Abstract
EXAFS spectroscopy has been used to examine the effect of boron on the structure of Co/Al<sub>2</sub>O<sub>3</sub> catalysts. The EXAFS results from unmodified Co/Al<sub>2</sub>O<sub>3</sub> catalysts indicated that Co<sub>3</sub>O<sub>4</sub> 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/Al<sub>2</sub>O<sub>3</sub> catalysts with 3 wt % B completely suppressed the formation of Co<sub>3</sub>O<sub>4</sub> for Co loadings of 6 wt % Co and lower. In the absence of Co<sub>3</sub>O<sub>4</sub>, 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 Co<sub>3</sub>O<sub>4</sub> 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/Al<sub>2</sub>O<sub>3</sub> catalysts. This two-phase analysis considered both the fraction of Co EXAFS from bulk Co oxide (Co<sub>3</sub>O<sub>4</sub>) and the remaining fraction from Co surface species. This allowed the amount of Co<sub>3</sub>O<sub>4</sub> to be determined quantitatively and enabled structural information to be obtained from the Co surface phase.
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