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Vikatakavi, Avinash, (2023)  - Interazione tra H2 e film ultrasottili di CeO2 modificati con rame  - , Tesi di dottorato - (, , Universitą degli studi di Modena e Reggio Emilia ) - pagg. -

Abstract: Proton exchange membrane fuel cells (PEMFCs) are attractive for applications in transportation, stationary and portable devices owing to their high-power density, high efficiency, ease of operation and low/zero emissions. Despite their salient features, PEMFCs large commercialization is impeded due to the utilization of expensive and scarce platinum-based electrocatalysts. Many non-noble materials have been explored so far to replace Pt and obtain similar or better catalytic activity. Among them, cerium oxide (CeO2), has been considered as the promising candidate as electrode material for PEMFCs because of its redox properties and the ability to dissociate H2 molecules in the absence of noble metals. In addition, modification of CeO2 with aliovalent cationic species, has been identified as a promising method to reduce the activation barrier for H2 dissociative adsorption on the surface of CeO2. Herein, we investigate the modifications induced in Cu-modified CeO2 epitaxial ultrathin films, used as well-controlled model systems, during the interaction with H2. CeO2 ultrathin films, doped with different concentrations of Cu, were grown by reactive molecular beam epitaxy and their chemical and morphological properties during thermal reduction cycles in UHV and H2 were studied, using in-situ X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). XPS spectra provided quantitative information on the concentration of Ce3+ ions, O-vacancies and OH-groups on the surface during the interaction with H2. The results are interpreted also with the help of density functional theory calculations performed by a group of collaborators. Also, Cu nanoparticles (NPs) supported on CeO2 ultrathin films were studied under the same conditions. Along with the reducibility of ceria, the stability of Cu NPs on these surfaces has been investigated using STM. The experimental results indicate that the Cu atoms as dopant and NPs supported on the surface of CeO2 tend to increase the reducibility of Ce atoms as the electrons released during the O-vacancy formation and H2 dissociation are acquired mostly by Ce ions. The increase in the concentration of OH groups during thermal treatments in H2 can be ascribed to the lower H2 dissociation barrier on the Cu-modified surfaces than on pure ceria. However, also Cu atoms are reduced from +2 to +1 along with the Ce atoms during dissociative adsorption of H2 on the surface. The formation of Cu1+ significantly decreases the H2 dissociation barrier. The surface morphology of the films is not altered significantly after the H2 thermal reduction cycles. In-operando NEXAFS studies on the Cu doped films at APE-HE beamline at the ELETTRA synchrotron allowed to study the evolution of the oxidation state of Ce and Cu during thermal treatments in H2 at near ambient pressure. The results confirmed a partial reduction of Cu atoms to the +1 state and its influence on the reduction of Ce as well as on H2 dissociation.