Ab initio Simulation of Clusters: Modeling the Deposition Dynamics and the Catalytic Properties of Pd _N on MgO Surface F-Centers

Summary

Nano-catalysts are studied in an ab inito framework by solving the Kohn-Sham equations of density functional theory for the supported clusters and a finite zone of the underlying surface. An efficient and accurate numerical parallel implementation of the Kohn-Sham solver using plane waves for the kinetic energy calculations and a real space grid for the potential energy evaluations permits first principle molecular dynamics simulations of the nano-catalyst formation process namely the low-energy deposition of neutral Pd _N clusters (N = 2–7 and 13) on a MgO(001) surface with oxygen vacancies (so called F-centers, FC). The main findings of this simulations are a steering effect by an attractive “funnel” due to the polarizing F-center. This results in strong adsorption of the cluster, with one of its atoms pinned atop of the FC confirming that corresponding experiments are performed with supported size-selected nano-clusters and not with larger structures grown by coalescence. Interestingly, the deposited Pd₂-Pd₆ clusters retain their gasphase geometries, while for N>6 the clusters adopt structures which maximize the contact area with the surface. Furthermore, we show that a large number of NO molecules can adsorbe on the low coordinated sites of the supported Pd clusters. For instance, the Pd ₄ was able to capture up to 5 NO in our simulations (4 on Pd-Pd bridges and one molecule on top of the tetrahedral cluster). In order to demonstrate the accuracy of our method, we report on an additional study of finite temperature photoelectron spectra for sodium cluster anions.