Characterization of Unsupported and Alumina-Supported Molybdenum Sulphides

Abstract

Hydrotreating catalysts play a major role in refining processes of crude oil. They are typically sulphided molybdenum-based materials using alumina as a support. The great technological importance of this class of catalysts has motivated significant research activities aiming at a characterization of the catalyst structure and the nature of the coordination sites [1–9]. It is now generally accepted that MoS2-like structures resembling dispersed platelets are found in the sulphided catalysts. This structural description is based on experimental evidence from laser Raman spectroscopy [10–12], X-ray absorption spectroscopy [13–15], and time differential perturbed angular correlation (TDPAC) spectroscopy [16, 17]. Electron microscopy has shown that single and stacked MoS2 slabs are present in sulphided Mo/Al203-catalysts, their dimensions and degree of stacking being dependent on sulphidation conditions [18–20]. The picture that emerges from these and other sulphidation studies can be briefly summarized as follows. Highly dispersed MoS2-like slabs, having typical lengths ≤ 3 nm, are the dominant molybdenum species in sulphided catalysts on Al2O3 supports. Mo atoms are thought to have trigonal prismatic or distorted octahedral coordination by sulphur atoms. These slabs are stabilized by Mo-O-Al bonds, which are formed after mild sulphidation at temperatures ≤ 670 K. In this state, the slabs are probably oriented in upright positions relative to the alumina support surface. At higher sulphidation temperatures, the average size of the MoS2 slabs grows and the degree of stacking increases. The stacks are then presumably detached from the Al2O3 support and lie flat on the surface.