On the formation of type I and type II NiMoS phases in NiMo/Al 2O 3 hydrotreating catalysts and its catalytic implications

Abstract

Two active phases can exist in sulfided NiMo/Al 2O 3 catalysts. Impregnation of an ammoniacal NiMo solution leads eventually to NiMoS phase I, which is strongly bonded to the alumina, while the use of nitrilotriacetic acid gives rise to the totally sulfided NiMoS phase II. Phase I is less active in gas-phase thiophene hydrodesulfurization (HDS) and trickle-flow quinoline hydrodenitrogenation (HDN), but more active in trickle-flow dibenzthiophene HDS. The presence of phosphoric acid in the impregnation solution results in partial formation of NiMoS II upon sulfidation; P itself does not have a direct effect in HDS, but does influence HDN performance, mainly through increasing the catalyst's affinity for quinoline. Extended X-ray absorption fine structure analysis data are consistent with this picture. In-situ Raman spectroscopy and NO chemisorption/infra-red spectroscopy cannot distinguish between NiMo, NiMoP, and NiMo(nitrilotriacetic acid) catalysts. In-situ fluoridation of the sulfided NiMo/Al 2O 3 catalyst leads to a fairly complete phase I å phase II transformation. Again, however, in-situ Raman spectroscopy does not detect the difference. The implications of these findings for catalyst design are discussed.