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Title: Oxidation of supported PtRh partcles:size and morphology effects
Authors: Dalmiglio, Matteo Maria
Supervisore/Tutore: Gregoratti, Luca
Issue Date: 8-Apr-2010
Publisher: Università degli studi di Trieste
The chemical transformations of supported PtRh particles ranging in size from a few micrometers to a few nanometres, and nanocrystalline films have been studied under identical oxidizing conditions by means of different chemical and structural characterization techniques; in particular the main technique used has been the scanning photoemission spectromicroscopy (SPEM) available at the EscaMicroscopy beamline of the Elettra Synchrotron Light Source. This novel experimental technique allows sample’s chemical mapping with a spatial resolution of 100nm and the acquisition of photoemission spectra on regions with the same dimension, and allow us to determine the chemical state of single micro-particles. In particular we studied PtRh cluster deposited by PLD (pulsed laser deposition) on a tungsten single crystal (W(110)) covered by a thin magnesium oxide film (MgO).
Significant variations of the Pt and Rh atoms reactivity have been revealed by comparing the oxidation states of particles with different dimensions and, for the micron-scale particles, also within the same island.
It was demonstrated that a selected oxidation occurs: rhodium atoms undergo stronger and faster oxidation than platinum ones. Furthermore, the oxidation process is composed by many intermediate steps, in which metastable oxides are formed. Very small cluster’s oxidation (<10nm diameter) is significantly faster then the bigger one (>100nm). Some morphological and structural clusters’ modifications after long oxidation treatments were also investigated using a high resolution SEM (<2nm lateral resolution).
Other measurements have been performed by using a Low Energy Electron Microscope (LEEM) that combines a high spatial resolution (<5nm) to a high sensitivity to surface structural modifications. In particular the behaviour of the clusters’ polycrystalline structure has been studied during oxidation-reduction treatments. It has been shown that the clusters’ surface is polycrystalline and that each nano crystals have different crystallographic orientation. After oxidation each nano-crystal undergoes a different oxidation rate. The diffraction pattern revealed that after a long oxidation the long range order of the particles’ surface is completely lost.
A characterization of the reactivity of the PtRh particles towards oxidation after an “ageing” process based on the repetition of many redox cycles has revealed a change in the stability of the oxides.
Other experiments have been realized with SEM and EDX for studying the clusters’ morphology at different annealing temperatures. The results have shown structural, chemical and morphology changes.
Ciclo di dottorato: XXII Ciclo
metadata.dc.subject.classification: SCUOLA DI DOTTORATO DI RICERCA IN NANOTECNOLOGIE
Keywords: Catalysis
Surface Science
Language: en
Type: Doctoral Thesis
Settore scientifico-disciplinare: FIS/03 FISICA DELLA MATERIA
NBN: urn:nbn:it:units-8882
Appears in Collections:Scienze fisiche

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