure. Hybrid system of Fe^IIITPPS₄/APTES/Si was prepared from a drop of Fe^IIITPPS₄ aqueous solution put on aminosilanized Si surface or by dipping the Si wafer in Fe^IIITPPS₄ aqueous solution. Nanostructures of size 50–250 nm were formed along with large rings of Ø50–100 μm which have been observed at chemisorption of highly concentrated (1 mM) Fe^IIITPPS₄ aqueous solution. Spectroscopic ellipsometry was used to characterize the APTES layer and to investigate the aggregation state of Fe^IIITPPS₄. The studies provided allowed one to presume that covalent bonds were formed between amino-groups of APTES and functional groups of sulfonic acid in porphyrin leading to immobilization of Fe^IIITPPS₄ on Si substrate.     

First-principles study on the electronic structures of iron phthalocyanine monolayer

The electronic band structure and magnetic properties of iron phthalocyanine (FePc) monolayer were investigated by using the first-principles all-electron full-potential linearized augmented plane wave energy band method. It is found that the ferromagnetic FePc monolayer is energetically more stable than the paramagnetic one. The exchange interaction, which splits the majority and minority bands, influences strongly on the electronic structure near the Fermi level (E_F). Magnetic moment of the central Fe atom is calculated to 1.95 μ_B. The range of the positive polarization of Fe site is larger in the out-of-plane than in the in-plane direction. The FePc ligand remains paramagnetic. The presence of states at E_F indicates the metallic character of FePc monolayer both for the paramagnetic and ferromagnetic states. However, the large density of states at E_F of the majority spins in the ferromagnetic state is expected to cause a phase transition to insulating antiferromagnetic state from the metallic ferromagnetic one.     

Short time synthesis of high quality carbon nanotubes with high rates by CVD of methane on continuously emerged iron nanoparticles

We report the variation of yield and quality of carbon nanotubes (CNTs) grown by chemical vapor deposition (CVD) of methane on iron oxide-MgO at 900-1000 °C for 1-60 min. The catalyst was prepared by impregnation of MgO powder with iron nitrate, dried, and calcined at 300 °C. As calcined and unreduced catalyst in quartz reactor was brought to the synthesis temperature in helium flow in a few minutes, and then the flow was switched to methane. The iron oxide was reduced to iron nanoparticles in methane, while the CNTs were growing.TEM micrographs, in accordance with Raman RBM peaks, indicate the formation of mostly single wall carbon nanotubes of about 1.0 nm size. High quality CNTs with I_G/I_D Raman peak ratio of 14.5 are formed in the first minute of CNTs synthesis with the highest rate. Both the rate and quality of CNTs degrades with increasing CNTs synthesis time. Also CNTs quality sharply declines with temperature in the range of 900-1000 °C, while the CNTs yield passes through a maximum at 950 °C. About the same CNTs lengths are formed for the whole range of the synthesis times. A model of continuous emergence of iron nanoparticle seeds for CNTs synthesis may explain the data. The data can also provide information for continuous production of CNTs in a fluidized bed reactor.     

Mössbauer and EXAFS studies of amorphous iron produced by thermal decomposition of carbonyl iron in liquid phase

Decomposition of iron carbonyl Fe(CO)₅ and Fe₂(CO)₉ in liquid phase gave amorphous and crystalline iron powders in the absence and presence of catalyst, respectively. The hyperfine fields were large in amorphous phases prepared from Fe(CO)₅ than from Fe₂(CO)₉. Crystalline iron, iron carbide and a trace amount of Fe₃O₄ were detected in the decomposition products of the amorphous phase prepared from Fe(CO)₅, and iron carbide was mainly included in the decomposition products of the amorphous phase prepared from Fe₂(CO)₉.