Adsorption and coadsorption of CO and H on ruthenium surfaces

The interaction of CO with the Ru(0001) surface at several coverages (11.1, 25.0, and 33.3%) is studied, as well as the interaction of CO with a stepped Ru(0001) surface. The preference for the adsorption site (atop versus hcp) is analyzed with density of states diagrams. Hydrogen layers can be densely packed; 1 ML could, in fact, correspond to more than 100% coverage, where 100% coverage would correspond to one addatom for each metal atom on the surface. Calculations are made for 1 ML of adsorbed hydrogen up to 300% coverage for 2 x 2 supercells. The H coadsorption with CO (2 x 2 (CO + nH), n = 1, 3, 4) is discussed for different adsorption sites. The lateral interaction H-CO is repulsive. H-ads and COads prefer to form islands rather than mixed structures. CO is little influenced by coadsorption, except when 1 ML of atomic hydrogen is preadsorbed. H is strongly affected by coadsorption. The H adsorption sites become highly asymmetrical if H and CO share one metal atom.