Structure and lateral interactions in binary and ternary coadsorbate layers of O, H and CO on Ru(001)

The mutual interactions and adsorption properties of the ternary Ru(001)-(2×2)-(O+H+CO) coadsorbate layer and some binary combinations thereof have been investigated using infrared absorption spectroscopy and thermal desorption spectroscopy. Common to all layers investigated is a (2×2)-O overlayer into which the other components have been incorporated afterwards. Hydrogen adsorption as well as H2 thermal desorption are found to be strongly influenced by a site-blocking effect by coadsorbed CO which prevents any hydrogen uptake (when CO has been preadsorbed), or increases the H2 desorption temperature by about 75 K (when CO has been post-adsorbed). This is why the preparation of the ternary system requires a definite adsorption sequence. For the binary Ru(001)-(2×2)-(O+nH) system well-ordered layers are found for n=1 and n=3. A revised structural model is proposed to explain the apparent zeroth to first order desorption kinetics of the α1 H2 desorption peak (ΘH>0.25 ML) which contradicts earlier subsurface hydrogen models; instead, the formation of localized high H-density areas accompanied by a site change (fcc→hcp) is suggested.