Showing papers on "Mixed oxide published in 1992"

Formation of mixed oxide powders in flames: Part I. TiO2−SiO2

Abstract: Mixed oxide powders, e.g., Al2O3−TiO2, SiO2−GeO2, and TiO2−SiO2, are used in industry to produce ceramics, optical fibers, catalysts, and paint opacifiers. The properties of these products depend upon the morphology of the powders. Ceramics and optical fibers are produced using either a uniform mixture of multicomponent particles or a uniform solution. The desired morphology for catalysts is a high surface area and many active sites. TiO2 coated with a layer of SiO2 is the desired structure for use as a paint opacifier. In this paper, TiO2−SiO2 mixed oxide powders were synthesized using a counterflow diffusion flame burner. TiCl4 and SiCl4 were used as source materials for the formation of oxide particles in hydrogen-oxygen flames. In situ particle sizes were determined using dynamic light scattering. A thermophoretic sampling method also was used to collect particles directly onto carbon coated grids, and their size, morphology, and crystalline form examined using a transmission electron microscope. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The effect of temperature and of Si to Ti concentration ratio on particle morphology was investigated. Strong temperature dependence was observed. At high temperatures, TiO2 particles were covered with discrete SiO2 particles. At low temperatures, the structure changes to TiO2 particles encapsulated by SiO2. TEM diffraction pattern measurements showed that the TiO2 is rutile and the SiO2 is amorphous silica. At high Si to Ti ratios, SiO2-encapsulated TiO2 particles form. At low Si to Ti ratios, one obtains TiO2 particles covered with discrete SiO2 particles.

Mixed Oxide Capacitor of CuO—BaTiO3 as a New Type CO2 Gas Sensor

Abstract: An oxide capacitor consisting of BaTiO3 and an oxide is studied as a new type CO2 sensor based on capacitance change. Sensitivity to CO2, as well as the optimum operating temperature, was strongly dependent on the particular oxide mixed with BaTiO3. Among the elements investigated in this study, CuO–BaTiO3 exhibited the highest sensitivity to CO2. In particular, the CuO–BaTiO3 mixed oxide at the equimolar composition is highly sensitive to CO2. The optimum operating temperature and frequency for CuO–BaTiO3 are 729 K and 100 Hz, respectively, and the 80% response time to 2% CO2 is within 25 s. The equimolar mixture of CuO and BaTiO3 can measure the CO2 concentration from 100 to 60 000 ppm. Carbonation of oxide seems to play a key role for the detection of CO2 on these mixed oxide capacitors. The optimum operating temperature of these mixed oxide capacitors for CO2 detection, therefore, correlates with the decomposition temperature of the carbonate corresponding to the oxide mixed with BaTiO3. The capacitance increase of CuO–BaTiO3 upon exposure to CO2 seems to result from the elevated height of the potential barrier at the grain boundary between CuO and BaTiO3. Carbonation of CuO in the element seems to bring about the elevation in the height of the potential barrier.

Preparation and Properties of Clays Pillared with SiO2-TiO2 Sol Particles.

Abstract: The silicate layers of montmorillonite clay were pillared with SiO2–TiO2 sol particles. Although only the SiO2 sols were negatively charged, the SiO2–TiO2 mixed oxide sols were positively charged and could be ion-exchanged with the interlayer cations of montmorillonite. The resulting pillared clays had a basal spacing expanded to more than 40 A; the spacing was maintained during calcination up to 773 K. The adsorption properties of the calcined samples were studied for nitrogen as well as organic vapors. A structural model for the arrangement of the intercalated sol particles is proposed in which small SiO2–TiO2 sol particles are packed between the silicate layers. Micropores with a slit width of 10–12 A are formed in the interstices between the sol particles and the silicate layers.

Properties of Acid Sites on TiO2–SiO2 and TiO2–Al2O3 Mixed Oxides Measured by Infrared Spectroscopy

Abstract: Both Lewis and Bronsted acid sites were found on the surface of TiO2–SiO2 mixed oxide, thouth no Bronsted acid sites were detected on the surface of pure TiO2 and SiO2 While on the surface of TiO2–Al2O3 mixed oxide, no Bronsted acid sites were generated The rate of isomerization of 1-butene was found to be enhanced on the Bronsted acid sites formed on the TiO2–SiO2 mixed oxide

Process for the transformation of vanadium/phosphorus mixed oxide catalyst precursors into active catalysts for the production of maleic anhydride

Abstract: Vanadium/phosphorus mixed oxide catalysts comprising vanadyl pyrophosphate, optionally containing a promoter component, are transformed from vanadium/phosphorus mixed oxide catalyst precursors comprising vanadyl hydrogen phosphate, optionally containing a promoter component, by subjecting the catalyst precursors to elevated temperatures in three stages: (a) an initial heat-up stage in an atmosphere selected from the group consisting of air, steam, an inert gas, and mixtures thereof, (b) a rapid heat-up stage at a programmed heat-up rate in a molecular oxygen/steam-containing atmosphere, and (c) a maintenance/finishing stage, first in a molecular oxygen/steam-containing atmosphere, and thereafter in a nonoxidizing, steam-containing atmosphere. Such catalysts are useful for the production of maleic anhydride via the partial oxidation of nonaromatic hydrocarbons, particularly n-butane, in the vapor phase with molecular oxygen or a molecular oxygen-containing gas.

Formation of mixed oxide powders in flames: Part II. SiO 2 −GeO 2 and Al 2 O 3 −TiO 2

Abstract: SiO2−GeO2 and Al2O3−TiO2 mixed oxide powders were synthesized using a counterflow diffusion flame burner. SiCl4, GeCl4, Al(CH3)3, and TiCl4 were used as source materials for the formation of oxide particles in hydrogen-oxygen flames. In situ particle sizes were determined using dynamic light-scattering. Powders were collected using two different methods, a thermophoretic method (particles are collected onto carbon coated TEM grids) and an electrophoretic method (particles are collected onto stainless steel strips). Their size, morphology, and crystalline form were examined using a transmission electron microscope and an x-ray diffractometer. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The formation of the mixed oxides was investigated using Si to Ge and Al to Ti ratios of 3:5 and 1:1, respectively. Heterogeneous nucleation of the SiO2 on the surface of the GeO2 was observed. In Al2O3−TiO2 mixtures, both oxide particles form at the same temperature. X-ray diffraction analysis of particles sampled at temperatures higher than 1553 K showed the presence of rutile, γ–Al2O3, and aluminum titanate. Although the particle formation process for SiO2−GeO2 is very different from that for Al2O3−TiO2, both mixed oxides result in very uniform mixtures.

Characterization of dispersion and surface states of NiO/γ-alumina and NiO/La2O3–γ-alumina catalysts

Abstract: A series of NiO/γ-alumina and NiO/La2O3–γ-alumina catalysts prepared by impregnation have been investigated by X-ray diffraction, diffuse reflectance spectroscopy and temperature-programmed reduction. For the NiO/γ-alumina catalysts with low nickel oxide content, a strong interaction between NiO and γ-alumina leads to the diffusion of Ni2+ ions into vacant surface sites of the support to form surface spinel NiAl2O4 and thus the reducibility of NiO was decreased. At high nickel oxide contents, NiO crystal appears after the surface vacancies have been filled. In this case, the counter-diffusion of Ni2+ and Al3+ ions at the interface was observed during calcination.For NiO/La2O3–γ-alumina catalysts with low La2O3 content, Ni2+ ions tend to interact with γ-alumina. The addition of the La2O3 has little effect on the state of the NiO species interacting directly with γ-alumina. For the catalysts with high La2O3 content, the dispersion of NiO was enhanced by the existence of La2O3, which can be ascribed to the formation of an La–Ni mixed oxide (or microcrystallites of NiO separated by La2O3). Strong interactions between components, such as NiO and Al2O3, and La2O3 and Al2O3, are influenced dramatically by the calcination temperature. As the temperature rises, the increasing interaction leads to the formation of bulk LaAlO3 and NiAl2O4. However, the calcination temperature has little effect on the interaction between NiO and La2O3.

Characterization of vanadium oxide catalysts supported on titania-zirconia by solid-state vanadium-51 and proton NMR spectroscopy

Abstract: A series of TiO{sub 2}-ZrO{sub 2} mixed oxide supported vanadia catalysts with various V{sub 2}O{sub 5} loadings ranging from 1 to 16 wt % were prepared by a wet impregnation method and were characterized by means of solid-state {sup 51}V and {sup 1}H NMR spectroscopic techniques. The solid-state {sup 51}V NMR spectra of V{sub 2}O{sub 5}/TiO{sub 2}-ZrO{sub 2} catalysts reveal the existence of two types of dispersed surface vanadium oxide complexes in a tetrahedral oxygen environment at lower vanadium loadings adn a third three-dimensional crystalline V{sub 2}O{sub 5} in distorted octahedral environment at higher vanadium contents. The proton NMR results provide evidence for the existence of metal oxide support interaction through the support hydroxyl groups. 33 regs., 4 figs.

Hydrogenation and Hydrogenolysis of Hydrocarbons on Perovskite Oxides

Abstract: A mixed oxide is one of a group of potentially important catalysts in heterogeneous catalysis, because different catalytic behavior would be expected to emerge with a combination of their components. For the fundamental research on the catalytic properties of the mixed oxides, a perovskite-type oxide with a chemical formula of ABO, has been used as a suitable catalyst because of their well-defined and stable structure. For some perovskite-type oxides, their electronic structures have been pointed out to be similar to those of transition metals on the basis of theoretical calculations of valence band structures (1–4). Thus, one of the catalytic features of perovskite-type oxides is to be active for hydrogen-involving reactions such as the equilibration between hydrogen and deuterium, the hydrogenation and isomerization of olefins, and in particular the hydrogenolysis of hydrocarbons (5–11).

XPS investigation on Co ? Cu mixed oxide catalysts

Abstract: Metastable and isomorphous cobalt-copper hydroxysalts with Co:Cu = 100:0, 70:30 and 50:50 atomic ratios have been prepared as precursors by coprecipitation at constant pH from solutions of cobalt and copper nitrates added to a solution of NaHCO3. Mixed oxides were obtained by decomposition of the precursors at 723 K for 24 h in air and in N2. The surface of both precursors and mixed oxides was investigated by XPS. Only Co2+ and Cu2+ species are revealed for the precursors. The mixed oxides present Co2+, Co3+, Cu+ and Cu2+ species depending on the composition and on the calcination atmosphere. Quantitative analysis has evidenced a cobalt enrichment at the surface for the Co:Cu = 70:30 and 50:50 precursors. With respect to mixed oxides, the samples calcined in N2 are characterized by a homogeneous distribution of cobalt and copper both in the bulk and on the surface; in contrast, for the corresponding samples calcined in air the original cobalt enrichment of the precursors is preserved.

Preparation of Perovskite-Type Oxide, LaCrO3, by Thermal Decomposition of La[Cr(CH2(COO)2)3]⋅6H2O

Abstract: The thermal decomposition of Lanthanum (III) tris (malonato) chromate (III) hexahydrate (La[Cr(CH2(COO)2)3]⋅6H2O) was studied from room temperature to 1000°C in order to investigate the formation process of perovskite-type oxides. The starting material and the decomposition products in plateau ranges of 150°-250°C, 400°-470°C, 560°-680°C and 720°C- were characterized by IR and X-ray diffraction analyses. The complex compound released almost all water molecules at about 200°C. Then, the anhydrated complex began to decompose at about 250°C, and the mixed oxide (LaCrO4) and perovskite-type oxide (LaCrO3) formed between 560° and 680°C and above 720°C, respectively. The specific surface area and particle size of the oxide (LaCrO3) calcined at 750°C were 2.4m2⋅g-1 and 0.2-0.5μm, respectively.

The relationship between mass transport and oxide chemistry in oxidation of Ni 3 Al alloys

Abstract: Isotope diffusion and surface analysis has been used to investigate mass transport processes and oxide chemistry during oxidation of Ni3Al alloys in oxygen between 450 and 800 °C. In all cases, the oxide has an outer layer rich in NiO that grows by outward metal diffusion. Below 650 °C, an inner mixed oxide layer allows fast inward oxygen diffusion and growth. A porous inner layer of alumina forms at 700 °C that also allows oxide growth by oxygen in-diffusion. At 800 °C, the alumina becomes protective and halts the inner oxide growth mechanism. When macroalloyed with chromium, oxide chemistry is similar, but with an additional chromium oxide in the outer layer. There is an increased oxidation protection at temperatures ⋚600 °C that results from the inhibition of oxygen in-diffusion as an oxidation process. Above 700 °C, however, protective alumina formation is retarded and faster outward metal diffusion enhances the oxide growth rate when compared to Ni3Al.

Selective formation of thin film iron garnet by complexing agent-assisted sol-gel processing

Abstract: Mixed oxides with Fe/Y ratio of 5/3 were prepared by a sol-gel method using organic polydentate ligands. The effect of the ligands, glycerine and five diols, on the structure of the metal oxides was investigated. When the ligands were long chain diols, 1,2-decanediol and 1,2-dodecanediol, garnet structure tended to appear as the main crystalline phase in the mixed oxide with calcination at 700°C. This tendency was present in films. It was shown that the transparent magnetic films can be prepared on mica and quartz substrates.

Metal–promoter interactions in vanadium oxide promoted Rh/SiO2 catalysts

Abstract: The influence of the calcination temperature on the structure and morphology of vanadium oxide promoted, silica-supported rhodium catalysts was studied. Pure Rh/SiO2 and VOx/SiO2 served as reference systems. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), FTIR spectroscopy of adsorbed CO and BET surface-area measurements. BET, XRD and TEM investigations revealed a strong sintering of the silica support at calcination temperatures at and above 973 K. There is a crystalline silica phase, which was found exclusively in the promoted Rh/SiO2 catalyst calcined at 1173 K. XRD, TPR and TEM gave evidence for the formation of rhodium–Vanadium mixed oxide on the catalyst surface at 973 K calcination. After high-temperature reduction (HTR = 773 K) highly dispersed metal is formed from this ternary oxide phase. A C- and O-bonded carbonyl species probably located at the metal/promoter interface was detected by FTIR spectroscopy at 83 K.

Identification and vibrational properties of the mixed oxide (1 - x)V2O5+xMoO3 (x ≤ 0.3)

Abstract: The mixed oxide (1-x)V2O5+xMoO3 (x ≤ 0.3) has been synthesized by melting the oxides, and its structures and vibrational properties are investigated by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy, Fourier transform infrared (IR) spectroscopy and Raman scattering. XRD revealed that the solid solutions of V2O5 with MoO3 are well formed except that the second phase V9Mo6O40 appeared at x ≥ 0.1. It turned out that the unit-cell dimension c of the solid solutions (orthorhombic) initially decreases whereas a and b increase with increasing x, and that all the lattice parameters tend to remain unchanged above x 0.1. The IR band due to the V-O stretching vibration at 1028 cm-1 shifted downwards, diffused and decreased in intensity with increasing x; similar changes are observed in the Raman spectra of the mixed oxide. It is shown that the V valence transition from V5+ to V4+ occurs as V is replaced by Mo. The V4+ to V5+ ratios obtained by deconvoluting the V 2p3/2 line showed some qualitative agreement with the prediction for x < 0.1 based on no Mo valence transition and the V5+ to V4+ transition as V is replaced by Mo. It is concluded that the V valence transition is responsible for the vibrational property changes of the mixed oxide.

Vanadium mixed oxide catalysts for the oxidative coupling of methane

Abstract: The improvement of methane conversion and C2+ selectivity for the oxidative coupling of methane has been pursued by evaluating a series of new polyfunctional oxide catalysts. The catalysts included: (i) a transition metal oxide, (ii) an alkaline earth metal oxide (CaO or MgO), and (iii) an inert oxide support (Al2O3, SiO2, TiO2, or ZrO2) to stabilize the active phases. Vanadium oxide was chosen from the set of transition metal oxides having a standard free energy change lower than zero for the reaction methane+metal oxide→1/2 ethene+water+reduced metal oxide. The molar composition of the catalysts was the following: 68Mg(Ca)/2V/30Al(Ti, Si, Zr). Several catalysts were tested before and after alkali impregnation (lithium with MgO and sodium with CaO). Impregnation of alkali caused an increase in the methane conversion in the CaO-based catalysts. The catalysts were examined by electron spin resonance and UV-visible spectroscopy to find relationships between vanadium species and catalytic activity. The spectroscopic analyses did not show a correlation between VIV species and catalytic activity, whereas the (V = O)3+ species seems to play an important role in the reaction.

Electrode layer for a fuel cell and method for producing this electrode layer

Abstract: A fuel cell air electrode with good adhesion can be applied to solid electrolytes or ceramic support plates. To produce the electrode layer, use is made of a mixed oxide of the type La1-xAxBO3 (A = Mg, Ca, Sr, Ba; B = Fe, Co, Ni, Mn, Cr; x = 0.1 to 0.2) to which 1 to 10% by weight in each case of aluminium oxide, bismuth oxide or chromium oxide or a combination of said oxides or a combination of said oxides with further oxides are added.

Limits of the Thermodynamic Stability of Cobalt—Iron—Manganese Mixed Oxides at 1200°C

Abstract: The thermodynamic stability of spinel and rock-salt structure phases has been investigated for the quasi-ternary mixed oxide system Co-Fe-Mn-O at 1200°C and at total pressures of the order of 1 atm (∼1.01 × 105 Pa) using thermogravimetric and electrical conductivity measurements. The results reveal the stability limits of the spinel and rock-salt structure phases with varying cationic composition and oxygen partial pressure at 1200°C. The oxygen partial pressure was varied over 12 orders of magnitude, from log10ao2= 0 to log10ao2= -12, by using CO/CO2 and N2/O2 gas mixtures and monitored by an electrochemical, stabilized zirconia-based EMF cell. The maximum cobalt mole fraction used in the investigation was about 0.33.

Investigations on counter electrode materials for solid state electrochromic systems

Abstract: Investigations have been carried out on mixed oxide systems (WO3)x(Li2O)y(MO)z, with M = Fe, Mn, V, Ce, Sb or Nb and 0.3 and 0.26 . Cyclic voltammetric measurements in a 1 N LiClO4- propylene carbonate electrolyte with an AgAgCl-reference electrode were used to study the insertion of Li+ ions into, and the extraction of the inserted ions from, the proposed compounds. However, the main problem is still the insufficient transparency of these materials. Problems concerning preparation of the thin film oxides are also noted.

Chromia-chrominum phosphates as solid acid catalysts

Abstract: A series of chromia-chrominum phosphate catalysts with various P/Cr ratios were prepared with the coprecipitation method. The surface acidic properties of these catalysts were examined by temperature-programmed desorption (TPD) of ammonia and the dehydration reaction of isopropanol. The dehydration reaction was carried out in a continuous-flow microreactor at 220 °C under atmospheric pressure. The results of TPD of ammonia indicated that the surface acidity of chromia-chrominum phosphate is medium strong. The acidic strengths are approximately the same for all the samples. However, the acid concentration is increased with increasing the P/Cr ratio of the catalyst. The dehydration activity of the catalyst is also increased with increasing the P/Cr ratio of the sample. The results indicated that the addition of phosphorus on chromia, even in a small amount, could significantly enhance the acidities of the catalysts. In addition, the acid amount of mixed oxide is decreased in the order: iron oxide-iron phosphate > chromia-chrominum phosphate > alumina-aluminum phosphate.