Co-adsorption of oxygen and carbon monoxide on Ni(1 1 1)

The co-adsorption of oxygen and carbon monoxide on the Ni(1 1 1) surface was investigated by high resolution electron energy loss spectroscopy. We found that CO molecules adsorbed at 150 K on the oxygen pre-covered surface have two distinct internal vibrations, i.e. at 236 and 258 meV. The vibrations at 236 and 258 meV are the fingerprints of molecules adsorbed at threefold hollow sites and at on-top sites, respectively. At 150 K it was not possible to selectively populate one kind of the allowed sites. On the contrary, above 200 K the adsorption of carbon monoxide on a O-p(2 × 2) surface led to the occupation of only on-top sites. As regards 0.25 and 0.33 ML of oxygen pre-coverages, the heating of the co-adsorbed (CO + O) layer from 150 to 200 K gave rise to a site change of some CO molecules, moving from hollow to on-top sites.