Surface chemistry model of cobalt-molybdenum catalysts supported on planar γ-Al203

Abstract:

　　Cobalt-molybdenum catalysts supported on γ-Al₂O₃ are well known for their success in the hydrotreating of petroleum feedstocks and coal liquifaction products. Their surface structure has been studied by several methods including recent attempts by surface science techniques. Applications of the surface science techniques are limited due to the porosity and insulating character of the γ-Al₂O₃ supports. This study presents an alternative approach Dy replacing the porous γ-Al₂O₃ supports with planar γ-Al₂O₃ grown on aluminum substrates: Preparation of the model catalysts was performed completely in situ. The γ-Al₂O₃ was prepared by thermally oxidizing the planar, polycrystalline aluminum substrates. The molybdenum and cobalt were vapor deposited to approximately monolayer coverage and subsequently oxidized. The planar model catalysts were characterized by ESCA, SIHS and AES and their surface structure compared to that of industrial catalysts.

　　The ESCA Al 2p band was used to monitor the oxidation of the aluminum. Shifts in the binding energy enabled differentiation between amorphous and crystalline Al₂O₃. Subsequent deposition and oxidation of molybdenum resulted in broadening and binding energy shifts of the Al 2p and Mo 3d bands suggesting interaction between the molybdenum and the γ-Al₂O₃.

　　Addition of cobalt onto the molybdenum layer appeared to result in an interactive phase between the cobalt and the molybdenum. The Co 2p band indicated the cobalt was in tetrahedral coordination. Depth profiles, obtained with SIMS, indicated the cobalt was interacting primarily with the molybdenum. When the order of metals deposition was reversed, the cobalt appeared to interact primarily with the γ-Al₂O₃ in a CoAl₂O₄ type phase. The AES line scans suggested that both the cobalt and molybdenum were well dispersed as would be characteristic of a uniform monolayer

coverage.

　　The findings of this study indicate similarities between the surface structure of the planar model catalysts and the industrial catalysts.