Chemisorption of Organics on Platinum. 1. The Interstitial Electron Model

Using first principles quantum mechanics (nonlocal density functional theory), we studied the bonding and electronic states for clusters of Pt atoms. These calculations suggest the interstitial electron model (IEM) in which (i) the 6s valence orbitals from the four atoms of a tetrahedron combine to form an interstitial bonding orbital at the center of the tetrahedron that is occupied by two electrons to form the interstitial electron bond (IEB), (ii) the 5d valence orbitals from each atom form a band of bonding and antibonding states sufficiently dense that the optimum occupation is high spin (Hund's rule) or nearly so, and (iii) bonds of organics to the Pt surface lead to covalent σ bonds to d orbitals localized on individual Pt atoms. This simple model explains the bonding and lowest electronic state of essentially all clusters studied. The IEM suggests that the bonding in three-dimensional face-centered cubic (fcc) systems has two electrons from each atom in the IEB, leaving the remaining eight valence electrons in d-like orbitals. For bulk Pt, this leads to a 6s 2 5d 8 effective electronic configuration. The IEM suggests that the (111) surface of Pt would have a 6s 1 5d 9 effective electronic configuration. This suggests that to model the chemistry of the Pt(111) surface we should use clusters leading to the (6s) 1 (5d) 9 configuration. This suggests the interstitial electron surface model (IESM) in which a simple planar cluster with eight atoms serves to model the chemistry of the Pt(111) surface. This is used in the accompanying paper (part 2) to examine CH x and C 2 H x species chemisorbed on Pt(111).