RESUMEN
Adsorption of low-molecular adsorbates (nonpolar hexane, nitrogen, weakly polar acetonitrile, and polar diethylamine, triethylamine, and water) onto individual (silica, alumina, titania), binary (silica/alumina (SA), silica/titania (ST)), and ternary (alumina/silica/titania, AST) fumed oxides was studied to analyse the effects of morphology and surface composition of the materials. Certain aspects of the interfacial phenomena dependent on the structural characteristics of oxides were analysed using calorimetry, (1)H NMR, and Raman spectroscopies, XRD, and ab initio quantum-chemical calculations. The specific surface area S(BET,X)-to-S(BET,N(2)) ratio (X is an organic adsorbate) changes from 0.68 for hexane adsorbed onto amorphous SA8 (degassed at 200 degrees C) to 1.85 for acetonitrile adsorbed onto crystalline alumina (degassed at 900 degrees C). These changes are relatively large because of variations in orientation, lateral interactions, and adsorption compressing of molecules adsorbed onto oxide surfaces. Larger S(BET,X)/S(BET,N(2)) values are observed for mixed oxides with higher crystallinity of titania or/and alumina phases in larger primary nanoparticles with greater surface roughness and hydrophilicity. Polar adsorbates can change the structure of aggregates of oxide nanoparticles that can, in turn, affect the results of adsorption measurements.
Asunto(s)
Óxido de Aluminio/química , Nanopartículas/química , Óxidos/química , Dióxido de Silicio/química , Titanio/química , Acetonitrilos/química , Adsorción , Dietilaminas/química , Etilaminas/química , Hexanos/química , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Nitrógeno/química , Propiedades de Superficie , Agua/químicaRESUMEN
Fe2O3/SiO2 nanocomposites based on fumed silica A-300 (S(BET)=337 m2/g) with iron oxide deposits at different content were synthesized using Fe(III) acetylacetonate (Fe(acac)3) dissolved in isopropyl alcohol or carbon tetrachloride for impregnation of the nanosilica powder at different amounts of Fe(acac)3 then oxidized in air at 400-900 degrees C. Samples with Fe(acac)3 adsorbed onto nanosilica and samples with Fe2O3/SiO2 including 6-17 wt% of Fe2O3 were investigated using XRD, XPS, TG/DTA, TPD MS, FTIR, AFM, nitrogen adsorption, Mössbauer spectroscopy, and quantum chemistry methods. The structural characteristics and phase composition of Fe2O3 deposits depend on reaction conditions, solvent type, content of grafted iron oxide, and post-reaction treatments. The iron oxide deposits on A-300 (impregnated by the Fe(acac)3 solution in isopropanol) treated at 500-600 degrees C include several phases characterized by different nanoparticle size distributions; however, in the case of impregnation of A-300 by the Fe(acac)3 solution in carbon tetrachloride only alpha-Fe2O3 phase is formed in addition to amorphous Fe2O3. The Fe2O3/SiO2 materials remain loose (similar to the A-300 matrix) at the bulk density of 0.12-0.15 g/cm3 and S(BET)=265-310 m2/g.
Asunto(s)
Compuestos Férricos/química , Nanocompuestos/química , Dióxido de Silicio/química , 2-Propanol/química , Adsorción , Tetracloruro de Carbono/química , Oxidación-Reducción , Solventes/química , Propiedades de Superficie , Temperatura , Tolueno/químicaRESUMEN
Morphological, structural, adsorption, and catalytic properties of highly disperse titania prepared using sulfate and pyrogenic methods, and fumed titania-containing mixed oxides, were studied using XRD, TG/DTA, nitrogen adsorption, (1)H NMR, FTIR, microcalorimetry on immersion of oxides in water and decane, thermally stimulated depolarization current (TSDC) and catalytic photodecomposition of methylene blue (MB). Phase composition and aggregation characteristics of nanoparticles (pore size distribution) of sulfate and pyrogenically prepared titania are very different; temperature dependent structural properties are thus very different. Catalytic activity for the photodecomposition of MB is greatest (per gram of TiO(2) for the pure oxide materials) for non-treated ultrafine titania PC-500, which has the largest S(BET) value and smallest particle size of the materials studied. However, this activity calculated per m(2) is higher for PC-105, possessing a much smaller S(BET) value than PC-500. The activity per unit surface area of titania is greatest for the fumed silica-titania mixed oxide ST20. Calcination of PC-500 at 650 degrees C leads to enhancement of anatase content and catalytic activity, but heating at 800 and 900 degrees C lowers the anatase content (since rutile appears) and diminishes catalytic activity, as well as the specific surface area because of nanoparticle sintering.