Direct imaging of catalytically important processes in the oxidation of CO over RuO2(110)

H. Over1*, A.P. Seitsonen1, E. Lundgren2+, M. Schmid2, P. Varga2

1Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany
*New address: MPI für Festkörperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany
2Institut für Allgemeine Physik, Technische Universität Wien, A-1040 Wien, Austria
+ Present address: Department of Synchrotron Radiation Research, Institute of Physics, University of Lund, S-22100 Lund, Sweden

J. Am. Chem. Soc. 123 (2001) 11807-11808

Summary

Ruthenium dioxide (RuO2) reveals unique and promising redox properties, making RuO2 a potential candidate for a versatile oxidation catalyst. Recently Zhang and Kisch reported, for instance, that RuO2 is a robust and efficient catalyst for room temperature (RT) oxidation of CO by humid air; recall that typical metal oxides do not tolerate humidity. In this contribution we present scanning tunneling microscopy (STM) data which directly image the catalytically important processes occurring on the RuO2(110) surface after exposing the pristine surface to CO and O2. These data are complemented by density functional theory (DFT) calculations.

The following processes are governing the catalytic activity of RuO2 on atomic scale. The reactants from the gas phase encounter strongly binding adsorption sites on the RuO2(110) surface in the form of the under-coordinated Ru atoms. For instance, CO adsorbs on the stoichiometric RuO2(110) surface by 1.2 eV (over the 1f-cus-Ru), while on the reduced RuO2(110) surface the CO binding energies are 1.61 eV and 1.85 eV for adsorption over 1f-cus-Ru and 2f-cus-Ru atoms, respectively. The RuO2 surface provides an active oxygen species to react with CO, i.e. the under-coordinated (bridging) lattice oxygen atoms Obr. The recombination of adsorbed CO with Obr creates vacancies, which are identified with STM. At RT, oxygen molecules from the gas phase can efficiently dissociate on RuO2(110) via the molecular precursor state. This leads to weakly held O atoms, which are grouped in pairs as imaged with STM. It is argued that the weakly held oxygen plays an important role in replenishing the consumed (bridging) lattice oxygen atoms on the (partially) reduced RuO2(110) surface.

Corresponding author: H. Over (now at University Giessen). Reprints also available from M. Schmid (schmid< encoded email address >).

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Part of this work is on display in the IAP/TU Wien STM Gallery (see the non-metals page).