Preparation, characterization and catalytic properties of singly and doubly titanium-modified mesoporous silica gel

A series of titanium-modified mesoporous silica gel have been synthesized using tetrabutyl titanate. The samples were characterized by nitrogen adsorption-desorption, FT-IR and Raman spectroscopy, as well as by solid state diffuse reflectance UV-VIS spectroscopy. Physicochemical characterization of the materials showed that Ti atoms were part of the framework of silica gel, and it was probably in a tetrahedral coordination for low Ti contents. The resulting titania modified the inner walls of the mesoporous silica gel after hydrolysis and calcination. Actually, the titanium precursor reacted and condensed with the active silanol groups on silica gel via Si-O-Ti bonds. In addition, the titanium-modified mesoporous silica gel showed distinct activity behavior in the catalytic oxidation of cyclohexene and styrene with hydrogen peroxide as oxidant.     

Crystallization of silicalite‐1 from clear synthesis solutions: Effect of template concentration on crystallization rate and crystal size

Synthesis of silicalite‐1 powders and membranes from initially clear solutions with different tetrapropylammonium hydroxide or bromide concentrations was studied. While tetrapropylammonium bromide acts only as template, tetrapropylammonium hydroxide provides both the template and hydroxyl ions to the synthesis medium. The effects of template and hydroxyl ion concentration on the product yield, crystallization rate and crystal size were investigated. Pure and highly crystalline silicalite‐1 was obtained with all compositions. The nucleation time decreases from 100 h to 20 h and the crystal size decreases from 3.5 μm to 0.35 μm as the template amount x is increased from 5 to 30 moles at a batch composition of 80SiO₂.xTPAOH.1500H₂O at 95 °C. Yield of silicalite‐1 passes through a maximum at intermediate TPA concentration. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of ion exchange and calcinations on the structure and photocatalytic activity of hydrothermally prepared titanate nanotubes

Titanate nanotubes (TiNTs) were prepared by alkaline hydrothermal processing. The TiNTs are thermodynamically unstable and easily transformed to the titania phase by heat or acid treatment. These phase transformations are affected by the preparation conditions. In this study, we investigated the effects of using the washing process to modify the sodium content of the TiNTs. After an alkaline hydrothermal process was used to prepare the TiNTs, the resulting suspensions were washed with weak acid solution and distilled water until the pH value of the wash solution reached approximately 1 or 7, and the products were identified as H‐TiNTs or Na‐TiNTs, respectively. The characteristics and photocatalytic activities of the H‐TiNTs and Na‐TiNTs were compared for various calcination temperatures. The H‐TiNTs were transformed completely to anatase‐type TiO₂ by dehydration during calcination, while the crystallinity of the Na‐TiNTs increased with calcination temperature. However, the photocatalytic H₂ production rates on calcined H‐TiNTs were much higher than on Na‐TiNTs, which could be attributed to the crystalline anatase phase.     

Al2O3对低品位菱镁矿与天然硅石合成制备镁橄榄石的影响

以低品位菱镁矿与天然硅石为原料,以Al2O3为添加剂,通过反应烧结制备镁橄榄石。研究讨论Al2O3对镁橄榄石材料相组成、晶胞参数、微观结构及常温性能的影响。用XRD和SEM对烧后试样的相组成和显微结构进行表征。利用X’pert plus软件对试样中镁橄榄石相的晶胞参数进行计算。结果表明:以低品位菱镁矿轻烧氧化镁和天然硅石为原料,经1500℃煅烧合成制备的镁橄榄石中,由于Al2O3的加入,促进了系统中顽火辉石相的形成。Al2O3的加入导致镁橄榄石相的晶胞参数和晶胞体积的增大。随着Al2O3加入量增加,镁橄榄石的相对密度和常温耐压强度逐渐增加,说明了Al2O3对镁橄榄石的促烧结作用。     

Investigation on sol–gel silica coatings for the protection of ancient glass: Interaction with glass surface and protection efficiency

To hinder the phenomenon of weathering of ancient stained glass, the present work proposes the application of sol–gel coatings to the glass surface. Previous investigations [1], [2], [3], [4], [5] and [6], in fact, show that sol–gel silica coatings do not change the appearance of artistic glasses when deposited on their surface. Moreover, the film thickness is so small (around 200 nm) and its composition and structure so compatible with that of the original glass, that the characteristics of the coating and original glass are not distinguishable. In this work, several recipes used to produce sol–gel coatings have been tested in order to understand their behavior when adopted for covering ancient weathered glass. The coatings are made of sol–gel silica prepared with two different catalysts: H⁺, Pb²⁺ and without catalyst. All the investigated samples show a good adhesion of the coating to the glasses used to simulate the behavior of ancient artefacts. The sol–gel silica coatings have been studied before and after accelerated ageing to test the resistance of the protective coatings to weathering. Another important index to test of the efficiency of the sol–gel coatings for the protection of an ancient glass is the lead ion mobility. In ancient stained glass, in fact, this element is present in the metallic lead network, in the grisaille paintings and constitutes a main component of many glass tesserae. The action of water on this highly mobile ion involves the degradation of the glass itself and the release of the ion in the rain solution. Ageing tests show the efficiency of H⁺ and Pb²⁺ catalyzed coatings and the inefficiency of the non-catalyzed sol–gel layers.     

Growth and characterization of epitaxial layers on aluminum nitride substrates prepared from bulk, single crystals

A comparative study of epitaxy of AlN, GaN and their alloys, grown on c-axis and off-axis substrates of single-crystal aluminum nitride has been carried out. Growth on off-axis (>30°) substrates appears to result in rough surfaces and the absence of two-dimensional electron gas (2DEG). However, smooth morphologies were demonstrated for both homoepitaxial and heteroepitaxial growth on on-axis (<2°) substrates. On one of these oriented substrates a 2DEG, with a mobility of 1000 cm²/V s and a sheet density of 8.5×10¹² cm⁻² at room temperature, was also demonstrated for the first time.     

Studies on the structural,thermal and optical behaviour of solution grown organic NLO material: 8‐hydroxyquinoline

Single crystal of 8‐hydroxyquinoline (8HQ) having chemical formula C₉H₇NO, an organic nonlinear optical (NLO) material has been successfully grown by slow evaporation solution growth technique at room temperature. The crystal system has been confirmed from the powder X‐ray diffraction (PXRD) analysis. The crystalline perfection was evaluated by high resolution X‐ray diffractometry (HRXRD). From this analysis we found that the quality of the crystal is quite good. However, a very low angle (tilt angle 14 arc sec) boundary was observed which might be due to entrapping of solvent molecules in the crystal during growth. Its optical behavior has been examined by UV‐Vis. analysis, which shows the absence of absorbance between the wavelengths ranging from 400 to 1200 nm. From the thermal analysis it was observed that the material exhibits single sharp weight loss starting at 113°C without any degradation. The laser damage threshold was measured at single shot mode and the SHG behavior has been tested using Nd:YAG laser as a source. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations on nucleation,thermodynamical parameters and growth of Benzimidazole crystals from low temperature solution

Metastable zonewidth and induction period measurements of Benzimidazole (BMZ) are presented. The nucleation parameters such as interfacial tension, radius of the critical nucleus and critical free energy change have been calculated for the solution grown Benzimidazole (BMZ) single crystals at different supersaturation ratios. The grown BMZ crystals were characterized by the differential scanning calorimetric studies for analysis of thermal properties. The dielectric behaviour of the crystal was studied at different temperature and frequency. The laser damage threshold studies show that BMZ crystal has higher laser damage threshold. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)