1. Center for Atomic-scale Materials Design, Department of Physics, Building 311, Technical University of Denmark, DK-2800 Lyngby, Denmark, Science Institute, VR-III, University of Iceland, IS-107 Reykjavik, Iceland, Faculty of Science, VR-III, University of Iceland, IS-107 Reykjavik, Iceland, Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, and Department of Chemical Engineering, Stanford University, Stanford, California 94305