Stacking of Molybdenum Disulfide Layers in Hydrotreating Catalysts

Abstract Over the past several years there has been an intensive effort reported in the open literature addressing the nature of the active site(s) in supported hydroprocessing catalysts. Generally, these catalysts are either nickel- or cobalt-promoted molybdenum disulfide supported on an alumina carrier. Several different theories describing the promotion effect of nickel or cobalt have been proposed, each to some extent excluding the other theories. We have recently prepared a number of alumina based Ni/Mo and Co/Mo catalysts designed to aid us in understanding the roles played by both the molybdenum disulfide and the promoter metal. In addition, we were also interested in examining the effect of phosphorus addition to these catalysts. These catalysts have been examined by X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). Our results show that the primary difference between nickel promoted and cobalt promoted catalysts after sulfidation is the extent of stacking of the resulting molybdenum disulfide layers. Nickel promoted catalysts, generally used industrially to remove nitrogen from crude oil feedstocks, have molybdenum disulfide stacks of 5–6 layers while cobalt promoted catalysts, most useful industrially for sulfur removal, appear to contain molybdenum disulfide primarily as monolayers. If phosphorus is present in the impregnating solution then the number of stacks increases for both nickel- and cobalt-promoted catalysts while the length of the molybdenum disulfide crystallite decreases. A reason for this difference in behavior can be attributed to the preference of the nickel and the cobalt to be interlayer coordinated and tetrahedrally coordinated, respectively, when sulfided under hydrotreating conditions.