Electronic structure and chemical reactivity at solid surfaces: the role of under-coordinated atoms and bimetallic alloys.

The 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 promising depicters of chemical reactivity. This physical quantity cannot be easily accessed in the experiment 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 identification in the core level photoemission spectra, the photoemission contribution originated from bulk and surface atoms 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 and Density Functional Theory calculations. With this approach, highly under-coordinated Rh and Pt atoms, namely adatoms and addimers, on homo-metallic surfaces (Rh(100), Rh(111) and Pt(111)) has been probed. Rh-Pt mixed systems are also investigated in order to test the changes in Rh catalytic properties induced by interaction with the underlying Pt substrate. Surface atomic coordination changes can be induced also by surface reconstruction processes. Pt(100) represents a formidable example of how the same substrate can exhibit completely different catalytic properties by changing its surface structure from the more open high-reactivity (1 ? 1) bulk termination to the strained low-reactivity reconstructed quasi-hexagonal (HEX) surface. In this thesis the (1 ? 1)? (HEX) phase transition on a clean Pt(100) surface was investigated. Another example was the reconstruction of O(2 ? 2)pg/Rh(100). In this case the surface atomic rearrangement is caused by the presence of an atomic adsorbate. The understanding of the reconstructed phase is a fundamental step towards the understanding of the microscopic mechanisms, which permits the subsurface oxygen penetration during oxide formation. Finally, the structure of the Ni3Al(111) surface is examined by means of X-ray Photoelectron Diffraction. This is a preliminary study towards the investigation of the oxidation process which is responsible for the formation of a thin well-ordered alumina film in UHV conditions.