1 Department of Physics and Astronomy, Faculty of Science, Aarhus University, Aarhus University2 Interdisciplinary Nanoscience Center, Faculty of Science, Aarhus University, Aarhus University3 Teoretisk naturvidenskab, Aarhus University, Aarhus University4 Lund University5 ESRF6 Institut the Ciencia de Materials de Barcelona7 Technische Universität Wien8 Department of Physics and Astronomy, Science and Technology, Aarhus University9 Universität Wien10 University of St Andrews11 Department of Physics and Astronomy, Science and Technology, Aarhus University
Using a combination of experimental and theoretical techniques, we show that a thin RhO2 surface oxide film forms prior to the bulk Rh2O3 corundum oxide on all close-packed single crystal Rh surfaces. Based on previous reports, we argue that the RhO2 surface oxide also forms on vicinal Rh surfaces as well as on Rh nanoparticles. The detailed structure of this film was previously determined using UHV based techniques and density functional theory. In the present paper, we also examine the structure of the bulk Rh2O3 corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the CO oxidation reaction over Rh(1 1 1), Rh(1 0 0) and Pt25Rh75(1 0 0) at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the CO2 production coincides with the formation of the thin RhO2 surface oxide film. In the case of Pt25Rh75(1 0 0), our measurements demonstrate that the formation of bulk Rh2O3 corundum oxide poisons the reaction, and argue that this is also valid for all other Rh surfaces. Our study implies that the CO oxidation reaction over Rh surfaces at realistic conditions is insensitive to the exact Rh substrate orientation, but is rather governed by the formation of a specific surface oxide phase.
Catalysis Today, 2009, Vol 145, Issue 3-4, p. 227-235