In Situ X-ray Absorption of Co/Mn/TiO₂ Catalysts for Fischer−Tropsch Synthesis

Abstract

The reduction behavior of Co/TiO 2 and Co/Mn/TiO 2 catalysts for Fischer−Tropsch synthesis has been investigated by soft X-ray absorption spectroscopy (XAS). In situ XAS measurements of the L 2,3 edges of Co and Mn have been carried out during reduction treatments of the samples in H 2 at a pressure of 2 mbar and at temperatures up to 425 °C. The changes of Co and Mn 3d valences and the symmetries throughout the reduction have been determined by comparison with theoretical calculations based on the charge transfer multiplet code. Furthermore, bulk Co 3 O 4 has been reduced under the same conditions to evaluate the effect of TiO 2 as a support on the reducibility of Co oxides. The average Co valence at the various temperatures has been determined from a linear combination of the reference spectra. It was found that the unsupported Co 3 O 4 was easily reduced to Co 0 at 425 °C, whereas the Co 3 O 4 supported on TiO 2 catalysts was only reduced to a mixture of CoO and Co 0, even after 12 h reduction at 425 °C. The presence of Mn further retards the reduction of the supported Co 3 O 4 particles. The Mn III ions were easily reduced to MnO at temperatures lower than 300 °C, and they remained in this oxidation state even after further temperature increase. In addition, catalytic tests in the Fischer−Tropsch synthesis reaction at a pressure of 1 bar indicate that the selectivity of these catalysts might be related to the extent of Co reduced after the activation treatment (i.e., the reduction with H 2 ).