Metal cluster support interactions in the Cu/ZnO system: A QM/MM study

To study in detail the interaction of copper atoms and clusters in various, potentially important catalytic oxidation states (Cu, Cu+, and Cu2+) on polar surfaces, we have applied a hybrid QM/MM embedding model within the computational chemistry package ChemShell. Using the model, we study specific Cu/ZnO surface sites treated at a high ab initio level of theory while also maintaining the effects of steric bulk constraints, the influence of a long-range electrostatic field, and a self-consistent treatment of polarization of the ZnO support. The model has been applied to copper atoms, ions and clusters (Cu-n, n = 1, 4) on the zinc-terminated (0001) surface of ZnO. The anchoring sites for copper adsorption have been identified as vacant zinc interstitial surface sites that result from the reconstruction of polar surfaces. The undercoordinated oxygens at these sites strongly bind to the copper anchor in all three oxidation states whereas the preferred path for cluster growth is shown to be by addition of neutral copper on both Cu-0 and Cu+ species.