R. Campostrini

Bauxite 'red mud' in the ceramic industry. Part 1: thermal behaviour

Samples of red mud, by-products of alumina production from bauxite, are studied in the 120–1400°C interval. An extensive characterization was performed by thermal and X-ray diffraction analyses. The identification of gaseous species released upon heating was carried out by coupling the thermal analizer with a gas-chromatographic/mass spectrometer. Density evolution was also determined as a function of the heat treatment. Results indicate primary H2O release from aluminium hydroxides, followed by carbonate decomposition with CO2 evolution below 900°C. Alkaline oxides, mainly CaO and Na2O, lead to the formation of Ca3Al2O6 and NaAlSiO4 between 900 and 1100°C. At the highest temperatures, reduction of Fe3+ to Fe2+, involving O2 release, promotes the formation of Fe2TiO4, with the disappearance of the rutile-TiO2 phase. The various solid state reactions, ascertained at different stages of the heating process, and possible mass balances are discussed with reference to the state diagrams of principal red mud components.

Influence of tungsten oxide on structural and surface properties of sol–gel prepared TiO2 employed for 4-nitrophenol photodegradation

A set of tungsten oxide/TiO2 polycrystalline samples (W/Ti) prepared by the sol–gel method has been characterised using several techniques, namely thermogravimetric analysis, gas-chromatography, mass spectrometry, thermogravimetric analysis combined with mass spectrometry or with gas chromatography/mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, determination of specific surface area, scanning electron microscopy, Fourier-transform infrared spectroscopy and monitorisation of pyridine adsorption for surface acidity. Moreover the samples have been empolyed as catalysts for 4-nitrophenol photodegradation in aqueous suspensions used as a ‘probe’ reaction. Characterisation results indicate that the surface of the W/Ti particles is enriched with homogeneously dispersed tungsten as well as microcrystalline or amorphous species. Maximum photoactivity for 4-nitrophenol photodegradation was achieved for a sample containing 1.7 mol W per 100 mol Ti.

Pyrolysis study of methyl-substituted Si-H containing gels as precursors for oxycarbide glasses, by combined thermogravimetry, gas chromatographic and mass spectrometric analysis

Monolithic and transparent gels were prepared by mixing various ethoxide silicon precursors containing Si—CH3 and Si—H groups, the composition ensuring the same number of C—H and Si—H bonds. Pyrolysis of these samples was followed under helium flow by connecting thermogravimetry, gas chromatographic and mass spectrometric analysis, to study the conversion of the gels into oxycarbide materials. In addition to the usual direct thermal and mass spectra analysis (TG–MS), a TG–GC–MS arrangement, allowing gas chromatographic separation of the species simultaneously evolving during thermodecomposition followed by mass spectral analysis, was successfully achieved. Experimental results indicate that mass loss occurs in three steps, each characterized by specific reactions. At low temperatures, densification of the siloxane network derives from further condensation reactions. At intermediate temperatures, a remarkable rearrangement of the siloxane chains occurs, with the release of volatile silanes and several siloxane fragments due to Si—H and Si—O bond exchanges. At higher temperatures, the development of methane was detected and attributed to Si—C bond cleavage. Pyrolysis of gels containing only Si—CH3 or Si—H groups was also studied for comparison.

Pyrolysis study of sol—gel derived TiO2 powders: Part III. TiO2-anatase prepared by reacting titanium(IV) isopropoxide with acetic acid

A homogeneous TiO2 gel was obtained by hydrolysing titanium(IV) isopropoxide that was previously modified by reaction with acetic acid. The so stabilized precursor was hydrolysed under strong acidic medium (pH=0 by HCl). Dried TiO2 powders were characterized by FT-IR, XRD, N2 adsorption analyses, coupled thermogravimetric (TG) gas chromatographic (GC) and `mass spectrometric (MS) analyses. A semiquantitative analysis of the main evolved chemical species allowed to depict both the chemical rearrangements occurring in the TiO2 matrix during pyrolysis and the chemical composition of the initial gel.

Novel polysiloxane and polycarbosilane aerogels via hydrosilylation of preceramic polymers

We report new polysiloxane and polycarbosilane aerogels, which have been obtained by crosslinking Si–H-containing polymers with a CC-containing crosslinker via hydrosilylation reactions. The crosslinking reaction has been carried out in a highly diluted solution using up to 97 vol% of solvent. The obtained aerogels have a colloidal structure with meso- and macropores. Density as low as 0.17 g cm−3 has been reached, which implies a porosity of ca. 84 vol%.

Immobilization of yeast and bacteria cells in alginate microbeads coated with silica membranes: procedures, physico-chemical features and bioactivity

Salt alginate beads are used to entrap yeast or bacteria cells and exploited as protective environment for deposition of silica gel membranes. This inorganic layer is obtained through different methods: by dipping the beads in a silica sol (method A), by further consolidating them with tetraethoxysilane in an apolar solvent (method B), and by coating the beads with methyltriethoxysilane in the gas phase (method C). Physical–chemical and biological features are investigated. The pyrolysis study elucidates the features of the polymeric organic layer. Elemental analysis and 29Si solid state NMR prove the presence and the condensation degrees of the silica membranes. The bioactivity is studied by evaluating both glucose (Saccharomyces cerevisiae) and L-malic acid (Oenococcus oeni) fermentations. The fermentation performance is discussed considering possible limitations of mass transport across the silica gel layer. Method A produces an inorganic layer made by of 57.3% Si(OSi)4 and 42.7% HO–Si(OSi)3 units). In this case a linear relation is found among different contact times between silica sol and beads and both silica amount and average thickness. Method B appears to be successful in building up the layer, but it is also detrimental to cell viability. Method C is very efficient in terms of mass deposit, cell viability maintenance and leakage reduction. In this case the membrane is constituted by H3C–Si(OSi)2OH and H3C–Si(OSi)3 units in an almost 1 : 1 ratio.

Sol-Gel Synthesis and Characterisation of TiO2-Anatase Powders Containing Nanometric Platinum Particles Employed as Catalysts for 4-Nitrophenol Photodegradation

A set of TiO2 and Pt-TiO2 polycrystalline samples were prepared by sol-gel method hydrolysing a modified alkoxide titanium precursors under acidic conditions. The Pt-TiO2 samples gave an homogeneous nanometric metal dispersion after drying heat treatment forming platinum particles in the range 2–4 nm. All the samples have been characterised using several techniques, namely thermogravimetric analysis, gas-chromatography, mass spectrometry, thermogravimetric analysis combined with mass spectrometry or with gas chromatography/mass spectrometry, X-ray diffraction coupled with a Rietveld refinement procedure, X-ray photoelectron spectroscopy and determination of specific surface area. Moreover, the samples have been employed as catalysts for 4-nitrophenol photodegradation in aqueous suspension used as a “probe” reaction. Characterisation results indicate that the thermal and chemical treatments of the catalysts influenced the photocatalytic activity. In the Pt-TiO2 samples both Pt(0) and Pt(II) species are present in the catalyst particles and the most abundant phase is anatase for all of the samples. Doping with Pt beneficially influences the photo-oxidant properties of TiO2 while the presence of organic residual species on the surface, deriving from the preparation procedure of the catalyst particles interferes negatively in the kinetics of the photocatalytic process.

Characterization of methyl-substituted silica gels with Si–H functionalities

Gels have been prepared via hydrolysis–condensation reactions of triethoxysilane (TREOS) and methyldiethoxysilane (MDES) in various ratios. The hydrolysis–condensation reactions of the precursors have been investigated by 29Si and 17O solution NMR. Both precursors present a very high reactivity towards hydrolysis and condensation reactions, and formation of co-condensed species will be discussed. These highly reactive precursors lead to transparent gels over a large composition range, 0.1 ⩽ MDES:TREOS ⩽ 10. Their structure was investigated by 29Si MAS NMR, FTIR and DSC: the gels appear as highly condensed homogeneous systems in which the difunctional and trifunctional units are randomly distributed. The microstructure of the gels was also investigated through the measurement of specific surface area and skeletal density. Finally, some physical properties have also been measured such as thermal stability in O2 and inert atmosphere (Ar), coefficient of thermal expansion (α) and elastic modulus (E).

Sol-gel derived anatase TiO2: morphology and photoactivity

Abstract High-surface area TiO 2 (anatase) was prepared by the sol-gel method and characterized by various instrumental and analytical methods, including X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, porosimetry, specific surface area measurements, scanning electron microscopy, and titration of surface hydroxyl groups. The specimen was tested for photodegradation of phenol and nitrophenols carried out in aqueous suspension at pH = 3; photoactivity was compared with that of commercially available anatase TiO 2 . Results are discussed in terms of porosity, surface area, and availability and population of surface OH groups, consequent upon the particular method used for preparation. The observed partial deactivation in time of gel-derived anatase was studied, and almost complete restoration of photoactivity after thermal treatment at 573 K for 3 hours was observed.

Sol-Gel Synthesis and Pyrolysis Study of Oxyfluoride Silica Gels

Silicon oxyfluoride materials are synthesized by the sol-gel method using triethoxyfluorosilane as precursor, bearing the Si—F bond. SiO(2−0.5x) Fx gel preparation requires peculiar experimental control of hydrolysis and condensation reactions. Maintenance of the Si—F bond during gelling, heating and aging was studied in the case of processes carried out under an argon atmosphere or in air. Fluorine contents in resulting samples were quantified by FT-IR and X-ray photoelectron spectroscopy (XPS); specific surface area and porosity of powdered samples were determined by N2 adsorption. The thermal stability of oxyfluoride gels was studied by thermogravimetric-mass spectrometric (TG-MS) coupled analyses during heat treatment, under He flow. Mass spectra recorded during principal weight losses indicate the release of variously fluorinated silicon species resulting from Si—F/Si—O exchange reactions. The evolution of these species was observed at different temperatures, depending on gelling conditions. In particular, degradation of ≡Si—F moieties was prominent for gels aged in air, whereas samples processed under an argon atmosphere preserve the ≡Si—F bond up to 300°C.