Entropy-controlled site occupation of CO adsorbed on Ni(100)

The adsorption of CO on a Ni(100) surface has been studied by FT-IRAS in the temperature range from 85 K to 300 K. At 300 K and for =0.5, the CO molecules are predominantly adsorbed in on-top sites with only a minor fraction located at two-fold bridge sites. Measurements on a Ni(100) surface pre-covered with sulphur, oxygen and carbon indicate that the occupation of bridge sites may be caused by small amounts of surface impurities. The relative broadness of the infrared bands is explained by CO molecules occupying intermediate positions at domain walls. Upon lowering the temperature, the bridge sites are increasingly occupied at the expense of terminal sites. This process is completely reversible and is explained by a contribution of the hindered translations of the adsorbed CO molecules to the entropy. At 85 K and for low initial coverages, we observe an unusual high CO stretching frequency at 2205 cm^–1 which cannot be explained at present.