A new model describing the metal-support interaction in noble metal catalysts

The catalytic activity and spectroscopic properties of supported noble meta l catalysts are strongly influenced by the acidity/ alkalinity of the suppo rt but are relatively independent of the metal (Pd or Pt) or the type of su pport (zeolite LTL or SiO2), As the alkalinity of the support increases, th e TOF of the metal particles for neopentane hydrogenolysis decreases. At th e same time, there is a decrease in the XPS binding energy and a shift from linear to bridge bonded CO in the IR spectra. Analysis of the shape resona nce in XANES spectra indicates that in the presence of chemisorbed hydrogen the difference in energy between the Pt-II antibonding orbital and the Fer mi level decreases as the alkalinity of the support increases, Based on the results from the IR, XPS, and shape resonance data a new model is proposed in which the interaction between the metal and support leads to a shift in the energy of the metal valence orbitals, The EXAFS structural analysis in dicates that the small metal particles are in contact only with the oxide i ons of the support. Finally, a new spectroscopic characterisation, Atomic X AFS, is presented which provides new insights into the origin of the electr onic changes in the metal. As the alkalinity of the support increases, ther e is decrease in the metal ionisation potential. The primary interaction is a Coulomb attraction between metal particle and support oxygen ions, which affects the metal interatomic potential. This model for the metal-support interaction explicitly excludes the need for electron transfer, and it can account for all observed changes in the catalytic, electronic, and structur al properties of the supported metal particles induced by support acidity r anging from acidic to neutral to alkaline, (C) 1999 Academic Press.