Sol Gel Alumina Research Articles

Influence of the Synthesis Method on the Properties of Ceria-Doped Alumina

Ceria-doped alumina sol-gel materials were obtained by two synthesis methods at low temperature; using method A, 2-propanol-diluted cerium precursor was slowly added at the time of the aluminum sol formation in acidic environment; using method B, the cerium salt was mixed with the aluminum alkoxide before sol formation in a basic environment. The supports were characterized by N2 physisorption, thermogravimetric and thermal differential analyses (TGA and DTA), X-ray diffraction (XRD), 27Al Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR), 2-propanol reactions, and ammonia temperature-programmed desorption (NH3-TPD). The samples obtained by Method B present similar values in properties such as specific areas, pore volumes, pore size distribution, and acidity compared to those of pure alumina; the alumina structure was not modified, but segregated crystallites of CeO2 were found in samples calcined at 1000 ∘C, as observed by XRD. The ceria-containing materials synthesized by method A show a thermal behavior similar to that of alumina, with no appreciable segregation of CeO2 detected by XRD and modifications in the amounts of tetra, penta, and octa-hedral aluminum coordination as determined by NMR. 2-propanol reactions showed a good correlation with acid density determined by NH3-TPD. As the percentage of ceria in the material increases, surface area, pore volume, and acidity decrease. These changes can be correlated with an increase of pentacoordinated aluminum content. The results indicate that CeO2 is well dispersed in the alumina framework when method A is used, but synthesis method B does not have the same effect on the CeO2incorporation.

Sol–gel multicapillary columns for gas–solid chromatography

In this work, we report the method for the preparation of multicapillary columns (MCCs) for gas–solid chromatography. The porous layer adsorbent is formed on capillary walls by the hydrolysis of aluminum alkoxide in the presence of polypropylene glycol (PPG) and HCl. Porosity and selectivity of the adsorbent depend on reaction conditions and the concentration of PPG. Sol–gel MCCs are well suited for high-speed chromatographic analysis of light hydrocarbons by gas–solid chromatography. Nine-component mixtures of C1–C4 hydrocarbons are separated within 8–12 s. The efficiency of 25–30 cm long alumina sol–gel MCCs consisting of approximately 1400 capillaries of 40 μm diameter is up to 2500–3000 theoretical plates.

Strategy for Allosteric Analysis Based on Protein-Patterned Stationary Phase in Microfluidic Chip

An effective method is presented for the on-chip analysis of chiral interactions with a successful depression of nonspecific adsorption. The alumina gel-derived protein network on poly(methyl methacrylate) (PMMA) microchannel was explored to form a protein-stationary phase and then used to carry out electrophoresis for fast enantioseparation coupled with electrochemical detection. On the basis of the chemical modification of a synthesized copolymer containing silane-functionalized scaffold, alumina sol-gel could react readily with the silane groups and form steady microstructure on the chip surface achieving the encapsulation of functional biomolecules. Compared with the native PMMA microchannels, the modified surfaces exhibited much better wettability, more stable and enhanced electroosmotic mobility, and less nonspecific adsorption. The water contact angle and EOF of alumina-gel-derived PMMA substrate were 22 degrees and 4.3 x 10(-4) cm(2) V(-1) s(-1), compared to those of 73 degrees and 1.9 x 10(-4) cm(2) V(-1) s(-1) from the untreated one, respectively. Bovine serum albumin, acting as a target protein, could be stably and homogeneously immobilized in the modified PMMA microchannel to fabricate a protein-stationary phase. Under a mild condition, D- and L-tryptophan were efficiently separated with a resolution of 1.57. The as-prepared microchip can perform chiral separations within short time, indicating that the general protocol has the potential to provide a platform for high throughput screening of enantiomer candidates such as those biochemical drugs with protein targets and the research of receptor interactions.

Effect of strontia on the densification and mechanical properties of sol–gel alumina

Boehmite sol was prepared by hot water hydrolysis of aluminum iso-propoxide using nitric acid as the catalyst. Hydrolysis was carried out at 80 °C for 80 min. Calcium nitrate was added and the peptization was complete at 80 °C for 1 h. The sol was precipitated in ammonia, the precipitate was aged for 24 h, dried at 120 °C and calcined at 500 °C for 3 h. The calcined powder was milled at 230 rpm for 6 h. The amount of calcium oxide was varied from 1 to 20 vol.%. The particle size and particle size distribution of the calcined powders were studied. The average particle size was found to increase with calcia content. The powder was compacted into cylindrical pellets using uni-axial press at 180 MPa and sintered at 1600 °C for 6 h. Mechanical properties such as hardness, fracture toughness, diametrical tensile strength and flexural strength for compacts containing various concentrations of strontium oxide was studied. There was a slight decrease in the density with increase in strontia content. Increase in the mechanical properties such as hardness, fracture toughness and diametrical tensile strength was observed with increase in strontia content.

Abnormal Grain Growth During Rapid Annealing of Sol–Gel Alumina Thin Film Deposited on Ni‐Based Superalloy

A ∼50 nm thick alumina layer was deposited on an Ni‐based superalloy substrate by a sol–gel method. α‐AlOOH particles presented in the layer after drying at 140°C transformed mostly to α‐Al2O3 grains within ∼1 min at 1100°C under a low oxygen partial pressure annealing environment. During the same time period, the α‐Al2O3 grains grew significantly in the lateral direction, resulting in the aspect ratio of grain diameter to thickness of ∼20. The presence of a preferred orientation in the α‐Al2O3 layer suggested that the mechanism for the lateral growth was abnormal. The lateral growth mechanism appeared to become very slow when a critical thickness (∼100 nm) was reached.

Adsorption of Wells–Dawson tungsten heteropolyacid on sol–gel alumina: Structural features and thermal stability

The Wells–Dawson tungsten heteropolyacid, H 6P 2W 18O 62 supported on sol–gel and non-sol–gel alumina has been investigated by infrared spectroscopy (IR), thermal analysis (TGA/DSC), and X-ray diffraction (XRD). X-ray diffraction indicates that the heteropolyacid primary structure in bulk form holds up to 350 °C and by supporting it on the sol–gel alumina the thermal stability rose to 650 °C. UV–vis spectroscopy showed that the sol–gel alumina has a higher tendency to adsorb Wells–Dawson tungsten heteropolyacid than the non-sol–gel alumina. The heteropolyacid showed higher interaction with the sol–gel alumina than with the non-sol–gel. Esterification of propanoic acid with hexanol in the presence of alumina-supported heteropolyacid revealed that the acidic character of the heteropolyacid remains active to some extent.

Activation of Metal-Carbonyl Clusters by their Encapsulation Within Alumina Sol-Gel Matrices

Entrapment of Ru3(CO)12 and Os3(CO)12 within alumina sol-gel matrices, (obtained by hydrolysis and polycondensation of a THF solution of aluminum isopropoxide) converts these clusters into highly active, leach-proof and recyclable hydrogen transfer catalysts. Likewise Rh2Co2(CO)12 which, in its silica sol-gel entrapped form, neither promotes the isomerization of allylbenzene nor the disproportionation of dihydroarenes, becomes upon its entrapment in an alumina sol-gel matrix, an efficient and recyclable catalyst for these reactions. While the entrapped osmium cluster promotes at 100–110∘C and 20 bar only the hydrogenation of alkenes, the ruthenium analog catalyzes also the saturation of the aromatic C–C bonds. The hydrogenations take place via the hydride mechanism. XPS and TEM studies revealed that the carbonyl clusters do not decompose to give detectable amounts of (entrapped) metal particles during the encapsulation as well as during the catalytic processes.

Marvelous prolongation of lifetime of alumina sol from aluminum sec-butoxide with the presence of ethylacetoacetate in aqueous medium

A mol. ratio around 1-mol aluminum sec-butoxide(ASB) to 100-mol water has been generally used for the preparation of alumina sol by peptizing process. It was found that the lifetime of this alumina sol was only about half month. Due to its short lifetime, this alumina sol was sometimes limited to synthesize alumina sol–gel materials via such methods as spraying, moulding, electrophoresis deposition, spin- and dip-coating, etc which had much dependence on the viscosity condition of sols. On the industrial point of view, the short lifetime also limited this alumina sol to be a commercial product on market. In the present study with the addition of EAcAc the lifetime of this alumina sol was found to be prolonged for 24 times and reach about twelve months. The corresponding reaction mechanism was investigated by TEM and UV absorption spectra determinations of the sols. EAcAc was considered in this study to be reacted with the neighboring HO–Al groups exposed on the surface of the colloidal particles, but not as has been usually figured to be reacted directly with ASB precursor. Such a reaction results in the formation of an outside layer around the colloidal particle which contributed largely to decreasing the activity of Al2O3 colloidal particle. The present study would accelerate the full applications of this alumina sol both in lab and market scope.

Chemically Bonded Composite Sol–Gel Alumina Coatings

This communication introduces chemically bonded (CB) composite sol–gel (SG) technology. The composite includes majority (>80 vol%) particulate filler, the balance being SG‐derived bonding phase. Ceramic bond formation in such a composite is induced through chemical reaction rather than through sintering. In the particular variant of the process presented here, the reactivity of the SG‐derived alumina ceramics toward phosphates, such as aluminum phosphate or phosphoric acid, is explored to produce a ceramic (reaction) bond in the system without sintering. This allows attaining structural integrity of the composite at relatively low temperatures of 300°C. Several examples of 40–200 μm thick films of CB alumina on stainless steel, Mg and Al, Ni, and on carbon fiber‐reinforced epoxy are discussed.

Hydrogenation of edible oil over Pd-Me/Al 2O 3 catalysts (Me = Mo, V and Pb)

This work presents the performance of supported Pd and Pd-Me (Me = Mo, V and Pb) catalysts in the hydrogenation of sunflower oil. The catalysts were prepared using alumina as support, and were following two different methods of preparation: wet impregnating and sol–gel techniques. The samples were characterized by atomic absorption, N 2 adsorption isotherm, temperature-programmed reduction and hydrogen chemisorption. For Pd monometallic catalysts prepared by wet impregnation (WI) of γ-Al 2O 3 and alumina sol–gel (SG), for the same iodine value (IV) and Pd surface, Pd-SG generates more trans-isomers than Pd-WI sample. This could be attributed to differences in the support morphology. Regarding bimetallic catalysts, for a smaller amount of exposed Pd, the Pd-Mo/γ-Al 2O 3 and Pd-V/γ-Al 2O 3 catalysts show the same activity compared with the respective monometallic catalyst, and increase the selectivity to trans-isomers. The molybdenum and vanadium promoting effect could be a consequence of the formation of an adsorbed initial state, after which the hydrogenation would take place on the Pd surface. On the other hand, Pd-Pb/γ-Al 2O 3 showed the lowest hydrogenation activity. This would be result of a certain dimensional limitations on the space lattice of Pd for hydrogenation of double bonds due to the formation of a Pd–Pb alloy. The sol–gel Pd-Mo sample was the most effective catalyst regarding the cis-isomers selectivity and reveals the route for future research.

Screening of Pd and Ni supported on sol–gel derived oxides for dichloromethane hydrodechlorination

Abstract Several alumina, titania, zirconia and silica xerogels modified with approximately 0.05 wt.% Pd or 0.05 wt.% Ni were synthesized by cogellation and conventional impregnation method using Pd and Ni acetylacetonate as metal precursors. Thermolysis of supported complexes was conducted in static air at 300 and 350 °C for Pd and Ni loaded samples, respectively. The resulting catalysts were characterized by XRD, XRF, BET, TPR, and tested in the vapor phase hydrodechlorination of dichloromethane (DCM) with molecular hydrogen. Prior to reaction, catalysts were reduced at 300 °C in 5% H 2 /He. The synthetic mixture consisted of 1200 ppm CH 2 Cl 2 , 1.2% H 2 and balance He at a space velocity of 0.005 g min ml −1 . Palladium supported catalysts were more active than those containing nickel, i.e. DCM conversions at 200 °C over Pd(I)–Al 2 O 3 almost doubled those obtained over Ni(I)–Al 2 O 3 and no intermediate reaction products were observed over Pd supported catalysts whereas CH 3 Cl was formed over nickel catalysts. Comparison of the specific activities of selected catalysts expressed as TOF, indicate that the observed differences in activity are related to the nature of the support and the accessibility of the metal function to the fluid phase, even though textural properties also appear to play a role. The most active catalyst, palladium impregnated on sol–gel titania (Pd(I)–TiO 2 ) had an initial specific activity of 0.36 s −1 but, it strongly deactivated after a 48 h exposure to 1200 ppm DCM or by thermolysis of the Pd precursor in static air at 600 °C. Although palladium impregnated in sol–gel alumina exhibited lower activity it was much more stable than Pd(I)–TiO 2 .

Phase and microstructural development in alumina sol-gel coatings on CoCr alloy.

Phase transformation of gamma-Al(2)O(3) to alpha-Al(2)O(3) in alumina sol gel coatings on biomedical CoCr alloy was studied as function of heat treatment temperature and time. Transformation in unseeded coatings was significant only above approximately 1200 degrees C. Addition of alpha-Al(2)O(3) seed particles having an average size of approximately 40 nm lowered the phase transformation temperature to around 800 degrees C. These particles were considered to act as heterogeneous nucleation sites for epitaxial growth of the alpha-Al(2)O(3) phase. The kinetics and activation energy (420 kJ/mol) for the phase transformation in the seeded coatings were similar to those reported for seeded monolithic alumina gels indicating that the transformation mechanism is the same in the two material configurations. Avrami growth parameters indicated that the mechanism was diffusion controlled and invariant over the temperature range studied but that growth was possibly constrained by the finite size of the seed particles and/or coating thickness. The phase transformation occurred by the growth of alpha-Al(2)O(3) grains at the expense of the precursor fine-grained gamma-Al(2)O(3) matrix and near-complete transformation coincided with physical impingement of the growing grains. The grain size at impingement was approximately 100 nm which agreed well with that predicted from the theoretical linear spacing of seed particles in the initial sol.

Effect of potassium on the physicochemical properties of molybdenum oxide catalyst supported on sol–gel alumina–zirconia

Potassium-promoted molybdenum oxide/alumina–zirconia catalysts were prepared by the sol–gel method. The catalysts were characterized by nitrogen physisorption, X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and Fourier transform spectroscopy measurements. It was observed that potassium addition did not drastically disturb the textural properties and led to a redispersion of the molybdenum oxides composed both of octahedral and tetrahedral structures. Molybdenum oxide mainly led to the formation of Brönsted acid centers while alumina–zirconia support contained Lewis acid centers. Potassium led to a decrease in the number of Brönsted acid centers rather than that of Lewis centers.

Effect of deflocculants on the densification and mechanical properties of sol–gel alumina

Boehmite precipitate prepared at pH=8.5 was oven-dried and calcined at different temperatures and their powder characteristics were determined. The colloidal precipitate (20 wt.%) was dispersed in aqueous medium containing 0.1 wt.% of different deflocculants. The boehmite suspensions containing these deflocculants were centrifuged, oven-dried and calcined. The powder was then compacted into pellets using uniaxial press at 180 MPa and sintered at 1600 °C. The compacts prepared with deflocculants were found to give higher sintered density and mechanical properties than the compacts prepared without deflocculants. When the deflocculants were used, the densification and fracture toughness of the compacts obey the following order: polyamino benzoic acid>ammoinum polyacrylate>ammonium citrate>polyethylene amine>acrylic acid>ammonium carbonate>ammonium tartarate>citric acid.

Sol‐Gel Alumina Coating for Improved Cyclic Oxidation Resistance of Silicon Nitride

A 25 nm thick α‐alumina layer was deposited on a turbine‐grade silicon nitride by sol‐gel dip coating and subsequent heat treatment in air at 1200°C. This layer had a nanometer grain structure. Silicon nitride protected by this thin layer showed a significant improvement in oxidation resistance over its uncoated counterpart after 200 cyclic exposures in air at 1250°C. The oxide layer grown on the coated silicon nitride also exhibited superior surface morphology, compared with the uncoated silicon nitride.

Sol Gel Alumina Coating On Fe–Cr–Al–Y Fibre Media for Catalytic Converters

Sol gel coating of γ-alumina was applied to Fe–Cr–Al–Y fine fibre media to examine the feasibility of using alloy fibres as a substrate for automobile catalytic converters. Two sol gel routes: aqueous based colloidal and alcohol based polymeric sol were used for coating. Prior to deposition, the fibre was oxidised in air to allow a dense α-alumina layer to form. The effect of several coating parameters such as withdrawal speed, sol concentration, soaking time and firing scheme on the coating quality and on the thickness were studied. The morphology of the coating was investigated with scanning electron microscopy (SEM) and the thickness of the coating was measured by the focused ion beam (FIB) technique. The crystal structures of the coating and the thermally grown oxide were studied with photo stimulated luminescence spectroscopy (PSLS). The coated fibre media exhibited different behaviour compared to the ordinary flat substrate. The performance of the alcohol based sol was better than that of the aqueous based sol with regard to the quality of the coating. PSLS was effective in differentiating the γ- and α-alumina.

Glucose microbiosensor based on alumina sol–gel matrix/electropolymerized composite membrane

A procedure is described that provides co-immobilization of enzyme and bovine serum albumin (BSA) within an alumina sol–gel matrix and a polyphenol layer permselective for endogenous electroactive species. BSA has first been employed for the immobilization of glucose oxidase (GOx) on a Pt electrode in a sol–gel to produce a uniform, thin and compact film with enhanced enzyme activity. Electropolymerization of phenol was then employed to form an anti-interference and protective polyphenol film within the enzyme layer. In addition, a stability-reinforcing membrane derived from (3-aminopropyl)-trimethoxysilane was constructed by electrochemically-assisted crosslinking. This hybrid film outside the enzyme layer contributed both to the improved stability and to permselectivity. The resulting glucose sensor was characterized by a short response time (<10 s), high sensitivity (10.4 nA/mM mm 2), low interference from endogenous electroactive species, and a working lifetime of at least 60 days.

Alumina-Catalyzed Epoxidation with Hydrogen Peroxide: Recycling Experiments and Activity of Sol-Gel Alumina

Commercial alumina looses some activity after the first epoxidation reaction of (S)-limonene with hydrogen peroxide, but maintains a good activity and a very high selectivity in the subsequent three reactions. After this its activity is strongly reduced, probably due to structural modifications. Aluminas obtained by sol-gel methods are normally less active than the commercial alumina. However, the use of monomeric aluminum sec-butoxide and of oxalic acid to form stable alumina mesophases allows a very active alumina to be obtained, which catalyses the epoxidation of the less reactive cyclohexene with hydrogen peroxide in 98% yield. Close to 50% of the active oxygen is used up in the formation of molecular oxygen.

Ion beam induced luminescence of thin films

Precise control of ion range and ionisation rate makes ion beam induced luminescence (IBL) a useful analytical technique for studying thin films. Combinations of IBL and cathodo-luminescence (CL) offer complementary information. In the present studies of thin films of alumina produced by pulsed laser deposition (PLD), they reveal the influence of PLD growth conditions. Strong sensitisation from the presence of a layer of copper nanoparticles, changes in nanoparticles following thermal treatments and a range of intrinsic defects in the layers were observed. For multilayer films the IBL was particularly useful in noting signals from the deeper layers. Some data were also recorded for sol–gel alumina films grown on silica or borosilicate glass.

Decomposition of Nickel Formate on Sol–Gel Alumina and Characterization of Product by X-Ray Photoelectron and TOF-SIMS Spectroscopies

Samples of nickel-supported alumina were prepared from sol–gel and non-sol–gel alumina by impregnation of aqueous solution of nickel formate and the decomposition behavior of the nickel formate on alumina was then followed by combined thermal analysis and infrared spectroscopy. The decomposition products were characterized by X-ray photoelectron spectroscopy, time-of-flight secondary-ion mass- spectroscopy, and scanning electron microscopy. The surface areas and pore distributions were measured by BET. Nickel formate was found to decompose over a wide temperature range owing to its interaction with the alumina. The surface area of the sol–gel alumina after nickel loading increased whereas the non-sol–gel alumina remained unchanged. The products were amorphous, and they had a pore distribution in the mesoporous region. The nickel species existed as nickel aluminate, unsupported nickel oxide, stoichiometric nickel oxide, and metallic nickel. The nickel species were uniformly distributed on the surface. The particles agglomerated at the submicron level; no nickel species was found on the surface.