In heterogeneous catalysis, e. g., CO oxidation in automotive catalytic converters, the reactants are adsorbed on the surface of the catalyst, where they finally react to form the desired product. In the early days of catalysis studies, it was believed that only the metal surface acts as a catalyst – until it was discovered that (ultra-thin or bulk-like) oxides of these metals can be responsible for high catalytic reactivity! Thus, for about a decade (≈2000–2010), our line of research was mainly focused on oxygen on transition metal surfaces and its reaction with the simple molecules H2 and CO. All these activities were done in close collaboration with several other experimental and computational groups, a combined effort that turned out to be highly productive.

Ultra-thin oxides – a zoo of new phases at surfaces

STM image and structure model of an oxidized palladium surface

© Michael Schmid/TU Wien

Our group, together with Georg Kresse, opens an external URL in a new window, was the first one to solve the structure of a purely 2-dimensional oxide, i.e., an oxide that only exists as an ultrathin layer on a metal surface. After this initial discovery, we have found a large variety of further oxides on the Pd(111) surface, allowing us to derive the building rules that govern how these structures are formed. Rh, an immediate neighbor of Pd in the periodic table, forms a so-called tri-layer oxide, i.e., a layer sequence of O-Rh-O on top of the metal, and it turned out that this oxide is universal for all the low-index crystallographic orientations of Rh [i.e., (111), (100), (110)]. Most of our work on ultrathin oxides was in close collaboration with Edvin Lundgren, who was in our group when he discovered the 2D oxide on Pd(111), and is now at the Department of Synchrotron Radiation Research, opens an external URL in a new window of Lund University.

When it comes to reactions, it is still a highly controversial topic whether the surface oxides are extremely good catalysts for oxidation processes or catalytically dead. We could show that the trilayer Rh oxide reacts only at its borders at very low gas pressures [still in the ultrahigh vacuum (UHV) range]. This was expected since gases such as H2 and CO do not adsorb on the oxygen-terminated surface of the 2D oxide. We could directly describe the macroscopic reaction kinetics by the data derived form our STM experiments. At elevated temperatures and close to atmospheric pressure, recent results of our collaboration partner in this field, Edvin Lundgren, showed a 1:1 correlation between the existence of a surface oxide and high catalytic activity, in contrast to what one should expect from the UHV experiments. 

We have also discovered the structure of a surface that was probably the first one where the existence of a 2D oxide was suggested: the (4×4) phase of oxygen on Ag(111), again considered an active phase for a catalytic process in industry (ethylene epoxidation). Surprisingly, it turned out that this structure has to be considered metallic silver that only forms grooves to make space for the oxygen. Georg Kresse, who did the ab-initio calculations in this project, found that this structure is stable only due to van-der-Waals forces, quite unusual for a metal surface!

Here is only a small selection of our papers on these topics:

  • E. Lundgren, G. Kresse, C. Klein, M. Borg, J. N. Andersen, M. De Santis, Y. Gauthier, C. Konvicka, M. Schmid, P. Varga
    Two-dimensional oxide on Pd(111)
    Physical Review Letters 88, 246103 (2002); doi: 10.1103/PhysRevLett.88.246103

  • J. Gustafson, A. Mikkelsen, M. Borg, E. Lundgren, L. Köhler, G. Kresse, M. Schmid, P. Varga, J. Yuhara, X. Torrelles, C. Quirós, J. N. Andersen
    Self-limited growth of a thin oxide layer on Rh(111)
    Physical Review Letters 92, 126102 (2004); doi: 10.1103/PhysRevLett.92.126102

  • E. Lundgren, A. Mikkelsen, J. N. Andersen, G. Kresse, M. Schmid, P. Varga
    Surface oxides on close-packed surfaces of late transition metals
    Journal of Physics: Condensed Matter 18, R481 (2006); doi: 10.1088/0953-8984/18/30/R01

  • J. Gustafson, R. Westerström, A. Resta, A. Mikkelsen, J. N. Andersen, O. Balmes, X. Torrelles, M. Schmid, P. Varga, B. Hammer, G. Kresse, C. J. Baddeley, E. Lundgren
    Structure and catalytic reactivity of Rh oxides
    Catalysis Today 145, 227 (2009); doi: 10.1016/j.cattod.2008.11.011

  • J. Klikovits, M. Schmid, J. Gustafson, A. Mikkelsen, A. Resta, E. Lundgren, J. N. Andersen, P. Varga
    Kinetics of the reduction of the Rh(111) surface oxide: Linking spectroscopy and atomic-scale information
    The Journal of Physical Chemistry B 110, 9966 (2006); doi: 10.1021/jp0611875

  • M. Schmid, A. Reicho, A. Stierle, I. Costina, J. Klikovits, P. Kostelník, O. Dubay, G. Kresse, J. Gustafson, E. Lundgren, J. N. Andersen, H. Dosch, P. Varga
    Structure of Ag(111)-p(4 x 4)-O: No Silver Oxide
    Physical Review Letters 96, 146102 (2006); doi: 10.1103/PhysRevLett.96.146102

"Rough" surfaces

STM image and schematic view of an oxidized Rh(111) surface with steps

© Jan Klikovits & Michael Schmid/IAP

Catalysts often consist of metal nanoparticles, which have many steps and edges. Therefore stepped surfaces are of high interest, and we have identified one-dimensional oxides on stepped surfaces. Formation of these 1D oxides can lead to significant restructuring or roughening of the surface, and 1D oxides can show higher activity of catalytic processes than others form of oxygen bound on surfaces. It was also interesting to discover that 1D oxides impede further 2D oxidation, similar to 2D oxides protecting the underlying metal from further (3D) oxidation.

  • J. Klikovits, M. Schmid, LR. Merte, P. Varga, R. Westerström, A. Resta, J. Andersen, J. Gustafson, A. Mikkelsen, E. Lundgren, F. Mittendorfer, G. Kresse
    Step-orientation-dependent oxidation: From 1D to 2D oxides
    Physical Review Letters
    101, 266104 (2008); doi: 10.1103/PhysRevLett.101.266104

Catalysis on the oxide, not the metal

Until the end of the 1990ies, it was believed that reactions like CO oxidation on platinum-group metals always occur by simple adsorption of oxygen on the surface; the O adatoms would then react with adsorbed CO to form CO2, the so-called Langmuir-Hinshelwood mechanism. STM experiments conducted in our group have helped to find out that high catalytic activity can be due oxidation of the catalyst; it was shown that RuO2 formed at sufficient oxygen pressure catalyzes CO oxidation by the so-called Mars-van Krevelen mechanism, where the oxide itself provides oxygen for the reaction. We have continued these studies in collaboration with Herbert Over, opens an external URL in a new window and his group in Giessen (formerly Berlin).

  • H. Over, Y. D. Kim, A. P. Seitsonen, S. Wendt, E. Lundgren, M. Schmid, P. Varga, A. Morgante, G. Ertl
    Atomic-scale structure and catalytic reactivity of the RuO2(110) surface
    Science 287, 1474 (2000); doi: 10.1126/science.287.5457.1474

  • H. Over, A. P. Seitsonen, E. Lundgren, M. Schmid, P. Varga
    Direct imaging of catalytically important processes in the oxidation of CO over RuO2(110)
    Journal of the American Chemical Society 123, 11807 (2001); doi: 10.1021/ja016408t