INTERACTION OF CO MOLECULES WITH ELECTRON-DEFICIENT PT ATOMS IN ZEOLITES - A DENSITY-FUNCTIONAL MODEL CLUSTER STUDY

The coordination and electronic structure of monatomic Pt species in m ordenite have been investigated by scalar-relativistic density functio nal model cluster calculations using CO molecules as a probe. It was f ound that anchoring a Pt-CO moiety by the protons of one or two acidic hydroxyl groups increases the CO stretching frequency compared to tha t of free Pt-CO, but leaves the frequency still smaller than that of a free CO molecule, in line with experiment. The results for various mo lecular model complexes support the hypothesis that the platinum speci es in mordenite are electron-deficient. An alternative model comprisin g ''naked'' protons interacting with Pt-CO moieties can be ruled out s ince the calculated CO frequency is too large, The dependence of the C O stretching frequency on the acidity of the Bronsted groups and on th e electronic charge of the Pt species is discussed.