Adsorption studies on RuO2(110) and Ru(11-21) surfaces

Abstract

Adsorption and oxidation studies were performed on RuO2(110) and Ru(1121) surfaces by using vibrational spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). By using the latter method, information on the bonding strength of adsorbates can be derived. Bare RuO2(110) surfaces were prepared by exposing 10 7 L O2 to Ru(0001) at 700 K. The bare RuO2(110) surface is terminated by coordinatively unsaturated O (Obridge) and Ru (Ru-cus) atoms. By exposure of oxygen, a weakly bonded atomic oxygen called O-cus can be adsorbed at Ru-cus, creating a more O-rich surface. At 85 K, CO adsorbs at Ru-cus and is called CO-cus. In contrast, at 300 K, CO reacts with O-bridge first, and is then attached to the Ru underneath O-bridge; it is therefore called CO-bridge. CO is found to be able to react with the bare and the O-rich RuO2(110) surface. Two reaction channels are identified: CO may react with O-cus or with O-bridge. For the exposure values applied, only surface oxygen takes part in the CO oxidation. The oxygen depleted surface can be restored by O2 exposure at RT. Thus RuO2(110) + O2 + CO turned out to be a remarkable surface redox system operating at 300 K. This system is able to work under steady-state conditions. A remarkable agreement of the kinetic data was found between the RuO2(110) single crystal surface, typically operated at 10−7 mbar pressure, and small supported RuO2 particles working at atmospheric pressure. It is demonstrated that the former ”pressure gap” in CO oxidation on Ru actually is a ”material gap” which could be bridged in the present thesis. Hydrogen and CO adsorption on Ru(1121) were studied. Three adsorption states of hydrogen, α-, βand γ-H, were observed. At high coverage, three H atoms are suggested to be squeezed into one unit cell, in which α-H is adsorbed at fourfold symmetric sites, while βand γ-H are adsorbed at pseudo-threefold sites. For low coverage CO is adsorbed on Ru(1121) at on-top sites. For high coverage CO forms a compression phase, in which some CO molecules are somewhat shifted away from the on-top sites but remain linearly bonded to the substrate, and additional CO adsorbs on bridge sites, similarly as on Ru(1010). About 20 % CO can also be dissociated. The dissociation of CO is mainly determined by the CO occupation at the neighboring unit cells. This is explained here by the so-called ”bonding competition” effect between CO in neighboring unit cells.