A.R. Overweg

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mossbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Bronsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination.  2002 Elsevier Science (USA). All rights reserved.

A new approach to the determination of atomic-architecture of amorphous zeolite precursors by high-energy X-ray diffraction technique

The structure of amorphous precursor species formed under hydrothermal conditions, prior to the onset of crystallization of microporous aluminosilicate zeolites, is determined employing high-energy X-ray diffraction (HEXRD). The investigation, combined with the use of reverse Monte Carlo modelling suggests that even numbered rings, especially 4R (R: ring) and 6R, which are the dominant aluminosilicate rings in zeolite A, have already been produced in the precursor. The model implies that the formation of double 4Rs occurs at the final step of the crystallization of zeolite A.

The role of alpha-iron and cementite phases in the growing mechanism of carbon nanotubes: a 57Fe Mössbauer spectroscopy study

57Fe Mössbauer spectroscopy was used to study the reduction behavior at temperatures as high as 1073 K of an iron/silica catalyst, and also the carbonaceous materials isolated after acetylene decomposition over this catalyst at several temperatures (873-1073 K). The products were previously characterized by transmission electron microscopy and it was clearly proven that the concentration of carbon nanotubes increased when reaction reached highest temperatures. This was related with an increment in cementite concentration (generated from initial alpha-iron and the progressive reduction of the remnant Fe+2 caused by acetylene decomposition) as detected by 57Fe Mössbauer. These results undoubtedly revealed the role of alpha-iron as active center for acetylene decomposition and cementite as main carbide intermediate species in the catalytic growth of CNTs.

Synthesis and characterisation of microporous bimetallic Fe-Cr-Si-O materials derived from silsesquioxane precursors

Calcination of silsesquioxane mixtures of (c-C5H9)7Si7O9(OH)3, 1, (c-C5H9)7Si7O12Fe(CH3)2N(CH2)2N(CH3)2, 2, and (c-C5H9)7Si7O9(OSiMe3)O2CrO2, 3, led to microporous amorphous bimetallic Fe–Cr–Si–O materials with different Fe∶Cr ratios. A set of complementary characterisation techniques including N2 physisorption, XRD, XPS, RS, IR, HRTEM and Mossbauer spectroscopy were used to follow the variation of the textural properties, metal oxide dispersion and speciation with metal content. Fe–Cr–Si–O materials possess high surface areas and uniformly controlled micropores with an average pore size diameter of around 6–7 A. Metal oxide speciation of calcined silsequioxane mixtures appears to be significantly different from that observed for these metals in the individually calcined metal silsesquioxanes. The iron oxide and monochromates are the predominant species in the calcined precursors 2 and 3 while very small particles (2–4 nm) of bimetallic mixed oxides are the major species in the Fe–Cr–Si–O materials. This suggests that the metal oxide species are highly interdispersed and can come into close contact with each other during the calcination procedure thus favoring the formation of the bimetallic mixed oxide phase. In contrast, a silica reference material containing 7% Fe and 3% Cr prepared via the impregnation method showed only chromate species and large particles (10–30 nm) of iron oxide. This suggests that metallasilsesquioxane mixtures may be used as versatile precursors for the preparation of silica-based catalysts containing very small and well dispersed particles of mixed metal oxides.

Localization of ferrocene in NaY zeolite by powder x-ray and neutron diffraction

We study the inclusion of the metallocene ferrocene Fe(C5H5)2 molecules in the supercages of NaY zeolite. To find the exact location of the ferrocene molecules within the supercages we perform neutron and powder x-ray diffraction on bare NaY zeolite, and on NaY zeolite loaded with one or two ferrocene molecules per supercage. Using the complementary properties of both techniques we show that the ferrocene molecules are located just above a line joining two neighboring sodium ions at the SII positions in the zeolite supercage. The C5H5 rings are oriented towards the sodium ions in an ordered manner. This structure is confirmed by quantum chemistry calculations. The geometry of the ferrocene molecules barely changes, indicating that the increased reactivity of ferrocene upon adsorption in a zeolite is thus a result of the position of the molecules. The main interactions responsible for this position are Coulombic attraction and hydrogen bonding. The inclusion of ferrocene in a Y-type zeolite provides a homogeneous distribution of iron throughout the zeolite at well-defined locations.

Direct synthesis of mesoporous Fe-MFI zeolite

Abstract The direct synthesis of mesoporous Fe-MFI zeolite via a novel route is reported. A whole range of as-synthesised Fe-MFI zeolites with varying iron (0.075 – 0.6 wt%) and aluminium (0 – 1.1 wt%) concentrations contain mesopores. The main difference of the new method with respect to conventional synthesis of Fe-MFI zeolites is that instead of adding iron salts (eg Fe(NO 3 ) 3 ) to the solution gel, a synthesis gel starting from metallic iron foil dissolved in nitric acid is used. A broad mesopore-size distribution is observed with transmission electron microscopy for all as-synthesised Fe-MFI zeolite samples. The mesopores are retained after calcination as well as after subsequent mild steaming.

Evolution of the Active Phase of CoMo/Al2O3 Catalysts under Industrial Conditions: a High-Pressure MES Study

The behavior of CoMo/Al2O3 catalysts sulfided in H2S/H2 gas mixture, under industrial conditions, was investigated using Mossbauer emission spectroscopy (MES). An intermediate Co‐Mo phase is formed after increasing the sulfidation pressure to 4 MPa, favoring the Co‐Mo‐S phase formation. An increase in the quadrupole splitting value of the Co‐sulfide species after treatment at 573 K is proposed as a prerequisite for the formation of ideal Co‐Mo‐S structures.

The characterization of FeZSM-5 by 57Fe Mössbauer spectroscopy: Sensitivity towards nitrogen

Mössbauer spectra were recorded at 4.2 K of an FeZSM-5 zeolite that has been treated with nitrogen at 623 K. Changes in the sample environment clearly show the affinity of nitrogen for some of the iron centers in the material. Additionally, structural similarities have been found between this FeZSM-5 zeolite and that activated by a high-temperature vacuum treatment.

Probing the Redox States of Iron in Steam‐Treated Isomorphously Substituted [Fe,Al]MFI Catalyst

57Fe enriched isomorphously substituted [Fe,Al]MFI zeolites with varying aluminum content were prepared and subsequently calcined, proton‐exchanged, and steamed to render them active in the direct oxidation of benzene to phenol using N2O as oxidant. The different post‐treatment steps cause the migration of iron from framework to extra‐framework positions creating various Fe‐sites as deduced from 57Fe Mossbauer spectroscopy. An extraordinary high concentration of Fe2+ species (which were correlated to the formation of α‐sites that was claimed to be responsible in the catalytic benzene‐to‐phenol reaction) was obtained for the sample containing 1.1% w/w aluminum. On the contrary, for the aluminum‐free sample (Fe‐silicalite) only Fe3+ species were observed. In‐situ Fe K‐edge XANES were performed to study the reactivity of the Fe‐species in the steamed samples under reducing/oxidizing environment. Reduction‐oxidation (redox) behavior was observed only for aluminum‐containing samples, whereas the Fe3+ species i...