An amorphous magnetically ordered ultra-thin film of Fe2O3 on graphite: Mössbauer spectroscopy

An amorphous ultra-thin film of Fe₂O₃ was prepared on partially oriented graphite (Papyex) by surface oxidation of an adsorbed monolayer of Fe(CO)₅. Mössbauer spectra exhibit super-paramagnetism below 79 K. The Mössbauer fraction parallel to the film exceeds that perpendicular by 16% at room temperature.     

Structures,electronic properties and reaction paths from Fe(CO)5 molecule to small Fe clusters

The geometries, electrical characters and reaction paths from Fe(CO)₅ molecule to small Fe clusters were investigated by using all-electron density functional theory. The results show that in the decomposition process of pentacarbonyl-iron, Fe(CO)₅ molecule prefers to remove a carbon monoxide and adsorb another Fe(CO)₅ molecule to produce nonacarbonyldiiron Fe₂(CO)₉ then Fe₂(CO)₉ gradually removes carbon monoxide to produce small Fe clusters. As It can be seen from the highest occupied molecule orbital–lowest unoccupied molecule orbital gap curves, the Fe(CO)_n=3,?and?5 and Fe₂(CO)_n=3,?7?and?9 intermediates have higher chemical stability than their neighbors. The local magnetic moment of the carbon monoxide is aligning anti-ferromagnetic. The effect of external magnetic field to the initial decomposition products of Fe(CO)₅ can be ignored.     

Reaction mechanism of spinel CuFe2O4 with CO during chemical-looping combustion: An experimental and theoretical study

Spinel CuFe₂O₄ is a promising oxygen carrier due to its synergistic enhanced performance. A fundamental understanding of the reaction mechanism between oxygen carrier and fuels is important for a rational design of highly efficient oxygen carrier. The reaction mechanism of spinel CuFe₂O₄ with CO during chemical-looping combustion (CLC) was studied based on thermogravimetric analyses (TGA) and density functional theory (DFT) calculations. Two distinct reaction stages were clearly observed. CuFe₂O₄ was mainly transformed into Cu and Fe₃O₄ with a rapid reaction rate in the initial stage, and then product Fe₃O₄ was slowly reduced to FeO or even to Fe. The reactivity of CuFe₂O₄ is much higher than that of Fe₂O₃, which is ascribed to the existence of Cu. The enhanced oxygen evolution activity of CuFe₂O₄ at low temperature is validated by both the experimental and theoretical methods. Three types of surface oxygen coordinated with different metal atoms show different reactivity. Two kinds of reaction pathways are involved in CO oxidation over CuFe₂O₄. In the one-step reaction pathway, CO directly reacts with the oxygen bonding to two octahedral Cu and one octahedral Fe atoms to form a CO₂ molecule without an energy barrier, which corresponds to the surface oxygen consumption observed in TGA experiments. In the possible two-step reaction pathway, CO first adsorbs on the surface, and then reacts with the oxygen bound to one octahedral Cu and two octahedral Fe atoms to generate CO₂ by surmounting an energy barrier of 10.84 kJ/mol, which is the most kinetically and thermodynamically favorable pathway.     

Characterization of Fe states in dilute 57 Mn implanted SnO 2 film

States of dilute Fe in SnO₂ have been monitored using ⁵⁷Fe emission Mössbauer spectroscopy following implantation of ⁵⁷Mn (T _1/2 = 85.4 s) in the temperature range from 143 K to 711 K. A sharp annealing stage is observed at ~330 K where the Fe^3?+?/Fe^2?+? ratio shows a marked increase. It is suggested that this annealing stage is due to the dissociation of Mn-V_O pairs during the lifetime of ⁵⁷Mn; the activation energy for this dissociation is estimated to be 0.9(1) eV. Fe^3?+? is found in a paramagnetic state showing spin-lattice relaxation rates consistent with an expected T ² dependence derived for a Raman process. In addition, a sharp lined doublet in the Mössbauer spectra is interpreted as due to recoil produced interstitial Fe.     

Effect of Sn on methane decomposition over Fe supported catalysts to produce carbon

In this work, alumina-supported Sn containing Fe catalysts were investigated in CVD reactions (Chemical Vapor Deposition) using methane for carbon production. The catalysts were prepared with 10 wt.% of Fe (as Fe₂O₃) and 3, 6 and 12 wt.% of Sn (as SnO₂) supported on Al₂O₃ named hereon Fe10Sn3A, Fe5Sn6A and Fe10Sn12A, respectively. These catalysts were characterized by SEM, TPCVD, TPR, TG, Raman, XRD and ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectroscopy. Methane reacts with Fe10A catalyst (without Sn) in the temperature range 680?C900°C to produce mainly Fe⁰, Fe₃C and 20 wt.% of carbon deposition. TPR and TPCVD clearly showed that Sn strongly hinders the CH₄ reaction over Fe catalyst. ⁵⁷Fe Mössbauer suggested that in the presence of Sn the reduction of Fe^?+?3 by methane becomes very difficult. ¹¹⁹Sn Mössbauer showed Sn^?+?4 species strongly interact with metallic iron after CVD, producing iron-tin phases such as Fe₃SnC and FeSn₂. This interaction Sn?CFe increases the CVD temperatures and decreases the carbon yield leading to the production of more organized forms of carbon such as carbon nanotubes, nanofibers and graphite.     

The coadsorption of CO and H2 on Fe(100)

The coadsorption of CO and hydrogen on an Fe(100) surface was studied by temperature programmed desorption and X-ray photoelectron spectroscopy. It was found that CO adsorption blocked the subsequent dissociative adsorption of H₂, although it did not seem to affect the hydrogen binding energy. Preadsorption of hydrogen was observed to reduce the binding energy of CO subsequently adsorbed and to inhibit the dissociation of CO. A new surface species was identified in a coadsorbed layer of CO and hydrogen. This species was evidenced by the formation of a desorption peak for H₂ at 475 K when CO was adsorbed subsequent to H₂ adsorption.     

Laser deposition of iron on graphite substrates

Pulsed laser deposition of iron atoms on graphite substrates was performed to produce iron carbide films. Mössbauer spectra of the sample revealed that iron carbide was produced on the substrate surface and that an α-Fe layer was produced above the iron carbide layer. When the substrate temperature was maintained at 300 K, the iron carbide layer had a hyperfine magnetic distribution because it contained high density of defects. Laser deposition of Fe at 570 K produced cementite Fe₃C with fewer defects due to enhancement of thermal reactions or annealing of the films. The orientation of hyperfine field of the Fe₃C film was parallel to the substrate surface.     

Structural,electronic and catalytic performance of single-atom Fe anchored 3Si-doped graphene

By density functional theory (DFT) calculations, it is found that the single-atom Fe anchored three Si modified defective graphene (3Si-graphene-Fe) exhibits the high stability, and this system is semiconducting property and has non-magnetic moment. Besides the most stable configurations, electronic structures and magnetic properties of adsorbed species (O₂, CO, 2CO and CO/O₂) on 3Si-graphene-Fe systems are comparably discussed. The adsorption of O₂ is more stable than that of CO molecule and the coadsorption of 2CO and CO/O₂ has the larger adsorption energy than that of the isolated one. The adsorbed O₂, CO and CO/O₂ can induce the change in magnetic properties of 3Si-graphene-Fe system, and the coadsorbed CO/O₂ on system exhibits the metallic property. Among the reaction mechanisms, the CO oxidation reactions through Eley–Rideal (ER) reactions have lower energy barriers (<0.5?eV) than those of the Langmuir–Hinshelwood (LH) and new termolecular Eley–Rideal (TER) mechanisms, indicating that the ER reaction as starting step is an energetically favourable process. These results provide an important guidance on validating the catalytic activity of single atom on graphene-based materials.     

Water adsorption on the (001) plane of Fe2O3: An XPS,UPS, Auger,and TPD study

Thermal programmed desorption, ultraviolet photoelectron spectroscopy, and X-ray photoelectron spectroscopy have been used to study the reaction of H₂O with stoichiometric and partially reduced single crystals of α-Fe₂O₃. On the stoichiometric surface only ice condensation below 200 K was observed. Oxygen deficient surfaces were prepared by Ar bombardment giving rise to a decrease in the work function of the crystal of up to 1 eV. On these surfaces OH species were formed as detected by UPS that were stable up to 320 K. Annealing the defective surfaces between 475 and 700 K increased the work function by values between 0.5 and 0.7 eV respectively. These surfaces contained reduced Fe²⁺ species in subsurface layers as shown by UPS and XPS, but were inactive towards H₂O chemisorption. The Fe²⁺ species were stable for long periods of time at temperatures of up to 775 K. Potassium deposited on the surface forms a strongly bound monolayer compound. With H₂O it produced a complex that resulted in H₂ evolution upon annealing.     

Raman study of ultrathin Fe3O4 films on GaAs(001) substrate: stoichiometry,epitaxial orientation and strain

The growth and characterization of high‐quality ultrathin Fe₃O₄ films on semiconductor substrates is a key step for spintronic devices. A stable, single‐crystalline ultrathin Fe₃O₄ film on GaAs(001) substrate is obtained by post‐growth annealing of epitaxial Fe film with thicknesses of 5 and 12 nm in air. Raman spectroscopy shows a high ability to convincingly characterize the stoichiometry, epitaxial orientation and strain of such ultrathin Fe₃O₄ films. Polarized Raman spectroscopy confirms the unit cell of Fe₃O₄ films is rotated by 45° to match that of the Fe (001) layer on GaAs, which results in a built‐in strain of − 3.5% in Fe₃O₄ films. The phonon strain‐shift coefficient(−126 cm⁻¹) of the A_1g mode is proposed to probe strain effect and strain relaxation of thin Fe₃O₄ films on substrates. It can be used to identify whether the Fe layer is fully oxidized to Fe₃O₄ or not. Copyright © 2011 John Wiley & Sons, Ltd.     

Reduction of oxidized Fe(100) by hydrogen

The interaction of hydrogen with the oxide layer on Fe(100) has been studied with ellipsometry, AES and LEED. The oxide layer formed at room temperature on Fe(100) rearranges at elevated temperatures, resulting in a reconstructed oxide phase in deeper layers, plus a single monolayer of oxygen on top of the surface. This monolayer is unchanged upon heating. These surfaces are exposed to hydrogen pressures up to 2 × 10⁻² Torr at crystal temperatures between 473 and 643 K. The reduction proceeds via a mechanism of dissociative adsorption of hydrogen on an oxygen filled site. A continuous transport of oxygen from deeper layers to the surface region occurs on a time scale which is fast in comparison with the observed reaction rate. These oxygen containing reaction sites are related to the reconstructed oxide, since a single monolayer of oxygen on Fe(100) is inactive to hydrogen in the pressure range measured. The apparent activation energy for the reaction between the oxide overlayer on Fe(100) and hydrogen is 59 ± 4 kJ/mol at the initial oxygen coverage.     

Thermal stability of amorphous SiOC/crystalline Fe composite

We examined the thermal stability of amorphous silicon oxycarbide (SiOC) and crystalline Fe composite by in situ and ex situ annealing. The Fe/SiOC multilayer thin films were grown via magnetron sputtering with controlled length scales on a surface-oxidized Si (100) substrate. These Fe/SiOC multilayers were in situ or ex situ annealed at temperature of 600 °C or lower. The thin multilayer sample (~10 nm) was observed to have a layer breakdown after 600 °C annealing. Diffusion starts from low groove angle triple junctions in Fe layers. In contrast, the thick multilayer structure (~70 nm) was found to be stable and an intermixed layer (Fe_xSi_yO_z) was observed after 600 °C annealing. The thickness of the intermixed layer does not vary as annealing time goes up. The results suggest that the Fe_xSi_yO_z layer can impede further Fe, Si and O diffusion, and assists in maintaining morphological stability.     

Mössbauer study of Fe-oxide surface layers formed on small Fe particles

A Mössbauer study has been made of the iron-oxide formed on the surface of ultrafine Fe particles (about 293 Å in diameter) prepared by an aerosol method. Particular attention has been paid to the morphology of the oxide layers and to the magnetic structure. X-ray analyses indicate that the oxide layer is a mixture of Fe₃O₄ and γ-Fe₂O₃, and is composed of divided fine crystallites. Further a large non-collinearity in the spin structure of the oxide layer is found that is the likely origin of the low saturation magnetization observed for this type of system.     

The interaction of hydrogen and carbon monoxide with oxygen on Cu(111)-Fe Surfaces

The interaction of hydrogen and carbon monoxide with oxygen adsorbed on Cu(111)-Fe surfaces containing different amounts of iron has been studied with ellipsometry, Auger electron spectroscopy and low energy electron diffraction. With carbon monoxide copper can be reduced completely and if, at larger iron deposits, γ-Fe₂O₃ is present, γ-Fe₂O₃ can be reduced to Fe₃O₄. The maximum reaction rate is proportional to the square of the total copper surface. With hydrogen all oxygen can be removed. The reduction proceeds via a number of different stages. This is explained by the subsequent occurrence of γ-Fe₂O₃, Fe₃O₄ Fe_0.95O and Fe.     

Two-dimensional phase transition in xenon submonolayer films adsorbed on (0001) graphite

Ultra high vacuum molecular beam techniques coupled with LEED and Auger electron spectroscopy are particularly well suited to the study of surface chemical reactions because of the ability to assess the effect of the surface conditions on the reaction probability. Investigation of the hydrogen-deuterium exchange reaction on a series of low and high Miller index platinum single crystals has indicated that the steps present on the high index surfaces are necessary for the dissociation and subsequent recombination of hydrogen. We have undertaken a systematic study of a series of small molecule reactions on these stepped surfaces to determine the reaction probability on stepped platinum surfaces. Reactions involving dissociation of Ha₂, D₂, O₂, OH, NH, and CH bonds proceed on the stepped surfaces with much higher reaction probabilities than reactions requiring dissociation of N₂, or CO bonds. All of the reactions studied resulted in cosine product angular distributions except for the formation of CO₂, which exhibited a distribution more peaked at the normal to the surface.     

Ab initio study of structural and magnetic properties of cubic Fe4N(0 0 1) surface

First-principles calculations are employed to study the structural and magnetic properties of fully-relaxed cubic Fe₄N(0 0 1) surfaces with both Fe₂- and Fe₂N-termination. The results of surface stability calculations show that the (0 0 1) surface of Fe₄N is most possibly existing with Fe₂N-termination. Slab structures have more localized features in the density of states especially for the Fe₂N-terminated surface due to structure relaxation. The average magnetic moments of Fe atoms increase with increasing thickness of slabs. The calculated interlayer distances indicate that the decreases of d₁₂ and d₂₃ result in stronger hybridization and shorter bond distances between Fe2 atom in the second layer and other atoms in surface or the third layers, which lead to variation of magnetic moments with different slab thicknesses.     

On the reaction mechanism of CO2 reforming of methane over a bed of coal char

CO₂ reforming of methane was studied over a bed of coal char in a fixed bed reactor at temperatures between 1073 and 1223 K and atmospheric pressure with a feed composition of CH₄/CO₂/N₂ in the ratio of 1:1:8. Experimental results showed that the char was an effective catalyst for the production of syngas with a maximum H₂/CO ratio of one. It was also found that high H₂/CO ratios were favoured by low pressures and moderate to high temperatures. These results are supported by thermodynamic calculations. A mechanism of seven overall reactions was studied and three catalytic reactions of CH₄ decomposition, char gasification and the Boudouard reaction was identified as being of major importance. The first reaction produces carbon and H₂, the second consumes carbon, and the third (the Boudouard reaction) converts CO₂ to CO while consuming carbon. Equilibrium calculations and experimental results showed that any water present reacts to form H₂ and carbon oxides in the range of temperatures and pressures studied. Carbon deposition over the char bed is the major cause of deactivation. The rate of carbon formation depends on the kinetic balance between the surface reaction of the adsorbed hydrocarbons with oxygen containing species and the further dissociation of the hydrocarbon.     

Synthesis and electrochemistry of LiNi0.6Fe0.3−xMg0.1+xO2 (x=0, 0.5 and 1.0) materials for lithium-ion batteries

Lithium nickel oxide cathodes doped with Fe and Mg was synthesized by the solid state reaction method at 800 °C. Structural investigation of these materials was performed using XRD and EDAX. The electrochemical behavior was studied using galvanostatic charge/discharge in order to investigate the performance of LiNi_0.6Fe_0.3Mg_0.1O₂, LiNi_0.6Fe_0.25Mg_0.15O₂ and LiNi_0.6Fe_0.2Mg_0.2O₂ materials. It is shown that LiNi_0.6Fe_0.3Mg_0.1O₂ produced about 96 m Ah/g of discharge capacity. Paper presented at the International Conference on Functional Materials and Devices 2005, Kuala Lumpur, Malaysia, June 6 – 8, 2005.     

Mössbauer study of Fe powder mechanically alloyed by power ultrasonics

The Mössbauer and transmission electron microscopy (TEM) analysis of Fe-powder and Fe₄₆C₅₄-powder blend, mechanically milled by high power ultrasonics (USM) in He environment for 20–75 hours, have been carried out. As shown, the USM results in effective grinding of initial polycrystalline iron particles up to formation of single crystalline state, dissolution of carbon in iron particles, synthesis of carbides and possibly penetration of Fe atoms into graphite. Annealing of processed Fe₄₆C₅₄ powder causes carbide reaction.