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Downloaded by UNIV OF MARYLAND COLL PARK on October 14, 2014 | http://pubs.acs.org Publication Date: February 10, 1986 | doi: 10.1021/bk-1986-0298.ch006
Evidence for the Migration of MnO upon Reduction of Ni-MnOx and Its Effects on CO Chemisorption Y. W. Chung and Y. B. Zhao Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60201
Model Ni/MnOx catalysts were prepared by 150-Ånickel deposition onto an oxidized manganese disk under ultrahigh vacuum. The surface composition of the catalyst was followed as a function of time at 500°Κ in ultra high vacuum. Rapid diffusion of reduced manganese oxide onto the nickel surface was observed. This oxide migra tion phenomenon was observed for a wide range of metal/ TiO catalysts reduced at ≥ 700°K and is presently accepted as the key step in the induction of strong metal-support interaction (SMSI). The present observation appears to be the first case in which SMSI can be induced at normal (500°K) catalyst reduction temperatures. High resolution electron energy loss spectroscopy (EELS) experiments of CO on Ni/MnOx reduced at 500°K showed dramatic reduction of the C-O stretching frequency of adsorbed CO. These EELS results are discussed in light of similar data on Ni/TiO2 and infra-red data on Rh/MnOx. 2
After the 1978-publication of Tauster and coworkers on CO and Η chemisorption suppression on titania-supported group VIII cata lysts reduced at 500 C (1), many research studies were performed to elucidate the mechanism of this so-called strong metal-support interaction (SMSI) phenomenon. The f i r s t direct clue came from the work of Dumesic and Ha H e r . Work by Dumesic and coworkers (2) on ammonia symthesis using Fe/Ti02 showed that SMSI behavior per s i s t s even for iron p a r t i c l e s with diameter exceeding 200-Â. This finding excludes the delocalized charge transfer model as a viable explanation of SMSI since such charge accumulation w i l l be comp l e t e l y screened out i n one atomic spacing by the metal. Resasco and Haller (3) found that the ethane hydrogenolysis a c t i v i t y of Rh/titania decreases as the square-root of reduction time, the drop being faster with higher reduction temperature. Both groups proposed that during high temperature reduction to induce SMSI, t i t a n i a diffuses to the metal surface. 0097-6156/86/0298-0054S06.00/0 © 1986 American Chemical Society
In Strong Metal-Support Interactions; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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MnO Effect on Ni-MnO Reduction x
We provided a direct confirmation of this oxide migration model by starting with a single c r y s t a l Ni(111), depositing onto i t cont r o l l e d amounts of reduced t i t a n i a , and performing CO chemisorption and CO hydrogénation reactions on such a surface (4). Such a T i O / Ni(x
