Fe In Octahedral Sites Research Articles

Are Fe based catalysts an upcoming alternative to Ni in CO2 methanation at elevated pressure?

The raise of regenerative but unsteadily produced energy demands a highly flexible way to store the energy for time periods when less energy is produced than consumed. In the current study, it is investigated if catalysts based on environmentally more attractive and less hazardous to health Fe might be able to be considered as an alternative to Ni catalysts in the CO2 methanation at elevated pressure. For this a set of catalysts with 1–10 wt % Fe supported on the zeolite 13X is analysed in CO2 methanation at 1–15 bar. The trends of activity as well as selectivity with varying Fe loading and pressure are presented. Correlation with thorough characterization of the materials shows that a very high dispersion of Fe in octahedral sites within the zeolite is necessary to generate CH4 as the main reaction product and suppress the Fischer–Tropsch activity towards CC coupling reactions at elevated pressure. Especially with low Fe loading such as 1 wt % high reaction rates of 42 mmol(CO2)/(mol(Fe)∙s) with a CH4 selectivity of 76 % at 300 °C and 10 bar are obtained. In contrast to that, highly Fe loaded catalysts tend to form increasing amounts of Fischer–Tropsch products at increasing pressure. In addition, highly Fe-loaded catalysts are much more susceptible to destruction of the zeolite under reaction conditions. At the same time, highly loaded catalysts form a Fe3C shell around the remaining support. Hence, avoiding the formation of a Fe3C phase is crucial for high CH4 selectivity. The results presented here therefore show that catalysts with a very high Fe-dispersion in particular can gain considerably in importance as alternatives to Ni-methanation catalysts at elevated pressure.

Local structure analysis of Bi2(Ga1-xFex)4O9 mullite-type solid solutions synthesized via combination of ball milling and thermal treatment

Mullite-type Bi2(Ga1-xFex)4O9 solid solutions, with 0.1 ≤ x ≤ 0.9, have been synthesized by a combination of mechanical and thermal treatments of a Bi2O3/Ga2O3/α-Fe2O3 stoichiometric mixture. The microstructure of the as-prepared materials on the long-range and local atomic scales was investigated by X-ray diffraction and 57Fe Mössbauer spectroscopy, respectively. The XRD data analysis revealed in all cases linear dependence of the lattice parameters related on x. Due to the ability of the applied Mössbauer spectroscopy to probe the local environment of Fe cations, the local structural disorder in investigated solid solutions is provided. It was shown that the presence of Fe3+ cations in octahedral sites of the orthorhombic structure causes a local distortion of polyhedra in the material. The preferential occupation of Fe in octahedral site was revealed. Detailed quantitative information on both the cation distribution and the bond lengths provided is discussed in relation to the derived hyperfine parameters.

Temperature-induced iron oxidation in bafertisite Ba2Fe 4 2 + ${}_{4}^{2+}$ Ti2(Si2O7)2O2(OH)2F2: X-ray diffraction and Mössbauer spectroscopy study

The high-temperature behavior of bafertisite was studied by combination of techniques in order to characterize the temperature-induced iron oxidation associated with deprotonation of an octahedral layer. The chemical formula of bafertisite from Darai-Pioz alkaline complex (Tajikistan) determined by electron-microprobe analyzes and Mossbauer spectroscopy is Ba 2.11(Fe ${}_{2.70}^{2+}$ Fe ${}_{0.17}^{3+}$ Mn 1.09Zr 0.04Na 0.03) (Ti 1.96Nb 0.07)(Si 2 O 7)2 O 2(OH 1.29 O 0.65 F 0.06)F 2. In situ high-temperature powder X-ray diffraction revealed abrupt shift of reflections to the high-angle region and reduction of their intensity at T > 525 ∘C. The Mossbauer spectroscopy studies indicated that the crystal structure of bafertisite contains Fe in octahedral sites as predominantly ferric ions with Fe 3+/ ΣFe = 0.06, whereas bafertisite annealed at T = 600 ∘C has Fe in the same position with Fe 3+/ ΣFe up to 0.39. The differential scanning calorimetry and thermogravimetric analyzes reveal the occurrence of a broad exothermic effect at T ∼ 537 ∘C associated with the mass loss corresponding to deprotonation. Since in the studied sample of bafertisite, Fe 2+ apfu strongly prevails over OH apfu, the stoichiometric (charged-balanced) high-temperature oxidized modification cannot be obtained. In the paper, the high-temperature behavior of bafertisite is discussed and compared to that of astrophyllite.

Temperature trends for reaction rates, hydrogen generation, and partitioning of iron during experimental serpentinization of olivine

A series of laboratory experiments were conducted to examine how partitioning of Fe among solid reaction products and rates of H2 generation vary as a function of temperature during serpentinization of olivine. Individual experiments were conducted at temperatures ranging from 200 to 320°C, with reaction times spanning a few days to over a year. The extent of reaction ranged from <1% to ∼23%. Inferred rates for serpentinization of olivine during the experiments were 50–80 times slower than older studies had reported but are consistent with more recent results, indicating that serpentinization may proceed more slowly than previously thought. Reaction products were dominated by chrysotile, brucite, and magnetite, with minor amounts of magnesite, dolomite, and iowaite. The chrysotile contained only small amounts of Fe (XFe=0.03–0.05, with ∼25% present as ferric Fe in octahedral sites), and displayed little variation in composition with reaction temperature. Conversely, the Fe contents of brucite (XFe=0.01–0.09) increased steadily with decreasing reaction temperature. Analysis of the reaction products indicated that the stoichiometry of the serpentinization reactions varied with temperature, but remained constant with increasing reaction progress at a given temperature. The observed distribution of Fe among the reaction products does not appear to be entirely consistent with existing equilibrium models of Fe partitioning during serpentinization, suggesting improved models that include kinetic factors or multiple reaction steps need to be developed. Rates of H2 generation increased steeply from 200 to 300°C, but dropped off at higher temperatures. This trend in H2 generation rates is attributable to a combination of the overall rate of serpentinization reactions and increased partitioning of Fe into brucite rather than magnetite at lower temperatures. The results suggest that millimolal concentration of H2 could be attained in moderately hot hydrothermal systems like Lost City during fluid circulation on timescales of a few years.

Room-temperature photomagnetism in the spinel ferrite(Mn,Zn,Fe)3O4as seen via soft x-ray magnetic circular dichroism

We have used x-ray magnetic circular dichroism (XMCD) in conjunction with multiplet simulations to directly probe the origin of photomagnetism in nanocrystalline ${(\text{Mn},\text{Zn},\text{Fe})}_{3}{\text{O}}_{4}$. A photomagnetic effect at room temperature has been observed in these films with HeNe illumination. We have verified an intervalence charge transfer among octahedral Fe cations to account for the increase in magnetization observed at and above room temperature in small magnetic fields. Using XMCD, we demonstrate that the dichroism of Fe in octahedral sites increases by 18% at room temperature, while the dichroism of Fe in tetrahedral sites does not change.

Experimental study of germanium adsorption on goethite and germanium coprecipitation with iron hydroxide: X-ray absorption fine structure and macroscopic characterization

Adsorption of germanium on goethite was studied at 25 °C in batch reactors as a function of pH (1–12), germanium concentration in solution (10 −7 to 0.002 M) and solid/solution ratio (1.8–17 g/L). The maximal surface site density determined via Ge adsorption experiments at pH from 6 to 10 is equal to 2.5 ± 0.1 μmol/m 2. The percentage of adsorbed Ge increases with pH at pH < 9, reaches a maximum at pH ∼ 9 and slightly decreases when pH is further increased to 11. These results allowed generation of a 2-p K Surface Complexation Model (SCM) which implies a constant capacitance of the electric double layer and postulates the presence of two Ge complexes, > FeO – Ge ( OH ) 3 0 and > FeO – GeO ( OH ) 2 − , at the goethite-solution interface. Coprecipitation of Ge with iron oxy(hydr)oxides formed during Fe(II) oxidation by atmospheric oxygen or by Fe(III) hydrolysis in neutral solutions led to high Ge incorporations in solid with maximal Ge/Fe molar ratio close to 0.5. The molar Ge/Fe ratio in precipitated solid is proportional to that in the initial solution according to the equation (Ge/Fe) solid = k × (Ge/Fe) solution with 0.7 ⩽ k ⩽ 1.0. The structure of adsorbed and coprecipitated Ge complexes was further characterized using XAFS spectroscopy. In agreement with previous data on oxyanions adsorption on goethite, bi-dentate bi-nuclear surface complexes composed of tetrahedrally coordinated Ge attached to the corners of two adjacent Fe octahedra represent the dominant contribution to the EXAFS signal. Coprecipitated samples with Ge/Fe molar ratios >0.1, and samples not aged in solution (<1 day) having intermediate Ge/Fe ratios (0.01–0.1) show 4 ± 0.3 oxygen atoms at 1.76 ± 0.01 Å around Ge. Samples less concentrated in Ge (0.001 < Ge/Fe < 0.10) and aged longer times in solution (up to 280 days) exhibit a splitting of the first atomic shell with Ge in both tetrahedral ( R = 1.77 ± 0.02 Å) and octahedral ( R = 1.92 ± 0.03 Å) coordination with oxygen. In these samples, octahedrally coordinated Ge accounts for up to ∼20% of the total Ge. For the least concentrated samples (Ge/Fe < 0.001–0.0001) containing lepidocrocite, 30–50% of total co-precipitated germanium substitutes for Fe in octahedral sites with the next-nearest environment dominated by edge-sharing GeO 6–FeO 6 linkages ( R Ge–Fe ∼ 3.06 Å). It follows from the results of our study that the largest structural change of Ge (from tetrahedral to octahedral environment) occurs during its coprecipitation with Fe hydroxide at Ge/Fe molar ratio ⩽0.0001. These conditions are likely to be met in many superficial aquatic environments at the contact of anoxic groundwaters with surficial oxygenated solutions. Adsorption and coprecipitation of Ge with solid Fe oxy(hydr)oxides and organo-mineral colloids and its consequence for Ge/Si fractionation and Ge geochemical cycle are discussed.

Arsenic speciation in synthetic jarosite

Iron sulfate minerals may incorporate arsenic released from oxidizing arsenian pyrite and other sulfide minerals. For example, in the southern Mother Lode gold district of California, As is concentrated in jarosite, ideally KFe 3(SO 4) 2(OH) 6, that formed by the oxidation of sulfides in mineralized outcrops and mine tailings containing ∼100 to ∼2000 ppm As. These weathering crusts, which are vulnerable to storm erosion, are of concern as a source for the transport of As into watersheds. Both arsenatian jarosite and scorodite [FeAsO 4·2H 2O] provide only temporary storage mechanisms for As in the environment because of the limited pH and redox conditions over which they are stable. To evaluate the extent of incorporation of As into jarosite, a series of jarosites in the system K 2O–Fe 2O 3–As 2O 5–H 2SO 4–H 2O was synthesized at 95 °C and 1 bar. Potassium arsenate was included at relative molar proportions of 1–50% As T,aq (defined as 100*As/(As+S)) in the starting solutions. Arsenic was incorporated into all samples at higher proportions relative to S than in the starting solutions. Synchrotron X-ray diffraction results indicated that jarosite was formed from starting solutions that contained up to 20% As T,aq. An X-ray amorphous phase also formed in samples containing As and, at 25% As T,aq in the starting fluid, only an amorphous phase was obtained. Starting fluids with 33% and 50% As T,aq produced poorly crystalline scorodite. Scanning electron microscope (SEM) images show textural differences among the jarosite samples, from relatively uniform anhedral to subhedral particles 0.5–1 μm across in low-As jarosite samples, to smaller grains (<0.2 μm) dispersed in a groundmass in the samples richer in As. Results from SEM, SXRD and Raman spectroscopy suggest that most As in the synthetic samples enters the amorphous phase, but quantitative analytical electron microprobe (AEM) analyses showed that the grains of jarosite contain up to ∼30% As T,s. Iron K-edge and As K-edge extended X-ray absorption fine-structure (EXAFS) spectra of the arsenatian jarosite showed As and Fe environments consistent with substitution of As for S. SXRD indicated unit-cell expansion with increased substitution of As in jarosite. The extent to which As is accommodated in the jarosite structure may be limited by the charge-balance mechanism and deficiency of Fe in octahedral sites. The X-ray amorphous material produced in the syntheses exhibits short-range order and is similar to scorodite over small domains, on the order of 10–14 Å. Raman spectroscopy showed several broad peaks in the 750–950-cm −1 Raman shift spectral region, associated with As–O vibrations. The relative intensities of bands related to As–OFe and As–O vibrations are also consistent with substitution of As for S in jarosite, and with small, scorodite-like domains in the amorphous material. Collectively, these data show that the strong association between Fe and arsenate during nucleation of the solid materials leads to As retention in the oxidized Fe minerals produced during weathering of arsenian pyrite and other As-rich sulfide minerals.

Sandwich structures in the Etruscan-Padan type pottery

The sandwich (black core) structure in the production of Etruscan-Padan type pottery was investigated. Petrographic, X-ray diffraction (XRD) and Mössbauer data showed that colour changes from core to margin are related to differences in both Fe oxidation state and abundance of maghemite, hercynite and hematite. The occurrence of maghemite and hercynite and the higher quantity of Fe(II) in the cores suggest poor oxygen diffusion in the potsherds during firing. This is congruent with the occurrence of high fractions of paramagnetic Fe in octahedral sites, located in the amorphous phase probably derived from chlorite breakdown, and with the wide range of grain-size distribution of oxidic particles, indicated by the slowing down of superparamagnetic relaxation as temperatures are lowered.

Iron in Hydrothermal Clays from the Galapagos Spreading Centre Mounds: Consequences for the Clay Transition Mechanism

AbstractGlauconite and Fe-smectite, which can be distinguished by their peculiar morphology and stacking sequences, coexist in the Galapagos Spreading Centre hydrothermal mounds. Analytical electron microscopy (AEM) data suggest that Fe is entirely in octahedral sites in Fe-smectite whereas glauconite is K-rich with Fe in tetrahedral and octahedral sites. However, the Mossbauer spectra, recorded at various temperatures for samples containing both smectite and glauconite, were satisfactorily analysed with three overlapping doublets corresponding to Fe in octahedral sites. The contradictory results obtained with the two methods are explained by the presence of small particles of iron oxide intimately associated with glauconite. These particles were detected in Mossbauer spectra obtained at 77 K and 4·2 K and were observed by transmission electron microscopy. Iron oxide is a secondary phase formed by alteration of smectite. These data are in good agreement with the hypothesis that the smectite-glauconite reaction, which occurs at 30 m and low temperature in the Galapagos hydrothermal mounds, is a dissolution-precipitation process, dissolution of Fe-rich smectite being followed by precipitation of glauconite and iron oxides.

X-ray diffraction, infrared and Mössbauer studies of Fe-rich carbonates

Siderites and Fe-rich members of the dolomite-ankerite series have been studied by X-ray diffraction, FTIR and 57 Fe Mossbauer spectroscpy. X-ray diffraction data indicate a clear variation in unit-cell parameters with composition, although carbonate compositions close to pure siderite show some scattering. Infrared spectroscopy displays small shifts (<10 cm −1 ) in the three vibrational frequencies which appear near 1420 cm −1 (ν 3 ), 870 cm −1 (ν 2 ) and 730 cm −1 (ν 4 ). The relationship between the studied carbonate compositions and the bands at 870 cm −1 and 730 cm −1 is approximately linear for siderites, whereas for ankerites this is non-linear. Mossbauer data reveal the presence of one unique type of site occupancy as well as certain distortion in Fe O 6 octahedra in siderites. Quadrupole splitting (QS) and Fe content seeum to be independent from each other, but certain correlation between the amount of Fe and the band area exists. The variations observed for compositions close to pure siderite may be possibly due to the antagonic effect caused by the substitutions of Ca and Mg for Fe in octahedral sites

Artificial Weathering of Oxidized Biotite: III. Potassium Uptake by Subterranean Clover

AbstractPotassium uptake from biotites by subterranean clover decreased as the proportion of oxidized Fe in octahedral sites increased. The relative availability of K from sequentially oxidized biotites may be predicted by sodium tetraphenylboron extraction.No major changes in mica structure due to removal of interlayer K by plants were detected.

Etude par l'effet Mossbauer de MnFe 2− xSc xO 4

MnFe 2− x Sc x O 4 was studied by means of the Mössbauer effect. It was shown that Sc replaces Fe in octahedral sites only. Sc substitution results in the change of cationic distribution of a normal in the ferrite lattice. For the MnFe 1,3Sc 0,7O 4 sample the cationic distribution of a normal spinel was found.