Composition Of Mixed Oxides Research Articles

Coprecipitation of Arsenate with Metal Oxides. 3. Nature, Mineralogy, and Reactivity of Iron(III)−Aluminum Precipitates

Coprecipitation involving arsenic with aluminum or iron has been studied because this technique is considered particularly efficient for removal of this toxic element from polluted waters. Coprecipitation of arsenic with mixed iron-aluminum solutions has received scant attention. In this work we studied (i)the mineralogy, surface properties, and chemical composition of mixed iron-aluminum oxides formed at initial Fe/Al molar ratio of 1.0 in the absence or presence of arsenate [As/ Fe+Al molar ratio (R) of 0, 0.01, or 0.1] and at pH 4.0, 7.0, and 10.0 and aged for 30 and 210 days at 50 degrees C and (ii) the removal of arsenate from the coprecipitates after addition of phosphate. The amounts of short-range ordered precipitates (ferrihydrite, aluminous ferrihydrite and/or poorly crystalline boehmite) were greater than those found in iron and aluminum systems (studied in previous works), due to the capacity of both aluminum and arsenate to retard or inhibitthe transformation of the initially formed precipitates into well-crystallized oxides (gibbsite, bayerite, and hematite). As a consequence, the surface areas of the iron-aluminum oxides formed in the absence or presence of arsenate were usually much larger than those of aluminum or iron oxides formed under the same conditions. Arsenate was found to be associated mainly into short-range ordered materials. Chemical composition of all samples was affected by pH, initial R, and aging. Phosphate sorption was facilitated by the presence of short-range ordered materials, mainly those richer in aluminum, but was inhibited by arsenate present in the samples. The quantities of arsenate replaced by phosphate, expressed as percentages of its total amount present in the samples, were particularly low, ranging from 10% to 26%. A comparison of the desorption of arsenate by phosphate from aluminum-arsenate and iron-arsenate (studied in previous works) and iron-aluminum-arsenate coprecipitates evidenced that phosphate has a greater capacity to desorb arsenate from aluminum than iron sites.

Template-free sol–gel synthesis of microporous NiO–SiO2 composite with high surface area and narrow pore size distribution

Microporous NiO–SiO2 composites were synthesized by a new sol–gel chemistry strategy using propylene oxide as gelation agent. Simple procedure, using of cheap precursor and high quality of the synthesized target materials were recognized as the advantages of the process. The obtained maximum pore surface area of the composites is about 718 m2 g−1 with narrow pore size distribution around 9 A and micropore volume of 0.31 cm3 g−1. It was found that the surface area of the samples decreases with the increase of Ni/Si molar ratio. However, the micropore size distributions of the samples were not altered with the increase of Ni/Si molar ratio. The unique chemistry of this sol–gel route assures the effectivity, simplicity and low cost of the whole process, showing the characteristics for the potential large scale preparation of microporous mixed oxide composites with very high pore area and very narrow pore size distribution.

Combinatorial Discovery and Optimization of a Complex Oxide with Water Photoelectrolysis Activity

A ternary oxide containing cobalt, aluminum, and iron and not previously known to be active for the photoelectrolysis of water was identified using a high throughput combinatorial technique. The technique involves ink jet printing overlapping patterns of oxide precursors onto fluorine-doped tin oxide conductive glass substrates. Subsequent pyrolysis yields patterns of mixed oxide compositions that were screened for photoelectrolysis activity by scanning a laser over the material while it was immersed in an electrolyte and mapping the photocurrent response. The composition and optimum thickness for photoelectrochemical response of the newly identified material was further refined using quantitative ink jet printing. Chemical analysis of bulk and thin film samples revealed that the material contains cobalt, aluminum, and iron in a Co3O4 spinel structure with Fe and Al substituted into Co sites with a nominal stoichiometry of Co3−x−yAlxFeyO4 where x and y are about 0.18 and 0.30, respectively. The material is a p-type semiconductor with an indirect band gap of around 1.5 eV, a value that is nearly ideal for an efficient single photoelectrode for water photoelectroylsis. Photoelectrochemical measurements indicate that the onset of hydrogen evolution is about 0.9 V positive of the thermodynamic value but the photocurrent is limited by slow kinetics for hydrogen evolution.

Microwave plasmochemical synthesis of nanopowders in the system Pb-Zr-Ti-O

Conditions for building up the composition and structure of mixed Pb, Zr, and Ti oxides in a microwave plasma-chemical process were studied. As starting substances Pb(C2H5)4, Ti(C4H9O)4, and a 80% Zr(C4H9O)4 solution in 2-propanol were used. To avoid stratification of the components the initial temperature of the flow should not exceed 1000°C. Under these conditions the crystallization of a complex structure requires a combination of a plasma-chemical synthesis with subsequent thermal treatment of the resulting powders. After the thermal treatment an average particle size of lead zirconate-titanate nanopowder was 120 nm.

Study of the composition of ternary mixed oxides: Use of these materials on a hydrogen production process

Mixed oxides obtained by thermal decomposition of layered double hydroxides have been studied in the reaction of hydrocarbon steam reforming for producing hydrogen. The introduction of several elements on the structure could influence the metal electronic configuration, reducibility, particle size, dispersion and surface area modifying its catalytic performance. The effect of composition on structure and reactivity of these materials has been studied. Nickel, magnesium and aluminium from base catalyst have been substituted by different elements. Ni has been substituted by other active metals for this process like Co, Cu, Pd or Ru; Mg has been substituted by Mn, Zn, Ca or Li; and Al has been substituted by Fe, Cr, La or Ce. Samples have been characterized by N 2 adsorption (BET), X-ray diffraction (XRD), temperature-programmed reduction (TPR) and elemental analysis. Composition has an important effect on catalytic performance.

Influence of iron substitution on formation and structure of Cu-based mixed oxides derived from layered double hydroxides

Cu/Zn/Fe/Al mixed metal oxides were prepared by calcination of layered double hydroxides (LDHs) carbonate precursors. The iron content had significant influence on the crystallinity, purity and thermal stability of as-synthesized Cu/Zn/Fe/Al-LDHs as well as the structure and composition of resultant mixed oxides obtained at 773 K. The mixed oxide with the Fe/Al ratio of 0.3:0.7 exhibited highest catalytic activity for oxidation of phenol by hydrogen peroxide. Moreover, the Fe-containing mixed oxides had stronger oxidation abilities than the Fe-free ones. The difference of catalytic activity is probably related to the difference in the amounts and types of composite oxides containing Cu 2+ and/or Fe 3+ cations on the surface of calcined products depending on the pristine nature of the starting LDHs precursors.

Polymer complex solution synthesis of (YxGd1−x)2O3:Eu3+ nanopowders

Yttrium–gadolinium–europium oxide phosphors are regarded promising for many important optical applications, for example in computed tomography medical imaging and plasma displays. In this work, we investigated procedure for (YxGd1−x)2O3:Eu3+ (x= 0, 0.25, 0.5, 0.75 and 1) nanopowder synthesis using polymer complex solution method (PCS) based on polyethylene glycol fuel (PEG). Structural and emission properties of nanopowders are investigated by X-ray diffraction, electron microscopy and photoluminescence measurements. We obtained particles of about 40nm having excellent structural ordering in cubic bixbyte type of structure – space group Ia3 for all mixed oxide compositions. Luminescence emission measurements exposed characteristic transition of the trivalent europium ion incorporated into insulating host. 5D1 and 5D0 decay time values are measured to obtain information on different kinetic processes occurring for these two emitting levels.

Synthesis, characterization and humidity sensing properties of Cu–Sr–Al mixed metal oxide composites

Copper–strontium–aluminum mixed metal oxide composites prepared by sol–gel technique was investigated for humidity sensing properties. X-ray diffraction, scanning electron microscopy (SEM), FT-IR spectroscopy and nitrogen adsorption/desorption isotherm at 77 K was employed, respectively, to identify the structural phases, surface morphology, vibrational stretching frequencies and BET surface area of the composites. The composites were prepared with the mole ratios of Cu:Sr as (0.0:1.0, 0.2:0.8, 0.4:06, 0.6:0.4, 0.8:0.2 and 1.0:0.0) keeping the aluminium mole ratio as constant and were labeled as CuSA1, CuSA2, CuSA3, CuSA4, CuSA5 and CuSA6, respectively. The samples sintered at 900 °C for 5 h, were subjected to dc resistance measurements as a function of relative humidity (RH) in the range 5–98% and the results revealed that the sensitivity factor of CuSA5 possessed the highest humidity sensitivity of 42,000 while the pure composite copper aluminate (CuSA1) and strontium aluminate (CuSA6) possessed comparatively low sensitivities 10,000 and 20,000, respectively. The response and recovery characteristics of the composites CuSA1, CuSA5 and CuSA6 were studied.

Development of nano IrO2 composite-reinforced nickel–phosphorous electrodes for hydrogen evolution reaction

Electroless and electroplated nickel electrodes are extensively used for hydrogen evolution reaction (HER). In the present work, TiO2-supported IrO2 mixed oxide composite was prepared and used to reinforce Ni–P electroless plates to be used as catalytic electrodes for HER. The electrodes exhibited high electrocatalytic activity when the electrodes were used for HER. All the parameters including particle size of the catalyst, surface roughness, and surface active sites were studied. The particle size of the IrO2 catalyst in the mixed oxide was found to have high influence on the catalytic activity of the electrodes. Low overpotential as low as 70 mV at a current density of 200 mA cm−2 was achieved with the mixed oxide-reinforced Ni–P electrodes.

Selective reduction of NO by NH3 over manganese–cerium mixed oxides: Relation between adsorption, redox and catalytic behavior

Manganese–cerium mixed oxide catalysts with different molar ratio Mn/(Mn+Ce) (0, 0.25, 0.50, 0.75, 1) were prepared by citric acid method and investigated concerning their adsorption behavior, redox properties and behavior in the selective catalytic reduction of NOx by NH3. The studies based on pulse thermal analysis combined with mass spectroscopy and FT-IR spectroscopy uncovered a clear correlation between the dependence of these properties and the mixed oxide composition. Highest activity to nitrogen formation was found for catalysts with a molar ratio Mn/(Mn+Ce) of 0.25, whereas the activity was much lower for the pure constituent oxides. Measurements of adsorption uptake of reactants, NOx (NO, NO2) and NH3, and reducibility showed similar dependence on the mixed oxide composition indicating a clear correlation of these properties with catalytic activity. The adsorption studies indicated that NOx and NH3 are adsorbed on separate sites. Consecutive adsorption measurements of the reactants showed similar uptakes as separate measurements indicating that there was no interference between adsorbed reactants. Mechanistic investigations by changing the sequence of admittance of reactants (NOx, NH3) indicated that at 100–150°C nitrogen formation follows an Eley–Rideal type mechanism, where adsorbed ammonia reacts with NOx in the gas phase, whereas adsorbed NOx showed no significant reactivity under conditions used.

Structural aspects and porosity features of nano-size high surface area alumina–silica mixed oxide catalyst generated through hybrid sol–gel route

Alumina–silica mixed oxide nano-catalyst materials with compositions 83.6 wt.% Al 2O 3–16.4 wt.% SiO 2 (3Al 2O 3·1SiO 2), 71.82 wt.% Al 2O 3–28.18 wt.% SiO 2 (3Al 2O 3·2SiO 2), 62.84 wt.% Al 2O 3–37.16 wt.% SiO 2 (3Al 2O 3·3SiO 2) and 56.03 wt.% Al 2O 3–43.97 wt.% SiO 2 (3Al 2O 3·4SiO 2) have been prepared by a hybrid sol–gel technique using boehmite as the precursor for alumina and tetraethoxysilane as that for silica. The bonding characteristics and coordination features around Al and Si in the mixed oxide catalysts have been studied using FTIR and 27Al MAS NMR after calcination at 400 °C which is the temperature region where cross-condensation is seen to take place. A high BET specific surface area of 287 m 2 g −1 is obtained for 3Al 2O 3·1SiO 2 mixed oxide composition. The porosity features are further established by BET adsorption isotherms and pore size distribution analysis. The temperature-programmed desorption studies showed more surface active sites for the silica-rich composition, suggesting enhanced catalytic potential. The TEM features of the mixed oxides showed a homogeneous distribution of alumina and silica phases with particle sizes in the nano-range. The low silica-containing mixed oxide showed a needle-like morphology with a high aspect ratio of 1:50 and ∼10 nm particle size while the silica-rich composition had particle size in a wide range (∼20–75 nm).

HAADF Imaging: An Effective Technique for the Study of NonhomogeneousNanostructures

Atomic number contrast (Z-contrast) imaging using high-angle annular dark field (HAADF) detector, along with high resolution electron microscopy (HREM), is used to study the nanostructured metal, semiconductor, mixed oxide, and soft matter composites of inhomogeneous nature. A comparison between the HREM and HAADF images for the analysis of crystal structure, defects, and compositional inhomogenity in those nanostructures has been made. While the HREM technique is efficient in determining bulk crystallinity and defect structures, the HAADF imaging technique is superior in determining the surface inhomogenity, defect structures in the interior of the nanostructures, even at atomic resolution. The efficiency of the HAADF imaging technique in determining the surface inhomogenity and defect structures is demonstrated for the Au-Pt bimetallic clusters, CdSe nanofibers and nanowires, Nb16W18O94 mixed oxide, and polystyrene-mormorillonite clay nanocomposites.

Relevance of the Co 1− xNi xWO 4 wolframite-type mixed oxide compositions on the synthesis and catalytic properties of W-based carbides

A series of wolframite-type oxides (Co 1− x Ni x WO 4) with various compositions was prepared by urea-matrix combustion method and subsequently carburized using a temperature-programmed reaction (1 °C min −1) under a mixture of 10 vol.% C 2H 6/H 2, from room temperature to 700 °C, to obtain a mixed Co, Ni and W carbide catalysts. The catalytic performance was evaluated in a continuous flow reactor using hydrodenitrogenation of pyridine as model reaction. The wolframite-type oxides and the carbide catalysts pre- and post-HDN reaction were characterized using elemental analysis, X-ray diffraction (XRD), laser Raman spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetric (DSC), transmission electron microscopy (TEM) and BET surface area measurements. Urea-matrix combustion method is a convenient tool to prepare highly pure wolframite-type oxides, whose composition affects strongly the W-based carbide phase distribution and the HDN catalytic behaviour. At Ni compositions lower than Co contents the formation of Co 3W 3C and β-W 2C carbides is favoured, whereas at Ni compositions greater than those of Co the main phases were Ni and α-WC. At intermediate composition (Co 0.5Ni 0.5WC x ) bimetallic and monometallic carbides were formed. The CoWC x bimetallic catalyst showed greater activity in the steady state than Ni-containing catalysts. The HDN active phase present in CoWC x is different than that present in the Ni-containing catalysts, that is, carbon–metal bond strength of the bimetallic carbide, for the former, and metal nickel or weak Ni C bond, for the latter, play a very important role in the catalytic process.

Unique Dielectric Behavior of 0.6Pb(Ni1/2W1/2)O3·0.4PbTiO3 Derived from Mechanical Activation

0.6Pb(Ni1/2W1/2)O3·0.4PbTiO3(0.6PNW·0.4PT) of complex perovskite structure is successfully synthesized by mechanical activation of mixed oxide composition, followed by sintering at 950°C. It exhibits a considerably stable temperature dependence of dielectric constant over the wide temperature range of −120° to 20°C, although there occurs a dielectric peak at around 74°C. Raman spectroscopic studies show the coexistence of tetragonal and pseudocubic perovskite phases on sintering at 950°C, which are attributed to the inhomogeneous distribution of PbTiO3 arising from mechanical activation. The dielectric behavior can be fine tuned by thermal annealing at 750°C, leading to phase redistribution in PNW‐PT.

The structure - activity relationship in catalytic behavior of ThMnVO oxides

With an objective to elucidate the influence of structural changes on catalytic properties, the oxygen content, reducibility and chemical stability of substituted metavanadates of thorium, viz., Th(MnxV1-xO3-)4 with 0 × 1, were monitored using temperature programmed reduction / oxidation (TPR/TPO), powder XRD and FTIR techniques in conjunction with thermogravimetry. Catalytic activity was evaluated for oxidation of CO as a function of Mn content. The formation of new phases ThMn2(VO4)2O2 and ThMn4O8 was established and indexed on the basis of tetragonal unit cell with a0 = 7.124 A ̊ and c0 = 6.348 A ̊ and the hexagonal unit cell with a0 = 6.468 A ̊ and c0 = 11.188 A ̊ , respectively. The mixed oxide compositions generally exhibited better stability towards the repeated cycles of reduction and reoxidation compared to MnO2 or un-substituted vanadates or manganates. The reduction of an individual component in a mixed oxide depended on the extent of substitution and the catalytic activity of the mixed oxides was found to be very sensitive to resultant structural changes and to the oxygen non-stoichiometry. The compositions with the oxygen rich VO4 groups in general showed poorer activity for CO oxidation reaction compared to those comprising of VO3 groups.

Mechanically Activating Nucleation and Growth of Complex Perovskites

Mechanical activation of the constituent oxides at room temperature led to the formation of nanocrystalline Pb-based relaxor ferroelectrics and piezoelectrics of complex perovskite structure. The activation-triggered occurrence of complex perovskites, such as Pb(Mg1/3Nb2/3)O3, 0.9[0.4Pb(Mg1/3Nb2/3)O3– 0.6Pb(Zn1/3Nb2/3)O3]–0.1PbTiO3, and Pb(Zr0.52Ti0.48)O3, in the mixed oxide compositions and in an amorphous precursor derived from coprecipitation proceeds via a process involving nucleation and subsequent growth of perovskite crystallites with increasing degree of mechanical activation. This is fundamentally different from the one or more interfacial reactions and diffusions responsible for a thermally activated solid state reaction occurring at an elevated temperature. Nucleation occurs in a highly activated state, where a degree of amorphization has taken place, and the subsequent growth proceeds as a result of the constant collisions and rearrangement of perovskite nuclei.

Effects of redox thermal treatments and feedstream composition on the activity of Ce/Zr mixed oxides for TWC applications

The activity of cerium/zirconium mixed oxides for TWC applications submitted to different thermal treatments under oxidising, reducing and redox-cycled atmospheres has been investigated. The effect of each individual reactant in the reaction system has been studied using feedstreams in which some components had been removed and substituted by nitrogen. The conclusions extracted from the individual reactions have been used to describe the role and effect of each reactant when all the components are present in the feedstream, and to explain the differences observed in the catalytic performance after different treatments. Finally, a Ce/Zr mixed oxide composition has been proposed as the best support for TWC applications.

Dielectric Properties of Radio Frequency Magnetron Sputter Deposited Zirconium Titanate‐Based Thin Films

We have studied the dielectric properties of thin film zirconia‐titania‐based mixed oxide compositions. Films of different Zr/Ti ratios were developed by radio frequency magnetron sputtering, and the effect of dopant addition to the composition were also investigated. We present a comparison of the electrical properties of zirconium titanate thin film capacitors in metal‐insulator‐semiconductor and metal‐insulator‐metal structures, and discuss the effect of process parameters on film properties. Stoichiometric (1:1 Zr:Ti), 320 Å zirconium titanate films on silicon exhibited an effective dielectric constant of 17 at 1 MHz. This is equivalent to an thickness of 73 Å. For 2500 Å thin zirconium titanate films on silicon, the dielectric constant was determined to be 37 at 1 MHz. On platinized silicon, the dielectric constant of 450 Å zirconium titanate films was 33. This corresponds to an equivalent thickness of 53 Å. Direct current resistivities improved by nearly two orders of magnitude by modifying the film composition with an addition of tin oxide. Tin modification also resulted in a significant improvement in the film dc breakdown characteristics, and we have achieved dielectric breakdown field greater than 7 MV/cm. Zirconium titantate‐based thin films display high resistivity and high dielectric constant, and are highly promising for ultra‐large scale integration applications. These films provide large capacitance in small area, and hence, higher charge storage capacity.

Nickel molybdate as precursor of HDS catalysts: Effect of phase composition

Sulfided αand β-NiMoO4 have been employed as model HDS catalysts, aiming to investigate the effect of phase composition of the precursor mixed oxide on the physicochemical characteristics and activity of the sulfides. Both sulfides are highly amorphous or microcrystalline, and could not be differentiated by means of XRD. The differences in BET area were also found to be minimal. However, it was found by means of TPR that the stable α-isomorph is reduced at lower temperatures than the unstable β-phase in both the oxidic and sulfided states. Sulfided β-NiMoO4 was found to be a better catalyst for the HDS of thiophene than the sulfided α-isomorph. This could be related to the higher stability of the former in H2, as decomposition of the active, amorphous Ni-Mo-S structures results in less active and more crystalline phases, as found by XRD.

Waste Minimization In Semiconductor Processing

AbstractThe US semiconductor industry uses 5–7 thousand pounds of arsine annually. Fifty to eighty percent of the arsine used becomes a waste product, which requires abatement. Traditional methods of abatement are reviewed with an emphasis on dry chemical scrubbing. A variety of dry chemical scrubbing materials were evaluated for arsine capacity, using activated carbon as the baseline for comparison. A proprietary mixed oxide composition, employing copper oxide as the active ingredient was identified as having high capacity and efficiency. Disposal and possible reclamation options are discussed.