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 cavities. Both electron-enriched and electron-deficient encapsulated metal species have been proposed on the basis of experimental data. In this work, structure and adsorption properties of the cluster Pt₄, in both neutral and electronically modified forms, have been studied computationally with the help of a scalar-relativistic density functional method. The species Pt₄⁺ has been chosen to represent the case of a metal particle interacting with an electron attracting zeolite host; likewise, Pt₄^- has been taken to mimic the effect of an electron-donating host. Adsorption of CO probe molecules at on-top, bridge, and 3-fold hollow sites of the moieties Pt₄, Pt₄⁺, and Pt₄^- has been investigated to determine a relationship between the cluster charge and the C-O vibrational frequency shift Δω(CO). The chemical effect of electron-donor and electron-acceptor species on the electronic structure of the Pt₄ clusters and on the properties of adsorbed CO probes has been also explicitly taken into account by employing various models XPt₄CO (X = Na, Na⁺, NH₃). Properties of adsorbed CO probe molecules were calculated to be rather sensitive to the electronic state and the adsorption site of the Pt₄ particles, in line with experimental findings. A linear correlation between the effective charge of the metal cluster and the adsorption-induced vibrational frequency shift Δω(CO) has been found for CO adsorbed at on-top positions.