Small Platinum Clusters in Zeolites: A Density Functional Study of CO Adsorption on Electronically Modified Models

Abstract

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.