FTIR and UV–Vis characterization of Fe-Silicalite

The Role of In Situ/Operando IR Spectroscopy in Unraveling Adsorbate-Induced Structural Changes in Heterogeneous Catalysis

Heterogeneous catalysts undergo thermal- and/or adsorbate-induced dynamic changes under reaction conditions, which consequently modify their catalytic behavior. Hence, it is increasingly crucial to characterize the properties of a catalyst under reaction conditions through the so-called "operando" approach. Operando IR spectroscopy is probably one of the most ubiquitous and versatile characterization methods in the field of heterogeneous catalysis, but its potential in identifying adsorbate- and thermal-induced phenomena is often overlooked in favor of other less accessible methods, such as XAS spectroscopy and high-resolution microscopy. Without detracting from these techniques, and while aware of the enormous value of a multitechnique approach, the purpose of this Review is to show that IR spectroscopy alone can provide relevant information in this field. This is done by discussing a few selected case studies from our own research experience, which belong to the categories of both "single-site"- and nanoparticle-based catalysts.

Fire retardant properties enhancement of cotton twill fabric using pumpkin (Cucurbita maxima) extract

Health hazards and environmental pollution are major concerns in present world. So, it is high time to think about ecofriendly and sustainable production. In this study, pumpkin juice has been used as an ecofriendly flame retardant finish to enhance the functionality of cotton twill fabric. The pumpkin juice extracted from the fresh pumpkin without any chemicals. The cotton fabric was treated with pumpkin juice in exhaust method. The treated and untreated samples were characterized by TGA, FTIR, SEM, and EDX. The flame-retardant property of the samples were evaluated based on the LOI and vertical flame tester. The result demonstrated that the treated samples exhibited high fire-retardant properties after being finished with pumpkin juice. The LOI value of the treated samples increased to 29 from 19 after treatment. The main reason behind the increased flammability is the dehydration of pumpkin juice-treated fabric which was clarified from the TG analysis. Moreover, the FTIR, SEM, and EDX report ensured the presence of bound and unbound water molecules, different salt, and several atoms in the samples treated with pumpkin juice that enhanced the protection against the spreading of the fire and thus improved fire-retardant properties of the treated samples.

A MnOx@Eu-CeOx nanorod catalyst with multiple protective effects: Strong SO2-tolerance for low temperature DeNOx processes

A novel MnO_x@Eu-CeO_x catalyst with multiple protective attributes was designed and fabricated using a chemical precipitation method and tested for its low temperature SCR activity. The subject MnO_x@Eu-CeO_x nanorod catalyst exhibited superior SCR performance and strong SO₂-tolerance. The formation of the composite-shell structure enhanced the catalysts' surface acidity and redox performance, which resulted in excellent SCR performance. Moreover, the TG results suggested that the protective effect of the EuO_x-CeO_x composite-shell effectively reduced the deposition of the surface sulphates. The XPS, XRD analysis results of the subject catalyst together with theoretical calculations provided strong evidence that there was a strong interaction between Mn and Ce in the MnO_x@Eu-CeO_x. This significant interaction could provide maximum protection to the core from the effect of SO₂, which also contributed to the high SO₂ resistance of the catalyst. In situ FT-IR results also indicated that the chemisorbed species on MnO_x@Eu-CeO_x were much more stable in the presence of SO₂ compared to Eu-CeO_x/MnO_x, which resulted in the deposition of significantly less sulphates. This low temperature SCR catalyst with multiple protective attributes, including composite shell, strong interaction and core-shell structure, is the key to long-term resistance to SO₂.

Experimental Evidence of the Mechanism of Selective Catalytic Reduction of NO with NH3 over Fe-Containing BEA Zeolites

Various temperature-programmed techniques were used as tools in mechanistic studies of selective catalytic reduction (SCR) of NO with ammonia in the presence of Fe-containing BEA zeolites. Moreover, FTIR studies of adsorbed NH₃ and NO were conducted to determine the interactions of reactants with the catalyst surface. Iron was introduced into BEA zeolite by three different methods: i) two-step post-synthesis; ii) conventional wet impregnation; iii) ion exchange. The catalytic activity was dependent on the method used for iron introduction. The reactivities of NH₃ and NO adsorbed on iron-modified zeolites obtained by impregnation and ion-exchange methods were higher than those measured for the catalyst obtained by a two-step post-synthesis method. The activity of Fe-containing zeolites in SCR was related to the form of deposited iron species, as well as to the nature, strength, and concentration of acid sites. Possible reaction pathways of NO reduction over the FeBEA zeolite catalysts were presented and discussed.

Iron-exchanged high-silica LTA zeolites as hydrothermally stable NH3-SCR catalysts

Iron-exchanged high-silica LTA zeolites exhibit high activity for selective catalytic reduction of NO_x by NH₃, even after hydrothermal aging at 900 °C.

Fatty acid decarboxylation reaction kinetics and pathway of co-conversion with amino acid on supported iron oxide catalysts

Reaction kinetics and pathways of palmitic acid decarboxylation on Fe2O3/Al-MCCM-41 catalyst.

Effect of post-synthesis hydrogen-treatment on the nature of iron species in Fe-BEA as NH3-SCR catalyst

The relative amount of monomeric iron species increases after post-synthesis treatment with hydrogen, which improves the low-temperature activity during NH₃-SCR.

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.

Behavior of Extraframework Fe Sites in MFI and MCM-22 Zeolites upon Interaction with N2O and NO

We report on the characterization of an isomorphously substituted Fe-MCM-22 sample containing both Fe and Al in framework positions (Si/Fe = 44, Si/Al = 25). XANES spectroscopy was used to study the evolution of Fe sites as a consequence of thermal activation at high temperature (1073 K) and subsequent oxidation with N2O. The results were compared to those obtained in the same conditions on a well-known Fe-silicalite sample (Si/Fe = 68, Si/Al = infinity). In both samples, thermal activation causes migration of a fraction of Fe ions from framework to extraframework positions, this migration being accompanied by a reduction of Fe3+ to Fe2+. Upon oxidation with N2O at 523 K, the two samples show a different behavior. While in Fe-silicalite practically all of the Fe2+ sites formed by thermal activation are reoxidized to Fe3+, in Fe-MCM-22 only a fraction of the extraframework iron sites is involved in the reoxidation process. The accessibility of the extraframework Fe sites was also investigated by using the NO molecule as a surface probe. Upon NO dosage on the sample, the modification of the pre-edge peak and of the edge position suggests an important charge release from the extraframework Fe2+ ions to the adsorbed molecules. This could be formalized with the formation of Fe3+(NO-) complexes, compatible (on the basis of the simple molecular orbital theory) with a bent NO geometry. The formation of a complex family of Fe2+ mono-, di-, and trinitrosyl complexes was also confirmed by FTIR spectroscopy. Similarly to what was observed in the oxidation experiments, the fraction of extraframework Fe sites able to interact with NO in Fe-MCM-22 sample is smaller than that in Fe-silicalite treated in the same conditions. This trend is explained with a major clustering of extraframework Fe sites in Fe-MCM-22 sample, as was also suggested by FTIR experiments. These results suggest that the dispersion of iron in zeolitic matrixes prepared by isomorphous substitution could also depend on the zeolitic structure.

Template burning inside TS-1 and Fe-MFI molecular sieves: an in situ XRPD study

The high X-ray flux available at the European Synchrotron Radiation Facility (ESRF), combined with the use of a suitably designed area detector setup, allowed us to follow in real time the structural changes occurring during the template burning processes inside TS-1 and Fe-silicalite MFI zeolites with a X-ray powder diffraction technique (XRPD). Rietveld analysis of the XRPD patterns collected in the 350-1000 K interval, integrated each 15 K, yields to the determination of the template overall occupancy factor versus T with an accuracy comparable with that obtained by thermogravimetric measurements, routinely employed for this purpose. The evolution of the structural parameters (V, a, b, c, site occupancy factor of the template molecule) vs T has been obtained. These data allow us to have, for the first time, a complete view of the structural rearrangements induced by the template burning process on the zeolitic framework. The differences caused by the different heteroatom inserted in the MFI lattice (Ti or Fe) are discussed. For both TS-1 and Fe-MFI, the kinetics of the reaction were investigated, to obtain the activation energy of the calcinations process employing the nonisothermal data according to the theory recently proposed by Kennedy and Clark [Thermochim. Acta, 1997, 307, 27-35]. For TS-1 only, the time-resolved template burning experiment has been repeated in isothermal conditions at four different temperatures, to obtain the activation energy from isothermal data, according to the standard procedure. Comparison between Arrhenius plots obtained from isothermal and nonisothermal data demonstrates that the Kennedy and Clark method can be also applied to complex materials such as the MFI zeolites. This approach, when applied to time-resolved XRPD studies, is much less time consuming (requesting, in principle, one single nonisothermal run) with respect to the classic approach, which requests at least three isothermal runs. Moreover, it allows a remarkably lower associated error (151 +/- 11 versus 146 +/- 30 kJ mol(-)(1)) due to the much higher number of experimental points employed to perform the linear fit.

Anchoring Fe Ions to Amorphous and Crystalline Oxides: A Means To Tune the Degree of Fe Coordination

We report on an IR spectroscopic study on the room-temperature adsorption of NO on different iron(II)-containing siliceous matrices. Fe2+ hosted inside the channels of MFI-type zeolites (Fe-ZSM-5 and Al-free Fe-silicalite) exhibits pronounced coordinative unsaturation, as witnessed by the capability to form, at 300 K, [Fe2-(NO)], [Fe2+(NO)2] and [Fe2+(NO)3] complexes with increasing NO equilibrium pressure. Fe2+ hosted on amorphous supports (high surface area SiO2 and MCM-41) sinks more deeply into the surface of the siliceous support and thus exhibits less pronounced coordinative unsaturation: only [Fe2+(NO)2] complexes were observed, even at the highest investigated NO equilibrium pressures. Activation at higher temperature (1073 K) of the Al-free Fe-silicalite sample resulted in the appearance of Fe2+ species similar to those observed on SiO2 and MCM-41, and this suggests that local (since not detectable by X-ray diffraction) amorphisation of the environment around Fe2+ anchoring sites occurs. The fact that this behaviour is not observed on the Fe-ZSM-5 sample activated at the same temperature suggests that framework Al species (and their negatively charged oxygen environment) have an important role in anchoring extraframework Fe2+ species. Such an anchoring phenomenon will prevent a random migration of iron species, with subsequent aggregation and loss of coordinative unsaturation. These observations can thus explain the higher catalytic activity of the Fe-ZSM-5 system in one-step benzene to phenol conversion when compared with the parent, Al-free, Fe-silicalite system with similar Fe content. The nature of the support and the activation temperature can therefore be used as effective means to tune the degree of Fe coordination.