H. Over, ^1* Y. D. Kim, ¹ A. P. Seitsonen, ¹² S. Wendt, ¹ E. Lundgren, ³ M. Schmid, ³ P. Varga, ³ A. Morgante, ⁴ G. Ertl ¹

The structure of RuO₂(110) and the mechanism for catalytic carbon monoxide oxidation on this surface were studied by low-energy electron diffraction, scanning tunneling microscopy, and density-functional calculations. The RuO₂(110) surface exposes bridging oxygen atoms and ruthenium atoms not capped by oxygen. The latter act as coordinatively unsaturated sites--a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces--onto which carbon monoxide can chemisorb and from where it can react with neighboring lattice-oxygen to carbon dioxide. Under steady-state conditions, the consumed lattice-oxygen is continuously restored by oxygen uptake from the gas phase. The results provide atomic-scale verification of a general mechanism originally proposed by Mars and van Krevelen in 1954 and are likely to be of general relevance for the mechanism of catalytic reactions at oxide surfaces.

¹ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Physical Chemistry, Faradayweg 4-6, D-14195 Berlin, Germany.
² Istituto Nazionale per la Fisica della Materia, Unità di Roma, Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, I-00185 Rome, Italy.
³ Institut für Allgemeine Physik, Technische Universität Wien, Wiedner Hauptstr. 8-10, A-1040 Vienna, Austria.
⁴ Dipartimento di Fisica, Università di Trieste, Via Valerio 2, I-34127 Trieste and TASC-INFM Laboratory, Padriciano 99, I-34012 Trieste, Italy.
^*   To whom correspondence should be addressed. E-mail: over{at}fhi-berlin.mpg.de