Molecular adsorption and metal-support interaction for transition-metalclusters in zeolites: NO adsorption on Pd_n(n=1-6)clusters in mordenite

摘要

The adsorption of NO molecules on Pd_n clusters of varying size(n=1-6)located in the mainchannel of mordenite and the interaction of the metallic clusters with the zeolitic framework wereinvestigated using ab initio density-functional calculations under periodic boundary conditions. Thesupported clusters are created by binding Pd_n~2+_n~2cations to the inner cavity of a deprotonatedAl-exchanged zeolite with an Al/Si ratio of 1/11, such that a charge-neutral system is created.Compared to the highly symmetric structures of the gas-phase clusters, the clusters bound to thezeolitic framework undergo appreciable geometric distortions lowering their symmetry. Thedistortions are induced by strong interactions with "activated" framework oxygens located close tothe charge-compensating Al/Si substitution sites, but the cluster forms also weaker bonds to"nonactivated" oxygen atoms. The interaction with the framework also affects the electronic andmagnetic properties of the clusters. While in the gas phase all clusters (except the isolated Pd atomwith a closed dm ground state) have a paramagnetic moment of 2μB, in the zeolite clusters with twoto four atoms have zero magnetic moment, while the Pd_5cluster has a magnetic moment of2μBandfor the Pd_6cluster, it is even enhanced to 4μB(but the magnetic energy differences relative tolow-spin configurations are modest). Analysis of the magnetization densities shows that in allclusters with zero total moment (singlet ground state), there are sites with excess spin densities ofopposite sign. The influence of the cluster-support interaction on the chemical properties of theclusters has been tested by the adsorption of NO molecules. The results demonstrate the interplaybetween the molecule-cluster and cluster-framework interactions, which can lead to an increase ordecrease in the adsorption energy compared to NO on a gas-phase cluster. While on the gas-phasecluster adsorption in low-coordination sites (vertex or bridge) is preferred, for the cluster in thezeolite adsorption in threefold coordinated hollow or twofold bridge sites is preferred. The magneticproperties of the clusters and of the paramagnetic NO molecule play an important role. For thesupported clusters with zero magnetic moment, upon adsorption the spin of the molecule istransferred to the cluster (and induces also a modest polarization of the framework). For magneticclusters, spin pairing induces a reduced magnetic moment of the NO-Pd_n complex. The redshift ofthe NO stretching frequencies is reduced compared to the free clusters by the cluster-supportinteraction for the smaller clusters, while it remains essentially unchanged for the larger clusters. Adetailed electronic analysis of the cluster-support interactions and of the adsorption properties ispresented.