Sol–Gel Derived Rh/TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> Nanocomposite Membranes

A porous borosilicate substrate has been coated with base catalysed SiO₂ sol–gel nanoparticles. Onto these were deposited the vapour of titanium isopropoxide, where it reacted with the surface OH groups to give a TiO₂-overcoat. This nanocomposite sol–gel derived TiO₂/SiO₂ membrane was then doped with 1%Rh giving Rh/TiO₂/SiO₂ membranes. These are shown to be coherent and crack-free, to have good permeability and activity in the isomerisation of butanes. It appears that sol–gel chemistry will allow such membranes to be totally engineered at a nm level.     

Synthesis of ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript>–Cu(II) xerogels by joint hydrolysis in an aqueous ammonia atmosphere

ZrO₂–SiO₂ xerogels have been synthesized through hydrolysis of a mixture of tetrabutoxyzirconium and tetraethoxysilane in a desiccator in a vapor of a 15% aqueous NH₃ atmosphere. ZrO₂–SiO₂–Cu(II) xerogels were synthethized analogously through joint hydrolysis of a mixture of the organometallic precursors and copper(II) chloride. The effect of synthesis conditions on the physical and chemical properties of the resulting material has been studied.     

Sol–Gel Synthesis of Na<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> Glasses and their Characterization by NMR,SNMS, and AFM

Sodium aluminophosphate samples with composition 43.8Na₂O12.5Al₂O₃43.8P₂O₅ were prepared by the sol–gel route using different precursors and working in different pH ranges from pH < 1 up to pH > 10. The structures of the gels and of the corresponding glasses were investigated by solid state NMR and compared to that of a glass with the same composition prepared by a traditional melting process. In addition to bulk materials, thin films were deposited by dip coating on silica glasses. Applying secondary neutral mass spectrometry (SNMS), the expected elements and residual carbon were identified. The surfaces of the coatings and fracture surfaces of bulk material were investigated using atomic force microscopy (AFM). Solid state NMR revealed that samples prepared via a lactate route exhibited local Al and P environments closest to that of the melt-prepared glass, with the highest extent of Al-O-P connectivity.     

Effect of ZrO<Subscript>2</Subscript> on Catalyst Structure and Catalytic Sulfur-Resistant Methanation Performance of MoO<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts

A series of MoO₃/ZrO₂–Al₂O₃ catalysts was prepared and investigated in the sulfur-resistant methanation aimed at production of synthetic natural gas. Different methods including impregnation, deposition precipitation, and co-precipitation were used for preparing ZrO₂–Al₂O₃ composite supports. These composite supports and their corresponding Mo-based catalysts were investigated in the sulfur-resistant methanation, and characterized by N₂ adsorption–desorption, XRD and H₂-TPR. The results indicated that adding ZrO₂ promoted MoO3dispersion and decreased the interaction between Mo species and support in the MoO₃/ZrO₂–Al₂O₃ catalysts. The co-precipitation method was favorable for obtaining smaller ZrO₂ particle size and improving textural properties of support, such as better MoO₃ dispersion and increased concentration of Mo⁶⁺ species in octahedral coordination to oxygen. It was found that the MoO₃/ZrO₂–Al₂O₃ catalyst with ZrO₂Al₂O₃ composite support prepared by co-precipitation method exhibited the best catalytic activity. The ZrO₂ content in the ZrO₂Al₂O₃ composite support was further optimized. The MoO₃/ZrO₂–Al₂O₃ with 15 wt % ZrO₂ loading exhibited the highest sulfur-resistant CO methanation activity, and excess ZrO₂ reduced the specific surface area and enhanced the interaction between Mo species and support. The N₂ adsorption-desorption results indicated that the presence of ZrO₂ in excessive amounts decreased the specific surface area since some amounts of ZrO₂ form aggregates on the surface of the support. The XRD and H₂-TPR results showed that with the increasing ZrO₂ content, ZrO₂ particle size increased. These led to the formation of coordinated tetrahedrally Mo⁶⁺(T) species and crystalline MoO₃, and this development was unfavorable for improving the sulfur-resistant methanation performance of MoO₃/ZrO₂–Al₂O₃ catalyst.     

Sol gel derived Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> as catalysts for methane combustion: effect of zirconium loading

A series of Pd/Al₂O₃–ZrO₂ materials have been prepared via sol gel method as an attractive route to obtain more homogeneous binary oxides Al₂O₃–ZrO₂. A Zr loading between 2 and 15 wt% was used to investigate the Zr promotion of Pd/Al₂O₃ materials. The prepared catalysts were calcined at two different temperatures. Very interesting results have been obtained at low zirconium content. A small amount of Zr is seen to be sufficient to stabilize the activity and to obtain good catalytic performances with developed textural properties compared to conventional catalysts used to oxidize methane. The increase of the zirconium loading is seen to decrease the catalytic activity may be due to the development of tetragonal zirconia phase detected by XRD. Similar effect has been observed after heating catalysts at high temperatures. A loss in BET surface area and in metal dispersion has been also observed for zirconium rich catalysts. A contradictory effect on textural and structural properties is seen after their calcination at 700 °C.     

Sol–Gel Approach to Sr<Subscript>2</Subscript>CeO<Subscript>4</Subscript> Phosphor from Single Alkoxide Precursors

Syntheses and characterizations of sol–gel precursors of Sr₂CeO₄ were carried out. Each molecular precursor, [Sr₂Ce(OCH₂CH₂OCH₃)₈] (1), [Sr₂Ce(OⁱPr)₈] (2) and [Sr₂Ce₂(OⁱPr)₁₂(ⁱPrOH)₄] (3) was prepared from mixtures of Sr complexes and cerium(IV) alkoxides. The molecular structure of 3 showed that [CeO₆] octahedra are connected with distorted [SrO₆] octahedra by sharing edges with oxo bridges. X-ray powder diffraction patterns and spectrofluorometry were used to determine the evolution of structure from the precursor molecules to the luminescent oxides. The luminescent strontium cerium oxides were derived at relatively mild reaction conditions (700 °C for 1 h), and complete conversion was observed at 1000 °C for 1 h from these precursors. Comparing the spectra of the oxides derived from 2 and 3, the emission intensity of the oxide derived from 2 is much stronger.     

Preparation and Characterizations of ZnTiO<Subscript>3</Subscript> Powders by Sol–Gel Process

ZnTiO₃ powders were prepared by a sol–gel method. The gelation conditions and the gelation mechanism were investigated. The crystallization behavior and characteristics of the ZnTiO₃ were also investigated in detail. The experimental results show that homogenous and translucent gels can be prepared when the gelation conditions are appropriate and the gelation temperature remains constant. The gel structure can be described as a network of Ti and O, in which zinc ions are well distributed. The ZnTiO₃ phase can be detected by XRD in the powders calcined above 700 °C. The formation of ZnTiO₃ is a slow reaction process, which leads to the development of large ZnTiO₃ particles, with dimensions after calcination at 900 °C for 2 h in the range of 30–50 m.     

Sensor Performance of Nanostructured TiO<Subscript>2</Subscript>–SnO<Subscript>2</Subscript> Films Prepared by an Aqueous Sol–Gel Process

Nanostructured TiO₂–SnO₂ thin films and powders were prepared by a facile aqueous particulate sol–gel route. The prepared sols showed a narrow particle size distribution with hydrodynamic diameter in the range 17.2–19.3 nm. Moreover, the sols were stable over 5 months, since the constant zeta potential was measured during this period. The effect of Sn:Ti molar ratio was studied on the crystallisation behaviour of the products. X-ray diffraction analysis revealed that the powders were crystallised at the low temperature of 400 °C containing anatase-TiO₂, rutile-TiO₂ and cassiterite-SnO₂ phases, depending on annealing temperature and Sn:Ti molar ratio. Furthermore, it was found that SnO₂ retarded the anatase to rutile transformation up to 800 °C. The activation energy of crystallite growth was calculated in the range 0.96–6.87 kJ/mol. Transmission electron microscope image showed that one of the smallest crystallite sizes was obtained for TiO₂–SnO₂ binary mixed oxide, being 3 nm at 600 °C. Field emission scanning electron microscope analysis revealed that the deposited thin films had nanostructured morphology with the average grain size in the range 20–40 nm at 600 °C. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO gas at low operating temperature of 200 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption.     

Sol–gel synthesis and characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> nanocrystalline powder

Al₂O₃–TiO₂ nanocrystalline powders were synthesized by sol–gel process. Aluminum sec-butoxide and titanium isopropoxide chemicals were used as precursors and ethyl acetoacetate was used as chelating agent. Thermal and crystallization behaviors of the precursor powders were investigated by thermal gravimetric-differential thermal analysis, Fourier-transform infrared spectrum and X-ray diffraction. The average crystalline size of heat treated Al₂O₃–TiO₂ powders at 1,100 °C is ~100 nm.     

H<Subscript>2</Subscript>-Sensing Characteristics of Nanocrystalline La<Subscript>0.8</Subscript>Pb<Subscript>0.2</Subscript>FeO<Subscript>3</Subscript> Prepared by Sol–Gel Method

Nanocrystalline La_0.8Pb_0.2FeO₃ has been prepared by the sol-gel method. XRD patterns show that the nanocrystalline La_0.8Pb_0.2FeO₃ is a perovskite phase with the orthorhombic structure and its mean crystallite size is about 19 nm. The influence of Pb ions which replaced the La ions on A-sites can be directly observed from the electrical and sensing properties to H₂ gas. The conductance of La_0.8Pb_0.2FeO₃-based sensor is considerably higher than that of LaFeO₃-based sensor, and Pb-doping can enhance the sensitivity to H₂ gas. An empirical relationship of with α = 0.668 was obtained.     

Formation of nanocrystals in the ZrO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O system

Formation of zirconia nanocrystals in the course of thermal treatment of an X-ray amorphous zirconium oxyhydroxide was studied. It was shown that the formation of tetragonal and monoclinic polymorphs of ZrO₂ in the temperature range from 500 to 700°C occurs owing to dehydration and crystallization of amorphous hydroxide. An increase of the temperature up to 800°C and higher activates mass transfer processes and, as a result, activates the nanoparticle growth and increases the fraction of the phase based on monoclinic modification of ZrO₂ due to mass transfer from the nanoparticles with the non-equilibrium tetragonal structure. Herewith, formed ZrO₂ nanocrystals with monoclinic structure have a broad size distribution of crystallites, and the average crystallite size after thermal treatment at 1200°C for 20 min is about 42 nm.     

Photoresist Modification of Sol–Gel Solutions for Texturing of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> and Bi<Subscript>3</Subscript>TiNbO<Subscript>9</Subscript> Thin Films

Bismuth titanate (Bi₄Ti₃O₁₂) and bismuth titanium niobate (Bi₃TiNbO₉) c-axis textured thin films were fabricated using the sol–gel processing technique. Chemical modification of precursor solutions was performed using a proprietary photosensitive chemical (photoresist). Increases in crystallinity and texture of resulting films were seen over films that were made from unmodified solutions. Analysis by X-ray diffraction (XRD) revealed c-axis orientation factors for Bi₄Ti₃O₁₂ near 88% and Bi₃TiNbO₉ near 63% in “modified” films. Atomic force microscopy images of films show microstructural improvement between “modified” and “unmodified” films. Images generally show smaller randomly oriented grains in “unmodified” films, and larger oriented platelet structures related to growth due to crystal habit in “modified” films. Light scattering experiments show the addition of photoresist to the precursor solution initiates accelerated particle growth. AFM imaging of soft-baked films also suggests an enhancement of texture.     

Multicomponent systems LiCl–LiBr–Li<Subscript>2</Subscript>SO<Subscript>4</Subscript> and LiCl–LiBr–Li<Subscript>2</Subscript>SO<Subscript>4</Subscript>–Li<Subscript>2</Subscript>MoO<Subscript>4</Subscript>

Phase equilibria in the LiCl–LiBr–Li₂SO₄ ternary system and the LiCl–LiBr–Li₂SO₄–Li₂MoO₄ quaternary system were studied by differential thermal analysis. The compositions and temperatures of minima in the ternary and quaternary systems were determined to be (31.2 mol % LiCl, 46.8 mol % LiBr, 22.0 mol % Li₂SO₄, 460°C) and (25.2 mol % LiCl, 30.2 mol % LiBr, 14.6 mol % Li₂SO₄, 30.0 mol % Li₂MoO₄, 411°C), respectively.     

Ternary Systems LiBr–LiVO<Subscript>3</Subscript>–Li<Subscript>2</Subscript>CrO<Subscript>4</Subscript> and KBr–KVO<Subscript>3</Subscript>–K<Subscript>2</Subscript>CrO<Subscript>4</Subscript>

The binary system KVO₃–K₂CrO₄ and two ternary systems, LiBr–LiVO₃–Li₂CrO₄ and KBr–KVO₃–K₂CrO₄, were studied. In the ternary systems, the compositions and melting points of eutectic alloys were determined by differential thermal analysis: (49.0 mol % LiBr, 5.0 mol % LiVO₃, 46.0 mol % Li₂CrO₄, 400°C) and (17.0 mol % KBr, 78.0 mol % KVO₃, 5.0 mol % K₂CrO₄, 458°C), respectively.     

Preparation and Optical Properties of SiO<Subscript>2</Subscript> Sol–Gel Made Glass Colored with Carminic Acid

SiO₂ based films and bulk samples, containing dispersed carminic acid molecules, were prepared using the traditional sol–gel process. The precursor material was a mixture of water, TEOS, ethanol and carminic acid, the components were added at ratios known to provide material with good structure. The carminic acid in powder form was introduced in the solution. Films and bulk samples with intense red color were obtained. The optical absorption and emission spectra of this material contain a number of visible and UV bands corresponding to transitions between discrete electronic energy levels. It is found that optical and thermal stability of the coloration produced by the carminic acid is enhanced when the acid molecules are embedded into the SiO₂ matrix, especially in bulk samples. The energy spectrum of the carminic molecule was obtained from a quantum mechanical model based on the modified FEMO approach. This model provides a good agreement with the experiment without any adjustable parameters.     

CeO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Modified by Selective Doping with SrO for Improved Pd-Only Three-Way Catalyst

Composite CeO₂–ZrO₂–Al₂O₃ (CZA) modified by selective doping with SrO was prepared by simple coprecipitation method. The characterization results reveal that the structural property of the modified sample is fundamentally different from that of undoped CZA, the doped SrO can selectively combine with Al₂O₃ and act as a stabilizer, meanwhile homogeneous CZ solid solution is formed which appears insusceptible to Al₂O₃. Consequently more lattice defects are created in CZAS, which facilitate the improvement of oxygen mobility. The UV–Vis DRS and XPS results demonstrate that after doping with SrO, the supported catalyst Pd/CZAS possesses higher surface Pd and Ce³⁺ concentrations, which consequently leads to improved reducibility and catalytic performance. Furthermore, the synergistic effect between CeO₂–ZrO₂ and SrO–Al₂O₃ helps improve the thermal stability of Pd/CZAS. As a result, after aging treatment, Pd/CZASa still maintains improved structural, textural, morphological and reduction properties along with enhanced three-way catalytic performance.     

Sol–gel synthesis and transport property of nanocrystalline La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> thin films

La₂Mo₂O₉ films were successfully synthesized on silicon (100) and poly-alumina substrates via modified sol–gel method with inorganic salts of La(NO₃)₃ and (NH₄)₆Mo₇O₂₄ as precursors. Pure La₂Mo₂O₉ phase was confirmed by XRD if the annealing temperature was higher than 500 °C. Energy dispersive spectrometry (EDS) of TEM revealed that the molar ratio of La to Mo was nearly 1:1. Field-emission SEM characterization showed that the films were dense, crack-free and uniform. The grain size of the films ranged from 30 to 400 nm depending upon the calcination temperature and duration time. The roughness calculated from AFM topography varied in the range between 10 and 35 nm. The thickness of the films was more than 200 nm for single-layered films. The electrical conductivity of the films reaches 0.06 S/cm at 600 °C that was almost more than one order of magnitude higher than that of the corresponding bulk material.     

Ionic mobility and electrophysical properties of solid solutions in PbF<Subscript>2</Subscript>–SbF<Subscript>3</Subscript> and PbF<Subscript>2</Subscript>–SnF<Subscript>2</Subscript>–SbF<Subscript>3</Subscript> systems

Ionic mobility and electrical conductivity of solid solutions with fluorite structure, obtained with solid-state approach in PbF₂–SbF₃ and PbF₂–SnF₂–SbF₃ systems, are studied by 19F NMR and electrochemical impedance spectroscopy methods. The 19F NMR spectra parameters, types of ion motions in the fluoride sublattice, and the ionic conductivity magnitude are shown to be determined by the temperature and fluoride concentration in the solid solutions. The solid solution specific conductivity in the PbF₂–SbF₃ and PbF₂–SnF₂–SbF₃ systems at 420–450 K is as high as ~10^–2 S/cm, which allows accounting the solid solutions as a base for preparation of functional materials.     

Thermodynamic Properties and Thermal Expansion of Tm<Subscript>2</Subscript>O<Subscript>3</Subscript> · 2ZrO<Subscript>2</Subscript> Solid Solution

The isobaric heat capacity of Tm₂O₃ · 2ZrO₂ solid solution was measured by adiabatic calorimetry and differential scanning calorimetry (DSC), and smoothed values of the enthalpy changes, entropy, and reduced Gibbs free energy in the temperature range 8–1200 K were calculated. Thermal expansion was studied by X-ray diffraction in the temperature range 298–1173 K.