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 degree s C on the hydrogen reduction of a rhenium-promoted Co3O4/Al2O3 cataly st. The results show that reduction at this temperature yields a highl y dispersed system with small cobalt metal particles (<40 Angstrom). I n addition, a lower fraction of the cobalt content is randomly dispers ed over the tetrahedral vacancies of the alumina support. This dispers ion occurs during reduction and not calcination. The cobalt in these s ites cannot be reduced at 450 degrees C, a temperature that is too low to permit formation of the spinel CoAl2O4. 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 si ze of the metal particles and consequently a deviation from bulk prope rties. The effect is a manifestation of changes in shielding of the Is electrons by the valence orbitals relative to the situation pertainin g to the bulk metal. The actual crystal form of the metal particles co uld not be determined, but the recently reported metastable non-close- packed body-centered cubic form cannot be excluded, especially since g raphite was present and carbon is suggested to catalyze transformation to this structure.