The Direct Influence of the Support on the Electronic Structure of the Active Sites in Supported Metal Catalysts : Evidence from Pt-H Anti-Bonding Shape Resonance and Pt-CO FTIR Data

Abstract

The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by support properties such as the presence of protons, type of charge compensating cations, Si/Al ratio and/or presence of extra-framework Al. The metal support interaction is relatively independent of the metal (Pd or Pt) or the type of support (microporous zeolites such as LTL and Y or macroporous supports such as SiO_2). As the alkalinity of the support increases (i.e., with increasing electronic charge on the support oxygen ions), the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a systematic shift from linear to bridge bonded CO as indicated by the IR spectra. This is a strong indication of a change in the electronic structure of the catalytically active Pt surface atoms. Analysis of the Pt H anti-bonding shape resonance present in the Pt X-ray absorption spectra of the L_3 edge indicates that the difference in energy between the Pt H anti-bonding orbital and the Fermi level decreases as the alkalinity of the support increases. The results from the IR and Pt H shape resonance data directly show that the support influences the position in energy of the metal valence orbitals. The ionisation potential of the catalytically active Pt surface atoms decreases with increasing support alkalinity, i.e., with increasing electron charge on the support oxygen ions. This shift leads to a weakening of the Pt H bond.