Effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001)

Abstract

The effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001) were studied using laser-induced thermal desorption techniques. The surface mobility of hydrogen was measured as a function of CO surface coverage at 260 and 280 K. At both temperatures, the surface diffusion of hydrogen displayed an abrupt reduction at a coadsorbed CO coverage of ΘCO =0.12 ML. LEED studies revealed that a CO coverage of ΘCO =0.12 ML corresponded to the onset of the formation of ordered √3×√3 CO islands at 260 and 280 K. Temperature programmed desorption results demonstrated that a lateral repulsive interaction exists between hydrogen and carbon monoxide on Ru(001). Assuming that the lateral repulsive interaction leads to a hydrogen exclusion area around each CO admolecule, a hydrogen exclusion radius of rCO =2.2–2.7 Å was determined. The lateral repulsive interaction would also result in hydrogen exclusion from the interior of ordered √3×√3 CO islands and provide an explanation for the hydrogen surface diffusion results. According to this interpretation, the rigid √3×√3 CO islands obstruct the hydrogen surface diffusion pathways and thereby reduce the hydrogen surface mobility on Ru(001).