CO chemisorption over MnO_x-modified Ni(111): Importance of perimeter sites in strong metal-support interaction

CO chemisorption on both clean Ni(111) and MnO_x (x is between 0.2 and 0.5)-modified Ni(111) surfaces at 200 °K have been studied by Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and high-resolution electron energy loss spectroscopy (HREELS). The MnO_x Ni specimen was prepared by evaporating MnO₂ powder to the Ni surface and then reducing the specimen at 700 °K. TDS results showed that the presence of surface MnO_x species suppresses CO chemisorption and that the suppression is almost a linear function of the MnO_x coverage. The most prominent CO desorption peak on Ni occurs at 415 °K. This peak intensity decreased with increasing MnO_x coverage, indicating a site-blocking effect. An additional CO desorption peak at 305 °K was found on the MnO_x-modified Ni surface. This 305 °K peak intensity attains a maximum at some intermediate MnO_x coverage. HREELS showed a new C-O stretching frequency of 1620 cm^-1 for CO adsorbed on the MnO_x Ni surface. The disappearance of this 1620-cm^-1 energy loss peak upon heating the surface to 325 °K suggests that both the 305 °K TDS peak and the 1620-cm^-1 energy loss peak represent the same adsorbed CO state, which is attributed to CO adsorbed on MnO_x Ni perimeter sites. Surface morphology of the oxide islands and significance of the perimeter sites are discussed along with experimental results.