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 mordenite have been investigated by scalar-relativistic density functional model cluster calculations using CO molecules as a probe. It was found that anchoring a Pt-CO moiety by the protons of one or two acidic hydroxyl groups increases the CO stretching frequency compared to that 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 molecular model complexes support the hypothesis that the platinum species in mordenite are electron-deficient. An alternative model comprising "naked" protons interacting with Pt-CO moieties can be ruled out since the calculated CO frequency is too large. The dependence of the CO stretching frequency on the acidity of the Brønsted groups and on the electronic charge of the Pt species is discussed.