Low Fe Loading Research Articles

Nalidixic Acid and Fe(II)/Cu(II) Coadsorption at Goethite and Akaganéite Surfaces.

Interactions between aqueous Fe(II) and solid Fe(III) oxy(hydr)oxide surfaces play determining roles in the fate of organic contaminants in nature. In this study, the adsorption of nalidixic acid (NA), a representative redox-inactive quinolone antibiotic, on synthetic goethite (α-FeOOH) and akaganéite (β-FeOOH) was examined under varying conditions of pH and cation type and concentration, by means of adsorption experiments, attenuated total reflectance-Fourier transform infrared spectroscopy, surface complexation modeling (SCM), and powder X-ray diffraction. Batch adsorption experiments showed that Fe(II) had marginal effects on NA adsorption onto akaganéite but enhanced NA adsorption on goethite. This enhancement is attributed to the formation of goethite-Fe(II)-NA ternary complexes, without the need for heterogeneous Fe(II)-Fe(III) electron transfer at low Fe(II) loadings (2 Fe/nm2), as confirmed by SCM. However, higher Fe(II) loadings required a goethite-magnetite composite in the SCM to explain Fe(II)-driven recrystallization and its impact on NA binding. The use of a surface ternary complex by SCM was supported further in experiments involving Cu(II), a prevalent environmental metal incapable of transforming Fe(III) oxy(hydr)oxides, which was observed to enhance NA loadings on goethite. However, Cu(II)-NA aqueous complexation and potential Cu(OH)2 precipitates counteracted the formation of ternary surface complexes, leading to decreased NA loadings on akaganéite. These results have direct implications for the fate of organic contaminants, especially those at oxic-anoxic boundaries.

Quality or Quantity? How Structural Parameters Affect Catalytic Activity of Iron Oxides for CO Oxidation

The replacement of noble metal catalysts by abundant iron as an active compound in CO oxidation is of ecologic and economic interest. However, improvement of their catalytic performance to the same level as state-of-the-art noble metal catalysts requires an in depth understanding of their working principle on an atomic level. As a contribution to this aim, a series of iron oxide catalysts with varying Fe loadings from 1 to 20 wt% immobilized on a γ-Al2O3 support is presented here, and a multidimensional structure–activity correlation is established. The CO oxidation activity is correlated to structural details obtained by various spectroscopic, diffraction, and microscopic methods, such as PXRD, PDF analysis, DRUVS, Mössbauer spectroscopy, STEM-EDX, and XAS. Low Fe loadings lead to less agglomerated but high percentual amounts of isolated, tetrahedrally coordinated iron oxide species, while the absolute amount of isolated species reaches its maximum at high Fe loadings. Consequently, the highest CO oxidation activity in terms of turnover frequencies can be correlated to small, finely dispersed iron oxide species with a large amount of tetrahedrally oxygen coordinated iron sites, while the overall amount of isolated iron oxide species correlates with a lower light-off temperature.

3D N-doped ordered mesoporous carbon supported single-atom Fe-N-C catalysts with superior performance for oxygen reduction reaction and zinc-air battery

Single-atom Fe-N-C electrocatalysts have emerged as the most promising oxygen reduction reaction (ORR) catalyst. However, the low Fe loading and inaccessibility of Fe-N-C sites limit the overall ORR activity. Here, we report an efficient single-atom electrocatalyst (Fe-N-C/N-OMC) with Fe-N-C sites embedded in three-dimensional (3D) N-doped ordered mesoporous carbon framework. Fe-N-C/N-OMC shows high half-wave potential, kinetic current density, turnover frequency and mass activity towards ORR in alkaline electrolyte. Experiments and theoretical calculations suggest that the ultra-high ORR activity stems from the boosted intrinsic activity of FeN4 sites by graphitic N dopants, high density of accessible active site generated by high Fe and N loadings and ordered mesoporous carbon structure as well as facilitated mass and electron transport in 3D interconnected pores. Fe-N-C/N-OMC also shows comparable ORR activity to Pt/C in acidic electrolyte. As the cathode for zinc-air battery, Fe-N-C/N-OMC exhibits high open-circuit voltage, high power density and remarkable durability.

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.

Engineering Iron-Based Nanoparticles Spatially Dispersed on Mesoporous Carbon and Its Catalytic Activity for the Direct Oxidization of Benzene to Phenol

We report engineered iron-based nanoparticles supported on cagelike mesoporous carbon that leaves its most mesopores empty to retain an open pore network and are expected to be efficient catalyst with fast molecular diffusion/transportation. The nano-scale iron-based particle inlayed in mesoporous carbon catalyst was obtained via the introduction of N atoms as an anchor. Results of X-ray diffraction, N2 sorption and transmission electron microscopy showed that the cagelike mesoporous structure of the carbon matrix was retained during catalyst preparation and iron-based nanoparticles were spatially dispersed on the mesoporous carbon. Importantly, it was found that the obtained iron-based nanoparticles inlayed into mesoporous carbon with a low Fe loading of 1.26[Formula: see text]wt.% was an appropriate catalyst for the benzene hydroxylation to phenol using H2O2 as the oxidant. At a low temperature of 30∘C, 19.4% conversion to benzene and 14.6% phenol yield were obtained; in addition, the catalyst could be recycled at least four times.

Investigating the Influence of Fe Speciation on N2O Decomposition Over Fe\u2013ZSM-5 Catalysts

The influence of Fe speciation on the decomposition rates of N2O over Fe–ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe–ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N2O decomposition at 600 °C improved to 3.99 × 103 s−1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60 × 103 s−1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N2O decomposition when propane is present. When only N2O is present, low metal loading Fe–ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe3+ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as FexOy, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of FexOy nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.

Polyformamidine‐Derived Non‐Noble Metal Electrocatalysts for Efficient Oxygen Reduction Reaction

AbstractA facile approach for the template‐free synthesis of highly active non‐noble metal based oxygen reduction reaction (ORR) electrocatalysts is presented. Porous Fe−N−C/Fe/Fe3C composite materials are obtained by pyrolysis of defined precursor mixtures of polyformamidine (PFA) and FeCl3 as nitrogen‐rich carbon and iron sources, respectively. Selection of pyrolysis temperature (700–1100 °C) and FeCl3 loading (5–30 wt%) yields materials with differing surface areas, porosity, graphitization degree, nitrogen and iron content, as well as ORR activity. While the ORR activity of Fe‐free materials is limited (i.e., synthesized from pure PFA), a huge increase in activity is observed for catalysts containing Fe, revealing the participation of the metal dopant in the construction of active electrocatalytic sites. Further activity improvement is achieved via acid‐leaching and repeated pyrolysis, a result which is attributed to the creation of new active sites located at the surface of the porous nitrogen‐doped carbon by dissolution of the Fe and Fe3C nanophases. The best performing catalyst, which was synthesized with a low Fe loading (i.e., 5 wt%) and at a pyrolysis temperature of 900 °C, exhibits high activity, excellent H2O selectivity, extended stability, in both basic and acidic media as well as a remarkable tolerance toward methanol.

On the performance optimisation of Fe catalysts in the microwave - assisted H2 production by the dehydrogenation of hexadecane

Microwave - assisted catalysis has previously been shown to offer unique advantages in the rapid production of hydrogen from hydrocarbons fuels. In this work, we have prepared a series of supported catalysts with varying iron content and support materials, and evaluated the resulting H2 production from hexadecane, a key representative component of diesel fuel. These catalysts, under microwave activation, exhibit high selectivity for H2 over hydrocarbons in the effluent gas. Microwave-assisted Fe catalysts supported on SiC at low Fe loading levels (<10 wt.%) are found to be the most effective catalytic system for rapid and high-purity H2 production from hexadecane.

Iron and pH Regulating the Photochemical Mineralization of Dissolved Organic Carbon.

Solar radiation mineralizes dissolved organic matter (DOM) to dissolved inorganic carbon through photochemical reactions (DIC photoproduction) that are influenced by iron (Fe) and pH. This study addressed as to what extent Fe contributes to the optical properties of the chromophoric DOM (CDOM) and DIC photoproduction at different pH values. We created the associations of Fe and DOM (Fe-DOM) that cover the range of loadings of Fe on DOM and pH values found in freshwaters. The introduced Fe enhanced the light absorption by CDOM independent of pH. Simulated solar irradiation decreased the light absorption by CDOM (i.e., caused photobleaching). Fe raised the rate of photobleaching and steepened the spectral slopes of CDOM in low pH but resisted the slope steepening in neutral to alkaline pH. The combination of a low pH (down to pH 4) and high Fe loading on DOM (up to 3.5 μmol mg DOM–1) increased the DIC photoproduction rate and the apparent quantum yields for DIC photoproduction up to 7-fold compared to the corresponding experiments at pH >6 or without Fe. The action spectrum for DIC photoproduction shifted toward the visible spectrum range at low pH in the presence of Fe. Our results demonstrated that Fe can contribute to DIC photoproduction by up to 86% and produce DIC even at the visible spectrum range in acidic waters. However, the stimulatory effect of Fe is negligible at pH >7.

The texture evolution of g-C3N4 nanosheets supported Fe catalyst during Fischer-Tropsch synthesis

Graphitic carbon nitride (GCN) nanosheets decorated with iron nanoparticles (Fe/g-C3N4) were synthesized and used as catalysts in FTS. The crystal structure and morphology of Fe/g-C3N4 were characterized with XRD, FT-IR, HRTEM, EXAFS, BET, and its texture evolution during Fischer-Tropsch synthesis was monitored. It was shown that there were two types of iron species in Fe/GCN, i.e., “dissolved iron”, in which irons dissolved into g-C3N4 matrix as single atoms, and formed mainly at low Fe loadings. In contrast, when the loading exceeded the dissolved saturation, the irons would coordinate with (-N-CN-)2− to form the second type of iron species, Fe(NCN) like species. The “dissolved iron” was more difficult to thermally decompose and convert into iron carbide than Fe(NCN) like species. Such a slow transformation gave birth to unique induction period for low loading Fe catalysts. Moreover, the N-containing bases or defects of g-C3N4 might play a critical role in enhancing O/P ratio. As a result, g-C3N4 supported 40wt% Fe sample exhibited highest activity and C2=-C4=selectivity. Besides, the g-C3N4 possessed a “structure memory effect” analogous to hydrotalcite.

An Iron-based Film for Highly Efficient Electrocatalytic Oxygen Evolution from Neutral Aqueous Solution.

An ultrathin Fe-based film was prepared by electrodeposition from an Fe(II) solution through a fast and simple cyclic voltammetry method. The extremely low Fe loading of 12.3 nmol cm(-2) on indium tin oxide electrodes is crucial for high atom efficiency and transparence of the resulted film. This Fe-based film was shown to be a very efficient electrocatalyst for oxygen evolution from neutral aqueous solution with remarkable activity and stability. In a 34 h controlled potential electrolysis at 1.45 V (vs NHE) and pH 7.0, impressive turnover number of 5.2 × 10(4) and turnover frequency of 1528 h(-1) were obtained. To the best of our knowledge, these values represent one of the highest among electrodeposited catalyst films for water oxidation under comparable conditions. The morphology and the composition of the catalyst film was determined by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, and X-ray photoelectron spectroscopy, which all confirmed the deposition of Fe-based materials with Fe(III) oxidation state on the electrode. This study is significant because of the use of iron, the fast and simple cyclic voltammetry electrodeposition, the extremely low catalyst loading and thus the transparency of the catalyst film, the remarkable activity and stability, and the oxygen evolution in neutral aqueous media.

Al/Fe isomorphic substitution versus Fe2O3 clusters formation in Fe-doped aluminosilicate nanotubes (imogolite)

Textural, magnetic and spectroscopic properties are reported of Fe-doped aluminosilicate nanotubes (NTs) of the imogolite type, IMO, with nominal composition (OH)3Al2−x Fe x O3SiOH (x = 0, 0.025, 0.050). Samples were obtained by either direct synthesis (Fe-0.025-IMO, Fe-0.050-IMO) or post-synthesis loading (Fe-L-IMO). The Fe content was either 1.4 wt% (both Fe-0.050-IMO and Fe-L-IMO) or 0.7 wt% (Fe-0.025-IMO). Textural properties were characterized by High-Resolution Transmission Electron Microscopy, X-ray diffraction and N2 adsorption/desorption isotherms at 77 K. The presence of different iron species was studied by magnetic moment measurements and three spectroscopies: Mössbauer, UV–Vis and electron paramagnetic resonance, respectively. Fe3+/Al3+ isomorphic substitution (IS) at octahedral sites at the external surface of NTs is the main process occurring by direct synthesis at low Fe loadings, giving rise to the formation of isolated high-spin Fe3+ sites. Higher loadings give rise, besides IS, to the formation of Fe2O3 clusters. IS occurs up to a limit of Al/Fe atomic ratio of ca. 60 (corresponding to x = 0.032). A fraction of the magnetism related to NCs is pinned by the surface anisotropy; also, clusters are magnetically interacting with each other. Post-synthesis loading leads to a system rather close to that obtained by direct synthesis, involving both IS and cluster formations. Slightly larger clusters than with direct synthesis samples, however, are formed. The occurrence of IS indicates a facile cleavage/sealing of Al–O–Al bonds: this opens the possibility to exchange Al3+ ions in pre-formed IMO NTs, a much simpler procedure compared with direct synthesis.

Properties and catalytic performance in phenol hydroxylation of iron on zeolite beta prepared by different methods

Iron (Fe) was loaded on zeolite beta (BEA) in ammonium (NH4BEA) or proton (HBEA) forms by various methods. Fe/HBEA-LSIE and Fe/NH4BEA-LSIE were prepared by liquid state ion exchange (LSIE). This method resulted in low Fe loading and a good dispersion. The form of Fe was Fe2O3 mainly located at the ion exchange position in the zeolite channels. Fe/HBEA-IWI and Fe/HBEA-PM were prepared by incipient wetness impregnation (IWI) and physical mixing (PM), respectively. Both catalysts had higher Fe loading and larger Fe2O3 particles located on both external surface and in the zeolite channels. Despite the different loadings, all samples gave similar phenol conversion in phenol hydroxylation, around 60 % and yielded products catechol (CAT) and hydroquinone (HQ) with the CAT/HQ mole ratio 2:1. After the catalytic testing, all catalysts had a dark color from coking. A further investigation revealed a formation of coke and organic acids. According to temperature programmed oxidation (TPO), heavy coke was produced from Fe/HBEA-LSIE and Fe/NH4BEA-LSIE. The coke from Fe/HBEA-LSIE was heavier than that from Fe/NH4BEA-LSIE. Only light coke was produced from Fe/HBEA-IWI and Fe/HBEA-PM.

Fe K-Edge X-ray Absorption Fine Structure Determination of γ-Al2 O3 -Supported Iron-Oxide Species.

The structure of FeOx species supported on γ-Al2 O3 was investigated by using Fe K-edge X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) measurements. The samples were prepared through the impregnation of iron nitrate on Al2 O3 and co-gelation of aluminum and iron sulfates. The dependence of the XRD patterns on Fe loading revealed the formation of α-Fe2 O3 particles at an Fe loading of above 10 wt %, whereas the formation of iron-oxide crystals was not observed at Fe loadings of less than 9.0 wt %. The Fe K-edge XAFS was characterized by a clear pre-edge peak, which indicated that the FeO coordination structure deviates from central symmetry and that the degree of FeOFe bond formation is significantly lower than that in bulk samples at low Fe loading (<9.0 wt %). Fe K-edge extended XAFS oscillations of the samples with low Fe loadings were explained by assuming an isolated iron-oxide monomer on the γ-Al2 O3 surface.

Effect of preparation method of palygorskite-supported Fe and Ni catalysts on catalytic cracking of biomass tar

In this study, the effect of catalyst preparation and additive precursors on the catalytic decomposition of biomass using palygorskite-supported Fe and Ni catalysts was investigated. The catalysts were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is concluded that the most active additive precursor was Fe(NO3)3·9H2O. As for the catalyst preparation method, co-precipitation had superiority over incipient wetness impregnation at low Fe loadings.

Novel method for doping nano TiO2 photocatalysts by chemical vapour deposition

Highly active photocatalytic Fe-doped nano TiO2 was successfully synthesised by chemical vapour deposition (CVD) method using FeCl3 as Fe source. CVD was carried out by evaporating FeCl3 at 350°C in nitrogen flow during 30–90 min. The amount of Fe incorporated into TiO2 framework is adjusted by the amount of FeCl3 used and the evaporation time. The obtained sample was characterised by X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), energy dispersive X-ray spectroscopy (EDS), UV-Vis, Fourier transform-infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). Photocatalytic activities of the samples were tested in photocatalytic decomposition of 2-propanol in liquid phase using visible light instead of UV light irradiation. Non-doped TiO2 and high Fe loading TiO2 samples showed very low photocatalytic activity, whereas the low Fe loading TiO2 sample exhibited high photocatalytic activity under visible light. The high photocatalytic activity of this sample was rationalised by the existence of defects (Ti–OH groups) as the active sites.

Direct Hydrothermal Synthesis of Iron-Containing Mesoporous Silica SBA-15: Potential as a Support for Gold Nanoparticles

The preparation of mesoporous silica SBA-15 with high iron loadings (14−90 wt % Fe2O3) as a suitable support for gold nanoparticles to be used in CO oxidation catalysis has been investigated. The support materials were prepared by a direct hydrothermal two-step pH adjusting method which consisted of the formation of the silica mesophase at low pH and the inclusion of Fe3+ at varying pH in the range 2−7. The materials were characterized by XRD, SEM, and TEM/EDX, N2 porosimetry, and 57Fe Mössbauer spectroscopy. At relatively low Fe loading, the SBA-15 structure is maintained and iron is predominantly surface grafted to the silica surface. Such a mesoporous silica can accommodate up to 40 wt % Fe2O3 with a surface area of 460 m2/g. With increasing Fe content, precipitation of iron hydroxides competes with the surface grafting process and the resulting materials are an intimate mixture of hematite particles embedded in an Fe/SBA-15 matrix. A too high Fe3+ content in the synthesis gel results in a high rate of precipitation and impedes the formation of the silica mesophase. The stabilization of the mesophase at pH 7 is proposed to involve interactions of the surfactant with a surface grafted Fe3+ silica phase. The use of an SBA-15 which contains mainly surface grafted Fe3+ as a support for gold nanoparticles results in a more active catalyst for CO oxidation than gold supported by SBA-15 or iron oxide particles.

Nanoparticles of Fe 2O 3 inserted in SBA-15 silica at micropore mouth level: An experimental evidence of the confinement effect

Nanometric Fe 2O 3 particles could be inserted inside the internal pore volume of SBA-15 mesoporous silica when Fe(III) chelates (EDTA, gluconate and citrate) were used as impregnating precursors. Fe(III) nitrate preferentially yields 8 nm uniformly sized Fe 2O 3 clusters that selectively plug the SBA-15 channels through a geometric confinement effect. An oxidative degradation of Fe-chelate precursors yielded Fe(III) oxidic particles of various sizes and dispersion, depending on the nature and geometry of chelate anion. Fe(EDTA) precursors specifically generated Fe 2O 3 nanoparticles that selectively migrate towards two types of positions where the silica surface exhibits a high curvature. In samples involving low Fe loadings, Fe 2O 3 particles first creep towards the silica micropore mouths (nests), in which they are readily confined and stabilized. For higher Fe loadings, when most of the micropore nests were filled, oxidic particles eventually settle as a superficial film on the mesopore walls and undergo stabilization onto the surface roughness that can also favors their confinement, as theoretically predicted by Derouane and co-workers. Upon further reduction, as selectively followed by combined cyclic voltammetry and TPR, Fe 2O 3 readily yield FeO nanoslabs that remain even more efficiently confined within their respective nest positions through a further acid–base type stabilization.

Monodisperse magnetic iron colloids grafted with polyisobutene: a model system for Fischer-Tropsch catalysts?

Iron colloids stabilised with polyisobutene with a radius ranging from 2.1 to 8.2 nm have been prepared and characterised by Mossbauer absorption spectroscopy. Comparison with results reported in the literature on sarcosyl and oleic acid stabilised colloids reveals that the stabilising ligand influences the properties of the colloids. By impregnating an alumina support with a solution of the r=2.1 nm Fe colloids a model catalyst has been prepared. The first results show that the impregnation was successful. However, due to the low Fe loading, the signal-to-noise ratio is too low to perform good measurements on this model system. Therefore, Fe-colloids will be prepared that are enriched in 57Fe.

Active Sites in Fe/MFI Catalysts for NOx Reduction and Oscillating N2O Decomposition

Fe/MFI catalysts prepared either by sublimation of FeCl3 onto H-MFI or by solid state ion exchange were characterized by XRD, FTIR, and ESR spectroscopy. Ligated Fe3+ ions, such as (Fe(OH)2)+ or (Fe=O)+ in distorted tetrahedral coordination, multinuclear oxocations, such as (HO-Fe-O-Fe-OH)2+, and Fe2O3 oxide particles have been identified. Their relative abundances depend on the Si/Al ratio and the Fe loading. Nitro and nitrate groups, identified by their FTIR bands, are formed upon exposing Fe/MFI with high Fe loading to N2O. These adsorption complexes are reduced with benzene at 200°C but not at room temperature. Their reduction with CO occurs at a lower temperature than that of the bridging oxygen. Exposure to N2O at 320°C and 0.5 kPa pumps 30% more active oxygen into Fe/MFI than calcination in O2 of 100 kPa at 500°C. Thermal decomposition releases NO and O2 at the same temperature. The catalytic decomposition rate of N2O to N2+12O2 in diluted feed streams over Fe/MFI is first order in N2O. The rate constant is very low for samples with low Fe loading, but jumps to a much higher value for high Fe loadings, indicating the important role of multinuclear Fe clusters in accordance with recent EXAFS results. Stable isothermal oscillations are observed with overexchanged Fe/MFI (Fe/Al=1; Al/Si=1/14), in the presence of H2O vapor, showing that two different states of catalytic activity exist. No oscillation is observed with a dry feed or over Fe/MFI with low loading.