(Co)MoS2/Alumina Hydrotreating Catalysts: An EXAFS Study of the Chemisorption and Partial Oxidation with O2

The adsorption of O2 on alumina-supported (Co)MoS2 catalysts and the subsequent mild oxidation of the (Co)MoS2 by O2 have been studied by extended X-ray absorption fine-structure (EXAFS) spectroscopy. By analyzing the difference between spectra before and after O2 exposure, small changes in the structure could be determined, which were not resolved using standard methods. At 20°C on MoS2/alumina and (Co)MoS2/alumina, O2 is chemisorbed at the edge of the MoS2 particles at a Mo–O distance of 1.73(2) Å. The O2 chemisorption results at the Mo edge indicate that, despite the large fraction of Co at the surface of the MoS2 crystallite in (Co)MoS2/alumina, some of the Mo atoms are exposed to the reacting gases. At 100°C, there is partial substitution of S by O atoms in the Mo coordination sphere. The resulting decrease in both the Mo–S and Mo–Mo coordination numbers indicates partial disruption of the MoS2 crystallites. At 20°C, O2 chemisorption on (Co)MoS2/alumina also leads to displacement of the terminal Co–S bond and the formation of one Co–O bond at a distance of about 2.01(5) Å. The terminal Co–S bond distance is 2.26(2) Å and is significantly longer than the four bridging Mo–S–Co bonds, which are 2.18(2) Å. At 100°C, the latter are unreactive to O2, although the Coion coordination increases to about six, i.e., four bridging Co–S and two terminal Co–O bonds. The Co chemisorption results suggest that the terminal Co–S is the reactive bond that has been displaced by the oxygen adsorbate.