Low Iron Loading Research Articles

The nature of the active site in the Fe-ZSM-5/N 2O system studied by (resonant) inelastic X-ray scattering

Resonant inelastic X-ray scattering (RIXS) was applied to investigate whether the iron–oxygen species created upon reaction of Fe-ZSM-5 with N 2O have a (partial) 3d 4 configuration, i.e. oxidation state Fe(IV). Fe-ZSM-5 samples with low iron loading were pre-treated at high temperatures before reaction with N 2O. Then Fe K-edge XANES spectra were recorded either by selecting the fluorescence of the main Kβ 1,3 line or the satellite Kβ′ line in the Kβ emission spectrum. The experiments were performed in situ and imitated the experiments performed in the laboratory reactor. The absence of a pre-edge in the Kβ′ line spectrum proves that the configuration of the active iron–oxygen species is purely 3d 5. The result was confirmed for several differently treated Fe-ZSM-5 catalysts. The high reactivity of Fe-ZSM-5 is therefore not due to the formation of high oxidation states of iron, but must be ascribed to the particular oxygen species that is created by the reaction of N 2O with the catalyst.

The positive effect of NO on the N 2O decomposition activity of Fe-ZSM-5: A combined kinetic and in situ IR spectroscopic study

NO-assisted N 2O decomposition over four different Fe-ZSM-5 samples prepared by wet ion exchange (WIE) or chemical vapor deposition (CVD) of FeCl 3 was investigated by steady-state kinetics, in situ infrared (IR) spectroscopy, and transient response methods. Despite their lower iron loading, the samples prepared by WIE had the highest activity for N 2O decomposition in the presence of NO. The in situ IR experiments showed that the most active sample was characterized by a high concentration of adsorbed NO, as well as adsorbed NO 2 under reaction conditions. Step response experiments proved that NO 2 is an intermediate of the catalytic cycle and functions as intermediate oxygen storage. IR and transient kinetic experiments showed that WIE catalysts behave qualitatively different in NO-assisted N 2O decomposition than CVD catalysts. These differences are discussed in terms of the different structure of the iron species in the two types of samples.

Active Sites Characterization in Mixed Vanadium and Iron Antimonate Oxide Catalysts for Propane Ammoxidation

Ammoxidation of propane was investigated over FeVSbO mixed oxide catalysts. The results obtained show that in VSbO4 mixed oxide catalyst systems, substitution of vanadium by small quantities of iron has a strongly positive effect on the activity of the catalysts. With such a system, the selectivity to acrylonitrile decreases while the selectivity to propene increases. At high levels of vanadium substitution by iron, the activity was low but the selectivity towards acrylonitrile was enhanced. The characterization of the catalysts carried out using several techniques, such as X-ray diffraction, Mössbauer spectroscopy, X-ray absorption, and electron microscopy, has shown that a continuous solid solution was formed between VSbO4 and FeSbO4. At low iron loading, Fe3+ substituted for V4+ in the cationic-deficient structure V4+0.64V3+0.28Sb5+0.92♢0.16O4, leading to a structure with both cationic and anionic vacancies, Fe3+xV3+0.33V4+0.41−xSb5+0.92♢0.18O4−x, and generating very active sites for alkane activation. At high iron loading, only V3+ was present in the bulk of the solids and only Fe3+ contributed to the isolation of the vanadium sites, which are selective to acrylonitrile.

EXAFS study of the Fex/ZrO2 composite nanomaterials obtained by sol-gel synthesis.

The local Zr and Fe arrangements in the nanocomposite system Fe(x)/ZrO2 (x=0-0.2) obtained by sol-gel method were studied by EXAFS. The phase composition was found to vary significantly on iron loading. For the samples calcined at 500 or 600 degrees C, at x< or =0.01, the formed ZrO2 is a mixture of monoclinic and tetragonal structures. Only tetragonal structure is revealed at x > or = 0.05 with the exception of Fe0.2/ZrO2, calcined at 500 degrees C, where new non-typically short Zr-Zr distances (3.29, 3.43A) were observed. At low iron loading (less than one monolayer) Fe3+ ions were found to localize within the pre-surface layers of ZrO2 nanoparticles showing property of isolated ions. For greater Fe loading, iron exhibits either two-dimensional layer structure or two-phase system.

Hydrogen peroxide formation during iron deposition in horse spleen ferritin using O2 as an oxidant.

The reaction of Fe2+ with O2 in the presence of horse spleen ferritin (HoSF) results in deposition of FeOH3 into the hollow interior of HoSF. This reaction was examined at low Fe2+/HoSF ratios (5-100) under saturating air at pH 6.5-8.0 to determine if H2O2 is a product of the iron deposition reaction. Three methods specific for H2O2 detection were used to assess H2O2 formation: (1) a fluorometric method with emission at 590 nm, (2) an optical absorbance method based on the reaction H2O2 + 3I- + 2H+ = I3- + 2H2O monitored at 340 nm for I3- formation, and (3) a differential pulsed electrochemical method that measures O2 and H2O2 concentrations simultaneously. Detection limits of 0.25, 2.5, and 5.0 microM H2O2 were determined for the three methods, respectively. Under constant air-saturation conditions (20% O2) and for a 5-100 Fe2+/HoSF ratio, Fe2+ was oxidized and the resulting Fe3+ was deposited within HoSF but no H2O2 was detected as predicted by the reaction 2Fe2+ + O2 + 6H2O = 2Fe(OH)3 + H2O2 + 4H+. Two other sets of conditions were also examined: one with excess but nonsaturating O2 and another with limiting O2. No H2O2 was detected in either case. The absence of H2O2 formation under these same conditions was confirmed by microcoulometric measurements. Taken together, the results show that under low iron loading conditions (5-100 Fe2+/HoSF ratio), H2O2 is not produced during iron deposition into HoSF using O2 as an oxidant. This conclusion is inconsistent with previous, carefully conducted stoichiometric and kinetic measurements [Xu, B., and Chasteen, N. D. (1991) J. Biol. Chem. 266, 19965], predicting that H2O2 is a quantitative product of the iron deposition reaction with O2 as an oxidant, even though it was not directly detected. Possible explanations for these conflicting results are considered.

An Hermite expansion method for EXAFS data treatment and its application to Fe K-edge spectra

A new Hermite expansion method has been developed for EXAFS data treatment. This transformation technique involves an expansion into a series of odd Hermite functions, which are eigenfunctions of the sine Fourier transform. In this communication, we report mainly the use of the method to filter iron K-edge X-ray absorption data. Unlike the other numerical Fourier techniques, the new method takes into consideration additional physical information on the experimental data, i.e. both the length of the interval of the measurements and an estimation of the length of the interval of significant structure changes in real space are parameters of the method. The method was validated with iron K-edge fluorescence spectra of magnetite and applied to iron exchanged ZSM-5 zeolite with low iron loadings (0.75 wt.%). We are able to show that, compared to the same data unfiltered or spectra Fourier filtered with a large Gaussian window, the noise level is strongly reduced and artefacts at higher distances disappear. The validity of the method was proved by applying it to magnetite (Fe3O4), where it is possible to fit the two Fe–O distances (octahedral and tetrahedral sites) known from X-ray diffraction. With the new method, these shells are statistically significant and are observed as individual contributions in the Fourier transforms. We also show that the new method, unlike conventional Fourier techniques, retains the statistical independence of the data.

Direct spectroscopic and kinetic evidence for the involvement of a peroxodiferric intermediate during the ferroxidase reaction in fast ferritin mineralization.

Rapid freeze-quench (RFQ) Mössbauer and stopped-flow absorption spectroscopy were used to monitor the ferritin ferroxidase reaction using recombinant (apo) frog M ferritin; the initial transient ferric species could be trapped by the RFQ method using low iron loading (36 Fe2+/ferritin molecule). Biphasic kinetics of ferroxidation were observed and measured directly by the Mössbauer method; a majority (85%) of the ferrous ions was oxidized at a fast rate of approximately 80 s-1 and the remainder at a much slower rate of approximately 1.7 s-1. In parallel with the fast phase oxidation of the Fe2+ ions, a single transient iron species is formed which exhibits magnetic properties (diamagnetic ground state) and Mössbauer parameters (DeltaEQ = 1.08 +/- 0.03 mm/s and delta = 0.62 +/- 0.02 mm/s) indicative of an antiferromagnetically coupled peroxodiferric complex. The formation and decay rates of this transient diiron species measured by the RFQ Mössbauer method match those of a transient blue species (lambdamax = 650 nm) determined by the stopped-flow absorbance measurement. Thus, the transient colored species is assigned to the same peroxodiferric intermediate. Similar transient colored species have been detected by other investigators in several other fast ferritins (H and M subunit types), such as the human H ferritin and the Escherichia coli ferritin, suggesting a similar mechanism for the ferritin ferroxidase step in all fast ferritins. Peroxodiferric complexes are also formed as early intermediates in the reaction of O2 with the catalytic diiron centers in the hydroxylase component of soluble methane monooxygenase (MMOH) and in the D84E mutant of the R2 subunit of E. coli ribonucleotide reductase. The proposal that a single protein site, with a structure homologous to the diiron centers in MMOH and R2, is involved in the ferritin ferroxidation step is confirmed by the observed kinetics, spectroscopic properties, and purity of the initial peroxodiferric species formed in the frog M ferritin.

Study of cumene oxidation over zirconia-, titania- and alumina-based complex oxides obtained by sol-gel methods: activity-structure relationships

Activity-structure relationships have been studied in the low temperature liquid phase cumene oxidation over Fe O/ZrO 2, Fe O/TiO 2 and Fe O/Al 2O 3 complex oxides obtained by sol-gel methods. The study has been carried out by the measurements of overall kinetics, product distribution and by X-ray diffraction and Mössbauer spectroscopic methods. Kinetic study in the presence of homogeneous initiator (azo-bis-diazobutyronitrile) has led to the conclusion that the active surface of complex oxides participates in chain initiation most probably via R H bond rupture. Three groups of complex oxides are described: (1) two-phase polycrystalline zirconia-based catalysts showing high activity at low iron loading; (2) one-phase polycrystalline zirconia- and titania-based catalysts exhibiting either activity drop at some critical iron content (zirconia) or monotonic activity dependence (titania) on iron loading; (3) amorphous alumina-based catalysts containing γ-ferric oxide clusters and showing monotone activity dependence. Electronically excited terminal Fe O groups related to the surface FeO 6 polyhedra and iron-containing species located at the interface have been suggested as active centers acquiring the anion-radical behavior. When comparing thermodynamically stable titania (anatase) with metastable one-phase zirconia (cubic or tetragonal) as host matrices, the suggestion was introduced that greater activity of zirconia-based catalysts is due to more energy transfer from the host matrix to the particular active center.

Iron core formation in horse spleen ferritin: Magnetic susceptibility, pH, and compositional studies

Horse spleen ferritin (HoSF) reconstituted with small iron cores ranging in size from 8 to 500 iron atoms was studied by magnetic susceptibility and pH measurements to determine when the added Fe 3+ begins to aggregate and form antiferromagnetically coupled clusters and also to determine the hydrolytic state of the iron at low iron loading. The Evans NMR magnetic susceptibility measurements showed that at iron loadings as low as 8 Fe 3+/HoSF, at least half of the added iron atoms were involved in antiferromagnetic exchange interactions and the other half were present as isolated iron atoms with S = 5 2 . As the core size increased to about 24 iron atoms, the antiferromagnetic exchange interactions among the iron atoms increased until reaching the limiting value of 3.8 Bohr magnetons per iron atom, the value present in holo HoSF. HoSF containing eight or more Fe 3+ to which eight Fe 2+ were added showed that the Fe 2+ ions were at sites remote from the Fe 3+ and that the resulting HoSF consisted of individual, noninteracting Fe 2+ and the partially aggregated Fe 3+. pH measurements for core reduction showed that Fe(OH) 3 was initially present at all iron loadings but that in the absence of iron chelators the reduced iron core is partially hydrolyzed. Proton induced x-ray emission spectroscopy showed that Cl − is transported into the iron core during reduction, forming a stable chlorohydroxy Fe(II) mineral phase.