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.
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