Characterization of Boron-Modified Co/Al 2 O 3 Catalysts by EXAFS Spectroscopy

Abstract

EXAFS spectroscopy has been used to examine the effect of boron on the structure of Co/Al₂O₃ catalysts. The EXAFS results from unmodified Co/Al₂O₃ catalysts indicated that Co₃O₄ was formed at Co loadings of 1.5 wt % Co and higher. The lowest-loading (0.7 wt % Co) catalyst showed evidence of both tetrahedral and octahedral Co surface species. Modification of the Co/Al₂O₃ catalysts with 3 wt % B completely suppressed the formation of Co₃O₄ for Co loadings of 6 wt % Co and lower. In the absence of Co₃O₄, the Co species formed on the 3 wt % boron-modified catalysts were in a highly disordered state with large first shell Co-O coordinations and large Co-O nearest-neighbor distances. Results from a constant Co loading series (3 wt % Co) indicated that Co₃O₄ formation decreased as the boron loading increased. The distribution of supported Co species determined by EXAFS was compared with previous characterization by ESCA and gravimetric measurements; good agreement was found. A two-phase procedure was used to analyze the EXAFS data from the Co/B/Al₂O₃ catalysts. This two-phase analysis considered both the fraction of Co EXAFS from bulk Co oxide (Co₃O₄) and the remaining fraction from Co surface species. This allowed the amount of Co₃O₄ to be determined quantitatively and enabled structural information to be obtained from the Co surface phase.

Keywords

Surface analysis EXAFS x-ray absorption Catalysts Bond lengths Coordination numbers Cobalt

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