PREPARATION AND THERMAL EXPANSION OF FE2-xYxW3O12 POWDER BY CITRATE SOL-GEL PROCESS

Fe_2-xY_xW₃O₁₂ powder has been synthesized by the citrate sol-gel process. A model was proposed to calculate the concentration of species in a citric solution. The calculated results could provide valuable information for determining the optimal molar ratio of cation to citric acid and pH value of solution for Fe_2-xY_xW₃O₁₂ preparation. The predicted parameters derived from this model are in good agreement with the experimental results. The prepared gel and the Fe_2-xY_xW₃O₁₂ powder were characterized by X-ray diffraction (XRD) and differential thermal analysis-thermogravimetry (DTA-TG). The results show that it is very difficult to obtain pure Fe₂W₃O₁₂ powder by the citrate sol-gel process in the temperature range 500°–1000°C, however, Y₂W₃O₁₂ can easily be prepared even at 500°C. Y₂W₃O₁₂ annealed at 1000°C for 10 h is favorable for absorbing moisture in air to form Y₂W₃O₁₂·3.3H₂O. The thermal expansion coefficients of Y₂W₃O₁₂·3.3H₂O are: α_a = ? 8.01 × 10^?6°C^?1, α_b = ? 2.51 × 10^?7°C^?1, and α_c = ? 5.55 × 10^?6°C^?1 in 473–1173 K.     

A Novel Method for Measuring Active Sites of Fe3O4 for WGS Reaction

Phase transformation among iron oxides was investigated. Pure magnetite material was obtained by reducing iron oxide with diluted hydrogen in a narrow temperature window and with steam to prevent over-reduction. A pulse chromatographic method with N₂O decomposition over magnetite surface to determine active sites of iron oxide based catalysts for water gas shift reaction has been developed. N₂O decomposes over activated Fe₃O₄ surface to N₂ and leaves oxygen species at oxygen vacancy on the catalyst surface, which is the same site for water gas shift reaction. Lower temperature for N₂O decomposition is required to avoid magnetite bulk oxidation. An oxygen coverage on the active sites θ = 1 corresponded to a surface stoichiometry of N₂O/Fe²⁺ = 0.5 was estimated. A linear correlation between water gas shift reaction rate and the quantity of decomposed N₂O over the corresponded catalysts was observed.     

Effect of Hydrogen Treatment on the Catalytic Activity of Iron Oxide Based Materials Dispersed Over Activated Carbon: Investigations Toward Hydrogen Peroxide Decomposition

The effect of hydrogen treatment (400 °C/1 h) on the catalytic properties toward H₂O₂ decomposition of iron oxide based materials dispersed over activated carbon were investigated. Two different supports were evaluated: a commercial activated carbon (ACM) and an activated carbon produced from spent coffee grounds (ACR). The catalysts were characterized using XRD, SEM, N₂-sorption, XPS and TPR analysis. The main results suggest the formation of composites with high surface area (>800 m² g^?1) and the hydrogen treatment resulted in a great increase in the catalytic activity, probably as a function of the reduced iron species (Fe²⁺ and Fe⁰) formed with the treatment. Moreover, the catalyst prepared with ACR showed to be more active than that prepared from ACM.     

Iron(II) Sulfate(VI) from Titania Production as a Raw Material for Preparation of Hydrogen Sulfide Sorbents

A hybrid sorbent material for removal of hydrogen sulfide from air was developed. The material is based on activated carbon and iron compounds obtained from waste iron(II) sulfate(VI) heptahydrate. The iron salt is deposited on the carbonaceous support and subjected to oxidation (Fe²⁺ to Fe³⁺) using atmospheric oxygen under alkaline conditions. An effect of H₂O₂ addition to the process on the composition of the resultant material was also examined. X-ray diffraction (XRD) analyses confirmed easy conversion of waste FeSO₄·7H₂O to iron oxides Fe₃O₄ and FeOOH. The activated carbon supporting iron oxides revealed a higher efficiency in H₂S elimination from air compared to the commercial activated carbon, without any modification.     

Synthesis of Pure VSH-2 in Large Quantities and Its Characterization

A method to produce pure VSH-2 in large quantities (~20 g) was developed. The reagents were silica sol, V₂O₅, H₂SO₄, CsOH, and ethanol. Despite the fact that V₂O₅ was used as the vanadium source the oxidation states of most of the vanadium atoms in the produced VSH-2 were 4+, indicating that ethanol acts as a reducing agent. The crystals adopted individual octahedral shapes or aggregated states depending on the gel composition. The total surface area of the pristine VSH-2 with the general formula of Cs₂(VO)(Si₆O₁₄)·3H₂O was only 40 m²/g, indicating that the pores are blocked by the large Cs⁺ ions. The surface area increased to 149 m²/g upon exchanging Cs⁺ with Na⁺. Analyses of the diffuse reflectance UV–Vis spectra of Mⁿ⁺–VSH-2 (Mⁿ⁺ = Cs⁺, Na⁺, Ca²⁺, and Pb²⁺) revealed that the 215, 250, and 313 nm bands arise due to the V⁴⁺ to O^2? metal-to-ligand charge transfer (MLCT) and the 437, 590, and 914 nm bands arise due to the d–d transition of V⁴⁺. This reveals an unprecedented interesting situation that in dehydrated VSH-2 the framework oxide plays the role of both acceptor to V⁴⁺ and donor to Mⁿ⁺. The measured atomic magnetic moment (μ) was 1.64 BM, indicating that most of the V atoms exist in V⁴⁺. The ESR spectrum of VSH-2 showed a strong signal due to V⁴⁺ with the g value of 1.959 with ΔH_pp value of 168 G. The Raman spectra of Mⁿ⁺–VSH-2 revealed the existence of strong V=O stretching at 960 cm^?1, and other weak peaks. The V=O stretching band shifted to a higher energy region upon increasing the Sanderson’s electronegativity of Mⁿ⁺. The thermogravimetric (TGA) analysis showed that VSH-2 is thermally stable up to 550 °C and above which the oxidation of V⁴⁺ occurs.     

Detailed Kinetics of the Fischer�CTropsch Synthesis on Cobalt Catalysts Based on H-Assisted CO Activation

A complete mechanistic kinetic model of the Fischer?CTropsch synthesis (FTS) over a Co/Al₂O₃ state-of-the-art catalyst is developed here under conditions relevant to industrial operation. On the basis of the most recent findings on the reaction mechanism, here described according to the H-assisted CO dissociation theory and the alkyl chain growth mechanism, and on the basis of the latest indications available on the rate determining step involved in the CO activation process, rate expressions for all the steps leading to CO and H₂ consumption and n-paraffins, ??-olefins and H₂O formation are derived. Such expressions are functions of the molar fraction of CO, H₂ and olefins in the liquid phase surrounding the catalyst pellets, that in turn are related to the gas-phase pressure and composition, and of the surface coverage of the adsorbed species. Thermodynamic and kinetic parameters involved in the model are estimated by non-linear multi-response regression on a complete set of FTS experimental data collected in a lab-scale tubular reactor at steady-state conditions (P = 8?C25 bar, T = 210?C235 °C, H₂/CO feed ratio = 1.8?C2.7 mol mol^?1, GHSV = 2,000?C7,000 cm³(STP) h^?1 g _cat ^?1 ). Both the experimental CO conversion and the hydrocarbon distribution (up to N = 50) in FTS are well predicted by the model with 13 adaptive parameters, without the need of introducing any empirical parameter. Analysis of the model offers insight in the rates of the elementary steps associated with the reaction mechanism and in the surface coverages of the adsorbed species. Such information explains the peculiar reactivity observed over cobalt-based Fischer?CTropsch catalysts, and can provide guidelines for the design of more active and selective catalytic materials.     

Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

In this study, new Fe₂O₃ based materials are developed for the selective catalytic reduction (SCR) of NO_x by NH₃ in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO₂ is considered to be the most effective carrier for Fe₂O₃. The modification of the Fe₂O₃/ZrO₂ system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO₂ that is coated with 1.4 mol% Fe₂O₃ and 7.0 mol% WO₃ (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NO_x is obtained between 285 and 430 °C with selective formation of N₂. Here, the turnover frequency of NO_x per Fe atom is found to be 35 × 10⁻⁵ s⁻¹ that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H₂O and CO₂, whereas it is increased by NO₂.Temperature programmed reduction by H₂ (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe₂O₃, whereby relatively small oxide entities are also present. The strongly aggregated Fe₂O₃ species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH₃ that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N₂ and H₂O. In the final step, the reduced Fe sites be regenerated by oxidation with O₂. As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO₂ or O₂ into NO.     

Separation of Uranium from Aqueous Solutions Using Al3+‐ and Fe3+‐modified Titanium‐ and Zirconium Phosphates

Abstract

The ability of four amorphous Al³⁺‐ and Fe³⁺‐doped titanium and zirconium sorbents to separate U(VI) from acidic aqueous solutions (pH_init=3, ionic strength 0.1 M established by NaNO₃) was investigated using a batch technique and instrumental neutron activation analysis. All investigated sorbents were found to be chemically stable and remove considerable amounts of uranium from acidic aqueous solutions (pH_init=3). The scanning electron microscopic and powder‐X‐ray diffraction examination of the grains of the two investigated titanium phosphates after contacting the uranium solutions revealed the formation of sodium autunite (Na₂(UO₂)₂(PO₄)₂ · 6‐8H₂O) accompanied, in the case of the Fe³⁺‐doped titanium phosphate, by iron uranyl phosphate hydroxide hydrate (Fe(UO₂)₂(PO₄)₂(OH) · 7H₂O). No crystal formation was observed in the cases of uranium sorbed by zirconium phosphates indicating the different sorption mechanism involved.     

Degradation of an Anthraquinone Dye by Ozone/Fenton: Response Surface Approach and Degradation Pathway

A coupled O₃/Fenton process is applied to study the degradation ef?ciency of organic pollutants. The C.I. Acid Blue 80 (AB80), a kind of anthraquinone dye, is used as target contaminant. The results show that the combination of ozonation and Fenton process is a highly effective way of removing color from wastewater. Response surface methodology is applied to optimize the working conditions and the effects and interactions among initial pH (X₁), mole ratio of H₂O₂/Fe²⁺ (X₂) and ozone flux (X₃) are investigated. Regression equations determines that the best condition is that initial pH = 2.85, [H₂O₂]/[Fe²⁺] = 18.10 and ozone flux = 55.70 L.h^?1. It turns out the relative error of 1.32% with the predicted model when the actual value which is 88.76% in the best condition, compared to the predictive value of 88.95% under same condition. UV-Vis and FT-IR analysis are used as an assisted technique to study degradation mechanism during the oxidation process. The intermediate products are determined by gas chromatography/mass spectrometry (GC/MS) analysis and the plausible degradation pathway is proposed.     

Development of iron oxide sorbents for Hg removal from coal derived fuel gas: Sulfidation characteristics of iron oxide sorbents and activity for COS formation during Hg removal

The aim of this study is to develop a process for the removal of Hg⁰ using H₂S over iron oxides sorbents, which will be located just before the wet desulfurization unit and catalytic COS converter of a coal gasification system. It is necessary to understand the reactions between the iron oxide sorbent and other components of the fuel gas such as H₂S, CO, H₂, H₂O, etc. In this study, the sulfidation behavior and activity for COS formation during Hg⁰ removal from coal derived fuel gas over iron oxides prepared by precipitation and supported iron oxide (1 wt% Fe₂O₃/TiO₂) prepared by conventional impregnation were investigated. The iron oxide samples were dried at 110 °C (designated as Fe₂O₃-110) and calcined at 300 and 550 °C (Fe₂O₃-300 and Fe₂O₃-550). The sulfidation behavior of iron oxide sorbents in coal derived fuel gas was investigated by thermo-gravimetric analysis (TGA). COS formation during Hg⁰ removal over iron oxide sorbents was also investigated using a laboratory-scale fixed-bed reactor. It was seen that the Hg⁰ removal activity of the sorbents increased with the decrease of calcinations temperature of iron oxide and extent of sulfidation of the sorbents also increased with the decrease of calcination temperature. The presence of CO suppressed the weight gain of iron oxide due to sulfidation. COS was formed during the Hg⁰ removal experiments over Fe₂O₃-110. However, in the cases of calcined iron oxides (Fe₂O₃-300, Fe₂O₃-550) and 1 wt% Fe₂O₃/TiO₂, formation of COS was not observed but the Hg⁰ removal activity of 1 wt% Fe₂O₃/TiO₂ was high. Both FeS and FeS₂ were active for Hg⁰ removal in coal derived fuel gas without forming any COS.     

Ru Nanoparticles Entrapped in Mesopolymers for Efficient Liquid-phase Hydrogenation of Unsaturated Compounds

A chemical potential diagram of the V–Mg–O system was constructed for comparison in an in-situ experiment. A V–Mg–O catalyst used in the oxidative dehydrogenation of n-butane was prepared by the impregnation method and was characterized by in-situ X-ray diffraction (XRD). Mg₃V₂O₈ and MgO were detected on the in-situ XRD pattern under the oxygen pretreatment at 600 °C, and the in-situ XRD data under working conditions showed that Mg₃V₂O₈ is reduced to MgV₂O₄, having a cubic spinel structure with a lattice constant of a = 8.427 Å. The observed reduction of V⁵⁺ in Mg₃V₂O₈ to V³⁺ in MgV₂O₄ under the working conditions could be well understood through a chemical potential diagram.     

Iron exchanged ZSM-5 and Y zeolites calcined at different temperatures: activity in N2O decomposition

ZSM-5 and Y zeolites were modified with iron by an ion-exchange method and then calcined at 773, 873, 973 and 1,073 K. The obtained materials were characterized with respect to textural parameters (low-temperature N₂ sorption), structure (X-ray diffraction, UV–vis–DRS), redox properties (H₂-temperature programmed desorption, TPD) and surface acidity (NH₃-TPD). The obtained results have shown that the structure of zeolites influenced form, aggregation and content of the introduced iron species. In case of the FAU type structure characterized by wide pores (max. ring size, T-atoms—12) mainly iron in form of mononuclear Fe³⁺ cations and Fe _x ³⁺ O_y oligonuclear clustered species was found. On the other hand for the MFI type structure characterized by smaller pores (max. ring size, T-atoms—10) significant contribution of iron in the form of bulky Fe₂O₃ clusters located possibly on the outer surface of ZSM-5 was detected. Such significant differences in distribution of iron species is probably related to various mobility of iron species in the pore systems of both zeolites. The obtained materials were tested as catalysts in the process of N₂O decomposition. Calcination of zeolites at different temperatures influenced neither the properties nor the activity of the obtained catalysts.     

Magnetically Separable Fe3O4 Nanoparticles-Decorated Reduced Graphene Oxide Nanocomposite for Catalytic Wet Hydrogen Peroxide Oxidation

Fe₃O₄ nanoparticles-decorated reduced graphene oxide magnetic nanocomposites (Fe₃O₄/rGO NCs) were prepared by a facile one-step strategy, and further used as heterogeneous Fenton-like catalysts for catalytic wet hydrogen peroxide oxidation (CWHPO) of methylene blue (MB) at 25 °C and atmospheric pressure. The effects of variables such as the Fe₃O₄/rGO with the mass ratio of rGO, initial pH, MB concentration and H₂O₂ dosage were investigated. The Fe₃O₄/rGO NCs with rGO mass ratio of 10.0 wt % showed the highest H₂O₂-activating ability, which was six-fold than that of pure Fe₃O₄ nanoparticles (NPs). The resulting catalysts demonstrated high catalytic activity in a broad operation pH range from 5 to 9, and still retained 90.5 % catalytic activity after reuse in five cycles. Taking advantage of the combined benefits of rGO and magnetic Fe₃O₄ NPs, these Fe₃O₄/rGO NCs were confirmed as an efficient heterogeneous Fenton-like catalyst for CWHPO to treat organic pollutants. And a reasonable catalytic mechanism of Fe₃O₄/rGO NCs was proposed to interpret the degradation process.     

Selective ammoximation of ketones and aldehydes catalyzed by a trivanadium-substituted polyoxometalate with H2O2 and ammonia

The ammoximation of different ketones and aldehydes to their corresponding oximes catalyzed by K₆[PW₉V₃O₄₀]·4H₂O was carried out with hydrogen peroxide and ammonia in isopropanol at room temperature. High yields of oximes were obtained in this catalytic system. This catalytic system was proved to be heterogeneous by the ammoximation activity of removal of catalyst and the elemental analysis of the filtrate after reaction. A possible procedure for the ammoximation catalyzed by K₆[PW₉V₃O₄₀]·4H₂O with H₂O₂ and NH₃·H₂O was proposed. The fresh and used catalysts were characterized by IR and ³¹P MAS NMR, which revealed the good stability of the catalyst.     

Synthesis, characterization, and catalytic phenol hydroxylation of a novel complex oxide HxV2Zr2O9.H2O

A novel type of complex oxide H_xV₂Zr₂O_9.H _2O with V⁴⁺ and V⁵⁺ mixed valence has been hydrothermally synthesized in a V₂O₅-ZrO₂-H₂O system at 240°C for 5 days in presence of NaF. The catalytic data over these complex oxides show that these complex oxides are catalytically very active in phenol hydroxylation by 30% aqueous hydrogen peroxide, and their catalytic activity is dependent on crystal size of the catalysts. The phenol conversion over the catalyst with crystal size of 7 μm is twice that over the catalyst with crystal size of 35 μm. The V⁵⁺ species are suggested to be the catalytic active sites. Some other factors which influence the catalytic activity were also investigated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Vanadium Oxide Based Nanostructured Materials: Novel Oxidative Dehydrogenation Catalysts

Novel vanadium oxide based catalyst derived from the open-framework solid, [Co₃V₁₈O₄₂(H₂O)₁₂(XO₄)]·24 H₂O (X = V, S) (1) catalyses oxidative dehydrogenation of propane to propylene. Catalyst activity was evaluated in the temperature range 250–400 °C with varying gas hourly space velocity (GHSV). At 350 °C and GHSV of 9786 h^?1 and at 1.3% propane conversion the selectivity to propylene was 36.8%. The major products obtained were propylene and CO _x (CO₂ and CO). The ratio of the propylene to CO _x depended directly on the catalytic sites present. Thus, as the amount of the catalyst was decreased, the conversion decreased with an increase in the propylene selectivity and a decrease in the selectivity to carbon oxides—CO _x . The catalyst has been characterized by temperature programmed reduction and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).     

The Effect of Proton Hydration in Wells-Dawson and Keggin Type Heteropolyacids on their Catalytic Activity in Gas Phase ETBE Synthesis

The influence of proton hydration in Wells-Dawson H₆P₂W₁₈O₆₂ (HP2W) and Keggin H₃PW₁₂O₄₀ (HPW) type heteropolyacids on their catalytic activity in ethyl-tert-butyl ether (ETBE) synthesis in gas phase was investigated. In the case of samples with the content of crystallisation water per one proton below 0.8 ([H₂O]/[H⁺] < 0.8), the catalytic activity related to the mass unit of the HP2W was higher than that of the HPW in accordance with the order of proton concentration in mass unit. On the other hand, the activity related to one mole of protons indicated higher activity in the HPW than in the HP2W in accordance with the order of acid strength. In the series of samples with [H₂O]/[H⁺] > 0.8 the activity was much higher for the HP2W than the HPW and maximum of both catalytic activity and ethanol sorption capacity was observed for the HP2W. The possible role of the secondary structure of the hydrates was discussed.     

Microwave Assisted Crystal Growth of a New Organic—Decavanadate Assembly: [V10O27(OH)] · 2(C6N2H14) · (C6N2H13) · (C6N2H12) · 2H2O

Microwave synthesis was used to grow single crystals of a new organic–inorganic supramolecular assembly, [V₁₀O₂₇(OH)] · 2(C₆N₂H₁₄) · (C₆N₂H₁₃) · (C₆N₂H₁₂) · 2H₂O, in a very short time compared to the conventional solution technique. In order to generate crystals suitable for single crystal experiments, an equimolar mixture of reactants and a few hours of microwave heating are required. Although non-merohedral twinning is an inherent problem, the crystal structure can be solved and refined in the orthorhombic space group Pna2 ₁ with a = 20.972(4) Å, b = 10.3380(14) Å, c = 20.432(3) Å, Z = 4, with an excellent result, R(F²) = 0.0431. The assembly is hydrogen bond-assisted and built up of the monoprotonated decavanadate and 1,4-diazabicyclo[2.2.2]octane of various degrees of protonation. The number and location of protons on both the inorganic and organic motifs govern the formation of the extensive hydrogen bonding network, which in turn regulates the assembly architecture.     

The electrochemical behaviour of iron oxides in dilute sulphuric acid and the interpretation of the flade potential of iron

The cathodic reduction of Fe₂O₃, FeOOH, and Fe₃O₄ in dilute sulphuric acid has been directly investigated using graphite paste electrodes containing the oxides. It has been found that these oxides sustain redox reactions at a high positive potential which is stabilised by the presence of ferrous sulphate in the solution and, at saturation, tends to coincidence with the Flade potential. The redox process can be formulated, for example, as Fe₂O₃(its)+2H⁺+2H₂SO₄+2e^?=2FeSO₄s+3H₂O(1) and the potential derived from the free energy change is approximately right. γ-oxides and hydroxyoxides of iron enter into reaction more readily than their α-analogues, and this would be expected on structural grounds.