EXAFS determination of the structure of cobalt in carbon-supported cobalt and cobalt-molybdenum sulfide hydrodesulfurization catalysts

Abstract

The structure of the cobalt present in carbon-supported Co and Co-Mo sulfide catalysts was studied by means of X-ray\nabsorption spectroscopy at the Co K-edge and by X-ray photoelectron spectroscopy (XPS). Thiophene hydrodesulfurization\nactivities were used to measure the catalytic properties of these catalysts. By comparison of the EXAFS and XANES spectra\nof the catalysts with those of c09sS and Cos2 model compounds, it was concluded that all Co atoms in a catalyst prepared\nwith nitrilotriacetic acid as complexing agent were in the 'Co-Mo-S' state, while the Co atoms in a conventionally prepared\ncatalyst 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'\nstate have a distorted 5- to 6-fold sulfur coordination, and on the average, every Co atom is in contact with two Mo atoms\nat 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\nfaces of the MoS6 trigonal prisms along the edges of the MoS, crystallites with two additional sulfur atoms or H2S molecules\nattached. The Co atoms in the sulfided Co/C catalyst have Co-S and Co-Co coordinations as in c09sg, although the sulfur\ncoordination number is higher.