Patrizia Canton

Nucleation and crystallization behavior of glass-ceramic materials in the Li2O-Al2O3-SiO2 system of interest for their transparency properties

Abstract We investigated the nucleation and crystallization behavior of multi-phase glass-ceramic materials (in the Li2O–Al2O3–SiO2 system to which low amounts of oxides such as TiO2, ZrO2, P2O5, BaO, Sb2O3 and ZnO were added), with interest in their transparency properties as glazes for industrial tiles with high scratch resistance. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used. XRD showed that the nucleating oxides produce, during the nucleation stage (at 953 K for 20 h), an orthorhombic ZrTiO4 phase, which maintains a similar crystallite size (about 4 nm) during the subsequent crystallization processes. We succeeded in determining the concentration of nuclei by using XRD data only. This quantity resulted close enough to the corresponding one, determined using the classical TEM method. The formation of each crystalline phase, developed during the crystallization stage, as a function of the thermal treatment, was quantitatively measured using the Rietveld method. In particular, the isothermal transformations at 1003 and 1023 K of the characteristic main phase, i.e., β-eucryptite s.s., which is richer in silica with respect to the relevant stoichiometric phase, into β-spodumene s.s., also richer in silica, were found to be consecutive solid state reactions. Moreover, this preliminary investigation aims at improving knowledge of the mechanisms leading to the development of transparent/opaque materials during the thermal treatment of Li2O–Al2O3–SiO2 glass-ceramics.

Effects of gold nanoparticles deposition on the photocatalytic activity of titanium dioxide under visible light

The effects of gold nanoparticles deposited on titanium dioxide on the photocatalytic oxidative degradation of two organic substrates, i.e. formic acid and the azo dye Acid Red 1, and on the parallel O(2) reduction yielding hydrogen peroxide have been investigated under visible light irradiation. The method employed to reduce Au(iii) to metallic gold in the preparation of Au/TiO(2) photocatalysts was found to affect their photoactivity, also by modifying the properties of TiO(2). The presence of gold on TiO(2) facilitates both the electron transfer to O(2) and the mineralization of formic acid, which mainly proceeds through direct interaction with photoproduced valence band holes. The so-formed highly reductant CO(2)*(-) intermediate species may contribute in maintaining gold in metallic form. The controversial results obtained in the photocatalytic degradation of the dye were rationalised by taking into account that with this substrate, which mainly undergoes oxidation through a hydroxyl radical mediated mechanism, the photogenerated holes may partly oxidise gold nanoparticles, which consequently act as recombination centres of photoproduced charge carriers.

Consecutive hydrogenation of benzaldehyde over Pd catalysts: Influence of supports and sulfur poisoning

Abstract Pd catalysts on different supports (active carbon, silica, alumina) were studied for the selective hydrogenation of benzaldehyde to benzyl alcohol. They were characterized by TPR, CO chemisorption and XRD. Batch hydrogenation tests were performed before and after sulfur poisoning. Strong metal–support interaction was found for Pd/alumina, giving high Pd dispersion. Chemisorption stoichiometry Pd/CO=2 was confirmed again. While Pd/C has the highest activity for benzaldehyde hydrogenation, a satisfactory selectivity to benzyl alcohol requires an oxidic carrier or proper sulfur poisoning of Pd/C.

Gold nanoparticles incarcerated in nanoporous syndiotactic polystyrene matrices as new and efficient catalysts for alcohol oxidations.

The controlled synthesis of gold nanoparticles (AuNPs), incarcerated in a semicrystalline nanoporous polymer matrix that consisted of a syndiotactic polystyrene-co-cis-1,4-polybutadiene multi-block copolymer is described. This catalyst was successfully tested in the oxidation of primary and secondary alcohols, in which we used dioxygen as the oxidant under mild conditions. Accordingly, (±)-1-phenylethanol was oxidised to acetophenone in high yields (96%) in 1 h, at 35 °C, whereas benzyl alcohol was quantitatively oxidised to benzaldehyde with a selectivity of 96% in 6 h. The specific rate constants calculated from the corresponding kinetic plots were among the highest found for polymer-incarcerated AuNPs. Similar values in terms of reactivity and selectivity were found in the oxidation of primary alcohols, such as cinnamyl alcohol and 2-thiophenemethanol, and secondary alcohols, such as indanol and α-tetralol. The remarkable catalytic properties of this system were attributed to the formation, under these reaction conditions, of the nanoporous ε crystalline form of syndiotactic polystyrene, which ensures facile and selective accessibility for the substrates to the gold catalyst incarcerated in the polymer matrix. Moreover, the polymeric crystalline domains produced reversible physical cross-links that resulted in reduced gold leaching and also allowed the recovery and reuse of the catalyst. A comparison of catalytic performance between AuNPs and annealed AuNPs suggested that multiple twinned defective nanoparticles of about 9 nm in diameter constituted the active catalyst in these oxidation reactions.

Quantitative Phase Analysis in Semicrystalline Materials Using the Rietveld Method

A new procedure to perform quantitative analysis without any internal standard has been developed within Rietveld analysis. This new approach permits solution of the problem due to the presence of an amorphous phase when its chemical composition or the global sample composition is known. The equations developed, based on a study of the global intensities diffracted by each phase in reciprocal space, reduce to the well known formula of Hill & Howard [J. Appl. Cryst. (1987), 20, 467–474] when applied to completely crystalline materials.

Synthesis and catalytic activity of metal nanoclusters inside functional resins: an endeavour lasting 15 years

Cross-linked functional polymers (functional resins) are versatile, designable and useful supports for metal nanoclusters that are able to provide reasonably thermally and mechanically stable multi-functional metal catalysts characterized by good activity and selectivity. The paper reviews authors’ contributions to the field from the early 1990s to the present.

Wustite as a new precursor of industrial ammonia synthesis catalysts

Contradictory results about the best oxidic precursor of Fe ammonia synthesis catalyst prompted the present comparative investigation on wustite- and magnetite-based catalysts. Many physical (density, porous texture, crystalline phases, reduction rate, metal surface, abrasion loss) and catalytic (kinetic constants, thermoresistancy) properties have been determined on both catalysts. The wustite-based catalyst proved to be much more active, especially at lower temperatures, approaching the performances of Ru/C catalyst, except at high conversion. Possible reasons for such a behavior of the wustite-based catalyst are discussed, suggesting that a reconsideration of the present consolidated knowledge on Fe ammonia synthesis catalyst might be convenient.

ASAXS study of Au, Pd and Pd–Au catalysts supported on active carbon

Abstract Among the techniques commonly used, small angle X-ray scattering (SAXS) is, in principle, particularly suited for the analysis of nanostructured systems. However, catalysts supported on porous materials are three phase systems (support, voids and metal), so that the resulting spectrum contains more information than a single conventional SAXS measurement allows to extract. The use of synchrotron radiation source allows to circumvent this difficulty and to separate the scattering of the support from the one of metal by taking advantage of the so-called anomalous or resonant behavior of the atomic scattering amplitude of an element near its absorption edge. The results so far obtained on some Au, Pd and Pd–Au samples supported on active carbon are reported here.

Calcium hydroxide nanoparticles from solvothermal reaction for the deacidification of degraded waterlogged wood

HYPOTHESIS A combination of acid and iron ions inside the wood has been corroding the cellulose matrix of the Swedish warship Vasa, imposing its deacidification. Past deacidification treatments displayed poor penetration inside the wood matrix with limited efficacy. A vacuum assisted treatment of wood using newly developed calcium hydroxide nanoparticle dispersions represents a possible candidate for the treatment of acidic waterlogged wood objects such as sculptures and decorative artifacts. EXPERIMENTS A solvothermal process was used for the synthesis of calcium hydroxide nanoparticle dispersions. Before the application on waterlogged wood, the physico-chemical characterization of these systems was carried out using several techniques. The efficacy of the deacidification treatment of wood samples from the Vasa was assessed by determination of pH and Differential Thermal Gravimetric (DTG) measurements. FINDINGS The proposed solvothermal reactions can be used to produce stable and highly concentrated calcium hydroxide nanoparticle dispersions in alcohols, needing no further purification before the application. This process has also the advantage to be upscalable to industrial level. Both pH and DTG measurements showed that the newly developed dispersions can homogenously penetrate inside the wood up to 20cm, neutralizing acidity and creating an alkaline buffer inside the wooden matrix, to hinder the degradation of residual cellulose.

Structure and size of poly-domain Pd nanoparticles supported on silica

A polydomain internal structure was evidenced by means of High Resolution Transmission Electron Microscopy (HRTEM) in palladium nanoparticles of two 5% Pd/SiO2 catalysts prepared by impregnation technique. Such poly-domain structure biased the line broadening analysis of the XRPD patterns, from which highly underestimated average diameters were obtained in comparison to those measured by means of Transmission Electron Microscopy (TEM).In both the investigated catalysts, a Pd/CO average chemisorption stoichiometry close to 2 was found, in good agreement with previous results obtained for Pd/C and Pd/γ-Al2O3 catalysts. The consistency of this result, in cases with different particle sizes and different support materials, suggests that such average stoichiometry is of general validity for palladium, when chemisorption measurements of CO are made using pulse flow technique.