Electronic structure and chemical reactivity of transition metals' pseudomorphic layers and supported nanoclusters

2008/2009The importance of heterogeneous catalysis in chemical industry and its economic impact in today‟s society motivate the continuous research effort in this field. Transition metals are among the main ingredients of commercial catalysts due to their chemical properties which depend on their surface morphological and electronic structure. It is well known that their catalytic properties can be further improved by tuning particle size in the nanometre range or by alloying different transition metals. Nowadays it is possible to predict the variation of surface chemical properties on the basis of the d-band centre energy position, which is actually considered as one of the most reliable depicters of chemical reactivity. This physical quantity cannot be easily accessed by experimental measurement and is typically calculated using a theoretical approach. A promising approach to establish an experimental relationship between electronic structure and chemical reactivity relies on the use of X-ray Photoelectron Spectroscopy with third generation synchrotron radiation sources. Indeed, the high resolution achieved in the recent years has allowed the identification of the contributions originated from bulk and surface atoms in the core level photoemission spectra,thus determining what is usually named Surface Core Level Shift (SCLS). It has been shown that SCLS is a valuable probe of surface electronic structure, since the core level binding energy of an atom depends strongly on the local structural and chemical environment. In this thesis, the electronic structure modification induced by reduced coordination, surface strain, atomic rearrangement and ligand effects are investigated in different systems by means of High Energy Resolution Core Level Spectroscopy experiments, on several systems with different complexity. Pseudomorphic states of Pd, grown on a Ru(0001) surface, have been studied by comparing the calculated d-band center shifts of the differently coordinated atoms to the measured core level shifts of the same species, finding a strong relationship between these two physical quantities. For two of structures (1- and 2-Pd MLs/Ru(0001)) we tested the chemical reactivity by exposing the surfaces to oxygen. The results confirmed the relationship between d-band center shift, CLS and chemical reactivity. A more complex system is the one composed by Pd nanoclusters on Single Walled Carbon NanoTubes (SWCNTs), Highly Ordered Pyrolythic Graphite (HOPG) and Ir supported graphene sheet. The most relevant feature is the formation of a high BE component in Pd 3d5/2 spectrum, induced by those Pd atoms coordinated with surface defects. This interpretation has been suggested by the comparison of experimental results with the calculations on Pd/HOPG core level shifts for many different atomic species. For Pd/SWCNTs we studied also the oxidation mechanisms, ranging on a wide spectrum of oxidation conditions. Our results confirm the presence of a 2D oxide phase, as previously found on Pd single crystal, with some different behaviour induced by the reduced size of our particle Finally, the growth mechanisms and the chemical reactivity of Pt nanoclusters, supported on a MgO thin film has been studied by both energy and time resolved x-ray photoemission spectroscopy. The CO oxidation reaction has been investigated for different temperatures and different clusters‟ size, resulting also in a deeper comprehansion of the clusters‟ morphology.XXII Ciclo197