Alvise Benedetti

Electrosynthesis and Physicochemical Properties of PbO2 Films

An electrochemical and X-ray diffraction study has been conducted on the formation of lead dioxide deposits on platinum, from nitric acid solutions, as a function of potential and temperature. It has been shown that these parameters strongly influence the morphology and electrocatalytic activity of the PhO 2 films. The electrodeposition process is satisfactorily described by an electrochemical, chemical, electrochemical mechanism: (i) H 2 O → OH ads + H + + e; (ii) Ph 2+ + OH ads → Pb(OH) 2+ ; (iii) Pb(OH) 2+ + H 2 O → PbO 2 + 3H + + e; the second electron transfer stage and Pb 2+ diffusion control the dioxide formation in the lower and higher overpotential range, respectively. Temperature and potential (or current) are important parameters in the electrodeposition process. Depending on the potential region, the process can he kinetically or diffusion controlled. In an acid electrolyte, where mainly the β-PbO 2 modification is electrodeposited, the amount of α-phase impurity increases with increasing potential in the kinetically controlled region and decreases in the diffusion controlled domain. In addition, relatively low electrodeposition potentials and high temperatures favor an increase of the crystallite size with preferred crystallographic orientation for both α- and β-PbO 2 modifications. The temperature of the growth solution affects the crystallinity of the resulting oxide deposits and has a marked effect on their performance as anodes in processes at high positive potentials such as ozone generation.

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.

Preparation, structural characterization, and luminescence properties of Eu3+-doped nanocrystalline ZrO2.

Eu 3+ -doped zirconia nanopowders were prepared by the sol-gel technique using two different methods, based on the hydrolysis of zirconium n -propoxide, producing tetragonal and monoclinic zirconia under different preparation conditions. A detailed microstructure characterization was performed through wide angle x-ray scattering, small angle x-ray scattering, trasmission electron microscopy, and nitrogen physisorption measurements. The possible influence of the zirconia crystalline phases and particle sizes on the luminescence properties of the lanthanide ion was investigated. A detailed analysis of the emission spectra of the samples suggested that the dopant Eu 3+ ions replace the Zr 4+ ions in the zirconia crystal lattice. Moreover, samples prepared by the two different methods were characterized by different decay times of the Eu 3+ ion luminescence.

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.

Renewable H2 from Glycerol Steam Reforming: Effect of La2O3 and CeO2 Addition to Pt/Al2O3 catalysts.

Glycerol is the main byproduct of biodiesel production and its increased production volume derives from the increasing demand for biofuels. The conversion of glycerol to hydrogen-rich mixtures presents an attractive route towards sustainable biodiesel production. Here we explored the use of Pt/Al(2)O(3)-based catalysts for the catalytic steam reforming of glycerol, evidencing the influence of La(2)O(3) and CeO(2) doping on the catalyst activity and selectivity. The addition of the latter metal oxides to a Pt/Al(2)O(3) catalyst is found to significantly improve the glycerol steam reforming, with high H(2) and CO(2) selectivities. A good catalytic stability is achieved for the Pt/La(2)O(3)/Al(2)O(3) system working at 350 degrees C, while the Pt/CeO(2)/Al(2)O(3) catalyst sharply deactivates after 20 h under similar conditions. Studies carried out on fresh and exhausted catalysts reveal that both systems maintain high surface areas and high Pt dispersions. Therefore, the observed catalyst deactivation can be attributed to coke deposition on the active sites throughout the catalytic process and only marginally to Pt nanoparticle sintering. This work suggests that an appropriate support composition is mandatory for preparing high-performance Pt-based catalysts for the sustainable conversion of glycerol into syngas.

Encapsulation of submicrometer-sized silica particles by a thin shell of poly(methyl methacrylate)

Polymer encapsulation of submicrometer-sized silica particles by synthesis of the polymer shell, poly(methyl methacrylate) under static conditions in a reaction medium free of surfactants and stabilizing agents is described. The Stöber method, a base-catalyzed hydrolysis and condensation of tetraethyl orthosilicate is used for the synthesis of the monodisperse colloidal dispersion of silica particles. The silica particles are subsequently modified in situ with the surface grafting of the silane coupling agent, 3-(trimethoxysilyl)propyl methacrylate. Encapsulation is achieved using tetraethyl orthosilicate as a reaction medium, in which a thermally initiated radical polymerization of methyl methacrylate is promoted in the presence of the modified particles by a seeding method which leads to a thin coating of poly(methyl methacrylate), and hence silica core-shell particles. The complete encapsulation of individual silica spheres by poly(methyl methacrylate) is visually evidenced by TEM microscopy which reveals the presence of a polymer shell coating up to 10 nm. Evidence for the presence of a poly(methyl methacrylate) shell is further corroborated by DSC/TGA, DRIFT-IR and NMR measurements.

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.

MICROSTRUCTURE AND ELECTRICAL-PROPERTIES OF IRO2 PREPARED BY THERMAL-DECOMPOSITION OF IRCL3.X H2O - ROLE PLAYED BY THE CONDITIONS OF THERMAL-TREATMENT

Abstract Some features of the thermal decomposition of IrCl 3 ·3 H 2 O in an oxygen atmosphere are presented. The formation of IrO 2 phases has been followed by X-ray diffractometry, and the onset of definite reflections has been documented for samples pyrolyzed at 350° C, although moderate weight losses are observed by thermogravimetry at this temperature. The effect of the thermal history of iridium oxide coatings on their structure has been studied by means of electrical resistance measurements. An explanation of the results has been attempted in terms of impurity segregation to the oxide grain boundaries.

Physicochemical properties of thermally prepared Ti-supported IrO2+ ZrO2 electrocatalysts

Abstract Thermally prepared mixed-oxide IrO2 + ZrO2 films have been studied by Rutherford backscattering spectrometry (RBS), wide-angle X-ray scattering (WAXS) and cyclic voltammetry. Concentration depth profiling by RBS has shown that electrode films containing less than 50 mol.% of iridium dioxide have layered structures where noble metal oxide and zirconium oxide enrichments alternate. The outermost layer is enriched with iridium oxide. By WAXS analysis it was possible to prove the existence of an IrO2 and a ZrO2 phase. From cell parameters, very limited solubility could be ascertained, restricted at the two limits of the composition coordinate. In the range 0–20 mol.% of iridium dioxide, a tetragonal ZrO2 phase is formed. For samples richer in IrO2, the ZrO2 phase becomes amorphous. The microstructural features of the tetragonal IrO2-rich phase do not change significantly with the film composition. The effective surface area of the samples, as determined by cyclic voltammetry, exhibits a maximum in the composition range 50–80 mol.% IrO2. This result has been interpreted on the basis of WAXS and RBS data.

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.