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

Published online by Cambridge University Press:  28 February 2011

Yong-Bo Zhao ,
Tzer-Shen Lin  and
Yip-Wah Chung
Yong-Bo Zhao
Affiliation:
Northwestern University, Department of Materials Science and Engineering, and Catalysis Center, Evanston, Illinois 60208
Tzer-Shen Lin
Affiliation:
Northwestern University, Department of Materials Science and Engineering, and Catalysis Center, Evanston, Illinois 60208
Yip-Wah Chung
Affiliation:
Northwestern University, Department of Materials Science and Engineering, and Catalysis Center, Evanston, Illinois 60208
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Abstract

MnOx (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.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 111: Symposium O – Microstructure and Properties of Catalysts , 1987 , 435
Copyright
Copyright © Materials Research Society 1988

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