Characterization of Boron-Modified Co/Al2O3 Catalysts by EXAFS Spectroscopy

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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.