New precursor for the post-synthesis preparation of Fe-ZSM-5 zeolites with low iron content

Structure Features and Physicochemical Performances of Fe-Contained Clinoptilolites Obtained via the Aqueous Exchange of the Balanced Cations and Isomorphs Substitution of the Heulandite Skeletons for Electrocatalytic Activity of Oxygen Evolution Reaction and Adsorptive Performance of CO2

Fe(III)-modified clinoptilolites (Fe-CPs) were prepared by hydrothermal treatment. The collapse of the heulandite skeletons was avoided by adjusting the pH value using HCl solution, showing the maximum relative crystallinity of the Fe-CPs at an optimal pH of 1.3. The competitive exchange performances between Fe³⁺ ions and H⁺ with Na⁺ (and K⁺) suggested that the exchange sites were more easily occupied by H⁺. Various characterizations verified that the hydrothermal treatments had a strong influence on the dispersion and morphology of the isolated and clustered Fe species. The high catalytic activity of the oxygen evolution reaction indicated the insertion of Fe³⁺ into the skeletons and the occurrences of isomorphic substitution. The fractal evolutions revealed that hydrothermal treatments with the increase of Fe content strongly affected the morphologies of Fe species with rough and disordered surfaces. Meanwhile, the Fe(III)-modified performances of the CPs were systematically investigated, showing that the maximum Fe-exchange capacity was up to 10.6 mg/g. Their thermodynamic parameters and kinetic performances suggested that the Fe(III)-modified procedures belonged to spontaneous, endothermic, and entropy-increasing behaviors. Finally, their adsorption capacities of CO₂ at 273 and 298 K were preliminarily evaluated, showing high CO₂ adsorption capacity (up to 1.67 mmol/g at 273 K).

Light alkane oxidation over well-defined active sites in metal–organic framework materials

We review structure–catalytic property relationships for MOF materials used in the direct oxidation of light alkanes, focusing specifically on the elucidation of active site structures and probes for reaction mechanisms.

Characterization of Fibrous Wollastonite NYAD G in View of Its Use as Negative Standard for In Vitro Toxicity Tests

Today, despite considerable efforts undertaken by the scientific community, the mechanisms of carcinogenesis of mineral fibres remain poorly understood. A crucial role in disclosing the mechanisms of action of mineral fibres is played by in vitro and in vivo models. Such models require experimental design based on negative and positive controls. Commonly used positive controls are amosite and crocidolite UICC standards, while negative controls have not been identified so far. The extensive characterisation and assessment of toxicity/pathogenicity potential carried out in this work indicate that the commercial fibrous wollastonite NYAD G may be considered as a negative standard control for biological and biomedical tests involving mineral fibres. Preliminary in vitro tests suggest that wollastonite NYAD G is not genotoxic. This material is nearly pure and is characterized by very long (46.6 µm), thick (3.74 µm) and non-biodurable fibres with a low content of metals. According to the fibre potential toxicity index (FPTI) model, wollastonite NYAD G is an inert mineral fibre that is expected to exert a low biological response during in vitro/in vivo testing.

Beyond Radical Rebound: Methane Oxidation to Methanol Catalyzed by Iron Species in Metal-Organic Framework Nodes

Recent work has exploited the ability of metal-organic frameworks (MOFs) to isolate Fe sites that mimic the structures of sites in enzymes that catalyze selective oxidations at low temperatures, opening new pathways for the valorization of underutilized feedstocks such as methane. Questions remain as to whether the radical-rebound mechanism commonly invoked in enzymatic and homogeneous systems also applies in these rigid-framework materials, in which resisting the overoxidation of desired products is a major challenge. We demonstrate that MOFs bearing Fe(II) sites within Fe₃-μ₃-oxo nodes active for conversion of CH₄ + N₂O mixtures (368-408 K) require steps beyond the radical-rebound mechanism to protect the desired CH₃OH product. Infrared spectra and density functional theory show that CH₃OH_(g) is stabilized as Fe(III)-OCH₃ groups on the MOF via hydrogen atom transfer with Fe(III)-OH groups, eliminating water. Consequently, upon addition of a protonic zeolite in inter- and intrapellet mixtures with the MOF, we observed increases in CH₃OH selectivity with increasing ratio and proximity of zeolitic H⁺ to MOF-based Fe(II) sites, as methanol is protected within the zeolite. We infer from the data that CH₃OH_(g) is formed via the radical-rebound mechanism on Fe(II) sites but that subsequent transport and dehydration steps are required to protect CH₃OH_(g) from overoxidation. The results demonstrate that the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, that the selectivity-controlling steps involve both chemical and physical rate phenomena, as well as offering a strategy to mitigate overoxidation in these and similar systems.

NOx reduction by CO over Fe/ZSM-5: A comparative study of different preparation techniques

Fe/ZSM-5 catalysts were prepared by three kinds of ion exchange methods: aqueous ion-exchange (AI), hydrothermal ion-exchange (HI) and solid-state ion-exchange (SI). Their catalytic activities were tested for NOx reduction by CO in a separated NOx adsorption-desorption process. In this paper, performances of adsorption, reduction and dynamic adsorption-reduction were all investigated. All three catalysts exhibited good reduction activity at above 300 °C. Fe/ZSM-5(SI) exhibited excellent NOx removal efficiency in the dynamic adsorption-reduction experiments. However, in the dynamic process the adsorption efficiency of Fe/ZSM-5(AI) and the reduction efficiency of Fe/ZSM-5(HI) is not very good. The catalysts were further characterized by SEM, BET, XRD, XRF, XPS and TPD. It was found that the Fe content of the Fe/ZSM-5(SI) was the highest. Further, Fe is supported in the form of Fe2O3 particles. Bronsted acid sites were also playing a major role in the high catalytic activity. TPD and in situ DRIFT experiments show that more Fe loading in α acid sites could result in a higher NOx removal efficiency.

On the Chemistry of Iron Oxide Supported on γ-Alumina and Silica Catalysts

Catalysts of iron oxide on γ-alumina and silica which were prepared by an incipient wetness impregnation technique have been investigated in an effort to understand how the surface chemical properties are influenced by the nature of the supports. Surprisingly, this is the first study to compare in depth the influence of the supports on physicochemical parameters such as acidity, site nuclearity, and reducibility. In this study, surface characterisation techniques including N₂ physisorption at -196 °C, ammonia temperature-programmed desorption, inductively coupled plasma optical emission spectrometry, temperature-programmed reduction with hydrogen, CO-chemisorption, scanning electron microscopy, transmission electron microscopy, and NO adsorption by in situ Fourier transform infrared spectroscopy have been performed to understand the different surface reactions occurring over the two different supports. The aim of this study is to ascertain the primary differences between these two catalysts using several catalyst characterization techniques and correlate their chemical and structural differences to their catalytic activity in the conversion of 2-chlorophenol. The results disclose a higher density of acid sites, a smaller particle size of iron oxide, stabilization of Fe(II) aluminate after reduction on the alumina surface, and finally, the formation of isolated iron cations on the surface of alumina which are notably absent on the silica-supported catalyst.

DeNOx Abatement over Sonically Prepared Iron-Substituted Y, USY and MFI Zeolite Catalysts in Lean Exhaust Gas Conditions

Iron-substituted MFI, Y and USY zeolites prepared by two preparation routes-classical ion exchange and the ultrasound modified ion-exchange method-were characterised by micro-Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet (UV)/visible diffuse reflectance spectroscopy (UV/Vis DRS). Ultrasound irradiation, a new technique for the preparation of the metal salt suspension before incorporation to the zeolite structure, was employed. An experimental study of selective catalytic reduction (SCR) of NO with NH₃ on both iron-substituted reference zeolite catalysts and those prepared through the application of ultrasound conducted during an ion-exchange process is presented. The prepared zeolite catalysts show high activity and selectivity in SCR deNO_x abatement. The MFI-based iron catalysts, especially those prepared via the sonochemical method, revealed superior activity in the deNO_x process, with almost 100% selectivity towards N₂. The hydrothermal stability test confirmed high stability and activity of MFI-based catalysts in water-rich conditions during the deNO_x reaction at 450 °C.

Infrared photodissociation spectroscopy of iron nitrosyl cation complexes: Fe(NO)n⁺ (n = 1-5)

Infrared spectra of mass-selected mononuclear iron nitrosyl cations Fe(NO)n(+) with n = 1-5 and their argon tagged complexes are measured via infrared photodissociation spectroscopy in the nitrosyl stretching frequency region in the gas phase. The structures are established by comparison of the experimental spectra with the simulated spectra derived from density functional calculations. Two IR active bands were observed for the argon-tagged Fe(NO)2(+) and Fe(NO)3(+) complexes, consistent with theoretical predictions that these complexes have bent C(2v) and nonplanar C(3v) symmetry, respectively. The Fe(NO)4(+) complex was characterized to have a completed coordination sphere with 17 electrons containing a bent one-electron NO ligand and three three-electron NO ligands. The Fe(NO)5(+) complex was determined to involve a Fe(NO)4(+) core ion that is solvated by an external NO molecule.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

Structure and nuclearity of active sites in Fe-zeolites: comparison with iron sites in enzymes and homogeneous catalysts

Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe(2+) homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe(2+) grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe(2+) presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N(2)O decomposition on isolated sites, whose properties are more similar to those of the (FeO)(2+) in cationic complexes (included the [(H(2)O)(5)FeO](2+)"brown ring" complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.

Formation of the Surface NO during N2O Interaction at Low Temperature with Iron-Containing ZSM-5

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.