Preparation and Pretreatment Temperature Influence on Iron Species Distribution and N2O Decomposition in Fe–ZSM-5

Fe–ZSM-5 catalysts are prepared by FeCl₃ sublimation between 320 and 850 °C. The catalysts are characterised by XRD, H₂–TPR, NH₃–TPD, NO adsorption by DRIFTs, and catalytic activity is evaluated for N₂O decomposition. The influence of high temperature (850 °C) and pretreatment environment (air, He, He+H₂O and H₂) on the nature of iron species in Fe–ZSM-5 is further investigated by DRIFTs. High temperature FeCl₃ sublimation results in decreased FeO_x formation, easily reducible and narrow distribution of iron species in close proximity to alumina in Fe–ZSM-5. High temperature FeCl₃ sublimation or pretreatment results in isolated hydroxylated iron species, –Fe(OH)₂, which are not significant in Fe–ZSM-5 prepared by 320 °C FeCl₃ sublimation followed by calcination below 600 °C. Fe–ZSM-5 prepared by high temperature FeCl₃ sublimation show high N₂O decomposition activity and the improved performance can be correlated to –Fe(OH)₂ species in close proximity to alumina.     

Catalytic decomposition of nitrous oxide over Fe-BEA zeolites: Essential components of iron active sites

The catalytic decomposition of N₂O was investigated over Fe-BEA zeolites treated with various methods such as reduction, steaming and dissolution with potassium nitrate and nitric acid solutions in order to deduce the essential components of the active sites for the decomposition. The iron species were characterized by XPS, XANES, ESR, NO adsorption, and linear sweep voltammetry. The reduction-treated Fe-BEA zeolite with the large amounts of Fe(II) and Fe(III) species showed the highest activity. On the contrary, the dissolution treatment with the potassium nitrate solution seriously deteriorated the catalytic activity of the Fe-BEA zeolite by agglomerating iron oxide clusters and interaction between iron and potassium atoms. The catalytic roles of Fe(II)/Fe(III) species and the negative effect of potassium on the catalytic activity of the Fe-BEA zeolites were discussed.     

Investigating the Influence of Fe Speciation on N<Subscript>2</Subscript>O Decomposition Over Fe–ZSM-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 Fe_xO_y, 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 Fe_xO_y 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.     

Catalytic activity of Fe ions in iron-based crystalline and amorphous systems: role of dispersion,coordinative unsaturation and Al content

Different Fe-based materials, where iron ions are anchored on crystalline and amorphous siliceous supports are studied by FTIR spectroscopy of adsorbed NO and in the N₂O decomposition reaction test. The influence of Al concentration is also considered. We show that the coordination state of Fe dramatically changes when Fe is anchored on crystalline or amorphous matrices. In crystalline Fe-ZSM-5 and Fe-silicalite samples highly coordinatively unsaturated and isolated Fe sites are present, which form Fe²⁺(NO)₂ and Fe²⁺(NO)₃ complexes upon NO contact. The presence of oxidic clusters, forming Fe²⁺(NO) complexes, whose relative concentration strictly depends upon the presence of Al and the concentration of Fe, is also evidenced. Fe-ZSM-5 sample (Si/Fe = 1120) mainly contains isolated Fe²⁺ ions characterised by high coordinative unsaturation. This sample also shows the highest activity in N₂O decomposition, indicating that isolated and coordinatively unsaturated Fe sites are the most active precursors for the catalytic reaction. It is thought that adsorbed oxygen is formed, upon N₂O decomposition, on both isolated and oxidic clusters, forming ferryl groups in the former case and bridged structures in the latter.     

Isothermal “light-off” during catalytic N2O decomposition over Fe/ZSM-5

The catalytic decomposition of N₂O over Fe/ZSM-5 was studied at pressures up to 1 atm. As the partial pressure of N₂O in the feed to a packed bed reactor was varied, the catalytic activity was observed to abruptly increase at inlet partial pressures above some critical value. This abrupt increase in activity was not due to the reaction exotherm, but is believed to be a kinetic phenomenon. In similar experiments where the temperature was varied the activity did not jump to a higher level. The dual levels of activity were observed for several Fe/ZSM-5 catalysts that spanned a range of SiO₂/Al₂O₃ ratios from 30 to 280, irrespective of whether the iron was introduced into the zeolite by ion exchange or by sublimation of iron chloride. When the inlet partial pressure of N₂O to the packed bed reactor was sufficiently high to cause the catalyst to operate in the high activity regime, the high activity state was sustained throughout the catalyst bed, even though the N₂O partial pressure in the latter part of the bed had dropped below the critical level. Microkinetic modeling shows that this kind of behavior is possible in an isothermal catalytic system. The microkinetic model includes two redox cycles. In one cycle the cation oxidation state alternates between Fe²⁺ and Fe³⁺. In the second, more active redox cycle there is alternation between a surface nitrite and nitrate. The former redox cycle predominates at low partial pressures and the latter at high N₂O partial pressures.     

Effect of precursors on surface and catalytic properties of Fe/TiO2 catalysts

Titania‐supported iron (5 wt%) catalysts were prepared by a sol–gel method using different gelation pH and metal precursors (Fe(II) and Fe(III)). Characterization data of calcined catalysts revealed that, irrespective of the nature of the metal precursor, iron is present in all cases as ferric oxide. However, the crystalline phase exhibited by titania does depend on the metal precursor used. The catalytic activity of the catalysts, tested in the combustion of methane at atmospheric pressure, is not related to the dispersion of iron oxide. Thus, Fe³⁺ ions may be obtained in two extreme situations; one highly dispersed in which Fe³⁺ ions are placed in the titania network and another in which large Fe₂O₃ crystals are located on the surface of the catalyst. The former exhibits the best performance in the combustion of methane. © 2002 Society of Chemical Industry     

The influence of precursors on Rh/SBA-15 catalysts for N2O decomposition

Series of Rh/SBA-15 catalysts were prepared by impregnation and grafting method applying different Rh precursors. The catalytic behaviors of N₂O decomposition over these catalysts were tested in an automated eight flow reactor system. The catalysts were characterized by X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), N₂ adsorption/desorption, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results showed that the dispersion of Rh species on the catalysts is closely related to the molecular size and the hydrophobic property of the precursors comparing to the hydrophilic support, better dispersion results were found in catalysts by impregnation of smaller precursors, while by grafting better dispersion resulted from big precursor. On the other hand, the activities of the catalysts match well with the Rh dispersion status. Rh/SBA-15-CDCR starting from [(CO)₂RhCl]₂ showed good dispersion and gave the best N₂O decomposition activity.     

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

A new method for preparing ZSM-5 catalysts containing highly dispersed iron species is presented. Iron(III) oxalate was used as the iron precursor in an aerobic aqueous exchange process. The presence of different iron species inside the zeolite pores was investigated by IR spectroscopy of adsorbed NO, which revealed an excellent iron dispersion. The results were compared with those obtained on a Fe-ZSM-5 catalysts prepared by iron(II) chloride sublimation and on samples prepared by controlled migration of framework iron in isomorphously substituted samples.     

N2O Decomposition over Liquid Ion-Exchanged Fe-BEA Catalysts: Correlation Between Activity and the IR Intensity of Adsorbed NO at 1874 cm−1

Various Fe-BEA catalysts have been synthesized by liquid ion-exchange, varying the iron precursor (nitrate, sulfate or chloride), the iron content (0.2–1.5 wt% Fe), and the conditions of activation, i.e. atmosphere (air or inert) and heating rate (5 or 20 K min^?1). The catalysts were tested in direct N₂O decomposition in the temperature range of 625–825 K. An optimal iron loading of 0.8 wt% Fe was found, with no significant influence of the used Fe-precursor. Activation of the ion-exchanged catalysts in inert gas yields significantly better performance than activation in air. NO adsorption combined with infrared analysis was used to characterize the various Fe²⁺ species present in the differently prepared Fe-BEA zeolites. A correlation exists between the absorption intensity of NO at 1874 cm^?1 and the activity of the catalysts in N₂O decomposition. This relationship, which can be used as a reliable and fast assessment of catalyst performance, suggests an important role of ferrous ions in the activity of these catalysts. Based on these results and previous mechanistic studies using transient techniques, the NO absorption band at 1874 cm^?1 is tentatively assigned to oligonuclear oxocations in the zeolite channels, with general formula Fe _x O _y .     

Metal Exchanged ZSM-5 Zeolite Based Catalysts for Direct Decomposition of N2O

Co+Pt/ZSM-5 and Ag+Pt/ZSM-5 type catalysts were prepared by ion exchange method followed by calcination. These Co and Ag based catalysts, promoted by a small amount of Pt have been studied for their catalytic activity towards N₂O decomposition. Both the catalysts show high catalytic activity, however, cobalt–platinum based catalyst shows relatively better activity at higher temperature. At 550 °C almost 100% conversion of N₂O is achieved over Co+Pt/ZSM-5 with a maximum of 0.08479 mmole of N₂O decomposed per gram of the catalyst per unit time. These catalytic materials have been characterized for their structure, composition, morphology and other details, using XRD, SEM, EDX, ICP, BET techniques. Much improved catalytic activity for the bimetallic zeolite than the mono-metal containing compositions clearly demonstrate the synergistic effect of these transition metals, while high surface area of ZSM-5 is also responsible for the improved N₂O decomposition activity.     

Reactivity of oxygen species formed upon N2O dissociation over Fe–ZSM-5 zeolite: CO oxidation as a model

The oxidative power toward CO of α-oxygen formed upon N₂O dissociation over isolated and binuclear Fe/ZSM-5 zeolite is investigated by means of DFT calculations. The two α-sites [Fe–O]⁺ and [Fe–(μO)–(μOH)–Fe]⁺ exchanged in ZSM-5 were considered since their activity in the N₂O decomposition was recently shown. Computed electronic properties, charge transfers and frequency analysis of α-oxygen and iron in [O–Fe–O]⁺ and [OFe–(μO)–(μOH)–FeO]⁺ suggest a Fe^II character for the isolated and Fe^IV for the binuclear Fe–ZSM-5 sites. Addition of CO on oxygen atoms reveals that along the oxidation reaction the valence state for the isolated iron is II and remains relatively constant while a clear change from IV to II is calculated for the binuclear iron. According to DFT calculations CO addition on the α-oxygen from the iron active sites induces a significant length increase of the Fe–α-oxygen bond. Whatever the α-sites, the addition of CO is strongly exothermic and leads to stable minima resembling an adsorbed CO₂ on iron active site. This reactivity is in line with the well known high reactivity of α-oxygen and the rapid CO₂ formation at low temperatures. Based on the calculated enthalpy values, the adsorption of CO is slightly more favourable on binuclear [OFe–(μO)–(μOH)–FeO]⁺ than over isolated [O–Fe–O]⁺ iron site. A comparison of the entropic parameters suggests the opposite with a stronger oxidative power of α-oxygen from isolated over those from binuclear iron site.     

Transesterification of tributyrin with methanol over calcium oxide catalysts prepared from various precursors

Calcium oxide catalysts were prepared by calcining various precursors such as calcium acetate, carbonate, hydroxide, nitrate and oxalate and their catalytic activities were examined in the transesterification of tributyrin with methanol. The prepared calcium oxide catalysts were characterized using thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption and temperature programmed desorption (TPD) of CO₂. The calcium oxide catalyst obtained by calcining calcium hydroxide at 600–800 °C showed the highest tributyrin conversion and methyl butyrate yield. The large desorption peak of CO₂ TPD confirmed that its numerous basic sites were responsible for its high activity. The low-temperature decomposition of calcium hydroxide provided many nano-sized pores with strong basic sites. Although the activity of the calcium oxide catalyst prepared from calcium hydroxide was high, its activity was one order of magnitude less than that of sodium hydroxide catalyst. The dissolution of calcium oxide catalysts in products and their repeated uses were also investigated to discuss their advantages as heterogeneous catalysts in the production of biodiesel.     

Aromatization of Methane Over Mo-Fe/ZSM-5 Catalysts

Two groups of samples were studied for the aromatization of methane over Mo-Fe/ZSM-5 catalysts. The first group contains fixed loading (5 wt%) and variable Mo/Fe ratio. The second group was prepared with fixed Mo/Fe ratio and variable loading. The samples were characterized by TGA, S _BET, XRD, NH₃-TPD and TPO analysis. NH₃-TPD results indicate that the strength of strong acid sites increases when Mo/Fe ratio decreases for samples with fixed loading. The amount of strong sites decreases and new intermediate acid sites appear on samples containing both Mo and Fe. XRD results show the presence of iron molybdate for samples impregnated with both Mo and Fe. The catalytic properties of these samples were related not only to the amount and strength of acid sites but also to the iron molybdate phase.     

Nature and Reactivity of the Active Species Formed After NO Adsorption and NO + O2 Coadsorption on an Fe-Containing Zeolite

Reactivity of the NO adspecies on Fe-ZSM-11 was studied by FTIR in situ. The effect of Fe content and the oxidation state of Fe in the samples were correlated with the catalytic activity. The relation between the adsorbed species, the Brønsted sites and catalytic activity in the SCR of NO_x to N₂ was also investigated. Moreover, FTIR allowed us to identify the active sites and the adsorption complexes present in FeMFI. Samples prepared by the sol–gel method with different Fe content displaying vastly different activity and selectivity in the reduction of NO to N₂ with isobutane in excess of O₂. Thus, in contact with pure nitric oxide, NO ions, mononitrosyl groups, nitro groups and nitrate ions have been identified. Feⁿ⁺ active sites are the most probable centers for NO oxidation to NO₂ and its further conversion to adsorbed nitro groups and nitrate ions, steps that are crucial for NO reduction. The concerted action of Feⁿ⁺ and H⁺ sites of the catalysts over the NO conversion to N₂ and isobutane conversion was analyzed.     

Synthesis, characterization and acidic properties of nanopowder ZSM-5 type ferrisilicates in the Na/K alkali system

In this study Aluminium-free ZSM-5 type nanopowder ferrisilicates (as K-FZ or Na-FZ) were synthesized successfully from silicic acid, potassium (or sodium) carbonate and iron (III) nitrate using hydrothermal procedure (in relatively low crystallization time) in the presence of tetrapropylammonium bromide (TPABr) as a template. Prepared samples have been characterized by XRD, TEM, EDAX, NH₃-TPD, BET surface area, FTIR, TGA/DTA, SEM and ICP analysis. FTIR spectra confirm the production of ZSM-5 type ferrisilicates by the zeolite pentacycle peak at 547 cm^− 1 and the symmetric stretching peak of Si-O-Fe at 656 cm⁻¹. XRD diffractograms confirmed ZSM-5 crystal phase. The results clearly show that K⁺ cation (like Na⁺) plays a role of structure directing as well as a role of charge balancing agent. This could be assigned from NH₃-TPD analysis that ferrisilicate pretreatment procedure and iron-exchanging can significantly affect the strength of acidic sites. NH₃-TPD indicates that using potassium instead of sodium, leads to an increasing of the global acidity. Furthermore Na-FZ exchanging with iron ions (as Fe-NaFZ) leads to the same results. Thermal analysis revealed 14.7% weight loss due to the decomposition of TPABr template. Scanning electron microscope revealed that there is clustering of spherical particles in dry state. The TEM images confirm the particle size of the ferrisilicates in the range of 15-30 nm and show that iron-exchanged ferrisilicate has well-dispersed nano-sized iron oxide particles with various sizes in the range of 5-15 nm.     

The effect of sodium on the catalytic activity of ZnO–Al2O3/ZSM-5 and SnO–Al2O3/ZSM-5 for the transesterification of vegetable oil with methanol

In order to elucidate the effect of sodium on the activity of ZSM-5 supported metal oxides catalysts (ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5) for the transesterification of soybean oil with methanol, ZSM-5 supported metal oxides were prepared with and without sodium hydroxide by impregnation. The metal compositions of the ZSM-5 supported metal oxide catalysts and the metal concentrations dissolved from the catalysts to the methylester phase were measured by SEM-EDS and inductive coupled plasma spectroscopy, respectively. The catalytic activity of ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5 containing sodium did not originate from surface metal oxides sites, but from surface sodium sites or dissolved sodium leached from the catalyst surface.     

Co–ZSM-5 catalysts in the decomposition of N2O and the SCR of NO with CH4: Influence of preparation method and cobalt loading

Co–ZSM-5 prepared via different methods with Co/Al ratios ranging from 0.03 to 0.83 are investigated in both the direct N₂O decomposition and the selective catalytic reduction (SCR) of NO with CH₄. UV–vis and H₂-TPR are used to get an insight in the active species in these reactions. It is observed that in catalysts with low Co loadings (Co/Al < 0.3) Co is predominantly present as mono-atomic Co species, located at ion exchange positions in ZSM-5. Higher Co loadings result in the formation of different kinds of Co-oxides, which constitute the majority of species in the over-exchanged catalysts (Co/Al > 0.5). The mono-atomic species show the highest activity in the direct decomposition of N₂O, whereas the oxidic Co species do not seem to contribute much to the overall decomposition. In the SCR, the Co-oxide species catalyze the combustion of CH₄ whereas the selectivity towards NO reduction is much increased at low Co loadings. Therefore, over-exchange of Co–ZSM-5 does not seem to be favorable for both the direct N₂O decomposition and the SCR of NO with CH₄. Co/Al ratios <0.3 give the best results both in terms of conversion and activity per Co atom in both reactions.     

Steam tolerance of Fe/ZSM-5 catalyst for the selective catalytic reduction of NOx

This article reports the effects of steam on the activity and stability of Fe/ZSM-5 for the selective catalytic reduction of NO withiso-butane. When the feed contained 10% of H₂O, the de-NO_x activity was maintained if the temperature was above the maximum conversion temperature. However, when the temperature was below the maximum conversion temperature, the catalytic activity decreased. The effect of high temperature steam treatment on the stability was also examined. After the steam treatment, the activity of Fe/ZSM-5 decreased due to the dealumination of ZSM-5 and the migration of Fe ion isolated in the ion exchange site to form ferromagnetic iron agglomerate. The physicochemical properties of the fresh and deactivated catalysts were monitored by ESR,²⁷Al MAS NMR, XPS, XRD, TPR and FT-IR spectroscopy. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Oxygen reduction by Fe-based catalysts in PEM fuel cell conditions: Activity and selectivity of the catalysts obtained with two Fe precursors and various carbon supports

Fe-based catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cell conditions have been prepared by adsorbing two Fe precursors on various commercial and developmental carbon supports. The resulting materials have been pyrolyzed at 900 °C in an atmosphere rich in NH₃. The Fe precursors were: iron acetate (FeAc) and iron tetramethoxy phenylporphyrin chloride (ClFeTMPP). The nominal Fe content was 2000 ppm (0.2 wt.%). The carbon supports were HS300, Printex XE-2, Norit SX-Ultra, Ketjenblack, EC-600JD, Acetylene Black, Vulcan XC-72R, Black Pearls 2000, and two developmental carbon black powders, RC1 and RC2 from Sid Richardson Carbon Corporation. The catalyst activity for ORR has been analyzed in fuel cell tests at 80 °C as well as by cyclic voltammetry in O₂ saturated H₂SO₄ at pH 1 and 25 °C, while their selectivity was determined by rotating ring-disk electrode in the same electrolyte. A large effect of the carbon support was found on the activity and on the selectivity of the catalysts made with both Fe precursors. The most important parameter in both cases is the nitrogen content of the catalyst surface. High nitrogen content improves both activity towards ORR and selectivity towards the reduction of oxygen to water (4e⁻ reaction). A possible interpretation of the activity and selectivity results is to explain them in terms of two Fe-based catalytic sites: FeN₂/C and FeN₄/C. Increasing the relative amount of FeN₂/C improves both activity and selectivity of the catalysts towards the 4e⁻ reaction, while most of the peroxide formation may be attributed to FeN₄/C. When FeAc is used as Fe precursor, iron oxide and/or hydroxide are also formed. The latter materials have low catalytic activity for ORR and reduce O₂ mainly to H₂O₂.     

Improved ORR activity of non-noble metal electrocatalysts by increasing ligand and metal ratio in synthetic complex precursors

In an effort to improve oxygen reduction reaction (ORR) activity by increasing the catalytic active site density in carbon-supported non-noble metal catalysts, several nitrogen-containing catalysts were synthesized through a heat treatment process at 900 °C using precursor complexes of Fe(II) and tripyridyl triazine (TPTZ). Fe to TPTZ mole ratios of 1:2, 1:3, 1:4, 1:5, 1:6, and 1:7 were used to prepare the precursor complexes. X-ray diffraction and surface electrochemical techniques were used to characterize these catalysts (Fe–N_x/C), and revealed that when the amount of TPTZ in the precursor complex was increased, the decomposition of Fe–N_x sites, which are considered active sites for the ORR, was effectively reduced, resulting in higher Fe–N_x site density and thus improving the catalysts’ ORR activity. This beneficial effect was validated through rotating disk electrode tests and analysis of the ORR kinetics catalyzed by these catalysts. The obtained results showed that as the Fe to TPTZ mole ratio in the precursor complex was decreased, the catalytic ORR activity of Fe–N_x/C increased monotonically in the mole ratio range of 1:2–1:6. Therefore, increasing the amount of ligand in the precursor metal complex was demonstrated to be an effective way to reduce the decomposition of ORR active site density and thereby enhance the ORR activity of non-noble metal catalysts.