Pre-Eutectic Densification of Calcium Carbonate Doped with Lithium Carbonate

Pressureless sintering of CaCO₃ was carried out, with Li₂CO₃ (from 0.5 to 8 wt%) as an additive, under different pressures of CO₂. Densification occurs between 600 and 700°C. Sintering above the eutectic temperature (T>662°C) leads to the decomposition of calcium carbonate and the materials become expanded. At 620° under 1 kPa of CO₂, a relative density of 96% is reached. Li₂CO₃ enhances the densification process and grain growth of calcium carbonate. CO₂ pressure slows down densification and grain growth kinetics. These results are explained by the influence of carbonate and calcium ion vacancies on the sintering mechanisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low temperature complete combustion of dilute propane over Mn-doped ZrO2 (cubic) catalysts

Combustion of dilute propane (0.9 mol%) over Mn-doped ZrO₂ catalysts prepared using different precipitating agents (viz. TMAOH, TEAOH, TPAOH, TBAOH and NH₄OH), having different Mn/Zr ratios (0.05—0.67) and calcined at different temperatures (500—800°C), has been thoroughly investigated at different temperatures (300—500°C) and space velocities (25,000–100,000 cm³ g⁻¹ h⁻¹) for controlling propane emissions from LPG-fuelled vehicles. Mn-doped ZrO₂ catalyst shows high propane combustion activity, particularly when its ZrO₂ is in the cubic form, when its Mn/Zr ratio is close to 0.2 and when it is prepared using TMAOH as a precipitating agent and calcined at 500—600°C. Pulse reaction of propane in the absence of free-O₂ over Mn-doped ZrO₂ (cubic) and Mn-impregnated ZrO₂ (monoclinic) catalysts has also been investigated for studying the relative reactivity and mobility of the lattice oxygen of the two catalysts. Both reactivity and mobility of the lattice oxygen of Mn-doped ZrO₂ are found to be much higher than that of Mnimpregnated ZrO₂. Propane combustion over Mn-doped ZrO₂ catalyst involves a redox mechanism     

Preparation of ZrO2-TiO2-CeO2 and Its Application in the Selective Catalytic Reduction of NO with NH3

TiO₂,ZrO₂-TiO₂,andZrO₂-TiO₂-CeO₂ were prepared by co-precipitation method and characterized by X-ray diffraction (XRD), specific surface area measurements (BET), temperature programmed desorption (NH₃-TPD), oxygen storage capacity (OSC), and temperature programmed reduction (H₂-TPR). The results showed that ZrO₂-TiO₂-CeO₂ exhibited large number of surface strong acid, possessed some oxygen storage capacity, and strong redox property. The three materials were used as supports and the monolith catalysts were prepared with 1% (w) V₂O₅ and 9% (w)WO₃ for selective catalytic reduction (SCR) of NO with ammonia in the presence of excessive O₂, and the results of catalytic activity showed that the catalyst used ZrO₂-TiO₂-CeO₂ as support yielded nearly 100% NO conversion at 275 °C at a gas hourly space velocity (GHSV) of 10000 h⁻¹, and it had the best catalytic activity and showed great potential for practical application.     

Preparation,Characterization, and Catalytic Behavior of Nanostructured Mesoporous CuO/ZrO2 Catalysts for Low-Temperature CO Oxidation

High-surface area mesoporous 20 mol% CuO/ZrO₂ catalyst was prepared by a surfactant-assisted method of nanocrystalline particle assembly, and characterized by x-ray powder diffraction (XRD), N₂ adsorption, transmission electron microscopy (TEM), H₂-TPR, TG-DTA, and x-ray photoelectron spectra (XPS) techniques. The catalytic properties of the CuO/ZrO₂ nanocatalysts calcined at different temperature were evaluated by low-temperature carbon monoxide oxidation using a CATLAB system. The results showed that these mesoporous nanostructured CuO/ZrO₂ catalysts were very active for low-temperature CO oxidation and the CuO/ZrO₂ catalyst calcined at 400°C exhibited the highest catalytic activity.     

The state of metals in the supported bimetallic Pt−Pd/SO4/ZrO2 system

The behavior of the metals in the Pt−Pd/ZrO₂ and Pt−Pd/SO₄/ZrO₂ systems was studied by DRIFT spectroscopy. After reduction of Pt−Pd/ZrO₂ at 100 °C, the states of the metals are mainly Pt⁰ and Pd⁰ with a minor admixture of positively charged forms of Pt⁺ or Pd²⁺. An increase in the temperature of reduction leads to the formation of a bimetallic alloy. In the Pt−Pd/SO₄/ZrO₂ system, the effects of alloy formation and the interaction of the surface SO₄ groups superimpose. At low reduction temperatures, the surface SO₄ groups interact mainly with palladium. The influence of the surface sites on both supported metals increases with increasing reduction temperature. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1265–1270, July, 1999.     

Synthesis and characterization of submicron size particles of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> by microemulsion route

Among the various positive electrode materials investigated for Li-ion batteries, spinel LiMn₂O₄ is one of the most important materials. Small particles of the active materials facilitate high-rate capability due to large surface to mass ratio and small diffusion path length. The present work involves the synthesis of submicron size particles of LiMn₂O₄ in a quaternary microemulsion medium. The precursor obtained from the reaction is heated at different temperatures in the range from 400 to 900 °C. The samples heated at 800 and 900 °C are found to possess pure spinel phase with particle size <200 nm, as evidenced from XRD, SEM, and TEM studies. The electrochemical characterization studies provide discharge capacity values of about 100 mAh g⁻¹ at C/5 rate, and there is a moderate decrease in capacity by increasing the rate of charge–discharge cycling. Studies also include charge–discharge cycling and ac impedance studies in temperature range from −10 to 40 °C. Impedance data are analyzed with the help of an equivalent circuit and a nonlinear least squares fitting program. From temperature dependence of charge-transfer resistance, a value of 0.62 eV is obtained for the activation energy of Mn³⁺/Mn⁴⁺ redox process, which accompanies the intercalation/deintercalation of the Li⁺ ion in LiMn₂O₄.     

Short Range Order Evolution in the Preparation of SnO2 Based Materials

The evolution of Eu³⁺ doped SnO₂ xerogels to the cassiterite structure observed during sintering was studied by means of Eu³⁺ spectroscopy, XRD and EXAFS at the Sn K-edge. Eu³⁺ ions adsorbed at the surface of colloidal particles present a broad distribution of sites, typical of oxide glasses. With sintering at 300°C, this distribution is still broadened. Crystallization is clearly observed by the three techniques with increasing sintering temperature. It is found that the addition of Eu³⁺ limits the crystallite growth.     

Silica-Zirconia Sol–Gel Coatings Obtained by Different Synthesis Routes

Homogeneous xSiO₂-(1−x)ZrO₂ coatings have been prepared onto glass-slides, monocrystalline Si and stainless steel (AISI 304) using sols prepared via acid and basic catalysis. Zirconium tetrabutoxide (TBOZr), zirconium n-propoxide (TPZ), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) were used as precursors of zirconia and silica, respectively. The different parameters involved in the synthesis procedure, as molar ratios H₂O/alkoxides, NaOH/alkoxides, and sintering temperature have been analysed, correlating the stability and rheological properties of the sols. The evolution and structure of the sols and coatings have been studied by FTIR. Coatings have been prepared by dipping from acid and basic sols. Electrophoretic Deposition (EPD) technique has also been used to prepare coatings onto stainless steel from basic particulate sols in order to increase the critical thickness. A maximum thickness of 0.5 μ m was reached by both dipping and EPD process for 75SiO₂: 25 ZrO₂ composition. The critical thickness decreases with ZrO₂ amount depending strongly of the drying conditions. Si–O–Zr bonds have been identified by FTIR, indicating the existence of mixed network Si–O–Zr in the coatings obtained by the different routes. Crystallisation of ZrO₂(t) was only observed at high sintering temperature (900^∘C) by FTIR and confirmed by DRX.     

Planar ZrO2 Waveguides Prepared by the Sol-Gel Process: Structural and Optical Properties

ZrO₂ waveguides are prepared by the sol-gel process from a solution containing zirconiumn-propoxide and acetylacetone in propanol-2. Structural characterizations are investigated for different annealing temperatures using suitable techniques including Waveguide Raman Spectroscopy, Electron Microscopy and Atomic Force Microscopy. Films are amorphous at 300°C and the pure ZrO₂ tetragonal crystalline phase appears beyond 400°C. Crystallized films present a dense, uniform and polycrystalline structure made up by randomly oriented nanocrystallites, the diameter of which increases from 38 Å at 400°C to 53 Å at 600°C. Waveguides are at least monomode TE₀ at 632.8 nm. At this wavelength, optical losses are about, 0.8±0.2dB/cm for amorphous layers and increase up to 2.5±0.4 dB/cm for 600°C heat-treated waveguides.     

Influence of composition on corroding process of Na<Subscript>2</Subscript>O-K<Subscript>2</Subscript>O-CaO-ZrO<Subscript>2</Subscript>-SiO<Subscript>2</Subscript> glasses

The corroding process of six glasses of the Na₂O-K₂O-CaO-ZrO₂-SiO₂ system with ZrO₂content 0–2.13 mass % by water was observed during static tests at 121°C and pressure of 0.25 MPa in steam sterilizer. Significant increase of Na⁺ and K⁺ content in leachates was observed after the addition of ZrO₂ into glass. Further increase of the content of ZrO₂ in glasses slowed down the rate of Na⁺ and K⁺ leaching. The leaching process of SiO₂ as well as Na⁺, K⁺, and Ca²⁺ ions was evaluated on the basis of comparison with model leaching processes. Variation of the concentrations of Na⁺, K⁺, Ca²⁺, and SiO₂ in leachates with time was described by empirical equation. Observed changes in the initial leaching rates of Na⁺, K⁺, Ca²⁺, and SiO₂ can be ascribed to the content of ZrO₂ in glasses. The presence of ZrO₂ in glasses reduced the overall rate of glass dissolution.     

Thermo-Raman Studies on Dehydration of Na4P2O7⋅10H2O and Phase Transformations of Na4P2O7

In this work, dehydration of sodium diphosphate decahydrate Na₄P₂O₇⋅10H₂O and phase transformations of Na₄P₂O₇ in open air have been studied in detail by thermo-Raman spectroscopy. The spectra were measured continuously in a temperature range from room temperature up to 600°C for the bands of P₂O₇ ^4- and H₂O. The spectral variation showed one step of dehydration and four-phase transformations. The thermo-Raman intensity(TRI) and differential thermo-Raman intensity (DTRI) curves calculated from the characteristic bands of H₂O also showed one step of dehydration with the loss of all hydrated water in the temperature interval from 45 to 69°C. Thermogravimetric measurements supported this result. The thermo-Raman investigation indicated the transformation of Na₄P₂O₇ from low temperature phase to high temperature phase proceed through pre-transitional region from 75 to 410°C before the major orientational disorder at 418°C and minor structural modifications at 511,540 and 560°C. The results from differential scanning calorimetry and differential thermal analysis on Na₄P₂O₇ showed endotherms at 407,517, 523, 548, 557°C and 426, 528, 534, 555, 565°C, respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sol–gel Al2O3/ZrO2 duplex films for protection of a γ-TiAl based alloy against high temperature oxidation

Al₂O₃/ZrO₂ duplex films were deposited on a γ-TiAl based alloy by sol–gel processing starting from aluminum isopropoxide (Al(OC₃H₇)₃) and zirconium (IV) oxychloride octahydrate (ZrOCl₂ · 8H₂O) as raw materials. Isothermal oxidation at 900 and 1,000 °C in 0.1 MPa O₂ and cyclic oxidation at 1,000 °C in air of the coated and uncoated specimens were performed to investigate the effect of the duplex films on the oxidation behavior of the γ-TiAl alloy. The results of the isothermal oxidation tests indicated that the parabolic rate constants of the alloy were decreased due to the applied thin film. Additionally, the present film exhibited a beneficial effect on the cyclic oxidation resistance of the alloy in air. The duplex film could restrain the growth of TiO₂, causing an increase of the Al₂O₃ content in the oxide mixture and thus decreased the oxidation rate.     

Sol-coated preparation and characterization of macroporous α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> membrane support

A simple approach namely sol-coated technique has been developed for the low cost fabrication of macroporous ceramic under a far below common sintering temperature of alumina with large dimension grains. The prepared green support shows higher sinteractive than the one treated by wet impregnation method under the same sintering conditions. The support possesses great potential applications with 6.63–7.71 μm in pore size, 39% open porosity as well as >45 MPa mechanical strength at the sintering temperature range of 1350°– 1500°C. The results indicate that the nitrogen gas flux and pure water permeation value was 51 252.35 m³ m⁻² h⁻¹ bar⁻¹, 98.43 m³ m⁻² h⁻¹ bar⁻¹, respectively, which were more dependent on the pore structure and pore size distribution than open porosity.     

Synthesis of high surface area nanocrystalline anatase-TiO2 powders derived from particulate sol-gel route by tailoring processing parameters

Stabilised titania sols were prepared using an additive free particulate sol-gel route, via electrostatic stabilisation mechanism, with various processing parameters. Peptisation temperature, 50°C and 70°C, and TiO₂ concentration, 0.1, 0.2 and 0.4 molar, were chosen as processing parameters during sol preparation. Results from TiO₂ particle size and zeta potential of sols revealed that the smallest titania hydrodynamic diameter (13 nm) and the highest zeta potential (47.7 mV) were obtained for the sol produced at the lower peptisation temperature of 50°C and lower TiO₂ concentration of 0.1 M. On the other hand, between the sols prepared at 70°C, smaller titania particles (20 nm) and higher zeta potential (46.3 mV) were achieved with increasing TiO₂ concentration up to 0.4 M. X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) results of produced powders annealed at different temperatures showed that the 300°C annealed powder made from 0.1 M sol prepared at 50°C was a mixture of anatase and brookite, corresponding to a major phase of anatase (∼95% estimated), with the smallest average crystallite size of 1.3 nm and the highest specific surface area (SSA) of 193 m²/g. Furthermore, increasing TiO₂ concentration up to 0.4 molar for the sols prepared at 70°C resulted in decreasing the average crystallite size (1.9 nm at 300°C) and increasing SSA (116 m²/g at 300°C) of the powders annealed at different temperatures. Anatase-to-rutile phase transformation temperature was increased with decreasing peptisation temperature down to 50°C, whereas TiO₂ concentration had no effect on this transition. Anatase percentage increased with decreasing both peptisation temperature and TiO₂ concentration. Such prepared powders can be used in many applications in areas from photo catalysts to gas sensors.     

Synthesis and thermal decomposition of potassium peroxotitanate to K2Ti2O5

Potassium peroxotitanate was synthesized by the peroxo method. During the thermal decomposition K₂Ti₂O₅ can be obtained. The isothermal conditions for decomposition of K₂[Ti₂(O₂)₂(OH)₆]·3H₂O were determined on the base of DTA, TG and DSC results. DTA and TG curves were recorded in the temperature range 20 and 900°C at a heating rate of 10°C min^–1. The obtained intermediate compounds were characterized by means of quantitative analysis and IR spectroscopy. The mechanism of thermal decomposition of K₂[Ti₂(O₂)₂(OH)₆]·3H₂O to K₂Ti₂O₅ was studied. The optimal conditions for obtaining K₂Ti₂O₅ were determined (770°C for 10 h).     

Investigation of Solid-Solid Interactions between Pure and Li2O-Doped Magnesium and Ferric Oxides

The results obtained showed that the addition of small amounts of LiNO₃ to the reacting mixed solids, consisting of equimolar proportion of Fe₂O₃ and basic MgCO₃ much enhanced the thermal decomposition of magnesium carbonate. The addition of 12 mol% LiNO₃ (6 mol% Li₂O) decreased the decomposition temperature of MgCO₃ from 525.5 to362°C. MgO underwent solid–solid interaction with Fe2O3 at temperatures starting from800°C yielding MgFe₂O₄. The amount of ferrite produced increased by increasing the precalcination temperature of the mixed solids. However, the completion of this reaction required prolonged heating at elevated temperature above 1100°C. Doping with Li₂O much enhanced the solid–solid interaction between the mixed oxides leading to the formation of MgFe₂O₄ phase at temperatures starting from 700°C. The addition of 6 mol% Li₂O to the mixed solids followed by precalcination at 1050°C for 4 h resulted in complete conversion of the reacting oxides into magnesium ferrite. The heat treatment of pure and doped solids at 900–1050°C effected the disappearance of most of IR transmission bands of the free oxides with subsequent appearance of new bands characteristic for MgFe₂O₄ structure. The promotion effect of Li2O towards the ferrite formation was attributed to an effective increase in the mobility of the various reacting cations. The activation energy of formation (ΔE) of magnesium ferrite was determined for pure and variously doped solids and the values obtained were 203, 126, 95 and 61 kJ mol⁻¹ for pure mixed solids and those treated with 1.5, 3.0 and 6.0 mol% Li₂O, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Structural characterization and photocatalytic activity of hollow binary ZrO2/TiO2 oxide fibers

The formation of hollow binary ZrO₂/TiO₂ oxide fibers using mixed precursor solutions was achieved by activated carbon fibers templating technique combined with solvothermal process. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ adsorption, X-ray photoelectron spectroscopy (XPS), UV-vis, and infrared (IR) spectroscopy. The binary oxide system shows the anatase-type TiO₂ and tetragonal phase of ZrO₂, and the introduction of ZrO₂ notably inhibits the growth of TiO₂ nanocrystallites. Although calcined at 575 °C, all hollow ZrO₂/TiO₂ fibers exhibit higher surface areas (>113 m²/g) than pure TiO₂ hollow fibers. The Pyridine adsorption on ZrO₂/TiO₂ sample indicates the presence of stronger surface acid sites. Such properties bring about that the binary oxide system possesses higher efficiency and durable activity stability for photodegradation of gaseous ethylene and trichloromethane than P25 TiO₂. In addition, the macroscopic felt form for the resulting materials is more beneficial for practical applications than traditional catalysts forms.     

Synthesis of high surface area nanometer magnesia by solid-state chemical reaction

Nanometer MgO samples with high surface area, small crystal size and mesoporous texture were synthesized by thermal decomposition of MgC₂O₄ · 2H₂O prepared from solid-state chemical reaction between H₂C₂O₄ · 2H₂O and Mg (CH₃COO)₂ · 4H₂O. Steam produced during the decomposition process accelerated the sintering of MgO, and MgO with surface area as high as 412 m² · g⁻¹ was obtained through calcining its precursor in flowing dry nitrogen at 520°C for 4 h. The samples were characterized by X-ray diffraction, N₂ adsorption, transmission electron microscopy, thermogravimetry, and differential thermal analysis. The as-prepared MgO was composed of nanocrystals with a size of about 4–5 nm and formed a wormhole-like porous structure. The MgO also had good thermal stability, and its surface areas remained at 357 and 153 m²·g⁻¹ after calcination at 600 and 800°C for 2 h, respectively. Compared with the MgO sample prepared by the precipitation method, MgO prepared by solid-state chemical reaction has uniform pore size distribution, surface area, and crystal size. The solid-state chemical method has the advantages of low cost, low pollution, and high yield, therefore it appears to be a promising method in the industrial manufacture of nanometer MgO. Translated from Chinese Journal of Catalysis, 2006, 27(9): 793–798 (in Chinese)     

A New Type of Photocatalysis Initiated by Photoexcitation of Adsorbed Carbon Dioxide on ZrO2

ZrO₂ has been found to be an effective photocatalyst for reduction of CO₂ by hydrogen or methane at room temperature. The effective photon energy is less than the band gap energy of ZrO₂ (5.0 eV), indicating that photoexcitation of bulk ZrO₂ is not involved. The reaction is initiated by photoexcitation of surface carbonates derived from adsorption of CO₂ to convert it to a CO₂ ^– radical, which in turn reacts with hydrogen or methane to form surface formate. The formate is stable at temperatures below 573 K, but works as a reductant of CO₂ under photoirradiation. A new type of reaction mechanism is proposed.     

Preparation of nanostructured porous SiO2-Al2O3 oxycarbonitride materials obtained by a new chemical precursor method

Nanostructured hybrid materials containing Al₂O₃ were synthesized via a sol-gel method through hydrolysis and co-condensation reactions using trimethylsilyl isocyanate (TMSI) as a new silica source in the presence of tetramethoxysilane (TMOS) and three different quantities (10, 20 and 30 wt.%) of aluminum sec-butoxide (Al(OBu^sec)₃ as a modifying agent. The xerogel nanostructured materials are pyrolyzed in nitrogen atmosphere in the temperature range from 400°C to 1100°C. The transformation of the xerogel hybrid networks into Al-Si oxycarbonitride materials has been investigated by XRD, FTIR, SEM, AFM, and ²⁹Si MAS-NMR. To the best of our knowledge, the work reported here is the first synthesis of porous di-urethanesils modified with aluminum and one of the few examples of alumosilica oxycarbonitride materials