Energy barriers and chemical properties in the coadsorption of carbon monoxide and oxygen on Ru(0001)

Using density-functional theory we investigate the interactions and chemical properties of the coadsorption of carbon monoxide and oxygen on ruthenium (0001). For the adsorption phases that occur in nature, where CO occupies the top site, we find that with increasing oxygen coverage, the adsorption energy of CO can remain practically unchanged or even exhibit a slight increase. We attribute the increase to an O-induced lateral weakening of Ru-Ru bonds of non-O-bonded surface Ru atoms. Thus, these non-O-bonded Ru atoms can form stronger bonds to an on-top CO adsorbate. In contrast, a more expected behavior of a notable decrease in CO adsorption energy with increasing O coverage is observed only if the O atoms bond to the same Ru atoms as CO as, for example, is the case when CO occupies hollow sites. Furthermore, for some of the structures, we find that there is a manifestation of small activation energy barriers for CO adsorption well above the surface.