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

Abstract

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).

In situ X-ray absorption spectroscopy as a unique tool for obtaining information on hydrogen binding sites and electronic structure of supported Pt catalysts: towards an understanding of the compensation relation in alkane hydrogenolysis

“…In contrast, the bonding of Pt with various alkane radicals (i.e., alkanes with one H removed) appears to be generally determined by localized bonding between the carbons and the Pt 5d orbitals. This is consistent with reported GVB-MO calculations [49], which also indicate that the Pt-H bond has a strong overlap with Pt 6sp orbitals. In the valence-bond picture, an interstitial bond orbital (IBO) exists in the interstitial region of a Pt tetrahedral, as illustrated in the insert of Fig.…”

Nature of the Metal–Support Interaction in Supported Pt Catalysts: Shift in Pt Valence Orbital Energy and Charge Rearrangement

“…The MO results for an isolated metal cluster indicate that the Pt 6sp orbitals overlap strongly forming bonding and antibonding MOs or bands which are positioned on both sides of the 5d band (35,38,40,41). Consistent with this, Kua and Goddard (35,38) suggest that the 6s orbitals form "interstitial bond orbitals" (IBO and IBO*), in contrast to the 5d orbitals that are almost nonbonding, giving a narrower overall d band.…”

“…Consistent with this, Kua and Goddard (35,38) suggest that the 6s orbitals form "interstitial bond orbitals" (IBO and IBO*), in contrast to the 5d orbitals that are almost nonbonding, giving a narrower overall d band. In bulk metal or in clusters, one IBO forms at the center of half of the Pt 4 tetrahedra in the solid (35). In a tetrahedral Pt 4 cluster, the four Pt 6s orbitals would form symmetry-adapted linear combinations (SALCs) of A 1 and T 2 symmetry.…”

“…High level ab initio molecular orbital (MO) calculations on Pt clusters (35,36) (38), and NH 3 (41)] on the "surface" of these clusters have recently been published. These calculations use different levels of approximation (e.g., density functional theory, DFT (40,41,43), or generalized valence bond, GVB (35,38), theory).…”

“…These calculations use different levels of approximation (e.g., density functional theory, DFT (40,41,43), or generalized valence bond, GVB (35,38), theory). However, they all use some approximate linear combination of atomic orbitals to construct the molecular orbitals (LCAO-MO).…”