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 ).

A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer–Tropsch Catalyst

“…Yet, this value increased to 82% after 2 h of FT synthesis (data not shown). The lower degree of reduction might be attributed to the retarding effect of MnO on the extent of reduction, as has been shown earlier e.g., for Co/MnO/TiO 2 catalysts [16].…”

“…-selectivity values were found, however at a significant expense of the activity. This might be due to blockage of the cobalt surface [21] and/or a lower degree of reduction due to the reduction-retarding effect of MnO [16].…”

“…Examples of the effectiveness of this oxide in increasing the C 5? -selectivity in the cobalt catalyzed FT reaction have been reported [11][12][13][14][15][16][17][18][19][20][21][22][23]. As an example, a study from Bezemer et al [21] showed an increase in C 5?…”

Influence of the Cobalt Particle Size in the CO Hydrogenation Reaction Studied by In Situ X-Ray Absorption Spectroscopy

“…The presence of this cobalt oxide is evidenced by the extra features at 776.5 and 780 eV. During the heating treatment in hydrogen from 250ºC to 330ºC, the features in the X-ray absorption spectra changed to those of the metallic state characterized by two asymmetric absorption white lines at 778 and 794 eV as it was described by Morales et al [34].…”

Size-Controlled Model Co Nanoparticle Catalysts for CO2 Hydrogenation: Synthesis, Characterization, and Catalytic Reactions

“…The oxidation states of the surface Co (typically a few nm 22 of depth) can be judged by comparing to reference spectra for Co 0 (metal), Co 2+ , and Co 3+ , 23,24 as we have previously demonstrated. 25 The difference between oxidized and reduced Co is particularly distinct around the L 3 peak at 778 eV, which is sharp for the metallic feature but has significant shoulders on either side (most noticeably for the higher photon energy side).…”