Exafs Studies of Calcined and Sulfided CO‐Mo HDS Catalysts

Information regarding the structure of the cobalt containing phases in calcined as well as sulfided Co‐Mo/Al2O3 catalysts is obtained by analyzing the extended x‐ray absorption fine structure (EXAFS) above the Co K‐edge and comparing these results with those for model compounds. For the catalysts studied, the results show no evidence for any of the well‐ordered cobalt compounds which have been proposed to be present in such catalysts. In the calcined catalyst the Co atoms are observed to be present in quite disordered surroundings with distinct backscattering observed only from the nearest neighbor oxygen atoms. From in situ EXAFS data of the sulfided catalyst it is observed that sulfiding affects a majority of the cobalt atoms such that they now become surrounded by sulfur atoms. However, a well‐ordered cobalt sulfide does not form since only one strong backscatterer peak in the radial distribution function is observed. A comparison of the present Co EXAFS results with Mo EXAFS and Mössbauer spectroscopy measurements on the same catalysts suggests that in sulfided Co‐Mo/Al2O3 catalysts the cobalt promoter atoms seem to be located in surface positions of the Co‐Mo‐S phase. Furthermore, the present results show the necessity of carrying out experiments in situ.