Active sites in oxide-promoted metal catalysts: CO chemisorption on AlO_x-modified Ni(111)

CO chemisorption on both clean Ni(111) and AlO_x(x is between 0.5 and 1.1)-modified Ni(111) surfaces at 200 K has been studied by Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and high-resolution electron energy loss spectroscopy (HREELS). The AIO_x/Ni(111) specimen was prepared by evaporating AI onto the Ni surface and then oxidizing and reducing the specimen at 700 K. TDS results showed that the presence of surface AIO_x species suppresses CO chemisorption and that the suppression is almost a linear function of the AlO_x coverage. The most prominent CO desorption peak on Ni occurs at 415 K. This peak intensity decreases with increasing Al0_x coverage, indicating a site-blocking effect. An additional CO desorption peak at 300 K was found on the AIO_x-modified Ni surface. This 300 K peak intensity attains a maximum at some intermediate AIO_x coverage. HREELS results showed a new CO stretching frequency of 1613 cm^-1 for CO adsorbed on the A1O_x/Ni surface. The disappearance of this 1613-cm^-1 energy loss peak upon heating the surface to 325 K suggests that both the 300 K TDS peak and the 1613-cm^{- 1} energy loss peak represent the same adsorbed CO state, which is attributed to CO absorbed on A1O_x/Ni perimeter sites. Surface morphology of the oxide islands and significance of the perimeter sites are discussed.