Catalytic CO hydrogenation on potassic Fe/zeolite LTL

Zeolite LTL in the potassic form ( -LTL) was used as support of iron. The precursor was characterized by temperature programmed reduction (TPR) and Mössbauer spectroscopy (MS) at 298 and 15 K. The catalyst was obtained reducing the precursor with H₂ and its structural properties were studied by H₂ chemisorption, volumetric oxidation and MS under controlled conditions at 298 and 15 K. Measurements of activity and selectivity were carried out at 1 and 20 bar in the CO hydrogenation. Two different types of active sites were detected: Fe⁰ inside the zeolite channels and carburized iron on its external surface. The solid showed a high activity and selectivity towards light alkenes in comparison with other iron supported systems (Fe/SiO₂, Fe/Al₂O₃ and Fe/C). This behavior can be attributed to the high percentage of metallic iron promoted by potassium ions, located inside the zeolite channels. The external carbide crystals generate hydrocarbons in the diesel oil range.     

Mössbauer spectroscopy of iron compounds found in West Virginia coals

Mössbauer measurements were carried out in several coal samples from various seams in West Virginia. The presence of iron pyrite and iron (II) sulphate was observed in all the samples studied. The two different mineral compounds were clearly identified from their isomer shift and quadrupole splitting, and the Mössbauer spectral area was used to determine their amounts. This method is recommended as a complementary technique for the measurement of the amount of sulphur bonded to iron in coals. No evidence of organically bound iron in coal was found for more than forty different samples analysed. Using the Mössbauer effect, unequivocal evidence of the formation of a new mineral species during the low-temperature ashing process was found.     

Preparation of zeolite-entrapped iron clusters by alkali injection followed by reduction with dihydrogen gas

An alkali injection method was used to promote the formation of reduced iron clusters in zeolite Y. By injecting NaOH solution into the pores of iron ion-exchanged zeolite, the iron ions were exchanged back with the injected sodium ions to be precipitated as hydroxide forms. Reduction of these precipitates in hydrogen gas yielded reduced iron clusters inside the zeolite. All preparation procedures should be performed under oxygen-free atmosphere since the reexchange of the injected sodium ions with Fe³⁺ ions was much more difficult than that with the Fe³⁺ ions. Mössbauer spectra and ferromagnetic resonance spectra of the reduced catalysts revealed that the iron clusters were extremely small and uniform in size.     

Reactivity of generated oxygen species from nitrous oxide over [Fe,Al]MFI catalysts for the direct oxidation of benzene to phenol

The catalytic performance of various steam-activated [Fe,Al]MFI catalysts in the direct oxidation of benzene to phenol using N₂O as oxidant is described. All [Fe,Al]MFI catalysts contain ca. 90% of iron in the high-spin Fe²⁺ state, independent of the iron concentration (0.075–0.6 wt.% iron). In the presence of N₂O at 623 K, most Fe²⁺ ions (>90%) were oxidized to Fe³⁺ ions as deduced from Mössbauer spectroscopy. In the presence of benzene, subsequent reduction of Fe³⁺ to Fe²⁺ takes place. However, not all of the oxidized Fe²⁺ to Fe³⁺ ions were able to selectively oxidize benzene to phenol. This indicates that only a fraction of iron is catalytically active. For [Fe,Al]MFI catalysts with relatively high iron concentration, most of the extra-framework iron species formed are inactive in the direct oxidation of benzene to phenol. Finally, a more detailed in situ Mössbauer study for one sample, i.e. [Fe,Al]MFI (1:8) catalyst, was performed to illustrate the reduction/oxidation properties of the different iron species formed after steam-treatment.     

Comparison of microwave and radiofrequency low-temperature ashing of coal using Fe Mössbauer spectroscopy of sulphur forms

Low-temperature ashing of coal was carried out by the normal radiofrequency method and a new microwave technique to compare the effects of the two procedures on the Fe-S compounds in coal. Results from ⁵⁷Fe Mössbauer spectroscopy and chemical analyses indicate that the microwave technique, like the normal LTA procedure, effectively ashes the coal without the oxidation of pyritic sulphur. Dehydration of iron sulphates and oxidation of Fe²⁺ to Fe³⁺ in sulphates occurs in both ashing procedures under the conditions tested, the sequence being FeS0₄.4H₂O → FeS0₄.H₂0→Fe³⁺. Dehydration is caused by sample heating rather than by reaction with the activated oxygen. The combination of the Mössbauer method and chemical analysis has allowed the determination of the recoilless fraction ratio f(sulphate)/f(pyrite) = 0.55 ±0.15, which puts the Mössbauer results on a more quantitative basis than before. This is thought to be the first quantitative work on coal and coal products that has accounted for differences in f-values for different Fe compounds.     

Study of [Fe(bpy)3] complex encapsulated in zeolite Y

Tris complex of Fe^II with 2,2′ bipyridine (bpy) ligand, has been encapsulated in the supercages of zeolite Y and characterized by powder X-ray diffraction, Infrared spectroscopy, ⁵⁷Fe Mössbauer, effect measurements and multinuclear NMR spectroscopy. The complex was prepared in the supercages of Zeolite Y by exchanging the cations of zeolite Y with Fe^II and its subsequent complexing with bpy ligand. Based on the comparison of the results obtained for [Fe^II(bpy)₃](ClO₄)₂ and the [Fe(bpy)₃]Y complexes, it has been inferred that there is significant structural distortion for the encapsulated complex and all Fe is present as Fe^II in low spin state, which is characterized by isomer shift, δ = 0.37 mm s⁻¹ and quadrupole splitting, ΔE_q = 0.81 mm s⁻¹ as revealed by ⁵⁷Fe Mössbauer spectroscopic measurements.     

Solid state multinuclear NMR study of iron species in natural and modified clinoptilolite from Tasajera deposit (Cuba)

The present study has been conducted to elucidate the Fe-sites in natural clinoptilolite, and the possibility to produce a reversible replacement of these atoms in the zeolite framework. In order to achieve these results we have tried to introduce iron in clinoptilolite structure after the almost total extraction of the natural iron species. A sample of the purified natural clinoptilolite from Tasajeras deposit, Central region of Cuba, was modified by a hydrothermal treatment with orthophosphoric acid. Iron exchange forms of the orthophosphoric zeolite were obtained using FeSO₄ and Fe₂(SO₄)₃ solutions. The modified zeolites were studied by multinuclear magnetic resonance (²⁷Al, ²⁹Si, ³¹P MAS NMR and ¹²⁹Xe NMR). Other analytical techniques as Electron Paramagnetic Resonance, X-ray diffraction, Mössbauer and Infrared spectrometry, volumetric measurements and chemical analysis by Induced Plasma Coupled spectroscopy were used to support the results of NMR studies. The presence of iron species with tetrahedral and octahedral coordination in the structure of a natural clinoptilolite has been confirmed. The influence of iron in tetrahedral coordination in the thermal stability of the clinoptilolite structure was also confirmed.     

Mössbauer spectroscopic studies of Fe-substituted YIG

Compositional dependence of hyperfine parameters, determined through Mössbauer spectral analysis has been studied for Y_3−xFe_5+xO₁₂ (x = 0.0, 0.1, 0.3 and 0.5) garnet system at 300 K. The Mössbauer spectra have been fitted with three sextets in the ferrimagnetic state corresponding to Fe³⁺ ions at tetrahedral (d), octahedral (a) and dodecahedral (c) sites of the crystal structure. It is observed that isormershift, quadrupole shift and hyperfine field of d-site show no significant variation with Fe³⁺ concentration. The change in hyperfine fields of a- and c-sites with composition (x) has been explained on the basis of strength of exchange integrals, change in isomershift can be understood due to s-electron charge distribution and asymmetric displacement of oxygen ions surrounding the a- and c-sites seems to be responsible for observable quadrupole shift. The magneton number values obtained from magnetization and Mossbauer data are in agreement to those calculated using Neel's three sublattice model of ferrimagnetism.     

Oxidation of cobalt based Fischer–Tropsch catalysts as a deactivation mechanism

The oxidation of supported cobalt based slurry bed Fischer–Tropsch catalysts by means of water was studied. Water is one of the Fischer–Tropsch reaction products and can probably cause oxidation and deactivation of a reduced cobalt catalyst. Model experiments using Mössbauer emission spectroscopy and thermogravimetry as well as realistic Fischer–Tropsch synthesis runs were performed. It was demonstrated that Mössbauer emission spectroscopy can successfully be applied to the investigation of high cobalt loading Fischer–Tropsch catalysts. Strong indications were found that oxidation of reduced cobalt catalysts occurs under realistic Fischer–Tropsch conditions. Mössbauer emission spectroscopy and thermogravimetry results showed that the oxidation depends on the P_H2/P_H2O ratio, and that oxidation proceeds to less than complete extents under certain conditions. The formation of both reducible and less reducible cobalt oxide species was observed, and the relative ratio between these species depends on the severity of the oxidation conditions.     

Cobalt-, copper- and iron-containing monolithic aluminosilicate-supported preparations for selective catalytic reduction of NO with NH3 at low temperatures

Aluminium-doped mesoporous monolithic silica possessing fine mesopores has been prepared via the direct liquid crystal templating pathway using a non-ionic surfactant template and has been used as a support for cobalt-, copper-, and iron-based formulations in the selective catalytic reduction (SCR) of NO with ammonia in the presence of oxygen at low temperatures in the range of 373–723 K. The monolithic support was characterised by N₂ gas adsorption at 77 K, powder X-ray diffraction (XRD) and NH₃ adsorption at 373 K. Surface area, pore structure and surface acidity of the catalysts before and after being subjected to catalytic testing were determined, and good stability of pore structure and surface properties under SCR conditions was indicated. The NO conversions on aluminosilicate monolith-supported catalysts were compared with those observed on the reference catalyst, EUROCAT powder. The Co-functionalised catalysts appeared less relevant to DeNO_x purposes. Two impregnated and two ion-exchanged catalysts containing copper and iron showed catalytic performance comparable to that of the reference catalyst. They produced only small amounts of undesired product N₂O, the Fe-containing formulations being even more selective than EUROCAT. The nature of metal species in these catalysts was investigated with the aid of Cu 2p X-ray photoelectron spectroscopy (XPS) and ⁵⁷Fe Mössbauer spectroscopy.     

Low temperature oxidation of CO over tin-modified Pt/SiO2 catalysts

Low temperature oxidation of CO over alloy type Sn–Pt/SiO₂ catalysts with different Sn/Pt ratios has been investigated at different CO partial pressure using thermal programmed oxidation (TPO) technique and time on stream (TOS) experiments. The introduction of tin into platinum strongly increased the activity of the catalyst. The activity had a maximum, which depended on both the Sn/Pt (at./at.) ratio and the CO partial pressure. TOS experiments revealed the aging of the Sn–Pt/SiO₂ catalysts. FTIR and Mössbauer spectroscopy has been used to follow compositional and structural changes of Sn–Pt/SiO₂ catalysts during the catalytic run. The results show that the in situ formed, highly mobile “Snⁿ⁺–Pt” ensemble sites are responsible for high activity, while formation of relatively stable SnO_x type surface species are involved in the catalyst deactivation.     

The removal of iron and cobalt from aqueous solutions by ion exchange with Na‐Y zeolite: batch,semi‐batch and continuous operation

The removal of single component and binary mixtures of divalent cobalt and iron from water by ion exchange with synthetic Y zeolite has been studied in batch, semi‐batch and continuous modes of operation; the initial metal solution concentration did not exceed 2 mmol dm^?3. Binary Co/Na and Fe/Na ion exchange equilibrium isotherms (294 K) are presented wherein exchange site heterogeneity is evident in the case of the iron treatment. Under conditions of stoichiometric ion exchange, removal efficiencies for both cobalt and iron decrease with increasing metal concentration (0.2–2 mmol dm^?3) and the values were similar for both metals. Removal of cobalt under transient conditions was found to be temperature dependent. In the fixed bed operation, break‐through behavior was sensitive to changes in both flow rate and inlet concentration. The break‐through profiles for both metals under competitive and non‐competitive conditions are presented; iron removal is lower in the presence of cobalt and vice versa. An in situ regeneration of the fully loaded zeolite by back exchange with sodium is considered and the exchange capacity of the regenerated zeolite is reported. The feasibility of employing cycles of heavy metal uptake/zeolite regeneration is addressed. © 2002 Society of Chemical Industry     

Nanosized iron and iron–cobalt spinel oxides as catalysts for methanol decomposition

Nanosized iron and mixed iron–cobalt oxides supported on activated carbon materials and their bulk analogues prepared by thermal synthesis are studied by X-rays diffraction, Mössbauer spectroscopy, magnetic measurements and temperature programmed reduction. Their catalytic behavior in methanol decomposition to H₂, CO and methane is tested. Phase transformations in the metal oxides affected by the reaction medium are also investigated. Changes in the reaction mechanism of the methanol decomposition after the metal oxides deposition on the support as compared to the bulk phases are discussed.     

Characterization of Iron Species in Ex-Framework FeZSM-5 by Electrochemical Methods

Solid state electrochemistry is described as a method to characterize iron species in the different stages of the preparation of ex-framework FeZSM-5. The ex-framework method comprises the hydrothermal synthesis of isomorphously substituted FeZSM-5, followed by calcination at 823 K and steam treatment (300 mbar H₂O in N₂) at 873 K. Incorporation of FeZSM-5 samples in graphite-polyester composite (GPC) electrodes and identification of the electrochemical response provides information on the structural environment and oxidation state of electroactive iron species in the zeolite. ⁵⁷Fe Mössbauer spectroscopy and TEM analysis were additionally used to interpret the electrochemical responses. Tetrahedral iron atoms in framework positions were the only species observed in the as-synthesized material. After calcination, also isolated extra-framework iron ions and oligonuclear iron--oxo complexes were identified, as well as a minor amount of FeO_x nano-particles. The steaming procedure leads to extensive formation of the FeO_x nano-particles of 1-2 nm in size, which were also identified by TEM. Both oxidative as well as reductive dissolution processes have been observed for the nano-particles, suggesting the presence of both Fe(II) and Fe(III) oxidation states in the steamed FeZSM-5, which is confirmed by Mössbauer spectroscopy. Tetrahedrally coordinated iron atoms in framework positions, as well as the isolated extra-framework iron ions and oligonuclear iron-oxo complexes in octahedral positions, yield electron-transfer processes that approach that of strongly electrode-attached species with distinctive variations of the electrochemical parameters on the pH and the potential scan rate.     

The application of Mössbauer emission spectroscopy to industrial cobalt based Fischer–Tropsch catalysts

⁵⁷Co-Mössbauer emission spectroscopy (MES) has been used to study the oxidation of cobalt as a deactivation mechanism of high loading cobalt based Fischer–Tropsch catalysts for the gas-to-liquids process. It was reported previously [Catal. Today 58 (2000) 321; Proceedings of the International Symposium on the Industrial Applications of the Mössbauer Effect, 13–18 August, 2000, Virginia Beach, VA] that oxidation was observed at atmospheric pressure under conditions that were in contradiction with the bulk cobalt phase thermodynamics. A high-pressure MES cell was designed and constructed, which created the opportunity to study the oxidation of cobalt based Fischer–Tropsch catalysts under realistic synthesis conditions. The cobalt catalyst preparation procedure was investigated by means of ⁵⁷Fe-Mössbauer absorption spectroscopy, applying ⁵⁷Fe as a probe atom. Initial results indicate, although not yet conclusive, that a ⁵⁷Co-MES catalyst can be prepared from the industrial prepared standard Co catalyst by an additional simple incipient wetness impregnation procedure.     

Effect of additives on the stoichiometry of iron sulphides formed during liquefaction of a Powhatan No. 5 coal

Transformations of pyrite in a Powhatan No. 5 is studied using Mössbauer spectroscopy. Stoichiometry of the resultant pyrrhotites is shown to be influenced by the H₂S partial pressure in the reactor. Higher H₂S partial pressure yields pyrrhotites with more iron deficiency. Variation in the H₂S partial pressure is achieved through the addition of either pyrite, troilite or pyrrhotite. It is also shown that pyrrhotites having the same average at% Fe may have widely differing vacancy distribution on the Fe sites.     

Effect of iron counter-ions on the redox properties of the Keggin-type molybdophosphoric heteropolyacid Part I. An experimental study on isobutane oxidation catalysts

Keggin-type molybdophosphoric heteropolyacid with protons partially substituted by iron cations in a bulk form (Fe_0.85H_0.45PMo₁₂O₄₀) or supported on the cesium salt (Cs₂Fe_0.2H_0.4PMo₁₂O₄₀) have been synthesized and characterized by different techniques like the Mössbauer spectroscopy and the electron spin resonance (ESR). The effect of iron on the redox and catalytic properties for the oxidation of isobutane into methacrylic acid (MAA) has also been studied. Iron has been shown to have a different effect whether acts as a counter-cation in the bulk acid or in the acid supported on the cesium salt. In the first case, it increases both the selectivity in methacrylic acid and methacrolein (MA) and the activity of the acid phase whereas in the second case, it increases only the selectivity. This difference has been explained by the existence of an electron transfer between iron and molybdenum occurring only in the bulk acid. This electron transfer was related to a combined hydration-oxidation mechanism which promotes the reducibility of the solid and consequently its catalytic activity.     

Catalytic wet peroxide oxidation over mixed (Al–Fe) pillared clays

Mixed (Al–Fe) pillared clays are very efficient solid catalysts for oxidation of organic compounds in water by hydrogen peroxide. We have shown that in rather mild experimental conditions (atmospheric pressure, T≤70°C) and with a low excess (20%) of hydrogen peroxide, phenol was rapidly converted, mainly to CO₂, without significant catalyst leaching. The (Al–Fe) pillared clay catalyst (called FAZA) can be used several times without any change of its catalytic properties. According to the low leaching observed and a previous Mössbauer spectroscopy study, the iron species appear to be strongly bonded to the aluminium pillars.     

Catalysts by design: genesis and CO hydrogenation of novel Pd carbonyl clusters in zeolite 5A

The potential of catalyst synthesis by design is demonstrated by comparing Pd/5A to Pd/NaY. While supercage dimensions are similar for both zeolites, the diameter of the supercage windows not only determines the Pd nuclearity of entrapped Pd carbonyl clusters, but also restricts their growth under CO hydrogenation conditions. While Pd₁₃(CO) _x clusters prevail in zeolite Y as a result of migration and coalescence of primary Pd carbonyl clusters after CO exposure at room temperature, cluster growth in zeolite 5A is confined to Pd₆(CO) _x . Under the conditions of syngas conversion, small Pd clusters are stabilized in the supercages of 5A, in contrast to the agglomeration of Pd particles to size larger than 60 Å in NaHY. The catalytic activity of Pd/5A is twice that of Pd/NaHY. The selectivities of CO hydrogenation on both catalysts are also drastically different: on Pd/5A, methanol and dimethylether are the sole products besides methane, but on Pd/NaHY, production of C₂₊ hydrocarbons is significant.     

Changes in states of substituted iron in microporous (MFI analogue) and mesoporous (MCM-41) hosts exposed to redox conditions

Iron was incorporated into microporous MFI analogue (ZSM-5, Si/Fe  200 and FER, Si/Fe = 16) and mesoporous MCM-41 hosts of different preparations (Si/Fe  100 and 20, respectively). Samples were characterized by in situ Mössbauer spectroscopy under various conditions. Framework (FW) and extra-framework (EFW) sites can be distinguished in relation with strict three-dimensional (3D) crystallinity of the structure. In addition, in the microporous ZSM-5 system occurrence of Fe⁴⁺ oxidation state is hypothesized and reversible Fe⁴⁺ ↔ Fe³⁺ transformation is presumed for a minor portion of iron. Spectra of Fe-FER are interpreted in terms of dynamic formation and decomposition of Fe_FW–O–Fe_EFW dinuclear centres. In the mesoporous systems, the partly amorphous character of the pore wall is correlated with the redox behaviour and coordination of incorporated iron; larger portion of iron may take part in Fe³⁺ ↔ Fe²⁺ reversible processes and Fe²⁺ may be stabilized in distorted coordination (not appearing in microporous structures).