Characterization of Alumina Gel Catalysts by Emanation Thermal Analysis (ETA)

Alumina gels made from the metal alkoxide is known to have high catalyst activity for the selective reduction of NO _x by hydrocarbons. It is also reported that the fine structure of the gels effects the activity. In this study, the effect of the preparation method on the fine structure and catalyst activity of the gels was investigated. Monolithic gels were obtained by hydrolysis of Al(sec-C₄H₉O)₃. The wet gels were dried at 90°C (xerogels), supercritically dried (aerogels), or dried after immersion in an ethanol solution of methyltrimethoxysilane (modified xerogels). The changes in the microstructure during heating were discussed using the results of TG-DTA, ETA and N₂ adsorption. The ETA curves show the ²²⁰Rn-release rate, E, of the samples, previously labelled with ²²⁸Th and ²²⁴Ra, during heating. The decrease in E of the xerogel at temperatures higher than 400°C indicates a gradual decrease in the surface area and porosity. A remarkable decrease in the BET surface area of the xerogel was found after heat-treating at 500°C. On the other hand, constant E of the aerogels and modified xerogels above 450°C suggests high thermal stability. The pore radii, estimated by BJH method, and the catalyst activities at 500°C of the aerogels and the modified xerogels were higher than those of the xerogels. The temperature range in which the alumina gels are applicable as catalysts was determined.     

Porous silicon oxycarbide glasses from organically modified silica gels of high surface area

High surface area alkyl-substituted silica aerogels and xerogels were successfully prepared by sol-gel processing and supercritical drying. The gels were further heat treated in inert atmosphere to temperatures as high as 1000°C. Surface areas and pore structure of the gels and gels pyrolyzed at high temperatures were determined by multipoint BET surface area measurement. The aerogels and xerogels exhibited surface areas of about 1100 m²/g. No significant effect of pH was found on the surface areas of gels in the two step sol-gel process, but gels of low pH showed smaller pore diameter and higher density. Xerogels showed smaller surface area, pore size, and pore volume compared to aerogels. Upon pyrolyzing in inert atmosphere, the surface areas of all the gels decreased with temperature as a result of collapse of micropores and shrinkage of mesopores. Unlike pure silica gel, which loses almost all surface area and densifies at 1000°C, the advantage of the alkyl-substituted gels is that they maintained a high surface area of 400 m²/g at 1000°C.Also with the Department of Agronomy.     

Thermal Decomposition and Crystallization of Aqueous Sol-Gel Derived Zirconium Acetate Gels: Effects of the Additive Anions

Zirconia powders were prepared by forming gels by desiccation of aqueous precursor solutions of zirconium acetate containing nitric or sulfuric acid at pH 2.4 and 1.4 and pyrolyzing the gels to temperatures up to 825°C. The structure development in the gels and solid pyrolysis products was investigated. The crystalline zirconia structures produced monoclinic (m), metastable cubic (c) and tetragonal (t) polymorphs. The structure transition temperatures were strongly dependent on the pH, the anions and the stoichiometry of the zirconium complex in the precursor solution. The monoclinic polymorph fraction in the zirconia formed by pyrolyzing the gel formed from the precursor solution containing sulfuric acid at pH 2.4 to 750°C approaches zero while this ratio in the zirconia formed by pyrolyzing the gel formed from the precursor solution containing nitric acid at pH 1.4 to 825°C is 0.7.     

Raman Spectroscopic Investigations of the Effects of Ag+ and Ce3 + Doping on the Densification of Nanoporous Silica Xerogels

Doped and undoped 50 Å porous silica xerogels were heat-treated at various annealing temperatures ranging from 800°C to 1150°C. The heating was performed in air for 1 hour at each temperature. Raman spectroscopy was used to follow the structural changes occurring at various stages of the gel-to-glass transformation, as well as to investigate the effects of metal ions on the densification of these nanoporous silica xerogels. Raman data and density measurements showed that while densification is completely achieved at 1050°C for undoped xerogels, it occurs at 950°C for Ag⁺-doped samples. On the other hand, Ce^{3 +} doping was found to slow down the densification process, with complete densification occurring at 1100°C.     

Preparation and Characterization of Forsterite (Mg2SiO4) Xerogels

Mg2SiO4 gels were prepared from alkoxide precursors, and the formation of the forsterite crystal phase was studied after heat treatments up to 1200°C. Prehydrolyzed TEOS in solution with 2-methoxyethanol was mixed with Mg(OEt)2, and the solution was hydrolyzed using excess water. The resultant gels were dried at 100°C to form xerogels which were subsequently powdered. These powders were characterized using thermal analysis (DTA and TGA), surface area analysis (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM).DTA and XRD indicated that forsterite crystallized at 770°C, and by 1000°C the powders were predominantly crystalline. BET gave powder surface areas between 400 and 550 m2 g–1. TEM revealed angular particles with sizes between 0.2 and 2 m. The low temperature of crystallization of forsterite indicates a high degree of intimate mixing between the precursor alkoxides, although XRD indicated some degree of inhomogeneity.     

UV-Visible Absorption Characteristics of TEOS-Derived and Cr-Doped Silica Sonogels Aged in Different pH Solutions and Heat Treated up to 600°C

Cr-doped xerogels were obtained by sol-gel process from the acid-catalyzed and ultrasound-stimulated hydrolysis of tetraethoxysilane (TEOS) with addition of CrCl₃6H₂O in water solution during the liquid step of the process. The gels were aged immersed in different pH solutions for about 30 days, after that they were allowed to dry. The samples were annealed at temperatures ranging from 40 to 600°C and analyzed by UV-visible absorption spectroscopy. Cr³⁺ is the preferable oxidation state of the chromium ion in the gels annealed up to 250–300°C, in the case of aging in solutions of pH = 5 and 11. A high UV absorption below 320 nm, due to the host gel, and different absorption bands, depending on the temperature, due to the chromium ion were observed in the xerogels at temperatures below 250°C, in the case of aging in solutions of pH = 1 and 2. These absorption bands have not been assigned. Above 300°C up to 600°C, Cr⁵⁺, and possibly Cr⁶⁺, are the preferable oxidation states of the chromium ion independent of the pH of the aging solution, so the xerogels turn to a yellowish appearance in all cases.     

Surfactant Templated Silica Xerogels as Adsorbents for Rhodamine 6G

A series of silica xerogels was synthesized by using TEOS as the silica precursor and a non ionic surfactant Triton X100 (TX100) and a cationic surfactant Hexadecyltrimethylammonium Bromide (CTAB) as templates. The xerogels were synthesized through 2‐way catalysis using ultrasonic radiations for homogeneous mixing of the precursors and template. The surfactant template was later removed through calcination at 550 °C. It was found out that gels having CTAB as the template had higher surface area (612.08 m²/g) than the gels templated by TX100 (539.6 m²/g). High surface area xerogels were used in adsorption experiments for aqueous solutions of Rhodamine 6G (R6G). UV visible spectroscopy was used to find out the concentrations of dye solutions. The adsorption data of both the types of xerogels was found to follow Freundlich's adsorption model pointing towards the possibility of adsorption of the dye molecules on the heterogeneous surface of the adsorbent.     

Thermal evolution of TEOS-Al(secBuO)3-etac mullite gels

Mullite gels have been prepared using tetraethyl orthosilicate and aluminium tri-sec-butoxide chemically modified by ethylacetoacetate as precursors. The hydrolysis conditions are shown to have a very noticeable influence on the evolution of both phase and microstructure of the resulting xerogels. The optimization of thermal treatment allows well crystallized mullite samples with a density more than 90% of the theoretical value after sintering at 1200°C for 4 hours to be obtained.     

Development of two new approaches for preparation of zirconium silicate systems at low temperature

Homogeneous and stable zirconium silicate sols and gels were prepared via the sol-gel process. Two synthesis procedures were explored based on using acetylacetone (acac) and acetic acid (HOAc) as modifying agents. The homogeneous sols were characterized by small angle x-ray scattering (SAXS) and ²⁹Si NMR in the early stages of polymerization. Zr addition to the Si sols caused significant modification of the extent of condensation of the Si species as determined by ²⁹Si NMR. TGA and DTA of the two types of xerogels revealed differences attributed to the modifying agents. Fourier Transform Infrared (FTIR) spectroscopy of acac xerogels showed a band at 1600 cm^–1, assigned to acac bonded to Zr. FTIR spectra of the HOAc xerogels were consistent with OAc bidentate ligands bonded to Zr. UV-Vis spectroscopy results of the HOAc2.3 thin film presented a band assigned to OAc singly coordinated to Zr.     

The Structure Formation of Zirconium Oxide Gels in Alcoholic Solution

Zirconium oxide gels were prepared by controlled hydrolysis and condensation of zirconium-n-propoxide in alcoholic solution. After completion of the gelation the aging and drying of the alcogel was observed in situ by Small and Wide Angle X-ray Scattering experiments at room temperature. The xerogels obtained have been annealed at different temperatures to crystallization. The structural changes were observed by using SAXS and WAXS. Structural models reproducing the experimental WAXS data have been simulated by the reverse Monte Carlo method. A comparison between zirconium oxide xerogels obtained under different thermal treatments was made and evidence was found that thermal treatment at higher temperatures and preparation conditions affect the atomic arrangement of these amorphous gels.     

Synthesis and Physicochemical Properties of Pol(yzirconium methylsiloxane) Xerogels

A method of the synthesis of hybrid organo–inorganic materials (xerogels of pol(yzirconiummethylsiloxanes)) with a large specific surface area and a considerable volume of sorbing space is developed. It is shown that an increase in the content of zirconium ions in the polymethylsiloxane matrix is accompanied by an increase in its specific surface area and a decrease in both the volume and diameter of sorbing pores. It is assumed that the noticeable changes in the parameters of porous structure are caused by the enhancement of the hydrophilicity of the system upon introducing zirconium ions. Using differential thermal analysis, it is shown that methyl groups on the adsorbent surfaces are stable up to 350°C.     

Influence of the Er3+ Content on the Luminescence Properties and on the Structure of Er2O3-SiO2 Xerogels

Er₂O₃-SiO₂ xerogels doped with different Er/Si concentrations were annealed at 950°C for 120 h. The Er³⁺ doping level varied from 0 to 40000 Er/Si ppm. The effect of Er₂O₃ content on the sintering behavior of silica gels and on the luminescence properties was studied by Vis-NIR absorption, Raman and luminescence spectroscopies.     

Surface Energy Changes of Heat Treated TEOS Derived Silica Xerogels

The surface energy of monolithic silica xerogels was examined by measuring the interaction of organic probes with xerogels heated at temperatures close to the gel-to-glass transition temperature. Values of the dispersive component of the surface energy, , between 60 and 80 mJ m^-2 have been observed using n-alkanes for silica xerogels heated at 700, 800 and 900°C. At 1000°C, decreases to 8.37 mJ · m^-2. Also the differential heat of adsorption, variation of standard free energy and entropy of adsorption decrease when the silica xerogel is heated at 1000°C, showing a lower interaction potential of the organic probes with the silica surface. For the silica xerogels heated between 700 and 900°C, the acid character varies in accordance with the variation of the chemical nature of the silica. Upon heating at 1000°C, both acid and base characters are very close in accordance with a neutral surface. Within the experimental conditions used in this work, the surface of the obtained monolithic silica xerogels behaves as a glass surface when the treating temperature is 1000°C.     

ZrO<Subscript>2</Subscript> and Cu/ZrO<Subscript>2</Subscript> Sol–Gel Materials Spectroscopic Characterization

Copper-doped zirconia (1% mol) and zirconia powders were prepared by the sol–gel process, using zirconium n-butoxide and copper nitrate as precursors. The resulting xerogels are nanocrystalline and exhibit different properties from the corresponding microcrystalline materials. The copper nitrate salt was dissolved and co-gelled in situ at the initial stage of the reaction. The properties of the resulting materials were studied by XRD, FTIR and UV-Vis. The as-prepared samples were amorphous and crystallized to the tetragonal zirconia phase at 400 °C. At temperatures higher than 600 °C, the monoclinic phase was also obtained. No evidence of discrete crystalline copper compounds was observed, consistent with good dispersion of the dopant. Several bands were observed by FTIR in the 4400–3000 cm^–1 region, which diminishes in intensity and shifted to higher wavenumbers with heating. The bandgap energy (E_g) was strongly modulated by the presence of the dopant and heating temperature, with increasing temperature leading to a corresponding decrease in E_g.     

Indicator Tubes and Indicator Powders for Determining Fluoride and Chloride Ions

Noncovalent immobilization of Arsenazo I, Alizarin Red, Xylenol Orange, and diphenylcarbazone by incorporation into silicic acid xerogels and modification of silica gels was studied, and procedures for determining fluoride and chloride ions by solid-phase spectrophotometry and test methods were developed. Reactions of immobilized reagents with aluminum(III), zirconium(IV), and mercury(II) were studied. The possibility of using immobilized reagent–metal ion–halide ion systems for the determination of halide ions was assessed. Indicator powders were proposed for determining 0.5–10 mg/L fluoride ions and 1–30 mg/L chloride ions, and indicator tubes were developed for determining 20–200 mg/L chloride ions. The determination of fluoride and chloride ions is based on exchange complexation reactions proceeding in the systems immobilized Xylenol Orange–zirconium(IV) and immobilized diphenylcarbazone–mercury(II), respectively. Performance characteristics of the developed procedures were estimated. The procedures were verified by determining halide ions in Narzan mineral water.     

Preparation of Lithiated Co- and Ni-Oxide Powders and Thin Films by Wet Chemistry Processing

Layered lithiated Co- and Ni-oxide powders and thin films with rhombohedral (R m) structure were prepared by a peroxo wet chemistry route from Li(I), Co(II) and Ni(II) acetate precursors and the addition of H2O2. XRD analysis revealed that xerogel films and powders possessed a typical layered structure. Layered (R m) Li_0.99Co_1.01O₂ powder and Li_0.97Co_1.03O₂ films were formed around 500°C, while Li_0.93Ni_1.07O₂ powder and Li_0.94Ni_1.06O₂ oxide films were prepared around 550°C. The stoichiometry of the oxide films and powders was dependent on the heat-treatment temperature. The structure of sols, gels, xerogels and thin films was established from XRD spectra and from the FT-IR spectra, confirming their layered structure.     

Preparation of Transparent, Partially-Crystallized BaTiO3 Monolithic Xerogels by Sol-Gel Processing

Transparent, partially-crystallized nanostructured barium titanate (BaTiO3) monolithic xerogels (dried at 90°C) have been successfully synthesized via hydrolysis of Ba, Ti alkoxide precursor solutions in a concentration range of 1.0 mol/l with addition of water with a molar ratio of H2O/Ba 6.3. Transparent monolithic xerogels obtained from a precursor solution of 1.0 mol/l remained transparent even after firing at 500°C in oxygen, although the degree of their transparency was considerably decreased. Firing at temperatures above 500°C yielded translucent ceramics of BaTiO3, and ultimately, firing above 600°C resulted in normal opaque ceramic bodies. Those obtained from a more concentrated precursor solution of 1.2 mol/l were, on the other hand, still transparent after firing at 600°C in oxygen, and turned opaque at 700°C. The results demonstrate that the materials retained their transparency even after pyrolysis of organic compounds involving exothermic oxidation at temperatures in the range of 200 to 400°C. The densification behavior of transparent BaTiO3 monolithic xerogels obtained was found to be excellent; for example, those derived from a 1.0 mol/l precursor solution could be sintered to form monolithic BaTiO3 ceramics with a relative sintered density of about 94% and an average grain size of 1 m by firing at 1100°C for 2 h in oxygen.     

Lower critical solution temperature control and swelling behaviour of physically crosslinked thermosensitive copolymers based on N-isopropylacrylamide

In this contribution we have developed thermosensitive polymer matrices based on N-isopropylacrylamide (NIPAAm). Preparation of the hydrogels involved photopolymerisation of a combination of NIPAAm, 1-vinyl-2-pyrrolidinone (NVP) and distilled water, in appropriate amounts and contained a UV-light sensitive initiator called 1-hydroxycyclohexylphenylketone. As NIPAAm monomer could be readily dissolved in mixtures of liquid NVP and distilled water, the use of organic solvents was not required in the polymerisation process. Furthermore, chemical crosslinking agents are not needed in the synthesis. By alternating the feed ratio, hydrogels were synthesised to have lower critical solution temperatures (LCST) in the vicinity of 37 °C. This ability to shift the phase transition temperature of the gels provides excellent flexibility in tailoring transitions for specific uses. The transition temperature of the pseudo gels was established using cloud point measurement and modulated differential scanning calorimetry (MDSC). The chemical structure of the xerogels was characterised by means of Fourier transform infrared spectroscopy (Ftir), while swelling experiments in distilled water indicate that the swelling and dissolution behaviour of the gels is strongly temperature dependent.     

29Si MAS-NMR Study of Silica Gels and Xerogels: Influence of the Catalyst

Four silica gels were prepared by hydrolysis of tetraethoxysilane (TEOS) in ethanol, using different catalysts: HCl, NaOH, NH₃, and NBu₄F. Nitrogen adsorption-desorption isotherms indicated that the HCl-catalyzed xerogel was purely microporous, whereas the other samples exhibited a very broad distribution of mesopores and a variable amount of micropores. ²⁹Si MAS NMR spectroscopy of the wet gels (before drying) pointed to a low degree of condensation for the HCl-catalyzed gel, and to the presence of unhydrolyzed TEOS monomer in the NaOH-catalyzed gel. Comparison with the ²⁹Si MAS NMR spectra of the xerogels indicated a significant increase of the degree of condensation during the drying procedure (3 hrs at 120°C under vacuum) for the HCl-catalyzed gel.     

Influence of drying procedure on the mesoporosity and surface fractal dimensions of silica xerogels prepared with different agitation methods

Silica xerogels were prepared by thermal drying wet gels in an electric oven (70 degrees C) after certain duration of ambient drying, and the relevant effect is investigated on the mesopore structures and surface fractal dimensions of the resultant xerogels. The silica gels were derived from a hydrochloric acid-catalyzed TEOS (tetraethylorthaosilicate) system, and both magnetic stirring and ultrasonic vibration were adopted during sol preparation. The percentage mesoporosity and surface fractal dimensions are evaluated using image analysis methods, based on FE-SEM (field emission gun-scanning electron microscopy) images. The results show that the mesoporosity of the resultant xerogels decreases with the duration of ambient drying for samples prepared using magnetic stirring and low-intensity ultrasonic vibration, while samples subjected to high-intensity ultrasound show a somewhat reverse trend. Samples prepared with magnetic stirring have almost constant surface fractal dimensions (nearly 3), irrespective of the ambient drying before thermal drying. The surface fractal dimensions of samples prepared using ultrasound increase with the duration of ambient drying.