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Abstract  

The surface properties of supported gallium oxide catalysts prepared by impregnation of various supports (γ-Al2O3, SiO2, TiO2, ZrO2) were investigated by adsorption microcalorimetry, using ammonia and water as probe molecules. In the case of acidic supports (γ-Al2O3, ZrO2, TiO2), the acidic character of supported gallium catalysts always decreased in comparison with gallium-free supports; on very weakly acidic SiO2, new acidic centers were created when depositing Ga2O3. The addition of gallium oxide decreased the hydrophilic properties of alumina, titania and zirconia, but increased the amount of water adsorbed on silica. The catalytic performances in the selective catalytic reduction of NO by C2H4 in excess oxygenwere in the order Ga/Al2O3>Ga/TiO2>Ga/ZrO2>>Ga/SiO2. This order is more related to the quality of the dispersion of Ga2O3 on the support than to the global acidity of the solids.

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Solid solutions in the MgO-Al2O3-Cr2O3 system

Effects of polymorphism, temperature and pressure

Journal of Thermal Analysis and Calorimetry
Authors: A. Turkin and V. Drebushchak

Abstract  

Coexisting solid solutions with spinel and corundum structure were synthesized at 1773 K and two pressures, 1 bar and 25 kbar. Samples were analyzed by electron microprobe analysis and X-ray powder diffraction. Pressure and temperature were shown to affect the properties of the solid solutions in different ways. Pressure governs the composition of the defect spinel Mg1−xAl2O4, and temperature changes the cation distribution between coexisting phases. This allows one to separate the effects of cation exchange and magnetic contribution to the heat capacity in thermodynamic modeling. The defect spinel itself can form only because γ-Al2O3 exists, polymorph with spinel structure. Thermodynamic considerations argue in favor of eskolaite-spinel assemblages prevailing over corundum-picrochromite ones at very high temperatures deep in the Earth.

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Abstract  

Gamma-alumina membrane was prepared from anodic (amorphous) alumina (AA) obtained in a sulphuric acid electrolyte. The transformation scheme, i.e., the crystallization to form metastable alumina polymorphs and the final transition to α-Al2O3 with heating was studied by TG-DTA and X-ray diffraction (XRD) using fixed time (FT) method. When heating at a constant rate, the crystallization occurred at 900C or higher and the final formation of α-Al2O3 occurred at 1250C or higher, which temperatures were higher than the case of using anodic (amorphous) alumina prepared from oxalic acid electrolyte. Relative content of S of the products was obtained by transmission electron microscope (TEM)-energy dispersive spectroscopy (EDS). The proposed thermal change of anodic alumina membrane prepared from sulphuric acid is as follows: 1. At temperatures lower than ca 910C: Formation of a quasi-crystalline phase or a polycrystalline phase (γ-, δ- and θ-Al2O3); 2. 910–960C: Progressive crystallization by the migration of S toward the surface within the amorphous or the quasi-crystalline phase, forming S-rich region near the surface; 3. 960C: Change of membrane morphology and the quasi-crystalline phase due to the rapid discharge of gaseous SO2; 4. 960–1240C: Crystallization of γ-Al2O3 accompanying δ-Al2O3; and 5. 1240C: Transition from γ-Al2O3 (+tr. δ-Al2O3) into the stable α-Al2O3. The amorphization which occurs by the exothermic and the subsequent endothermic reaction suggests the incorporation of SO3 groups in the quasi-crystalline structure.

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Abstract  

The removal of heavy metal ion Co(II) from aqueous solution is studied using γ-Al2O3 by batch technique. The experiments are performed at T = 20 ± 2 °C, in 0.01 M NaNO3 solutions and under aerobic conditions. The effect of pH, ionic strength, fulvic acid (FA) and alumina concentration on the sorption of Co(II) on alumina are also respectively investigated. The pH affects the sorption of Co(II) significantly as compared with the effect of FA and ionic strength. The results indicate that strong chemical bonds are formed between Co(II) and the bare or FA coated alumina surface, and a transition from the adsorption to surface-induced precipitation of Co(II) on alumina surface takes place. The addition sequences of Co/FA on Co(II) sorption is also studied and the results indicate that the sorption of Co(II) in ternary system is independent of addition sequences. The results also suggest that the sorption of metal ions on mineral surface depends on the nature of mineral, nature of organic ligand and nature of metal ion.

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Reaction Kinetics, Mechanisms and Catalysis
Authors: Leonardo Maciel, Aleksándros de Souza, Valderio Cavalcanti-Filho, Augusto Knoechelmann, and Cesar de Abreu

Abstract  

The conversion of natural gas was carried out via tri-reforming of methane in a fixed bed reactor employing a Ni/γ-Al2O3 catalyst. The kinetic evaluations were performed in a temperature range from 923 to 1,123 K under atmospheric pressure. The effects due to water and oxygen addition to the feed of the process were examined in terms of the yields of hydrogen and carbon monoxide. Contributions of the reverse water–gas shift and oxidation reactions were evaluated. At temperatures above 1,000 K, methane and carbon dioxide conversions of 97.35 and 46.75% produced hydrogen and carbon monoxide with yields of 37.35 and 4.99%, respectively. A model was proposed to describe the kinetic behavior of the process considering the proposition of a four step reaction mechanism. The solutions of the equations of the model established predictions in terms of reactant and products concentration evolutions. The model predictions indicated that for operations at 1,123 K and 1.0 bar, with low spatial times (τ < 2.0 kg s/m3), a hydrogen yield as high as 75% was obtained.

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Abstract  

An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative conditions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Precipitated boehmites had high values of S BET determined from nitrogen adsorption (220–300 m2g–1), good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its dehydroxylation into γ-Al2O3, and delayed transformation of γ phase into α-Al2O3. Aluminium oxides derived from the hydroxides precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties.

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The aldol condensation of methyl acetate with formaldehyde to form methyl acrylate was studied in a continuous-flow reactor using a series of supported cesium basic catalysts with commercially available materials (ZSM-5, SiO2, and γ-Al2O3) as carriers prepared by vacuum impregnation. The catalysts were characterized by N2 adsorption-desorption, Fourier transform-infrared (FT-IR), X-ray diffraction (XRD), and temperature-programmed desorption of ammonia and carbon dioxide (NH3/CO2-TPD). The obtained results indicated that the selectivity of methyl acrylate was mainly influenced by the properties of supports. The formation of acetone is approximately proportional to the acidity of supports. The basicity of the catalysts was favorable to the formation of methyl acrylate according to the results of CO2-TPD. The hydrolysis of methyl acetate was inhibited over Cs-HT-SiO2 prepared by SiO2 after hydrothermal treatment. Furthermore, SiO2 with the large mesoporous volume is superior to other supports, which shows the best catalytic activity for the aldol condensation reaction. On the other hand, the catalytic performance of zeolite basic catalysts was strongly influenced by the effect of reactant diffusion. Internal diffusion resulted in the increase of conversion of methyl acetate with increasing specific surface area, while the conversion of methyl acetate decreased with increasing the weight hourly space velocity (WHSV) due to the external diffusion.

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A graphic method is proposed to determine all of the kinetic parameters in Wigner-Polanyi equation of desorption. A desorption rate curve from a single temperature-programmed desorption experiment is required by this method to determine the order of reaction (n), the activation energy (E d) and the pre-exponential factor (v) of the equation. The proposed method has been applied to the oxygen desorption from PdO/Al2O3 samples prepared by impregnating γ-Al2O3 with H2PdCl4 solution used as examples. From the graphic method, the values ofn=2, andv=1.37±0.80×109 s−1 were successfully determined for the desorption. The value ofE d depended on the dispersion of palladium (D) on PdO/Al2O3 samples, and was expressed by the equation:E d=175+174D kJ·mol−1. This graphic method is a direct and time-saving technique, on comparing with other methods suggested in the literature, for analysis of data from temperature-programmed desorption of simple desorption processes.

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Abstract  

Rice husk is a by-product of rice milling process and are a major waste product of the agricultural industry. They have now become a great source as a raw biomass material for manufacturing value-added silicon composite products, including silicon carbide, silicon nitride, silicon tetrachloride, pure silicon, zeolite, fillers of rubber and plastic composites, adsorbent and support of catalysts. The bulk and true densities of raw rice husk with different moisture and sizes were determined. The rice husk was subjected to pyrolysis in fluidized-bed reactor in air or nitrogen atmosphere. The products obtained were characterized by thermogravimetric and X-ray powder analysis, IR-spectroscopy, scanning electron microscopy and nitrogen adsorption at 77 K. The specific surface area of the products is comparable with this of γ-Al2O3. The kinetics of H2SeO3 adsorption out of aqueous solutions at 298 K was studied. The adsorption capacity of white rice husks ash was found to be higher than that of black rice husk ash and the adsorption kinetics obeyed the second order kinetic equation.

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Abstract  

The effect of γ-irradiation on surface and catalytic properties of CuO/Al2O3, NiO/Al2O3 and Fe2O3/Al2O3 was investigated. The techniques employed were XRD, nitrogen adsorption at −196 °C and catalytic conversion of ethanol and isopropanol at 250–400 °C using micropulse technique. The results showed that the supported solids being calcined at 400 °C consisted of well crystallized CuO, NiO, Fe2O3 and AlOOH phases. The AlOOH crystallized into a poorly crystalline γ-Al2O3 upon heating at 600 °C. All phases present in different solids calcined at 400 and 600 °C showed that these solids are of nanocrystalline nature measuring an average crystallite size between 6 and 85 nm. The crystallite size of crystalline phases present was found to be much affected by the dose of γ-rays and the nature of the metal oxide. This treatment resulted in a progressive increase in the specific surface area reaching to a maximum limit at a dose of 0.8 MGy. The dose of 1.6 MGy exerted a measurable decrease in the S BET. A radiation dose of 0.2 to 0.8 MGy brought about a progressive significant decrease in the catalytic activity of all the catalytic systems investigated. All the catalytic systems retained their high activity upon exposure to a dose of 1.6 MGy. The rise in precalcination temperature of the systems investigated from 400 to 600 °C brought about a measurable increase in their catalytic activity in the conversion of alcohols.

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