A New Model for the Metal-Support Interaction : Evidence for a Shift in the Energy of the Valence Orbitals

Abstract

The catalytic and spectroscopic properties of Pt supported on LTL zeolite are greatly affected by the acidity/alkalinity of the support. The turnover frequency (TOF) for neopentane hydrogenolysis and isomerization decreases from acidic to neutral to alkaline. In addition, in the infrared spectra, there is a decrease in the linear to bridging ratio of adsorbed CO which parallels the catalytic activity, indicating that the changes in TOF are due to a modification of the electronic properties of the Pt particles resulting from the metal-support interaction. The local structure of the Pt particles has also been determined by EXAFS spectroscopy. The Pt atoms are in contact with the oxygen ions of the support. None of the Al, Si or K ions are within bonding distance of the Pt. In addition, analysis of the L{I}{I}{I} and L{I}{I} near-edge spectra suggest that, contrary to the generally accepted model, the number of electrons in the valence band is unchanged by the support interaction. Furthermore, at the L{I}{I}{I} edge in the presence of chemisorbed hydrogen, a Pt-H antibonding orbital is observed near the Fermi level. Isolation of this shape resonance indicates that the energy difference between the antibonding orbital and the Fermi level increases with increasing acidity of the support and correlates with the TOF. Based on the analysis of the Pt-H shape resonance, a new model for the metal-support interaction is proposed where the binding energy of the Pt valence orbitals increase as the charge of the support oxide ion becomes more positive, i.e., becomes more acidic. The catalytic and spectroscopic properties are discussed in the context of this new model