X-ray Absorption Spectroscopic Studies at the Cobalt K-Edge on a Reduced Al₂O₃-Supported Rhenium-Promoted Cobalt Fischer−Tropsch Catalyst

In situ XAFS spectroscopic studies have been carried out at 450 °C on the hydrogen reduction of a rhenium-promoted Co 3 O 4 /Al 2 O 3 catalyst. The results show that reduction at this temperature yields a highly dispersed system with small cobalt metal particles (<40 Å). In addition, a lower fraction of the cobalt content is randomly dispersed over the tetrahedral vacancies of the alumina support. This dispersion occurs during reduction and not calcination. The cobalt in these sites cannot be reduced at 450 °C, a temperature that is too low to permit formation of the spinel CoAl 2 O 4 . A particularly significant feature of the spectrum of the reduced material is that the energy of the K-edge is increased by 3−4 eV. This is attributed to the small size of the metal particles and consequently a deviation from bulk properties. The effect is a manifestation of changes in shielding of the 1s electrons by the valence orbitals relative to the situation pertaining to the bulk metal. The actual crystal form of the metal particles could not be determined, but the recently reported metastable non-close-packed body-centered cubic form cannot be excluded, especially since graphite was present and carbon is suggested to catalyze transformation to this structure.