Influence of phosphorus on the structure and the catalytic activity of sulfided Co-Mo catalysts

The influence of phosphorus (added as phosphates) on the sulfiding rate of carbon-supported Co–Mo catalysts is studied by means of temperature-programmed sulfiding. Significant progress is made in understanding the influence of phosphates on the structure and thiophene hydrodesulfurization (HDS) activity (atmospheric pressure) of carbon-supported Co–Mo catalysts. The model compound Co3(PO4)2·8H2O is sulfided to minor extent below 800 K. Between 800 and 1000 K, instead of further sulfiding, this compound is reduced to a mixture of CoP and Co2P. Phosphates have a large influence on the structure and sulfiding rate of carbon-supported oxidic cobalt and Co–Mo catalysts. Addition of phosphates to these catalysts results in the formation of "Co–PO4" species which are, compared with other cobalt species, relatively difficult to sulfide. Between 650 and 800 K cobaltphosphate species are not sulfided but reduced to cobaltphosphides while disperse sulfided cobalt species react with P2O5 and hydrogen to phosphides, water and hydrogen sulfide. These cobalt sulfide species are formed upon sulfiding of CoO, Co3O4, cobalt nitrates or cobalt species surrounded by a small number of phosphates. The sulfiding of the Mo/C catalyst is hardly influenced by the presence of phosphates. The poisoning of the low-pressure thiophene HDS activity of carbon-supported cobalt and Co–Mo catalysts by phosphorus can be explained by either the formation of "Co–PO4" species or CoP and Co2P. Which of these explanations is valid depends on the reaction time and conditions. After short reaction times at 673 K, the major part of the cobalt species is present as "Co–PO4", whereas the amount present as phosphides increases with reaction time. The Mo/C catalysts are most likely poisoned by phosphorus due to chemisorption of elemental phosphorus (P or P4) on sulfur anion vacancies present on the edges of the MoS2 slabs