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

Abstract

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 CO 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.