Influence of the Preparation Conditions of Oxidic NiMo/Al₂O₃ Catalysts on the Sulfidation Ability: A Quick-XAS and Raman Spectroscopic Study

Abstract

The structure of oxidic precursors of supported NiMo hydrodesulfurization catalysts has been investigated in-depth by the combination of laser Raman spectroscopy and X-ray absorption spectroscopy measured at the Mo and Ni K edges at the different stages of the preparation. The oxidic catalysts were prepared by incipient wetness impregnation of δ-alumina with a solution obtained by dissolving MoO 3 in H 2 O 2 and subsequently adding Ni(NO 3 ) 2 ·6H 2 O in this as-prepared solution (8 wt % MoO 3; 2 wt % NiO). The formation of the 6-molybdoaluminate Anderson-type heteropolyanion (AlMo 6 O 24 H 6 ) 3– and of a mixture of oxo–hydroxo nickel species and bulk and/or surface NiAl-layered double hydroxide dispersed at the surface of the support has been identified after drying. Upon further thermal treatment at 723 K under dried air, calcined dehydrated catalyst is constituted of highly distorted isolated or partially condensed tetrahedral Mo units with terminal mono-oxo groups and of bulk and/or surface NiAl 2 O 4 -type and NiO-type species. After further exposure of the calcined catalyst to air moisture, a partial recovery of the Anderson-type molybdenum heteropolyanion and NiAl-layered double hydroxide species is evidenced by X-ray absorption spectroscopy. The nature and dispersion of active species formed after sulfidation under H 2 S/H 2 of the different oxidic catalysts (dried-NiMo, dehydrated-calcined NiMo, and calcined-NiMo samples) are finally discussed in light of the structure of the parent oxidic precursors.