Efficiency of Pd impregnated sol-gel derived γ-alumina porous spheres as catalyst

Porous γ-alumina spheres having surface area as large as 300 m ² /gm are obtained by using sol-gel process. γ-alumina supported Pd catalyst is synthesized by soaking it in PdCl ₂ solution followed by drying and heating at 300 ^° C/1 hr in hydrogen atmosphere. Specific Surface Area of Pd coated alumina remained unaltered. The scanning electron microscopy revealed uniform coating of Pd metal on the surface. The equivalent spherical diameter (ESD) of the coated Pd metal particles is estimated to be 50. The catalytic efficiency is evaluated by H ₂ -O ₂ combination reaction. The performance of this supported catalyst is found to be much superior in comparison with Pd catalyst realized on commercially available alumina having surface area of 80 m ² /gm. The improved efficiency of the catalyst could be co-related to enhanced surface of nano-structured Pd available for catalytic action. The added advantage of spherical shape of the support material appeared in uniform and homogeneous loading of the catalyst bed. The results highlight the important role played by support material in these type of catalysts. Received 24 November 2000     

Functionalization of γ-alumina cores by polyvinylpirrolidone: properties of the resulting biocompatible nanoparticles in aqueous suspension

A biocompatible polymer has been used to functionalize 45–50 nm diameter γ-alumina nanoparticles. Because the target was to use these systems in real applications, polyvinylpirrolidone (PVP) was chosen due to the characteristics of non-toxicity, biocompatibility, and feasibility of this polymer to form complexes with many cations and chemical species. This approach allows the use of these materials in medicine and food, textile, or pharmaceutical industry. The functionalization process required a previous attachment of an active group on the surface of the nanoparticles. Subsequently, a polymer chain was generated in situ, using vinyltrimethoxysilane (VTMS) and 1-vinyl-2-pyrrolidone (VP) as reactives. The morphology and topology of the nanocompound has been characterized in aqueous suspensions, attending to possible applications in this medium. The results obtained from the different techniques show that the polymer chain was successfully grafted to the nanoparticle surface, and allow an estimation of the size of the modified particle. Their electrical and conformational behavior have also been studied in different aqueous chemical environments.     

Surface morphology of c-plane sapphire (α-alumina) produced by high temperature anneal

A comparative study of the morphological surface evolution of c-plane (0001) α-Al₂O₃ upon annealing was investigated for non-miscut (i.e. substrates with 0° nominal miscut) and vicinal substrates. The samples were annealed in air at 1100 °C for different durations of time. Although non-miscut samples do not show any step bunching at this temperature, miscut substrates show a regular and ordered stepped morphology with clearly defined terraces as revealed by Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) image analysis. The surface morphology presents a number of coalescence points, i.e. locations where two steps merge and form a multiple step. Close to the coalescence points, parallel steps change direction to different low index direction.     

Experimental investigation into the pool boiling heat transfer of aqueous based γ-alumina nanofluids

This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations over the past few years. The work is motivated by the controversial results reported in the literature and the potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to formulate stable aqueous based nanofluids containing γ-alumina nanoparticles (primary particle size 10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼ ∼40% at a particle loading of 1.25% by weight. Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids under the nucleate pool boiling conditions be associated with the properties and behaviour of the nanofluids and boiling surface, as well as their interactions.This revised version was published online in August 2005 with a corrected issue number.     

Electrical conductivity of nonstoichiometric Ba β-alumina single crystals prepared by a floating zone method

Nonstoichiometric Ba β-alumina single crystals, Ba_1−0.25xMg_1−xAl_10+xO_17+0.25x (0≦x≦1), were prepared in the BaO–MgO–Al₂O₃ system by a floating zone method. The single crystals were cleaved parallel to a (001) plane. The electrical conductivity parallel to the cleavage plane (σ_//) was 10 to 20 times greater than that perpendicular to the cleavage plane (σ_⊥). The σ_// and σ_⊥ had the maximum at around x=0.5. The activation energy of σ_⊥, 240 to 270 kJ/mol, was slightly greater than that of σ_//, 170 to 190 kJ/mol. The σ_// was almost independent of oxygen partial pressure (P_O2), while σ_⊥ showed a P_O2^−1/6 dependence in a low P_O2 region of P_O2=10⁻⁴ to 10⁻¹³ Pa.     

Ionic conduction in Sr hexa-aluminate with β-alumina structure

A new type of strontium β-alumina was developed by a single solid state reaction. Polycrystals of Sr β-alumina with the chemical formula of SrMgAl₁₀O₁₇ were prepared and examined by a conductivity. The values of ionic conductivity in the Sr β-alumina were 5 to 3 orders of magnitude lower than those of polycrystalline Na β-alumina between 773 and 1273 K. The ion exchange of Sr β-alumina for sodium was also examined, and the values of conductivity for the partially ion-exchanged samples increased with the increase of the excess amount of Na⁺ up to 0.5–0.6 in comparison with that of the ideal Na β-alumina.     

Intrinsic luminescence centers in γ- and θ-alumina nanoparticles

In this study, we investigate the photoluminescence(PL) properties of γ and θ-alumina nanoparticles synthesized by the chemical wet method followed by annealing. The obtained experimental results indicate the presence of some favorable near ultraviolet(NUV)-orange luminescent centers for usage in various luminescence applications, such as oxygen vacancies(F, F~+_2, F~(2+)_2, and F_2centers), OH related defects, cation interstitial centers, and some new luminescence bands attributed to trapped-hole centers or donor–acceptor centers. The energy states of each defect are discussed in detail. The defects mentioned could alter the electronic structure by producing some energy states in the band gap that result in the optical absorption in the middle ultraviolet(MUV) region. Spectra show that photoionazation of F and F_2 centers plays a crucial role in providing either free electrons for the conduction band, or the photoconversions of aggregated oxygen vacancies into each other, or mobile electrons for electrons-holes recombination process by the Shockley–Read–Hall(SRH)mechanism.     

Comment on “formation and crystallization of β-alumina from precursor prepared by sol-gel method using metal alkoxide”

In the above [1] article, an earlier work “Low Temperature Synthetic Studies of β-Aluminas”, on this subject [2], with similar results but with much more interpretation of the X-ray patterns, was not cited. The readers are referred to it for more details.     

Effect of partial fluorination on the Lewis sites of microcrystalline γ-alumina studied by adsorption of pyridine as a probe molecule: A quantum chemical cluster model study

The influence of fluorination of microcrystalline γ-alumina on the acidity of Lewis sites was studied by quantum chemical cluster model approach. B3LYP and HF/6-31++G(d,p) levels of theory were employed, considering the standard and counterpoise-corrected potential energy surfaces (PESs). Explicit inclusion of dynamical electron correlation effects along with the elimination of the basis set superposition effects in geometry optimization and harmonic vibrational analysis were found to be crucial to reproduce the experimental trends in the shifts of the pyridine ν_19b and ν_8a modes upon fluorination. The acidity of surface-exposed Al-sites was found to increase upon fluorination, which is manifested as a ~ 10% increase of the interaction energies, and also in the characteristics of the electronic density and density Laplacian at the intermolecular bond critical point. Bader analysis of the electronic density has shown that pyridine adsorption on pure and fluorinated γ-alumina can be classified as non-covalent interaction.     

Estimation of higher-order magnetic spin interactions of Fe(III) and Gd(III) ions doped in α-alumina powder with multifrequency EPR

Fe(III) and Gd(III) ions in α-alumina (A1₂O₃) exhibit spin states ofS = 5/2 andS = 7/2 respectively. The magnitude of the zero-field interaction (ZFI) (D = 0.10?0.15 cm^?1) gives rise to an inter Kramers doublet splitting in the same order of magnitude as the X-band electron paramagnetic resonance (EPR) quantum (0.3 cm^?1). It is demonstrated that through a careful step-by-step analysis and spectral simulation of EPR spectra taken at D-band (130 GHz), Q-band (35 GHz), and X-band (9 GHz) at room temperature, the (relative) sign and magnitude of the ZFI parameters, b ₂ ⁰ , b ₄ ⁰ , and b ₄ ³ , can be reliably estimated.     

Sodium profiles in β″-alumina crystals: Modifications induced by argon bombardment

Abstract

Sodium depth profiles in implanted sodium β″-alumina single crystals have been measured by the nuclear resonance technique. A systematic investigation of the depth profile modifications as function of the implanted ion energy has been done using argon-ion (E = 50–600 keV) irradiation at fixed dose (Φ = 4 × 10¹⁶ions/cm²) and beam current (I = 1 μA/cm²). Argon doses were checked by Rutherford backscattering spectrometry. The changes in the sodium profiles are discussed in terms of transport equations which include three main processes: radiation enhanced transport, electric field assisted migration, and preferential surface sputtering of the alkali element. Special attention is devoted to the discussion of sputtering processes.     

Free-flowing, transparent γ-alumina nanoparticles synthesized by a supersonic thermal plasma expansion process

Nanoparticles of crystalline, phase-pure γ-alumina is synthesized in a supersonically expanded thermal plasma jet assisted experimental chemical reactor, with good control over the average particle sizes independently with respect to plasma current, oxygen flow rate and the ambient pressure in the sample collection chamber. Most of the synthesized particle samples were seen to be transparent, which gets more transparent with decrease in the particle sizes. The lowest achievable pressure in the chamber had produced particles with average 10 nm sizes, which was best also in terms of narrow size distribution. Another important observation was the absence of serious inter-particle agglomeration, producing free-flowing particles. Optical emission spectroscopic technique was used to study the plasma chemistry of the reaction zone as well as the plasma jet.     

Rare earth doped α-alumina thin films prepared by pulsed laser deposition: structural and optical properties

-Al₂O₃ films doped with about 1% Eu³⁺ were grown on sapphire (012) substrates by pulsed laser deposition using an ArF excimer laser under 10^-4 mbar oxygen pressure and at temperatures higher than 1050 °C. The Eu doping rate was measured by Rutherford backscattering spectroscopy. The crystalline structure was determined by X-ray diffraction and transmission electron microscopy. The films are grown epitaxially on the sapphire substrate. The Eu³⁺ fluorescence spectra are constituted of narrow lines. By means of pulsed laser deposition, Eu³⁺ ions may be introduced into a single site of the -phase lattice, with a concentration that cannot be reached using conventional crystal-growth methods. PACS 81.15.Fg; 68.37.Lp; 78.55.Hx     

Methane dehydrogenation on monomeric Rh center located on (100) γ-alumina — A theoretical study

The reaction mechanism of methane dehydrogenation on monomeric Rh center located on (100) γ-alumina has been theoretically investigated at the PBE0/cc-pVTZ, SDD level. The (100) face of γ-alumina support is represented by an Al₈O₂₆H₂₈ cluster, which is cut out from the ideal crystal structure. Then, two model Rh/γ-Al₂O₃ catalysts, in which Rh center interacts with one oxygen or two oxygen atoms of the (100) surface of γ-Al₂O₃, have been compared and denoted as Rh/Al₈O₂₆H₂₇ and Rh/Al₈O₂₆H₂₆, respectively. Toward methane activation, the model catalyst Rh/Al₈O₂₆H₂₇ exhibits better performance than Rh/Al₈O₂₆H₂₆. For the first CH bond cleavage of methane, the lowering of activation barriers on Rh/Al₈O₂₆H₂₇ is mainly caused by lower substrate activation strain ΔE^≠_strain[substr], which is from substrate equilibrium geometry to the geometry it adopts in the TS, in comparison with that on Rh/Al₈O₂₆H₂₆. These results are in qualitative agreement with the experimental results, where the partially reduced Rh⁺ is one of the active sites for methane dissociation.     

Photoluminescence, FTIR and X-ray diffraction studies on undoped and Al-doped ZnO thin films grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis

Undoped and aluminum-doped zinc oxide (ZnO) thin films have been grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis (USP) technique using zinc acetate dihydrate and aluminum chloride hexahydrate (Al source) dissolved in methanol, ethanol and deionized water. A number of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and photoluminescence (PL) were used to characterize the obtained ZnO thin films. It was seen that the orientation changed with increase in substrate temperature. During the ZnO deposition Zn source reacted with polycrystalline α-Al₂O₃ substrate to form an intermediate ZnAl₂O₄ spinel layer. It has been interestingly found that the intensity of green emission at 2.48 eV remarkably increased when the obtained ZnO:Al films were deposited at 380 °C. The FTIR absorbance intensity of spectroscopic band at 447±6 cm⁻¹ is very sensitive to oxygen sublattice disorder resulting from non-stoichiometry, which is consistent with the result of PL characterization.     

Influence of Self-Interaction ofσ Meson to the Equation of State of Nuclear Matter

In the relativistic σ-ωmodel, the influence of the parameters in self-interaction of a meson to the equation of state of normal nuclear matter, especially, to incompressibility, effective mass, and coupling constants, is studied in detail. We find that these parameters have an intense relationship to the property of nuclear matter. At the same time , we study the relation between the binding energy and pressure of relativestic △-resonance nuclear matter and temperature using using above results in the relativistic σ-ω-π model,and it is interesting to compare it to our prior work. In all these studies, the vacuum fluctuation on nucleon, △-isobar, and σmeson is considered.