Composition Of Mixed Oxides Research Articles

Surface composition and catalytic activity of La-Fe mixed oxides for methane oxidation

Four La-Fe oxide samples with La/Fe atomic ratio y=1.02∼0.68 (denoted as LayFe) were prepared by the citrate method. The samples had a decreased specific surface area with the La/Fe atomic ratio decreasing. XRD pattern proved that the sample La0.94Fe is single phase perovskite La0.94FeO3−d. Phase composition of the samples was estimated by the Rietveld refinement method. XPS analyses indicate that La3+ ions are enriched on surface of crystallites for all the samples, and surface carbonate ions are relatively abundant on the samples La1.02Fe and La0.94Fe. Catalytic activity for methane oxidation per unit surface area of the samples is in the order of La0.68Fe>La0.76Fe>La0.94Fe>La1.02Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe3+ ion on the surface of the samples La0.68Fe and La0.76Fe.

Libraries with Expanded Representation of Fission Product Yields for Calculations of the Nuclide Composition of Fuel in a Fast Spectrum

The calculation of the spent-fuel composition is the baseline problem of the analysis of the nuclear and radiological safety of objects with spent fuel assemblies. The use of fission product libraries formed on the basis of evaluated nuclear data with an enlarged number of energy points is especially important for highprecision calculations of the nuclide composition of fuel irradiated in a fast spectrum. The results of an analysis of the nuclide composition of mixed oxide and uranium-plutonium-zirconium metal fuel with burnup ~10% h.a. in a fast spectrum, which were obtained in high-precision calculations based on data from different fission product yield libraries, are presented. The use of new types of fuel in fast reactors necessitates more accurate and reliable prediction of the characteristics of spent fuel for the analysis of the nuclear and radiological safety of objects containing spent nuclear fuel. The accuracy of high-precision calculations largely depends on the reliability of the neutron-physical data libraries used. In addition, the libraries of the transport cross sections, fission product yields, and decay and the small-group library of the cross sections of transmutation transformations must be formed on a unified computational platform with matched physical models and methods of preparation, making it possible to validate the correctness and accuracy of the data [1]. To compile libraries of fission product yields, it is necessary to take account of the factors associated with different physical models of the representation of the initial data, for example, models of triple and double fission, the criteria for determining the main disintegration isotopes taking account of the excited states of the nuclides, and others [2, 3]. The evaluated nuclear data with an expanded energy grid of the yield of fission products for energies above 1 MeV and intermediate energy are needed primarily for the formation of fission yield libraries for systems with a fast spectrum [4‐6]. More precise determination of the composition of spent fuel with the reckoned maximum number of isotopes is important for problems associated with its reprocessing and the fabrication of recycled fuel. Higher computational accuracy of the nuclide composition of fuel during burnup is also needed in connection with the use of individual isotopes, for example, 137 Cs, 106 Ru, and 154 Eu, as indicators for evaluating the neutron-physical characteristics of spent fuel. The nuclide composition of spent fuel in a fast spectrum, calculated using the codes MONTEBURNS‐MCNP5‐ ORIGEN2 and different fission product yield libraries, are analyzed in the present work. Comparative results are presented for mixed uranium-plutonium oxide and uranium-plutonium-zirconium metal fuels with burnup ~10% h.a. [7]. Nuclear Data on Fission Product Yields. In evaluated nuclear data files, the fission product yield is conventionally presented for three points ‐ thermal (E = 2.53·10 ‐8 MeV), average point of the fission spectrum (E = 0.4 or 0.5 MeV), and

Nanostructured vanadium-molybdenum mixed oxides prepared by the solvothermal method

The specific features of the structure, morphology, and valence composition of nanostructured molybdenum-vanadium mixed oxides prepared by copolycondensation of relevant oxoacids under solvothermal conditions are investigated. It is shown that the resultant oxide phases are mixtures of hexagonal molybdenum trioxide and a V2O5 xerogel-based solid solution in which some vanadium ions in the lattice are substituted by molybdenum ions. It is demonstrated that vanadyl centers in thus obtained mixed oxides exhibit a high degree of magnetic association. The parameters of the spin Hamiltonian of V(IV) ions were determined from EPR spectra.

Preparation of Mixed Semiconductors for Methyl Orange Degradation

Attempts were made to compare the photocatalytic efficacy in between quasi‐fiber and near spherical commercial grade ZnO through the addition of monoclinic WO3 nanocuboid. Mixed oxide semiconductors were assessed for their dye degradation performance under irradiation of visible light energy. Surface area and the particle morphology pattern have an influence on the resultant photocatalytic features of these mixed oxide composites. The high porous quasi‐fibrous ZnO was successfully fabricated by a simple solution combustion method. It is deliberately made of clusters of primary near spherical particles that supports WO3 nanocuboid embedment and shows interactive characteristics in comparison to the counterpart commercial near spherical ZnO combined with WO3. The photocatalytic activity significantly increases up to 95% under visible radiation for 90 min due to high surface area imparted by unique quasi‐fiber morphology. The photogenerated electron‐hole pair interaction mechanism has been proposed to support the photocatalytic behavior.

Determining surface composition of mixed oxides with pH

In this study we have developed a 2-surface model with the EPHL (“Equilibrium pH at high oxide loading”) method for mixed and composite oxides. Oxide charging parameters, namely the protonation and deprotonation constants and the hydroxyl surface densities, can be established from measurements of the pure oxides and used in the 2-surface model to predict the point of zero charges (PZC) of mixed and composite oxides. The accuracy of these predictions has been demonstrated with diverse control samples of known surface composition (physical mixtures of silica and alumina of high and low surface area). The EPHL method has been extended to composite materials (bound catalysts) and can explain correlations of reactivity with catalyst surface composition. The “apparent surface coverage” (ASC) of a particular mixed or composite oxide sample may then be determined by comparing the PZC of the material to either the 2-surface model results, using parameters of the individual oxides, or a calibration curve of EPHL measurements of physical mixtures of the individual ingredients.

Improved overall water splitting with barium tantalate mixed oxide composites

The combination of effective charge carrier separation and improved electron transfer in highly crystalline barium tantalate composites modified with Rh–Cr2O3 core–shell co-catalyst systems induces enhanced activity for overall water splitting (OWS) with stoichiometric amounts of H2 and O2 (2 : 1). A sol–gel route employing complexing reagents was investigated to prepare selectively defined mixed oxide materials with improved surface areas and smaller particle sizes compared to the conventional solid state reaction (SSR). The catalytic activities of the materials are investigated in photocatalytic test reactions for hydrogen production and overall water splitting. The formation of Rh–Cr2O3 core–shell co-catalyst systems for water splitting is evidenced by transmission electron microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). Moreover, we developed new and highly active barium tantalate composites for hydrogen generation from aqueous methanol solutions under UV-light, which show the highest hydrogen evolution rate for a three-component composite consisting of Ba5Ta4O15/Ba3Ta5O15/BaTa2O6. Hydrogen rates of more than 6 mmol h−1 can be achieved without any co-catalyst. Using Rh–Cr2O3 core–shell co-catalysts on these three-component composites simultaneous generation of H2 and O2 from pure water splitting reaches rates up to 70% higher than for the pure Ba5Ta4O15.

Synthesis and Characterization of Nano CuO-ZrO<sub>2</sub> Mixed Oxide

Nano CuO-ZrO2 mixed oxides were prepared by wet chemical method by mixing equimolar solutions (0.45M) of cupric chloride (1.92g) and Zirconium oxychloride (3.63g) in aqueous Sodium hydroxide and refluxed at elevated temperature. The prepared nano CuOZrO2 mixed oxides were characterized by FT-IR, SEM, EDAX, XRD, DSC and CV studies. From XRD studies the size of the nano CuO and ZrO2 are found to be 25 and 9.5nm respectively through Debye-Scherrer's formula. The sizes of the CuO-ZrO2 mixed oxide particles have also been characterized and the average grain size of the particles is found to be 24nm in diameter. The nano particle composition and morphology of CuO, ZrO2 and mixed oxide have been analysed by EDAX set up attached with scanning electron microscope (SEM). EDAX analysis indicates the presence of Cu, Zr and O. SEM morphological studies of CuO, ZrO2 and mixed CuO-ZrO2 revealed the particle distribution with uniform granular structure. Cyclic Voltammetric studies exhibit good adherent behaviour on electrode surface and good electroactivity at pH 1.0. Nano CuO, ZrO2 and mixed CuO-ZrO2 under goes oxidation at 0.224V, 0.092V -0.072V and0.198V respectively. DSC thermogram of CuO, ZrO2 and mixed CuO-ZrO2 are recorded at the heating rate of 10o/ min. The glass transition temperature (Tg), the crystallization temperature (Tc) and melting point (TM) of the mixed oxide are determined from the DSC curve. The Tg value of CuOZrO2 mixed oxide is -50o C the Tc value is 20o C and melts at a temperature of 116o C.

Development of Fe2O3–TiO2 mixed oxide incorporated Ni–P coating for electrocatalytic hydrogen evolution reaction

Considering the electronic parameters and chemical characteristics, a synergistic catalytic effect of Fe2O3 along with TiO2 could be achieved for electrochemical reactions if both the oxides are produced in a mixed oxide form. The present study explored the mixed oxide composite viz; Fe2O3–TiO2, synthesized via thermal decomposition method, to increase the catalytic efficiency of Ni–P electrodes, the well known catalytic electrodes for hydrogen evolution reaction in alkaline medium. The incorporation of the Fe2O3–TiO2 mixed oxide into Ni–P matrix substantially reduced overpotential during hydrogen evolution reaction (HER) in 32% NaOH solution. A significant improvement on the electrochemical activity of the Ni–P coated electrodes was achieved as evidenced from the results of Tafel and impedance studies. The incorporation of Fe2O3–TiO2 mixed oxide composite into the Ni–P matrix has improved both metallurgical and electrochemical characteristics and hence its amount of incorporation should be optimum. The electrodes exhibited high stability under dynamic experimental conditions. The role of the composite and the possible mechanism are discussed in this paper.

Catalytic activity of alkali metal doped Cu-Al mixed oxides for N2O decomposition in the presence of oxygen

Three CuAl hydrotalcite-like compounds (CuAl-HLc) with Cu/Al atomic ratio of 2.0, 3.1 and 4.1 were prepared by a co-precipitation method and calcined to convert into CuAl mixed oxides. The CuAl mixed oxide with Cu/Al atomic ratio of 3.1 was incipiently impregnated by alkali metal (Na, K, Cs) salt solution to prepare the modified CuAl mixed oxide catalysts. The properties of the catalysts were characterized by AES, XRD, FT-IR, BET, H2-TPR and XPS techniques. The effect of CuAl mixed oxide compositions, alkali metal types and potassium precursors on catalytic activity for N2O decomposition in the presence of oxygen were investigated. It is revealed that the catalytic activity of CuAl mixed oxides impregnated by K2CO3 solution is higher than that of Na2CO3 and Cs2CO3 doped catalysts. In addition, the catalytic activity of CuAl mixed oxides can be largely enhanced by the addition of K species from the precursors of K2CO3 and K2C2O4, whereas KNO3- and CH3COOK-doped catalysts show a lower activity than sole CuAl oxide. The optimal K-doped catalyst exhibits a good activity for N2O decomposition in the presence of oxygen and steam.

Redox and catalytic properties of Ce–Zr mixed oxide nanopowders for fuel cell applications

Ce x Zr 1− x O 2 mixed oxides nanopowders with x = 0.10, 0.25, 0.50, 0.75 and 0.90, and pure ceria, all prepared by a low temperature citrate complexion technique, were investigated using a range of temperature programmed (TP) techniques in order to evaluate their suitability for use as anode catalyst materials in solid oxide fuel cells operating on hydrocarbon fuels. In TP reduction experiments, reduction peak temperatures and peak areas were related to the activity and amount, respectively, of available catalyst oxygen. Mixed oxides with higher Ce contents ( x ≥ 0.50) were found to be reduced at significantly lower temperatures than samples with x < 0.50. The oxide with x = 0.75 supplied the largest amount of labile oxygen. TP reaction experiments performed in dry methane on two samples indicated that susceptibility of the oxides to carbon deposition was inversely related to availability of catalyst oxygen. Light-off experiments in a stoichiometric CH 4/O 2 mixture were performed to investigate the catalytic activity of the mixed oxide compositions for methane oxidation. Catalytic activity increased with cerium content to a maximum at x = 0.75 before decreasing at x = 0.90. All the mixed oxides were considerably more active for methane oxidation than pure CeO 2 prepared by the same citrate route.

Preparation and characterization of magnetic nanoparticles with controlled magnetization

The effect of molar ratio of two hydrated iron salts used as precursors into a (co)precipitation-based synthesis method, on the composition, size and specific saturation magnetization of mixed iron oxides and oxyhydroxides magnetic nanoparticles as reaction products, was studied. The preparation procedure is based on a salt-assisted solid-state chemical reaction. The obtained products are magnetic multiphase components with the mean size ranging from 3 to 10 nm and specific saturation magnetization between 25 and 95.5 emu/g. The specific saturation magnetization modifies in a non-linear manner as the molar ratio of the iron salts varies. Excepting one sample, for which Fe2+/Fe3+ molar ratio was zero, all magnetic nanoparticles show a ferrofluid-like behaviour in the colloidal form. The small size, ferrofluid-like behaviour, and controlled specific saturation magnetization allow the use of new synthesized nanoparticles in specific biomedical or industrial applications.

Novel mixed-oxide ceramic for neutron multiplication and tritium generation

Beryllium and lithium titanate (Li2TiO3), have limited use in blankets due to the swelling of beryllium and low thermal conductivity of Li2TiO3. A novel mixed oxide composite of beryllium oxide and lithium titanate (BeO–Li2TiO3) is proposed, which utilizes the high thermal conductivity of BeO and its favourable neutronics. Li2TiO3 was prepared using two different routes, one employing a solid-state reaction and the other through sol–gel route. The sintered BeO–Li2TiO3 is found to have no intermediate products and its thermal conductivity decreased from 36 to 14W/m/K with the increase in temperature from 127°C to 927°C. The coefficient of thermal expansion (CTE) of BeO–Li2TiO3 is less than that of Li2TiO3. Thermodynamic calculations show that tritium cannot be trapped in BeO unless beryllium monotrioxide (BeOT) is formed. The merits of BeO are compared with beryllium metal and neutronic calculations on tritium production in this novel mixed oxide are also presented.

Al 2O 3–ZrO 2 mixed oxide composite incorporated aluminium rich zinc coatings for high wear resistance

Corrosion resistance and wear resistance are the two important parameters for high performance of zinc galvanic coating. In the present work, the improvement of these two characteristics was achieved by the incorporation of Al 2O 3–ZrO 2 mixed oxide composite in the coating. Al 2O 3–ZrO 2 mixed oxide composite was synthesized from ZrOCl 2·8H 2O. Aluminium rich zinc coatings with high sliding wear resistance was developed from a galvanic bath containing the mixed oxide. Based on the performance of the coating during physicochemical and electrochemical characterization, the concentration of mixed oxide composite in the bath was optimized as 0.50 wt% Al 2O 3–0.50 wt% ZrO 2. While rich in Al–metal content in the coating caused high corrosion resistance, the incorporation of the mixed oxide improved structural characteristics of the coating resulting in high wear resistance also. The coating was nonporous in nature and even the interior layers had high stability. The coatings have potential scope for high industrial utility.

Catalytic wet oxidation of phenol using membrane reactors: A comparative study with slurry-type reactors

The wet air oxidation of phenol over cerium mixed oxides has been carried in autoclave slurry-type reactor and also in a contactor type membrane reactor to assist about the benefits provided by the employment of the mesoporous top layer of a ceramic tubular membrane as catalyst (Ce mixed oxides) support. The effect of mixed oxide composition and use of Pt as dopant onto the phenol removal rate and selectivity towards mineralization have been studied on both types of reactor. For slurry-type reactors, two different autoclave reactors were used: one mechanically stirred highly pressurized, and the other magnetically stirred containing a porous stainless steel membrane as gas diffuser in an attempt to attain higher gas–liquid interfacial area. The performances of these reactors have been compared under similar reaction conditions (i.e. catalyst loading/liquid volume, temperature, phenol concentration) although the way in which reactants are fed to the reaction vessel (different among each other configuration) is clearly affecting the CWO phenol degradation route. From the catalytic systems studied, Pt doped Ce–Zr mixed oxides exhibit the best reaction performance in spite of the achieved phenol conversion levels are below 50%. For autoclave reactors, the gas feeding to the liquid volume by a membrane diffuser has almost no effect on phenol removal for the reaction conditions tested; whereas the catalytic membrane contactor type reactor clearly outperform autoclave reactor provided with membrane diffuser.

NOx storage and reduction properties of Pt/CexZr1−xO2 mixed oxides: Sulfur resistance and regeneration, and ammonia formation

The influence of the ceria-zirconia mixed oxide composition in Pt/CexZr1−xO2 catalysts was studied toward NOx storage capacity (NSC), including sulfur poisoning and sulfur regeneration, and NOx reduction efficiency in lean–rich cycling conditions. The results are compared with a Pt/Ba/Al model catalyst. The samples were characterized by N2 adsorption, XRD and H2-TPR. The behaviors of the ceria-zirconia supported catalysts are quite similar whatever their composition. They are sensitive to a reducing pretreatment which leads to an increase of (i) the cerium reducibility/oxygen mobility, (ii) the NO oxidation rate and (iii) the NOx storage capacity at 300 and 400°C. The sulfating treatment leads to a dramatic decrease of the NOx storage capacity for all catalysts, the decrease being more pronounced for the Zr-rich samples. H2-TPR experiments show that the sulfates amount and their stability tend to increase with the Zr content, but these sulfates are significantly less stable compared with Pt/Ba/Al. The sulfur elimination rates in rich mixture at 550°C are higher than 90% with the ceria-zirconia supported catalysts versus 56% with Pt/Ba/Al.The ceria-zirconia supported catalysts are able to convert NOx in lean–rich cycling condition. Compared with Pt/Ba/Al, the NOx conversions are a little lowered but the ammonia selectivity is significantly decreased with the Ce–Zr mixed oxides, with a beneficial influence of the cerium content.

Que penser de l’uranium sous forme appauvrie ?

It was demonstrated that chemical heat storage materials mixed with metal hydroxides were capable of storing heat at medium temperatures of approximately 200–300 °C. The performances of the developed materials were demonstrated in a thermo-balance and packed bed reactor. The mixed hydroxides can increase the operation heat storage temperature by changing the composition of mixed metal oxides in the material. MgαNi1−α(OH)2, which is a mixed hydroxide of magnesium hydroxide, Mg(OH)2, and nickel hydroxide, Ni(OH)2, is a candidate heat storage material. The mixed hydroxide was dehydrated, that is, it was capable of storing heats at 200–300 °C, at which pure Mg(OH)2 could not be dehydrated and could not store heat. The heat output performance of the material was examined in a packed bed reactor. It was shown that the mixed hydroxide material was potentially useful for chemically storing medium-temperature heat such as waste heats emitted from internal combustion engines, solar energy system and high-temperature processes.

XPS analysis of U xCe 1−xO 2±δ and determination of oxygen to metal ratio

The chemical states of U and Ce in the solid solutions of UO 2 and CeO 2 are studied using the X-ray photoelectron spectroscopy. A detailed analyses on U 4f and Ce 3d photoelectron peaks revealed the presence of Ce 3+ and U 5+/U 6+ states in the mixed oxides. The oxygen to metal ratios in different compositions of mixed oxides were estimated from the quantity of different chemical states of U and Ce present in mixed oxides.

High surface area sol–gel alumina–titania nanocatalyst

Alumina–titania mixed oxide nanocatalysts with molar ratios = 1:0.5, 1:1, 1:2, 1:5 have been synthesized by adopting a hybrid sol–gel route using boehmite sol as the precursor for alumina and titanium isopropoxide as the precursor for titania. The thermal properties, XRD phase analysis, specific surface area, adsorption isotherms and pore size details along with temperature programmed desorption of ammonia are presented. A specific surface area as high as 291 m2/g is observed for 1:5 Al2O3/TiO2 composition calcined at 400 °C, but the same composition when calcined at 1,000 °C, resulted in a surface area of 4 m2/g, while 1:0.5 composition shows a specific surface area of 41 m2/g at 1,000 °C. Temperature programmed desorption (of ammonia) results show more acidic nature for the titania rich mixed oxide compositions. Transmission electron microscopy of low and high titania content samples calcined at 400 °C, shows homogeneous distribution of phases in the nano range. In the mixed oxide, the particle size ranges between 10–20 nm depending on titania content. The detailed porosity data analysis contributes very much in designing alumina–titania mixed oxide nanocatalysts.

Catalytic synthesis of nitrogen-doped multi-walled carbon nanotubes using layered double hydroxides as catalyst precursors

The nitrogen (N)-doped carbon (CNx) nanotubes were synthesized by pyrolysis of ethylenediamine with Ni1·07Mg1·01AlO3·58, Ni1·99Mg0·29AlO3·78, and Ni2·31Mg0·08AlO3·89 mixed oxides as catalysts at 650°C. Those mixed oxides were obtained by calcination of corresponding layered double hydroxide precursors (LDHs). Structure and composition of LDHs and mixed oxides were characterized by X-ray diffraction (XRD) and Inductively coupled plasma spectrum. X-ray photoelectron spectroscopy and transmission electron microscope were used to characterize the N content, proportion of pyridine-like N structure and morphology of CN x nanotubes. The results showed that the tubes grown with Ni2·31Mg0·08AlO3·89 as catalysts had more obvious bamboo-like structure, larger diameter than those grown with Ni1·07Mg1·01AlO3·58 and Ni1·99Mg0·29AlO3·78. The N content and proportion of graphitic-like N structures increased with the content of Ni2+ increasing in LDH precursors. The morphology, N content and pyridine-like N structures for CN x nanotubes can be controlled to a certain extent by varying the content of Ni2+ in LDH precursors.

Study on medium-temperature chemical heat storage using mixed hydroxides

It was demonstrated that chemical heat storage materials mixed with metal hydroxides were capable of storing heat at medium temperatures of approximately 200–300 °C. The performances of the developed materials were demonstrated in a thermo-balance and packed bed reactor. The mixed hydroxides can increase the operation heat storage temperature by changing the composition of mixed metal oxides in the material. Mg α Ni 1− α (OH) 2, which is a mixed hydroxide of magnesium hydroxide, Mg(OH) 2, and nickel hydroxide, Ni(OH) 2, is a candidate heat storage material. The mixed hydroxide was dehydrated, that is, it was capable of storing heats at 200–300 °C, at which pure Mg(OH) 2 could not be dehydrated and could not store heat. The heat output performance of the material was examined in a packed bed reactor. It was shown that the mixed hydroxide material was potentially useful for chemically storing medium-temperature heat such as waste heats emitted from internal combustion engines, solar energy system and high-temperature processes.