Small platinum clusters in zeolites: A density functional study of CO adsorption on electronically modified models

Very small transition metal particles can be stabilized inside zeolite cavi ties. Both electron-enriched and electron-deficient encapsulated metal spec ies have been proposed on the basis of experimental data. In this work, str ucture and adsorption properties of the cluster Ptr, in both neutral and el ectronically modified forms, have been studied computationally with the hel p of a scalar-relativistic density functional method. The species Pt-4(+) h as been chosen to represent the case of a metal particle interacting with a n electron attracting zeolite host; likewise, Pt-4(-) has been taken to mim ic the effect of an electron-donating host. Adsorption of CO probe molecule s at on-top, bridge, and 3-fold hollow sites of the moieties Pt-4, Pt-4(+), and Pt-4(-) has been investigated to determine a relationship between the cluster charge and the C-O vibrational frequency shift Delta omega(CO). The chemical effect of electron-donor and electron-acceptor species on the ele ctronic structure of the Pt-4 clusters and on the properties of adsorbed CO probes has been also explicitly taken into account by employing various mo dels XPt4CO (X= Na, Na+, NH3). Properties of adsorbed CO probe molecules we re calculated to be rather sensitive to the electronic state and the adsorp tion site of the Pt-4 particles, in line with experimental findings. A line ar correlation between the effective charge of the metal cluster and the ad sorption-induced vibrational frequency shift Delta omega(CO) has been found for CO adsorbed at on-top positions.