Active Sites in Oxide-Promoted Metal Catalysts – Evidence for the Significance of Perimeter Sites in AlOx and MnOx/Ni(lll) Catalysts

Abstract

AbstractMnOx (0.2 < × < 0.5) and AlOx (0.5 < × < 1.1) /Ni(lll) model catalysts were prepared by depositing the oxides onto clean Ni(111) surfaces under ultrahigh vacuum conditions, with the average oxide coverage varying from 0 to 1.3 monolayers (ML) for MnOx and 0 to 2.8 ML for AlOx. CO chemisorption over these catalysts at 200 K was used to titrate different adsorption sites. Thermal desorption showed the usual prominent CO desorption peak at 415 K from the clean Ni(111) surface. This 415 K peak decreases with increasing oxide coverage in an almost linear fashion, indicating a siteblocking effect by the oxide. At the same time, a new thermal desorption peak appears at 305 K from MnOx/Ni(111) and 300 K from AlOx/Ni(111), reaching maximum intensity at some intermediate oxide coverage. For MnOx/Ni(111), this coincides with the appearance of a new C-0 stretching mode at 1620 cm−1, which disappears upon heating the surface to 325 K for a few minutes, suggesting that both the 305 K thermal desorption peak and the 1620 cm−1 loss peak represent the same adsorbed CO state. This is attributed to CO adsorbed on MnOx/Ni perimeter sites. Results from Monte Carlo simulation and scanning tunneling microscopy are presented and compared with CO chemisorption data. The significance of perimeter sites in strong metal-support interaction is discussed.