Search results for "Amorphous solid"
showing 10 items of 622 documents
Effect of Ti(IV) loading on CO oxidation activity of gold on TiO2 doped amorphous silica
2006
Abstract Amorphous SiO 2 , doped with various amount of TiO 2 , was used as support for gold nanoparticles. Gold was deposited through the controlled sol formation technique. The effect of the TiO 2 loading on the structural and surface properties was investigated by transmission electron microscopy, X-ray diffraction, Raman and X-ray photoelectron spectroscopy. The catalytic behavior was tested in the oxidation of CO. A significant enhancement in the activity was observed for the gold supported on mixed oxide with low TiO 2 loading. On the basis of characterization data, the positive effect is assigned to the electronic modification of the Ti(IV) of the newly formed Ti O Si bonds.
Effects of temperature on electron paramagnetic resonance of dangling oxygen bonds in amorphous silicon dioxide
2011
The properties of electron paramagnetic resonance (EPR) signal of oxygen dangling bonds in amorphous SiO2 ("non-bridging oxygen hole centers", NBOHC) in excimer laser-irradiated amorphous SiO2 were studied in the temperature range 20K to 295K. NBOHCs strongly affect optical and chemical properties of amorphous SiO2 -based (nano) structures and their surfaces. The behaviour of their EPR signal is complicated due to a nearly degenerate electronic ground state. It was found that EPR signal has a non-Curie (~1/T) T-dependence down to 40K, indicating that EPR-based concentration estimates routinely obtained at T = 77K underestimate the center concentrations at least by a factor of 1.7. The estim…
Properties of methyl radical trapped in amorphous SiO2 and in natural SiO2-clathrate Melanophlogite
2013
Abstract We report an experimental investigation by electron paramagnetic resonance (EPR) on methyl radical (CH3 ) observed in γ-ray irradiated high-purity amorphous silicon dioxide (a-SiO2) and in a polycrystalline sample of Melanophlogite, a rare natural form of SiO2-clathrate. From the analysis of the EPR spectra we estimate the correlation time of the hindered rotational motion of CH3 molecules at T = 77 K in the two different materials. This physical quantity gives a quantitative measure of the freedom of motion of CH3 molecules trapped in the two solid systems, putting forward relevant information on the properties of the cavities/interstices in which the radicals are confined. In par…
Effect of Alkali Ions on the Amorphous to Crystalline Phase Transition of Silica
2001
Abstract The effect of the addition of alkali ions to commercial amorphous silica, generally used as support for heterogeneous catalysts, has been investigated from the point of view of morphological and structural changes. Samples of alkali-doped silica were prepared by impregnation and subsequent calcination at various temperatures. The structural effect of Li, Na, K, and Cs was determined by use of techniques such as wide-angle (WAXS) and small-angle X-ray scattering (SAXS). The WAXS diffractograms, analyzed with the Rietveld method using the GSAS program, allowed qualitative and quantitative identification of the fraction of the different silica polymorphs like quartz, tridymite, and cr…
Effect of the Al/Si atomic ratio on surface and structural properties of sol-gel prepared aluminosilicates
2003
Abstract A series of aluminosilicates with an Al/Si ratio ranging from 0 to ∞ (0 for pure silica and ∞ for pure alumina) was prepared by sol–gel process and characterized by surface and structure techniques. Aluminum tri sec butoxide and tetramethylorthosilicate were used as precursors for the sol–gel synthesis. The acidic properties of the oxides were studied by determination of the zero point charges, through mass titration method, and, for selected samples, by FT-IR spectroscopy of adsorbed pyridine used as a probe for both Bronsted and Lewis acidity. A dependence of the acidity on the Al/Si atomic ratio was found. According to the X-ray diffraction patterns, all the oxides have an amorp…
Preparation of Pd coated anodic alumina membranes for gas separation media
2007
Different procedures of Pd electroless deposition onto anodic alumina membranes were investigated to form a dense metal layer covering pores. The main difficulty was related to the amorphous nature of anodic alumina membranes, determining low chemical stability in solutions at pH > 9, where Pd plating works more efficiently. As a consequence, it was necessary to find the operative conditions allowing Pd deposition without damaging the membrane: to reduce alumina dissolution, the plating bath was buffered at pH 8.5 by addition of either NaHCO 3 or Na 2 B 4 O 7 ·H 2 O. Acceptable conversion of Pd was found after a deposition time of 3 min. Single and multiple deposition steps (each lasting 3 …
New insights on the thermal analysis of low moisture composite foods.
2014
Abstract Low moisture baked products were investigated with a view to characterising the effect of both formulation and humidity on their physical stability. At the end of the baking process, the samples were in the amorphous state as a result of starch gelatinization and sugar melting. Their thermal properties were analyzed with differential scanning calorimetry and their glass transitions were studied. The DSC thermograms were thoroughly studied through a Gaussian deconvolution of the first derivative of their heat flow. This approach evidenced a multiple phase behavior with different glass transitions in composite systems. They were associated with either a polymer-rich phase and/or a pl…
Amyloid fibrils formation and amorphous aggregation in Concanavalin A
2007
We here report an experimental study on the thermal aggregation process of concanavalin A, a protein belonging to the legume lectins family. The aggregation process and the involved conformational changes of the protein molecules were followed by means of fluorescence techniques, light scattering, circular dichroism, zeta potential measurements and atomic force microscopy. Our results show that the aggregation process of concanavalin A may evolve through two distinct pathways leading, respectively, to the formation of amyloids or amorphous aggregates. The relative extent of the two pathways is determined by pH, as amyloid aggregation is favored at high pH values ( approximately 9), while th…
Exchange coupling in the correlated electronic states of amorphous GdFe films
2013
The bulk sensitivity of hard x-ray photoelectron spectroscopy in combination with circularly polarized radiation of the P09 beamline at PETRA III enables the investigation of the magnetic properties of capped films. We have determined the temperature dependence of the magnetic circular dichroism in the Fe 2$p$ and in the Gd 3$d$ states in amorphous GdFe films. The magnetic dichroism reflects the stronger temperature dependence of Gd moments compared to Fe moments in agreement with mean-field models. We resolved the exchange split Gd 3${d}_{5/2}$ substates and found a significant temperature dependence of the splitting which is attributed to a temperature dependent part of the exchange energ…
Structure and transport properties of amorphous aluminium silicates: computer simulation studies
2005
The structure and transport properties of SiO2-Al2O3 melts containing 13 mol% and 47 mol% Al2O3 are investigated by means of large scale molecular dynamics computer simulations. The interactions between the atoms are modelled by a pair potential which is a modified version of the one proposed by Kramer et al. [J. Am. Chem. Soc. 64, 6435 (1991)]. Fully equilibrated melts in the temperature range 6000 K >= T > 2000 K are considered as well as glass configurations, that were obtained by a rapid quench from the lowest melt temperatures. Each system is simulated at two different densities in order to study the effect of pressure on structural and dynamic properties. We find that the Al ato…