Cobalt-molybdenum catalysts EXAFS

Several groups [64-67,76] have reported EXAFS studies on sulfided cobalt-molybdenum catalysts. Figure 9.22 shows the Fourier transforms of M0S2 and of sulfided molybdenum and cobalt-molybdenum catalysts supported on carbon,... 

The Co/Mo = 0.125 catalyst has all the cobalt atoms present as Co-Mo-S and, therefore, the EXAFS studies of this catalyst can give information about the molybdenum atoms in the Co-Mo-S structure. The Fourier transform (Figure 2c) of the Mo EXAFS of the above catalyst shows the presence of two distinct backscatterer peaks. A fit of the Fourier filtered EXAFS data using the phase and amplitude functions obtained for well-crystallized MoS2 shows (Table II) that the Mo-S and Mo-Mo bond lengths in the catalyst are identical (within 0.01 A) to those present in MoS2 (R =... 

The HDS and HDN activities of a M0S2/AI2O3 catalyst both increase substantially on addition of cobalt or nickel. Several explanations for the promoter function of cobalt and nickel have been proposed (3-6). One model is based on the assumption that the promoter atoms induce a surface reconstruction of the edges of the M0S2 layers, leading to a greater number of exposed molybdenum atoms and, thus, to enhanced activity (24). On the other hand, infrared (19) and EXAFS (16, 20) investigations demonstrated that the molybdenum atoms are covered by the sulfur and the promoter atoms. Thus, if only the molybdenum atoms were active, the activity should decrease upon addition of cobalt or nickel atoms. 

An X-Ray absorption spectroscopy study has been carried out on a carbon-supported Co and Co-Mo sulfide catalyst, the latter consisting of a fully sulfided Co-Mo-S (type II) phase. Detailed information on the structure of the cobalt sulfide phase is obtained by comparing the EXAFS and XANES spectra of the catalysts with those of CogS8 and CoS2 reference compounds. It is shown that the cobalt atoms in the Co-Mo-S phase have an octahedral-like sulfur coordination while the cobalt atoms in the sulfided Co/C catalyst have a larger fraction of octahedral cobalt than Co Sg. On the basis of these results, the high HDS activity of a sulfided Co/C catalyst can be understood since it appears that the structure of the cobalt sulfide phase in Co/C is in agreement with that in Co-Mo/C. In the Co-Mo-S phase, approximately one cobalt atom is in contact with one molybdenum atom at a distance of 2.85 A. 

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