Contatto di riferimento: Laura Basiricò
Partecipanti: Dott. Stefano Pelli Cresi: Dipartimento FIM, Università di Modena e Reggio Emilia, Via G. Campi 213/a, Modena, Italy
Abstract
The widespread applicability of ceria (CeO2) has made this material one of the most attracting functional oxides in the last decades. Cerium dioxide in fact has the important property of easily forming, storing and transporting oxygen vacancies due to the low energy barrier needed to switch from Ce+4 to Ce3+ oxidation state. This feature makes this material perfect to support late transition metals (like Pd, Rh and Pt) or transition metal oxides (like CuOx or MnOx) in catalytic processes as active co-catalysts. In particular, ceria has a great impact in toxic emission treatment (as in methane oxidation) or in energy conversion systems like either hydrogen or methanol fuel cells and for the production of hydrogen. An important goal in this field is
the design of cerium oxide based materials with optimized reducibility at low temperature to optimize its reactivity in redox reactions. Obviously, the ability of repeatedly and rapidly pass through redox cycles is strongly related to the ease in forming and healing oxygen vacancies at the surface of ceria. Therefore, in order to optimize the reduction process, it is crucial to understand how the structure is modified during redox cycles and how it could be affected by the environment. To help in achieving this goal we started to study reducibility and the modification of the structure using XAFS (X-ray Absorption Fine Structure) in model system as ultrathin epitaxial films and then we will focus on real system as nanoparticle films. In fact, it was shown that the oxygen vacancy formation energy in cerium oxide in NP nanoparticles has a minimum at a specific size, and that it increases for smaller nanoparticle. The expansion of the cerium oxide lattice was often ascribed to a decrease of the electrostatic force due to reduction of surface Ce ions, rather than to an intrinsic dimensionality-related effect. However, the Ce3+ surface concentration in NP does not only depend on the NP size, but also on the different synthesis methods, on the NP shape and on the experimental conditions (vacuum, photon/electron beam exposure) and the possibility to control it is a rather difficult but
appealing task. We investigated first cerium dioxide ultrathin epitaxial film grown on Pt(111) and reduced by thermal treatments in vacuum. The structural and electronic modifications were studied using x-ray absorption spectroscopy (XAS) at the Ce L3 edge in the near and extended energy range.
Analysis of the spectra in near edge range shows that thermal treatment drastically modifies Ce oxidation state in the thinner sample (thickness less than 1nm), while the analysis of the extended energy range shows a contraction of the Ce-O bond by 2-3% compared to initial CeO2 cubic fluorite lattice. Differently, the same thermal treatments do not remarkably modify neither the structure nor the electronic state of thicker films (approximately 3nm~10 ML). This fact is consistent with the hypothesis that reduction involves only the topmost surface layers and does not influence the bulk structure of the film. It also suggests that the metal substrate has a key role in the process.