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.     

Low-Density Forsterite (Mg2SiO4) Powders Prepared from Modified Alkoxides

2MgO-SiO₂ low-density powders have been prepared from alkoxide precursors modified with acetic anhydride. The resultant solutions did not gel, but formed precipitates. These consisted of an organic magnesium compound and amorphous silica. Tapping densities comparable with those of forsterite aerogels were obtained, with the density increasing as the precursor solution concentration increased. Crystalline forsterite formed around 790°C. Higher initial solution concentrations led to greater amounts of MgO in the powders after heating to 1000°C. Transmission electron microscopy showed primary particles between 20 and 30 nm which formed loose fractal agglomerates. By altering the preparation conditions, powders with morphologies more like aerogels than xerogels could be produced.     

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.     

Application of Alumina Aerogels as Catalysts

Al₂O₃ gels prepared by hydrolysis of Al-alkoxide were applied as catalysts for NO reduction by hydrocarbon. Xerogels were obtained by drying at 90°C, for 24 h, in air. Aerogels were prepared by supercritical drying of the wet gels in ethanol, using an autoclave. The catalyst activity of the gels for NO reduction with C₃H₆ as determined at 200–600°C in a fixed bed flow reactor. NO conversion to N₂ was about 60% at 550°C with both catalysts, and at 600°C it was better with the aerogel than with the xerogel. The specific surface area of the xerogel, which was larger than that of the aerogel as-dried, decreased to 1/3 by annealing at >500°C. However, the surface area of the aerogel hardly changed by annealing at temperatures up to 700°C. The bimodal pore size distribution of the aerogel hardly changed by annealing, too. The microstructure of the aerogels is stable at high temperatures, and they are better catalysts at high temperatures.     

Ni／ZrO2-Al2O3制备表征及催化性能的研究

本文采用浸渍沉淀法制备了不同配比的ZrO2-Al2O3复合载体。并通过浸渍法制备Ni/ZrO2-Al2O3催化剂,以苯加氢制环己烷反应为探针,考察了ZrO2与Al2O3的配比对Ni催化剂催化加氢性能的影响;采用X射线衍射(XRD)、程序升温还原(TPR)、程序升温脱附(TPD)等技术考察复合载体对Ni催化剂的体相结构、还原性能以及表面吸附性能的影响。研究结果表明,ZrO2质量分数为20%的复合载体所负载的Ni催化剂有很好的加氢活性,优于单组分载体负载的Ni催化剂;采用浸渍沉淀法制备的ZrO2-Al2O3复合载体中ZrO2以非晶态形式存在,这是由于Al2O3的存在影响了ZrO2的内部结构;该载体负载的Ni催化剂较其他催化剂更容易被还原,吸附中心数量增加。     

MoO3/ZrO2纳米固体强酸催化剂的制备及其在异丁烷-丁烯烷基化反应中的应用

利用AOT/异辛烷反胶束体系制备了MoO3/ZrO2纳米粒子.TEM结果表明,反胶束法制得粒子的粒径均匀,95%以上处于38～60nm之间.将此纳米粒子负载于γ-Al2O3上,呈现高度分散状态.NH3-TPD和烷基化反应的测定结果表明,其酸量和反应活性明显高于浸渍法和溶胶-凝胶法制备的样品,烷基化产物中C8的含量在79%左右.     

Catalytic Combustion of Methane over MnOx／ZrO2-Al2O3 Catalysts

MnOx/Al2O3 and MnOx/ZrO2-Al2O3 catalysts were prepared by incipient wetness impregnation of Mn(CH3COO)2 on the corresponding supports, followed by the characterization using X-ray diffraction (XRD). temperature programmed reduction (TPR) and BET surface area techniques. The result shows the BET surface area of ZrO2-Al2O3 is lower than that of Al2O3 due to the loading of ZrO2.However tile resulted MnOx/ZrO2-Al2O3 catalyst exhibits higher activity for methane combustion than MnOx/Al2O3, because the addition of ZrO2 onto Al2O3 is beneficial for the dispersion of Mn species and the improvement of the lattice oxygen activity in MnOx. subsequently the activation of methane during combustion. The optimum loading of Zr in MnOx/ZrO2-Al2O3 is in the range of 5%-10% correlated with the calcination temperatures of catalyst supports.     

Sol-Gel Preparation of Fast Proton-Conducting P2O5-SiO2 Glasses

We have investigated the proton conductivities of the sol-gel-derived P₂O₅-SiO₂ glass at –50 to 120°C. The obtained glass is porous, where the surface area, pore volume and pore diameter are 740 m²/g, 0.5 cm³/g and <5 nm, respectively. The freezing temperature of water molecules adsorbed in the pores was –20°C, which is much lower than that of free liquid water due to the quantum size effect of the water confined in the pores. The electrical conductivities followed the Arrhenius equation in the temperatures between –20 and 120°C. Below –20°C, the adsorbed-water molecules were frozen, resulting in a rapid decrease of the proton conductivity. Considering the high conductivity, chemical and thermal stability, this oxide glass membranes have potential for the fuel cell membrane.     

Synthesis and Characterization of Hydrophobic Silica Aerogels Using Trimethylethoxysilane as a Co-Precursor

The hydrophobic property is one of the most important requirements for the long-term use of silica aerogels for transparent or translucent window insulation and opaque thermal insulating systems. Therefore, the present paper deals with the synthesis and characterization of hydrophobic silica aerogels using trimethylethoxysilane (TMES) as a co-precursor. Silica sol was prepared by keeping the molar ratio of tetramethoxysilane (TMOS) precursor, methanol (MeOH) solvent, water (H₂O) and ammonia (NH₄OH) catalyst constant at 1:12:4:3.7 × 10^–3 respectively throughout the experiments and the TMES/TMOS molar ratio (A) was varied from 0 to 2.35. The resulting silica alcogels were dried supercritically by high-temperature alcohol solvent extraction. Hydrophobicity of the aerogels was tested by measuring the percentage of water adsorbed by the aerogels after putting them directly on the surface of water under humid conditions. Alternately, the hydrophobicity was also tested by contact angle measurements. It was found that as the A value increased, the hydrophobicity of the aerogels increased but the optical transmission decreased from 93% to less than 5% in the visible range. The thermal stability of the aerogels was studied in the temperature range from 25 to 400°C. The hydrophobic nature of the aerogels was maintained up to a temperature of 300°C. The aerogels were characterized by infrared spectroscopy, optical transmittance, Scanning electron microscopy (SEM) and contact angle measurements. The results have been discussed by taking into account the hydrolysis and condensation mechanisms.     

氧化铝基表层ZrO2的相结构、孔结构及性能研究

近年来，双组份氧化物的性质和作用越来越受到人们的重视，国内外学者对TIOZ／A1203，Ceo。／AI。oa＊，MgO／AI。osRI等作了大量的研究．Zro。既可以做工业催化剂的载体，又可作添加剂或活性组份，它具有酸、碱、氧化和还原四种化学性质，在一氧化碳加氢合成甲醇中更具有重要作用·本文通过改变在7－AI。O3表面上Zro：负载量，研究了Zro。－AI。Oa体系的相结构及孔结构，并以CO氧化为模型反应，结合TPD－MS等实验技术研究并考察了添加Zroz对Alzoa基催化氧化及表面氧性能的影响，探讨Zroz的助催化作用·1实验部分（1）实验样…     

Crystallization Behavior of SiO2-TiO2 Ceramics Derived from Titanosiloxanes on Pyrolysis

The crystallization behavior of SiO2-TiO2 ceramics derived from titanosiloxanes was investigated in relation to the structure of the precursor and the pyrolysis temperature. The titanosiloxanes, [Si(OBut)2OTi(acac) 2O]2, [(ButO)3SiO] 2Ti (OPri)2, and [(ButO)3SiO] 3Ti(OPri), were pyrolyzed in an air atmosphere to form SiO2-TiO2 ceramics which crystallized to anatase at 600–650°C, 700–750°C, and 800–850°C, respectively. The crystallization temperature decreased with increased titanium content of the precursor. The average crystallite size of anatase increased with increased pyrolysis temperature and the titanium content. The crystallization temperature and the crystallite size for SiO2-TiO2 ceramics is controlled by the precursor structure, which may enable control of the physical properties of the ceramic materials.     

ZrO2在Rh/γ-Al2O3催化剂中对NO催化还原反应的作用

测定了含ZrO2的Rh/γ-Al2O3催化剂上NO+C2H4和NO+C2H4+O2的反应活性,并应用TPR、XRD、BET比表面等表征了ZrO2的加入方式和晶型对Rh/γ-Al2O3催化剂活性和结构的影响。结果表明,ZrO2的加入一定程度地抑制了Rh3+与γ-Al2O3之间的相互作用和γ-Al2O3的相变,提高了催化剂的热稳定性,明显提高了850℃老化样品的NO+C2H4反应活性。对于NO+C2H4+O2反应,含ZrO2样品的选择还原活性却较低,表明反应机理不同,而且ZrO2对C2H4的深度氧化有促进作用,但老化后活性下降幅度比不含ZrO2的样品小。     

Synthesis and properties of mullite and cordierite aerogels

This work deals with the preparation of aluminosilicate aerogels, especially mullite (3Al₂O₃·2SiO₂) and cordierite (2MgO·2Al₂O₃·5SiO₂) aerogels, from the cohydrolysis of tetraethoxysilane and chelated aluminum-secbutylate; in the case of cordierite magnesium nitrate was added. The influence of various preparation conditions on the aerogel synthesis is described. Crystallization and sintering behavior of mullite aerogels supercritically dried in acetone or alcohol differs from that one of mullite aerogels dried in CO₂. During non-isothermal heat treatment the former show a drastically reduced shrinkage compared to the latter. This behavior can be explained by a phase separation during the high temperature autoclaving process. In cordierite aerogels the crystallization of tetragonal mullite at about 1000°C is observed, while the correspondent xerogels show the crystallization of - and - cordierite between 1000 and 1100°C. On the other hand sintering is promoted in cordierite aerogels, which is due to the content of MgO.     

Measurement of mutual diffusion coefficients,densities, viscosities,and osmotic coefficients for the system KSCN-H2O at 25°C

Mutual diffusion coefficients measured on the volume-fixed frame of reference are reported for KSCN-H₂O at 25°C over the concentration range 0.0 to 10.26 mol-dm^–3. The diffusion coefficient at infinite dilution was obtained from limiting ionic equivalent conductances of K⁺ and SCN^–. Low concentration conductances of KSCN-H₂O at 25°C used to obtain the limiting ionic equivalent conductance of SCN^– are reported. Values of density and viscosity for this system are reported from 0.0 to 10.30 mol-dm^–3. Osmotic coefficienss of KSCN-H₂O at 25°C were measured by the isopiestic method. These are reported over the concentration range of 0.30 to 24.94 molal (saturation). Values of thermodynamic diffusion coefficients for the concentration range 0.0 to 10.26 mol-dm^–3 are tabulated. Results are compared to other potassium salts with monovalent anions at 25°C.     

Mechanism of the Formation of O2? Radical Anions on CeO2 and (0.5–10)%CeO2/ZrO2 during the Adsorption of an NO-O2 Mixture

It is established by ESR that the adsorption of an NO + O₂ mixture at 20°C on oxidized CeO₂ (O₂, T = 400–700°C) produces radical anions O ₂ ⁻ located both on isolated Ce⁴⁺ cations (O ₂ ⁻ (1)) and in associated anionic vacancies (O ₂ ⁻ (2)). These species differ in thermal stability. For example, O ₂ ⁻ (2) decomposes at 20°C, while O ₂ ⁻ (1) decomposes at 50°C. Only O ₂ ⁻ (1) species are observed at −196°C in ZrO₂-supported CeO₂. In the case of NO + O₂ adsorption at 20°C, O ₂ ⁻ is stabilized on Zr⁴⁺ cations and decomposes at 270°C. Increasing the cerium oxide content of the ZrO₂ surface from 0.5 to 10% only partially inhibits the formation of O ₂ ⁻ -Zr⁴⁺. The Zr⁴⁺ cation is shown to possess a higher Lewis acidity than the Ce⁴⁺ cation, and the ionic bond in O ₂ ⁻ -Zr⁴⁺ complexes is stronger than that in O ₂ ⁻ -Ce⁴⁺ complexes. ESR, temperature-programmed desorption, and IR spectroscopic data for various adsorption complexes of NO on CeO₂ suggest that, in the key step of O ₂ ⁻ formation, free electrons appear on the surface owing to the conversion of adsorbed NO molecules into nitrito chelates on coordinately unsaturated ion pairs Ce⁴⁺-O ₂ ⁻ .__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 414–422.Original Russian Text Copyright © 2005 by Il’ichev, Shibanova, Ukharskii, Kuli-zade, Korchak.     

Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths

Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol–gel reaction. Starting from AlCl₃·6H₂O and poly(ethylene oxide) (PEO) dissolved in a H₂O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H₂O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H₂O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO.     

Reactions of 2-Butylsulfanyl-2-alkenals with Alcohols and Water

2-Butylsulfanyl-2-alkenals react with alcohols at room temperature in the presence of acid catalysts to give 45–90% of the corresponding acetals. Acetals derived from 2-butylsulfanylpropenal readily undergo hydrolysis at the vinylsulfanyl group (20°C, catalysis by HCl or TsOH) with formation of 2-oxopropionaldehyde O,O- or O,S-acetals in 70–90% yield. Unlike 2-butylsulfanyl-2-propenal O,O-dialkyl acetals, the initial aldehydes and 2,4-dinitrophenylhydrazones derived therefrom are stable to hydrolysis under analogous conditions: the vinyl sulfide moiety remains unchanged even under considerably more severe conditions (100°C, 3 h; HCl, H₂SO₄, CF₃SO₂OH, or TiCl₄).__________Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 6, 2005, pp. 832–836.Original Russian Text Copyright © 2005 by Keiko, Chuvashev, Stepanova, Larina.     

Effects of temperature and pressure on the near-infrared spectra of HOD in D2O

The near-infrared absorption spectra (9500 to 11000 cm^–1) of HOD, 20 mol% in D₂O were measured at temperatures between 4 and 55°C and pressures up to 500 MPa. From the analysis of the spectra, the following conclusions are drawn. (1) At temperatures below about 38°C, the ice I-like bulky structure is destroyed to form the dense structure which reflects the high-pressure ice-like structure as the pressure is increased. (2) At temperatures above about 38°C, the bulky structure hardly remains at atmospheric pressure and the formation of dense structure proceeds monotonically with increasing pressure. The results and conclusion obtained in the present paper agrees with those obtained for pure H₂O water in the previous investigation.     

Organic Surface Modification of TEOS Based Silica Aerogels Synthesized by Co-Precursor and Derivatization Methods

The experimental results on the organic modification of tetraethoxysilane (TEOS) based silica aerogels synthesized by co-precursor and derivatization methods are reported and discussed. In order to obtain silica aerogels with better physicochemical properties in terms of higher hydrophobicity, optical transmission and thermal stability, eight organosilane compounds (hydrophobic reagents) of the type R _n SiX_4–n have been used. The molar ratio of tetraethoxysilane (TEOS), ethanol (EtOH), water (0.001 M oxalic acid catalyst) was kept constant at 1:5:7 respectively. The organically modified silica aerogels were produced by two different methods: (i) Co-precursor method and (ii) Derivatization method. In the former method, the molar ratio of hydrophobic reagent (HR) to TEOS was varied from 0.1 to 0.6. In the later method, derivatization of the wet gels was carried out using 20% hydrophobic reagent in methanol. The merits and demerits of both these methods have been presented. The organic surface modification of the aerogels was confirmed by the Fourier Transform Infrared (FTIR) spectroscopic studies and the contact angle measurements. In the co-precursor method, with the increase in hydrophobic reagent/TEOS molar ratio, the hydrophobicity increases ( = 136°) and the optical transmission decreases (5%), whereas in the derivatization method the optical transmission is very high (T 85%) but the hydrophobicity is low ( = 120°). The thermal stability of the hydrophobic aerogels (the temperature up to which the hydrophobicity is retained) was studied in the temperature range of 25–800°C. The aerogels based on the co-precursor method retained the hydrophobicity up to a temperature as high as 520°C and on the other hand, the derivatized aerogels are hydrophobic only up to a temperature of 285°C. For the first time, TEOS based hydrophobic silica aerogels have been obtained with negligible volume shrinkage using the trimethylethoxysilane (TMES) co-precursor. The aerogels were characterized by Fourier transform infrared spectroscopy (FTIR), optical transmittance, Scanning Electron Microscope (SEM), thermogravimetric (TG) and differential thermal (DT) analyses and the contact angle measurements.     

Phase Diagram of the Xe–H2O System up to 15 kbar

The phase equilibria in the Xe–H2O system have been studied by the DTA technique under hydrostatic pressures up to 15 000 bar in a temperature range from -25 °C to 100 °C. We have shown that the cubic structure I xenon hydrate forming at ambient pressure does not undergo any phase transitions under the conditions studied. The temperature of its decomposition into water solution and gas (fluid) increases from 27 °C at 25 bar to 78.2 °C at 6150 bar. At higher pressures the hydrate decomposes into water solution and solid xenon. In the temperature range from 6800 to 9500 bar the decomposition temperature (79.0–79.5 °C) is practically independent of pressure, while further pressure increase results in a slow decrease to 67 °C at 15 000 bar.