Ruthenium dioxide, a fascinating material for atomic scale surface chemistry

Abstract

Over the past few years, RuO2 has developed into one of the best-characterized late transition metal oxides in surface science, revealing unique and promising redox properties. The CO oxidation reaction over RuO2 (110) was intensively studied by low-energy electron diffraction, scanning tunneling microscopy, high resolution core level spectroscopy, and density functional theory calculations, connecting structural and electronic properties with chemical properties. On the atomic scale the presence of one-fold coordinatively unsaturated Ru sites (1f-cus Ru) is the primary reason for the high activity of stoichiometric RuO2 (110) towards the oxidation of CO and other small alcohols. On the stoichiometric RuO2 (110) surface, CO molecules adsorb strongly (adsorption energy exceeding 1.2 eV) on top of the 1f-cus Ru atoms, from where the actual oxidation reaction step takes place via recombination with under-coordinated lattice oxygen to form CO2 (the so-called Mars–van Krevelen mechanism); the conversion probability of this process is as high as 80%. This mechanism leads to a (partial) reduction of the RuO2 (110) surface, producing two-fold coordinatively unsaturated Ru sites (2f-cus Ru) via the removal of bridging O atoms. Therefore, equally important for being a good catalyst is the facile re-oxidation of the mildly reduced RuO2 (110) surface by oxygen supply from the gas phase. A weakly held oxygen species was found to adsorb on top of the 1f-cus Ru atoms and to actuate the restoration of the reduced RuO2 (110) surface. On the reduced RuO2 (110) surface, CO molecules adsorb in bridge sites above the 2f-cus Ru atoms by 1.85 eV, while the CO bond strength over 1f-cus Ru atoms is 1.61 eV.