高分子保护沉淀法制备超细纳米氧化镁

纳米氧化镁具有不同于本体材料的光、电、磁、化学特性,特别是超细纳米氧化镁由于其颗粒直径小、比表面积大,具有高纯度、高硬度和高熔点,高的反应活性,强吸附性,良好的低温烧结性,高电阻率等优良性质,可用于高绝缘材料,高质量的陶瓷材料,高性能阻燃纤维,环境保护的吸附剂、负载型甲醇和低碳醇合成的催化剂载体等领域,是一种有广泛应用价值的新型无机材料[1￣5]。已见报道的纳米氧化镁的制备方法有电子束蒸发法[6]、化学气相沉积法[7]、金属醇盐水解法,化学沉淀法[8,9],固相法[10],燃烧法[11],溶胶-凝胶法[12,13]等,然而由于氧化镁容易发生…     

Preparation and characterization of MgO/Al2O3 mixed oxides support for hydrotreating catalysts

The characteristics of pure MgO, Al₂O₃ and three binary mixtures of MgO-Al₂O₃ were studied by determining their BET surface areas, micropore surface area, total pore volume, adsorption-desorption isotherms, the x-ray diffractogram, surface acidity and catalytic functionality for cumene cracking.The XRD results show that the incorporation of alumina into the magnesia beyond 50% renders it amorphous. Furthermore, the mixed oxide containing 50% magnesia and 50% alumina had the highest BET surface area of 200 m²/g and is the only mixed oxide with bimodal pores.The results of the acidity measurements and cumene cracking functionality indicates that strong acid sites are found only in mixed oxides containing more than 50% alumina.     

Positron annihilation and electrical studies on the influence of loading magnesia nanoribbons on PVA-PVP blend

This work aims to study the effect of loading magnesia (0.5–2.0 wt%) on the positron annihilation parameters and electrical properties of the PVA-PVP blend. The films were synthesized by solution casting and checked by different techniques. XRD and HR-TEM of sol-gel prepared magnesia revealed that the average crystallite size was 14.29 nm with ribbon-like morphology with varying widths and lengths up to a few micrometers. SEM showed that the blend surface appeared smooth and homogenous and this confirmed the compatibility between PVA and PVP. However, loading magnesia increased surface roughness. TGA confirmed the thermal stability enhancement of blend film with magnesia incorporation. Ortho–positronium lifetime τ₃ and the free volume $V_f$ decrease with loading magnesia while the intensity I₃ is nearly constant. These features were interpreted in view of the hole-filling mechanism, the interaction between magnesia & blend and magnesia morphology. The equilibrium swelling ratio ESR was found for the studied films and a positive correlation between ESR and $V_f$ was reported. The current density-electric field, J-E, characteristics were of non-ohmic type. J increased with increasing magnesia levels and heating. The Richardson-Schottky effect was the dominant dc conduction mechanism at low temperature and low magnesia loading levels while it changed to Poole–Frenkel emission at higher values. Finally, a correlation between Log J and the intensity I₂ was reported.     

Precipitation and purification of uranium from rock phosphate

This study was carried-out to leach uranium from rock phosphate using sulphuric acid in the presence of potassium chlorate as an oxidant and to investigate the relative purity of different forms of yellow cakes produced with ammonia, magnesia and sodium hydroxide as precipitants, as well as purification of the products with TBP and matching its impurity levels with specifications of the commercial products. Alpha-particle spectrometry was used for determination of activity concentration of uranium isotopes in rock phosphate, resulting phosphoric acid, and in different forms of the yellow cake. Likewise, atomic absorption spectroscopy was used for determination of impurities. On the average, the equivalent mass concentration of uranium was 119.38 ± 79.66 ppm (rock phosphate) and 57.85 ± 20.46 ppm (phosphoric acid) with corresponding low percent of dissolution (48 %) which is considered low. The isotopic ratio (²³⁴U:²³⁸U) in all stages of hydrometallurgical process was not much different from unity indicating lack of fractionation. Upon comparing the levels of impurities in different form of crude yellow cakes, it was found that the lowest levels were measured in hydrated trioxide (UO₃·xH₂O). This implies that saturated magnesia is least aggressive relative to other precipitants and gives relatively pure crude cake. Therefore, it was used as an index to judge the relative purity of other forms of yellow cakes by taking the respective elemental ratios. The levels of impurities (Fe, Zn, Mn, Cu, Ni, Cd and Pb) in the purified yellow cake were found comparable with those specified for commercial products.     

Effect of the Support on the Structural Characteristics of Tin

Tin oxide was supported on alumina, titania, magnesia and silica, treated with hydrogen at different temperatures and characterized by Mössbauer spectroscopy and X-ray diffraction. For the samples calcined at 773 K, tin is present as SnO₂ on alumina, magnesia and silica, but it occupies Ti sites on titania. After hydrogen treatment at high temperatures, tin is reduced from Sn(IV) to Sn(II) on alumina and titania, from Sn(IV) to Sn(0) on silica, but practically not reduced on magnesia. These results show the different degree of interaction between tin and the supporting material.     

Heat-treated magnesia as an adsorbent for the class separation of aliphatic monocarbonyl compounds

In order to control the activity of magnesia as an adsorbent, the effect of temperature was studied. The activity of various temperature of treatment showed two regions. During the first, in the range 106–320°, the activity increased with increasing temperature, whereas at temperatures above 320° the activity decreased with increasing temperature.The iodine number of heat-treated magnesia correlated well with its affinity for 2,4-dinitrophenylhydrazones of carbonyls.By heating several batches of magnesia at graphically determined temperatures. it was possible to obtain adsorbents with the same activity for class separation.     

FT-IR study of the δ(OH) mode of surface hydroxy groups on metal oxides

Bands due to the in-plane bending mode of hydrogen-bonded surface hydroxy groups are observed on alumina, zirconia, magnesia and thoria samples. Their assignment is based on the effect of dehydration by heating under evacuation, and isotopic exchange using D₂O and H₂O¹⁸. The observed frequencies are apparently related to the position of the cut-off limit of the samples, suggesting a coupling of the δOH mode with the ν(MOH) mode.     

Multicomponent CoMgONaZSM-5 catalyst for hydrocarbon synthesis from CO + H2 mixtures containing CO2 and/or N2

The catalyst under study was prepared by kneading a physical mixture of precipitated cobalt hydroxycarbonate, magnesia and NaZSM-5 with water as a binder. This mixture was pretreated by drying at 120 °C or by calcination at 450 °C. Adsorption measurements revealed that NaZSM-5 was not affected by the solid state reaction with cobalt component and magnesia. When the catalyst was reduced, the metallic phase originated from different precursors depending on the thermal pretreatment. The composition of the unreduced part of the oxidic cobalt was also affected; after reduction, the surface of the dried sample contained both Co(II) and Co(III) in the ratio of 1.8 to 1.0, while that of the calcined sample consisted mostly of Co(III). Carbon monoxide was adsorbed in several forms, but only the most weakly bonded form desorbed without decomposition. Pretreatment by calcination caused a shift in the optimum synthesis temperature and a very marked decrease in the C₅⁺ fraction yield compared with the dried sample. The addition of nitrogen to the syngas caused an increase in the liquid hydrocarbon selectivity, while carbon dioxide, especially at higher concentrations, decreased the hydrocarbon yield substantially. The effect of reaction pressure depended on the feed gas composition.     

Photostimulated exoemission from the surface of magnesia

Photostimulated exoemission (PSE) from magnesia was studied along with the effect of the adsorption of active gases (H₂, O₂, CO₂, and water vapors) on the intensity and kinetics of the emission decay. Based on the literature data, it was shown that PSE was excited from the local surface centers including a pair of low-coordinated ions Mg_1c²⁺-O_1c²⁻ and V_s hole centers, which were also active centers of photosorption and photocatalytic oxidation of methane and some other hydrocarbons.     

Microstructure and oxygen permeability of a La0.6Sr0.4Fe0.9Ga0.1O3−δ membrane containing magnesia as dispersed second phase particles

Dense mixed-conducting membranes of La_0.6Sr_0.4Fe_0.9Ga_0.1O_3−δ (LSFG) with various contents of MgO as second phase particles were prepared to evaluate the influence of magnesia inclusions on LSFG stoechiometry, microstructure and oxygen permeation. XRD and EDS investigations on sintered pellets revealed that magnesia inclusions were quite inert with the LSFG matrix phase, the composition of which remained identical whatever the magnesia content. LSFG pure phase material was synthesized through a solid-state route and sintered between 1250 and 1350 °C. Sintering temperature strongly affected microstructure of the LSFG membrane since rapid grain growth and decreasing density were observed when temperature increased. Small amounts of fine particles of magnesia, from 2 to 10 vol%, were found to significantly reduce grain size of sintered samples and made it possible to obtain a high density on a large sintering temperature range. Average grain size experimental data of LSFG in function of the amount of second phase magnesia were also compared with numerical models from literature. Oxygen permeation rates of pure LSFG and composite LSFG/MgO dense membranes were measured in an air/argon gradient, in a temperature range from 825 to 975 °C and results were discussed to explain the flux improvement of composite membranes.     

An ESR study of electron transfer and oxygen adsorption on 95Mo supported catalysts

ESR studies using molybdenum enriched in ⁹⁵Mo have confirmed that (O₂^?)_s is stabilised at Mo⁶⁺ sites at 77 K on silica and alumina supported molybdenum. Warming the Mo/Al₂O₃ to 300 K leads to an increase in the number of (O₂^?)_s ions observed due to electron transfer from reduced molybdenum sites to give additional (O₂^?)_s adsorbed at Al³⁺ sites on the support. Molybdenum is not involved at the adsorption site of (O₂^?)_s on the magnesia supported catalyst.     

Separation of niobium and tantalum with cupferron

An attempt to separate niobium and tantalum by cupfcrron was only moderately successful at pH 4.5 to 5.5 in the presence of a magnesia mixture as a coagulating agent. A more satisfactory separation of niobium and tantalum from each other, tried out up to ratios of 30:1 and 1.30, is effected with Sn⁺² or Sn⁺⁴ as a co-precipitating agent under the conditions described niobium can be separated, in the presence of complexone III, from almost all the ions except U, Be, Ti and PO₄^-3. Iron and other tervalent elements, when present in 100 fold excess with respect to niobium, require double precipitation The method gives highly satisfactory results when applied to the analysis of niobium in niobium-molybdenum stainless steel.The use of titanium as a co-precipitant is less successful than that of tin     

Application of computer modeling to surface organometallic chemistry

Computer modeling studies of several surface organometallic complexes have been performed. These studies demonstrate the utility of the modeling technique as well as providing insight into the interaction between the surface organometallic species and the support surface. Models for hydroxylated oxide supports for silica, γ-alumina and magnesia are described. The surface structures for MORh(η³-C₃H₅)₂ (MAl, Si), Mg/[HFeOs₃(CO)₁₃]⁻, and (μ-H) (μ-OMM) Os₃(CO)₁₀ (MAl,Si) are modeled to determine the preferred arrangement of the organometallic species on the hydroxylated support. In the modeling procedures van der Waals interaction energies and non-bonded contacts as a function of orientation of the cluster with respect to the surface are considered. Relaxation of the cluster and Coulombic interaction energies are also considered where appropriate.     

Stereoselective single (copper) or double (platinum) boronation of alkynes catalyzed by magnesia-supported copper oxide or platinum nanoparticles

Copper(II) oxide nanoparticles supported on magnesia have been prepared from Cu(II) supported on magnesia by hydrogen reduction at 400 °C followed by storage under ambient conditions. X-ray photoelectron spectroscopy of the material clearly shows that immediately after the reduction copper(0)-metal nanoparticles are present on the magnesia support, but they undergo fast oxidation to copper oxide upon contact with the ambient for a short time. TEM images show that the catalytically active CuO/MgO material is formed of well-dispersed copper oxide nanoparticles supported on fibrous MgO. CuO/MgO exhibits a remarkable catalytic activity for the monoborylation of aromatic, aliphatic, terminal, and internal alkynes, the products being formed with high regio- (borylation at the less substituted carbon) and stereoselectivity (trans-configured). CuO/MgO exhibits complete chemoselectivity towards the monoborylation of alkynes in the presence of alkenes. Other metal nanoparticles such as gold or palladium are inactive towards borylation, but undergo undesirable oligomerization or partial hydrogenation of the C≡C triple bond. In contrast, platinum, either supported on magnesia or on nanoparticulate ceria, efficiently promotes the stereoselective diborylation of alkynes to yield a cis-configured diboronate alkene. By using platinum as the catalyst we have developed a tandem diborylation/hydrogenation reaction that gives vic-diboronated alkanes from alkynes in one pot.     

Manufacture of Boron-free Magnesia with High Purity from Residual Brine

A novel method for removing boron with ion exchange resin from residual brines tomanufacture boron-free magnesia is described. The concentration of boron in the target magnesiamanufactured thereby from Qinghai salt lakes is lower than 5 μg/g, and the typical D50 size ofproduct is 10.625 μm.     

Vacuum balance studies of phosphate-bonded oxide ceramics

Methods of phosphate bonding of oxide ceramics are summarised. Applications of vacuum balance techniques are described to study the sintering of lime, magnesia, alumina and kaolin in the presence of phosphoric acid and sodium polymetaphosphate additives. Changes in surface area, crystallite and aggregate sizes during sintering have been determined by gravimetric B.E.T. nitrogen gas sorption techniques, supported by X-ray diffractometry, optical and electronmicroscopy.Phosphoric acid has comparatively little influence on lime and magnesia sintering. There is no enhancement of sintering and even slight activation at temperatures between 800–1000 °C, when chemical reactions with the additive are completed. In contrast, sodium polymetaphosphate additive enhances sintering of lime and magnesia at temperatures between 500–1000 °C. When kaolin is calcined with sodium polymetaphosphate, the activated alumina in the intermediate metakaolinite readily forms aluminium phosphate to accelerate sintering.     

Novel preparation of nanocrystalline magnesia-supported caesium-promoted ruthenium catalyst with high activity for ammonia synthesis

A caesium-promoted ruthenium catalyst supported on nanocrystalline magnesia with high activity for ammonia synthesis was conveniently prepared by using hydrated ruthenium trichloride and hexahydrate magnesium nitrate as precursors, whereas dechlorination post-treatment and ready-made magnesia were not necessary.     

Investigation of the Reaction of Roasted Serpentine Ore with Some Ammonium Salts

The reaction between roasted serpentine ore and ammonium sulfate was studied at the range of temperature 250–1000°C using different molar ratios to determine the maximum extraction of magnesia and also to characterize the different reaction products. The maximum extraction of MgO from the roasted ore reached 92.4% at 400°C. It was found from XRD that ammonium magnesium sulfate [(NH₄)₂Mg₂(SO₄)₃] was produced as the main product at 400°C, which decomposes to magnesium sulfate at 500–600°C. The last compound decomposes to magnesium oxide at 900–1000°C. Thermal analysis of the reaction mixture confirmed the results obtained by XRD. Extraction of magnesia by ammonium chloride at 300–400°C showed low percentage of extraction (7.8%). Comparison was made between using ammonium chloride instead of sulfate taking into consideration the thermal decomposition products of both ammonium salts. Extraction of magnesia from the roasted ore by aqueous ammonium sulfate or ammonium chloride showed good results.     

Preparation and characterization of phosphatidylcholine‐coated zirconia–magnesia stationary phase for artificial membrane chromatography

Immobilized artificial membrane chromatography stationary phase was prepared by coating soybean phosphatidylcholine (PC) on zirconia–magnesia micro‐particles. The stability and chromatographic properties were investigated and compared with the PC‐coated silica chromatography stationary phase prepared by the same method. PC‐coated zirconia–magnesia chromatography stationary phase was more stable than the silica especially on resisting organic solvents. Hydrophobic action was the main factor for the retention of analyte on the new artificial membrane chromatography stationary phase, and electrostatic interaction had some contribution to retention. In addition, the special interaction between analyte and matrix affected retention greatly. Basic solutes were appropriate to be analyzed on PC‐coated zirconia–magnesia stationary phase and acidic solutes were appropriate to be done on the silica one. Hence, the two different matrices artificial membrane stationary phases were perfectly complementary.     

Influence of magnesia modification on the properties of copper oxide supported on gamma-alumina

XRD, BET, TPR, UV-vis DRS and in situ FT-IR were employed to investigate the dispersion, reduction and CO(2)-adsorption behaviors of copper oxide supported on magnesia modified gamma-Al(2)O(3) (Mg-Al) samples. The results indicate that magnesia could be highly dispersed on the surface of gamma-Al(2)O(3) to form a monolayer and the dispersion capacity is about 1.55 mmol/(100 m(2)gamma-Al(2)O(3)). For copper oxide supported on Mg-Al samples, both the dispersion capacity and the reduction temperature of surface CuO decrease with the MgO loading. CO(2)-adsorption IR results show that the surface strong basic amount for the catalysts increases with the dispersed MgO loading. In addition, the activity of CO oxidation suggests that the main active species in this system should be small CuO cluster and the existence of dispersed MgO enhances the activity of CO oxidation. The catalysts might be applied in pollution control devices for vehicle exhaust, CO gas sensors, catalytic combustion and gas purification of CO(2) lasers. All the results have been discussed by the consideration of the variation of gamma-Al(2)O(3) surface structure before and after magnesia modification.