Oxygen adsorbed on oxidized Ru(0001)

An atomic oxygen species adsorbed on Ru(0001) containing subsurface oxygen has been found and is characterized by the means of thermal description spe ctroscopy (TDS), ultraviolet photoemission spectroscopy, and reactive CO sc attering. The surplus oxygen can be adsorbed on an oxygen-rich ruthenium cr ystal at room temperature when mon than two monolayers of oxygen an deposit ed in the subsurface region (1 ML =1.58 x 10(15) cm(-2)). The maximum numbe r of oxygen atoms occupying this state depends on the oxygen content in the subsurface region. It saturates for oxygen contents higher than about 10 M L at a level of 0.25 ML. The initial binding energy of an adsorbed oxygen a tom, as derived from TDS data, is lower than 0.75 eV per atom. It continuou sly decreases with increasing lateral adatom density. The work function phi varies with the increasing population of this oxygen state. For saturating oxygen coverage it reaches a maximum that can be by 1.3 eV higher than the value known for a clean metallic Ru surface. The CO oxidation was performe d in a beam scattering experiment and it reveals a high reactivity of this weakly bound state. At room temperature the CO/CO2-conversion probability r eaches a value of 0.18, which is nearly by one order of magnitude higher th an the one found for subsurface oxygen at much higher temperatures. [S0163- 1829(99)12243-4].