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Alessandro Longo

Publications and source records attributed to Alessandro Longo.

6 recordsLinked to original sources
PubMed2006

Metal-support interaction and redox behavior of Pt(1 wt %)/Ce0.6Zr0.4O2.

The catalyst Pt(1 wt %)/Ce(0.6)Zr(0.4)O(2) is studied by CO-temperature programmed reduction (CO-TPR), isothermal oxygen storage complete capacity (OSCC), X-ray absorption spectroscopy (XAS) at the Pt L(III) edge, and in situ X-ray diffraction (in situ XRD), with the aim of elucidating the role of supported metal in CO oxidation by ceria-based three-way catalysts (TWC). The redox behavior of Pt(1 wt %)/Ce(0.6)Zr(0.4)O(2) is compared to that of bare ceria-zirconia. OSCC of redox-aged Pt/ceria-zirconia is twice that of bare ceria-zirconia, and the maximum of CO consumption occurs at a temperature about 300 K lower than redox-aged ceria-zirconia. XAS analysis allows one to evidence the formation of a platinum-cerium alloy in redox-aged samples and the stability of the metal particles toward oxidation and sintering during high-temperature treatments. Under CO flux at 773 K, bare ceria-zirconia shows a continuous drift of diffraction peaks toward smaller Bragg angles, due to a progressive increase of Ce(III) content. Under the same treatment, the structural rearrangement of Pt-supported ceria-zirconia starts after an induction time and takes place with an abrupt change of the lattice constant. The experimental evidence points to the role of supported Pt in modifying the redox properties of ceria-zirconia with respect to the bare support. It is proposed that the much faster bulk reduction observed by in situ XRD for redox-aged Pt/ceria-zirconia can be attributed to an easier release of reacted CO(2), producing a more effective turnover of reactants at the catalyst surface.

Journal Article↗
PubMed2005

A new cell for the study of in situ chemical reactions using X-ray absorption spectroscopy.

An in situ cell for reductive and oxidative treatments at different temperatures that allows the possibility of recording data as a function of temperature has been designed and constructed for X-ray absorption experiments at the GILDA beamline BM08 of ESRF. The cell is linked to a mass quadrupole spectrometer providing control of the reaction gases and monitoring of the products. The apparatus allows measurements to be performed both in transmission and fluorescence geometry. The cell was tested by studying the CO oxidation reaction promoted by a Pt/ceria-zirconia-supported catalyst. The CO(2) yield is correlated with the structural results confirming the existence of a strong metal-support interaction between the Pt metal clusters and the ceria-zirconia support.

Carbon Dioxide↗
PubMed2005

Relationship between structure and CO oxidation activity of ceria-supported gold catalysts.

Gold catalysts supported on cerium oxide were prepared by solvated metal atom dispersion (SMAD), by deposition-precipitation (DP), and by coprecipitation (CP) methods and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The catalytic activity was tested in the CO oxidation reaction. The structural and surface analyses evidenced the presence of a modified ceria phase in the case of the DP sample and the presence of pure ceria and gold metal crystallites in the case of the SMAD and CP samples. The DP sample, after a mild treatment in air at 393 K, exhibited only ionic gold, and it was very active below 273 K. By comparing the activities of the different catalysts, it is suggested that the presence of small gold particles, as obtained by the SMAD technique, is not the main requisite for the achievement of the highest CO conversion. The strong interaction between ionic gold and ceria, by enhancing the ceria surface oxygen reducibility, may determine the particularly high activity.

Journal Article↗
PubMed2004

On the use of grazing-incidence small-angle X-ray scattering (GISAXS) in the morphological study of ion-implanted materials.

Grazing-incidence small-angle X-ray scattering has become a widely used technique for the morphological analysis of surface systems. Here it is show how this technique can be applied to a buried system, like metallic clusters in glass obtained by ion implantation. The optimization of the data-collection geometry is described as well as the details of the quantitative data analysis. An experimental example on Cu + Au-implanted glasses shows the potentiality of the technique.

Journal Article↗
PubMed2004

Structural properties of nonionic cyclodextrin colloids in water.

The amphiphilic character in water of a novel class of chemically modified cyclodextrins has been investigated by means of small-angle X-ray scattering and light scattering. The introduction ofhydrophilic oligo(ethylene glycol) onto the secondary side of heptakis[6-alkylthio-6-deoxy-2-oligo(ethylene glycol)]-beta-cyclodextrins produces an enhanced water solubility of these molecules. Shape and dimensions of the generated micellar aggregates, analyzed in terms of a suitable core-shell model, remain stable in the wide concentration range explored. The highly associative behavior of these macromolecules is evidenced by the very low value of the critical micelle concentration (cmc), which is about 2 orders of magnitude smaller than the cmc usually obtained for traditional surfactant. Despite the complex geometry of this novel macromolecule, shape and dimensions of generated micellar aggregates can be properly described according to the thermodynamic approaches generally used for amphiphilic molecules and block copolymers. Results show how the modulation of hydrophobic and hydrophilic components sensitively influence the structural features of the generated aggregates thus offering the possibility to control molecular organization in a manner similar to that for traditional colloids. For all the classes of the investigated systems, the small micelles have been found in equilibrium with polydisperse large aggregates of entangled micelles. These novel nonionic colloidal systems combine inclusion and transport properties of host macrocycles, such as cyclodextrin, together with the increased stability of colloidal aggregates, and may be of interest for their potential application as innovative drug delivery systems.

Circular Dichroism↗