The structure of the cobalt present in carbon-supported Co and Co-Mo sulfide catalysts was studied by means of X-ray absorption spectroscopy at the Co K-edge and by X-ray photoelectron spectroscopy (XPS). Thiophene hydrodesulfurization activities were used to measure the catalytic properties of these catalysts. By comparison of the EXAFS and XANES spectra of the catalysts with those of c 0 9 s S and Cos2 model compounds, it was concluded that all Co atoms in a catalyst prepared with nitrilotriacetic acid as complexing agent were in the "Co-Mo-S" state, while the Co atoms in a conventionally prepared catalyst were partly present in a CO$8-like structure and partly in a "Co-Mo-S" structure. The Co atoms in the To-Mc-S" state have a distorted 5-to 6-fold sulfur coordination, and on the average, every Co atom is in contact with two Mo atoms at a distance of 2.80 A. On the basis of these data, the most likely position for the Co atoms is in front of the square sulfur faces of the MoS6 trigonal prisms along the edges of the MoS, crystallites with two additional sulfur atoms or H2S molecules attached. The Co atoms in the sulfided Co/C catalyst have Co-S and Co-Co coordinations as in c 0 9 s g , although the sulfur coordination number is higher. IntroductionCobalt-or nickel-promoted molybdenum sulfide catalysts supported on alumina are extensively used in the hydrotreatment of petroleum feedstocks. The increasing need for efficient removal of sulfur, nitrogen, and metal contaminants has led to a continuous drive to clarify the structure and the related catalytic activity of these complex catalyst systems. Especially the role and the chemical state of the promoter cobalt and nickel atoms in the sulfided catalysts is a subject of great interest, and numerous studies have been devoted to it.'v2 The introduction of in situ Mossbauer emission spectroscopy (MES) provided for the first time direct information regarding the nature of the cobalt phases present in a working Co-Mo hydrodesulfurization (HDS) catalyst. With the use of MES, Topsere et al.3 and Wivel et aL4 showed that most of the cobalt atoms are situated at MaS2 crystallite edges in a so-called "Co-Mo-S" structure and that this structure governs almost completely the HDS activity. However, the precise local structure of the cobalt promoter atoms is still unknown. Also, the high specific activity of the "Co-Mo-S" structure is not understood. In this respect, it has not been established whether the cobalt atoms are the active sites or whether the neighboring molybdenum atoms also play a direct role in the catalytic activit y . 5-7Detailed information on the chemical state of the cobalt atoms was obtained by Ledoux et al. with the use of 59C0 NMR.8 They argued that the promotion effect of cobalt was correlated with the concentration of cobalt sites having a distorted tetrahedral symmetry and that these sites were stabilized by so-called "rapid octahedral" cobalt atoms acting as a glue between the tetrahedral cobalt sites and the MoS, phase. Although the "rapid octahe...

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To study the local structure of the Ni promoter atom, the Ni and Mo K edge EXAFS spectra of Ni-MoSz/C hydrodesulfurization catalyst were measured in an in-situ EXAFS cell at 77 K. The Ni atom is situated in a square pyramid of five S atoms at a distance of 2.21 from the S atoms. In addition an EXAFS contribution due to a Mo atom at 2.82 A from the Ni atom could be identified. This local structure indicates that the Ni atoms are situated on top of the S 4 squares at the MoS 2 edges in millerite-type Ni sites. The Ni atoms are situated in the planes of the Mo atoms and not in the intercalation plane midway between successive MoS 2 sandwich layers.

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