Chemisorption and Dissociation of Carbon Monoxide on the Sm-Modified Ru(001) Surface

The adsorption of CO on the Sm-precovered Ru(001) surface has been studied by means of high-resolution electron energy loss spectroscopy (HREELS). A highly inclined CO species with an extremely low C−O stretching [ν(C−O)] frequency of ∼1060 cm-1 was identified in the Sm and CO coadsorbed layer. This unusual bonding geometry with a severely weakened C−O bond indicates a strong chemical interaction with neighboring Sm atoms. The Sm-induced dissociation of CO was found to result from this highly inclined precursor state when the surface was covered with more than ∼0.4 monolayer (ML) of Sm or annealed above ∼300 K. Several intermediate states with ν(C−O) modes in the range of ∼1120−1560 cm-1, whose C−O bonds were weakened but probably not tilted, also appeared along with a Sm-free CO species, depending on the coverages of both Sm and CO, as well as the thermal annealing temperature. Upon thermal annealing, a part of these intermediate states were converted into the highly inclined precursor state to dissociate, while the others desorbed without dissociation. In addition, a second-layer CO species with a high ν(C−O) frequency of ∼2160 cm-1 was observed and adsorbed very weakly onto a cationic Sm atom in the coadsorbed layer.