Giuliano Fagherazzi

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.

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.

An investigation on Pd/C industrial catalysts for the purification of terephthalic acid

Abstract Commercial 0.5% Pd/C catalysts for the production of purified terephthalic acid (PTA) have been investigated by several physical techniques (SEM-EDS, CO chemisorption, XRD) and by activity tests performed in conditions strictly similar to those of industrial operation. These catalysts show a strongly external Pd distribution, pointing to high Pd concentrations in the outer layers of the carrier. It was found that Pd sinters very easily and that a rough proportionality exists between catalytic activity and Pd surface area. Pd sintering was shown to be an important cause of deactivation for PTA catalysts. However, in some cases deactivation occurs by sulphur poisoning, with the formation of a Pd sulphide (Pd4S).

Structural investigation on the stoichiometry of β-PdHx in Pd/SiO2 catalysts as a function of metal dispersion

Structural investigations of dispersed Pd/SiO2 catalysts were carried out with XRD and SAXS techniques, supported by TPR and CO chemisorption measurements. The stoichiometry of Pd hydride, β-PdHx, was determined by measuring the shift of Pd 111 XRD reflection in the presence of hydrogen. It was confirmed thatx decreases when the metal dispersion increases. This behaviour could be quantified up to about 0.45 of Pd dispersion since the angular position of XRD peaks cannot be determined with the necessary precision at higher dispersions. TPR data, obtained up to dispersions of about 0.6, confirms such behaviour. The β-PdHx stoichiometry versus Pd dispersion relationship substantially agrees with that found with other techniques by Boudart and Hwang.

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.

Structural properties of ultra-fine zirconia powders obtained by precipitation methods

We have investigated various aspects of the stabilization of the tetragonal (t) and cubic (c) forms (metastable at room temperature) with respect to the monoclinic (m) form in ultrafine zirconia powders obtained by calcining amorphous hydrated zirconium oxides at various temperatures. They were prepared by precipitation from ZrOCl2 at different pH, using either NaOH or NH4OH solutions. We have clarified the importance of the role of Na+ ions in the initial zirconia gel as stabilizers of the cubic form. On the contrary, almost pure tetragonal phase was obtained at moderately low temperatures starting from precursors with a low content of sodium (⩽0.5wt%). By means of an X-ray diffraction (XRD) study and a suitable peak profile fitting procedure (convolutive method for the pattern decomposition followed by a straight-forward Fourier analysis) the amounts of the different crystallographic forms as well their microstructural properties such as crystallite size and lattice disorder (when present) were obtained. The thermal evolution of the systems were followed by both differential thermal analysis (DTA) and XRD, and we studied the martensitic t → m transformation obtained by pressing a zirconia powder containing a prevailing content of t form at room temperature. To obtain further microstructural and morphological information on the c → m transition, transmission electron microscopy and small angle X-ray scattering techniques were used.

Yttria-based nano-sized powders: A new class of fractal materials obtained by combustion synthesis

Small-angle x-ray scattering (SAXS) technique has been used to investigate nano-sized powders of Y 2 O 3 , Y 1.8 Er 0.2 O 3 , and Y 1.8 Nd 0.2 O 3 , obtained by means of combustion synthesis. The results show that the powders have a mass-fractal behavior, with fractal dimension, D f , in the 1.7–1.8 range, and average particle sizes in the 20–30-nm range. These results are supported by transmission electron microscope observations. Moreover, from SAXS data, it has been possible to establish a narrow particle size polydispersity, as well as the presence of particle-diffuse or “fuzzy” interfaces. These results show, for the first time, the fractal behavior of materials prepared by combustion synthesis and are in agreement with a recently developed model. The nanostructural features are discussed within the framework of the peculiar optical properties of the doped luminescent materials.

Influence of the valve metal oxide on the properties of ruthenium based mixed oxide electrodes: Part I. Titanium supported RuO2/Ta2O5 layers

Abstract RuO2/Ta2O5 mixed oxide electrodes have been investigated by XPS-AES techniques and X-ray diffractometry and their electrochemical behaviour for the oxygen evolution reaction has been studied. The surface analysis indicated a relative insensitivity of the surface composition to the changes of the Ru/Ta ratio in the bulk of the coatings. The diffractometric study showed the existence of a RuO2 rutile phase and a Ta2O5 amorphous phase in the electrode materials. no evidence for solid solutions was found. The influence of the noble metal concentration in the coatings on their catalytic activity has been studied for different oxide loadings. Maxima of Activity for a Ru/Ta atom ratio close to 1 have been found for coating thicknesses of 1 μm. Thicker layers exhibit an activity increasing with the noble metal content. The correlation between anodic charge density, q*, and current density, i, for the oxygen evolution reaction has been tested and definite trends related to the morphology of the electrode material have been assessed.

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.

Fractal aggregates of lanthanide-doped Y 2 O 3 nanoparticles obtained by propellant synthesis

Y 2-x Ln x O 3 (Ln 4 Ce, Pr, Nd, Eu, Gd, Ho, and Er) powders obtained by propellant synthesis have been characterized using small-angle x-ray scattering, wide-angle x-ray scattering, and transmission and scanning electron microscopy. All the samples showed a very porous, open microstructure with fractal scaling properties. The building blocks of the fractal aggregates are nanocrystallites of lanthanide-doped Y 2 O 3 , with variations in the cubic lattice constant proportional to the composition of the solid solution and to the lanthanide ionic radius. The particles had a narrow distribution of sizes with an average value in the 20–50 nm range. They are made of a core of 10–20 nm, consisting of almost perfectly ordered crystals and a “fuzzy” layer, characterized by either a growing lattice disorder or by a compositional gradient. From this dimension, up to at least 200 nm, the particle aggregate is a mass fractal with a fractal dimension, DM f , in the 1.6–2.0 range.