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Structure of a thin oxide film on Rh(100)

The surface oxide as a source of oxygen on Rh(111)

Understanding structural deactivation of supported and unsupported Ru catalysts on atomic scale

Structure of a thin oxide film on Rh(100)

We have studied the atomic arrangement of a very thin Rh oxide-like film formed on a Rh(100) surface after exposure to elevated oxygen pressures and elevated sample temperatures. The motivation for this study is the increasing conviction that such thin oxides may be of importance in catalytic reactions connected to the cleaning of car exhaust fumes. For a full understanding of the role of these oxides during the catalytic reaction, it is necessary to determine the structure on the atomic scale.

By using a combination of several advanced experimental and theoretical techniques provided by a number of groups in Europe, we could show that the thin oxide film consist of a so-called tri-layer of O-Rh-O, as shown in Fig. 1. The local arrangement within the oxide is hexagonal, despite the squared morphology of the substrate.

Figure 1: The structure of the thin oxide formed on the Rh(100) surface. Red balls are the oxygen atoms while the blue balls are Rh atoms.

Our detailed, combined, pan-european study ensures that the structural characterization has the highest quality despite the large number of atoms involved. The agreement between the various quantitative methods used is excellent, and is the basis for future studies of more realistic systems such as PtRh alloys which are used in catalytic car-exhaust converters under reaction conditions.

More information:
J. Gustafson, A. Mikkelsen, M. Borg, J. N. Andersen, and E. Lundgren, C. Klein, W. Hofer, M. Schmid, and P. Varga, L. Köhler and G. Kresse, N. Kasper, A. Stierle, and H. Dosch,
Structure of a thin oxide film on Rh(100),
Physical Review B 71, 115442 (2005)

The surface oxide as a source of oxygen on Rh(111)

Catalytic converters in cars are efficient in transforming poisonous gases such as CO and NO into more harmless ones such as CO2 and N2. In most cases the active materials are late transition metals such as Rh and Pt. It has for a long time been believed that the bare metal surfaces have been responsible for these catalytic reactions. Very recently, however, strong evidence has been found that it is actually the oxides of these materials formed during reaction conditions that are the most efficient phase in catalyzing CO and NO into less harmful gases. The structure and reaction properties of these oxides are virtually unknown, thus, in this paper we present an investigation on how a thin O-Rh-O film is interacting with CO. By combining several advanced experimental and theoretical methods provided by a number of groups across Europe, we have been able to propose a new mechanism for the CO induced reduction process, as described in Fig. 2.

Figure 2: Schematic illustration of the the reduction process of the thin oxide structure. (a) The initial adsorption of CO at defects and reaction with nearby O in the surface oxide. (b)The CO may easily adsorb and react on the exposed Rh in partly reduced areas. (c) Co-adsorbed O as well as O in the surface oxide participate in the reaction at a later stage in the reduction process. (d) In the final stage only CO is left on the surface.

Our main finding is that, under the experimental conditions used, the CO molecule does not adsorb on the surface of the oxide, but rather on oxygen under-coordinated sites such as steps. Despite this low adsorption probability, the oxide film is readily reduced, and the oxygen in the film is actively used in the catalytic reaction occurring on the metal surface. In fact, the surface oxide is acting as a source of oxygen, fuelling the reaction on the surface.

More information:
E. Lundgren, J. Gustafson, A. Resta, J. Weissenrieder, A. Mikkelsen, J.N. Andersen, L. Köhler, G. Kresse, J. Klikovits, A. Biedermann, M. Schmid, P. Varga,
The surface oxide as a source of oxygen on Rh(111)
J. Electron Spectrosc. Relat. Phen. 144-147 (2005) 367-372

Understanding structural deactivation of supported and unsupported Ru catalysts on atomic scale

This project was a collaboration between three partners of the NanO₂ consortium and the technical chemistry department at the Ruhr University of Bochum (Prof. Dr. Muhler). The surface-science-approach coupled with industrial catalyst research offers a synergistic strategy to improve the performance of industrial catalysts (see Fig. 3). The poorly understoood microscopic processes that determine the structural deactivation of ruthenium-based catalysts during CO oxidation have been elucidated. Based on these results measures are proposed to improve the performance of ruthenium catalysts. The most active Ru particle constists of the ultra thin surface oxide covering a metallic Ru core. This kind of core shell structure of Ru particle has been shown to reveal superior performance in terms of catalytic activity and stability under reaction conditions.

Figure 3

More information:
J. Aßmann, D.a Crihan, M. Knapp, E. Lundgren, E. Löffler, M. Muhler, V.Narkhede, H. Over, M. Schmid, P. Varga,
Understanding the structural deactivation of Ruthenium catalysts on atomic scale,
Angewandte Chemie Int. Ed. 44, 917 (2005)