Nature of cobalt species during the in situ sulfurization of Co(Ni)Mo/Al₂O₃ hydrodesulfurization catalysts

Abstract

The evolution in local structure and electronic properties of cobalt was investigated during in situ sulfurization. Using a combination of 1s X-ray absorption (XAS) and 1s3p resonant inelastic X-ray scattering (RIXS), the valence, coordination and symmetry of cobalt ions were tracked in two cobalt-promoted molybdenum oxide precursors of the hydrodesulfurization catalyst system, namely Co–Mo/Al2O3 and Co–Ni–Mo/Al2O3. Extended X-ray absorption fine structure shows that the Co—O bonds were replaced with Co—S bonds as a function of reaction temperature. The cobalt K pre-edge intensity shows that the symmetry of cobalt was modified from Co3+ O h and Co2+ O h to a Co2+ ion where the inversion symmetry is broken, in agreement with a square-pyramidal site. The 1s3p RIXS data revealed the presence of an intermediate cobalt oxy-sulfide species. This species was not detected from XAS and was determined from the increased information obtained from the 1s3p RIXS data. The cobalt XAS and RIXS data show that nickel has a significant influence on the formation of the cobalt oxy-sulfide intermediate species prior to achieving the fully sulfided state at T > 400°C.

RETRACTED: Hydrogenolysis of Polyethylene and Polypropylene into Propane over Cobalt-Based Catalysts

“…These results suggest that the state of the catalyst is evolving during hydrogenolysis, possibly forming carbide-like species that can be stabilized in MFI frameworks under hydrocarbon-rich reductive environments and that have been shown in Fischer–Tropsch synthesis to be active and to reduce CH 4 formation. 51 , 52 Furthermore, differences in intensity and broadening of the pre-edge XANES region were observed between the spectra of fresh and used Co/ZSM-5 ( Figure S17 ), suggesting changes in the coordination geometry, 53 − 55 which could be related to catalyst evolution. Contrarily, there was extensive reduction of the Co 3 O 4 during PE hydrogenolysis, as observed by a >100 K downward shift in the maxima of the H 2 -TPR profile and an order of magnitude lower H 2 consumption ( Figure 3 c) as well as by XAS, which detected almost exclusively metallic cobalt species with the main peak centered at 2.15 Å (not phase corrected) ascribed to Co–Co pairs ( Figure 3 d and Figures S14–S18 ).…”

Evidence of Octahedral Co–Mo–S Sites in Hydrodesulfurization Catalysts as Determined by Resonant Inelastic X-ray Scattering and X-ray Absorption Spectroscopy

“…Al Samarai and co-workers recently proposed a square pyramidal coordination of Co−Mo−S sites based on a 1s3p RIXS study of alumina supported Co−Mo HDS catalysts with high Co-loading (5 wt % Co, Co/Mo = 0.6). 13 We point out that the presence of noncentrosymmetric cobalt sulfide species is in line with our findings at high cobalt loading, and thus, it is likely that cobalt was not exclusively present as Co−Mo−S in their study. The two most likely coordination states of cobalt are therefore square planar or octahedral.…”

“…We can therefore rule out the presence of tetrahedral ( T d symmetry) or square pyramidal ( C 4v ) Co–Mo–S sites, which are noncentrosymmetric geometries that would lead to a stronger pre-edge as seen in the RIXS plane of Co 9 S 8 . Al Samarai and co-workers recently proposed a square pyramidal coordination of Co–Mo–S sites based on a 1s3p RIXS study of alumina supported Co–Mo HDS catalysts with high Co-loading (5 wt % Co, Co/Mo = 0.6) . We point out that the presence of noncentrosymmetric cobalt sulfide species is in line with our findings at high cobalt loading, and thus, it is likely that cobalt was not exclusively present as Co–Mo–S in their study.…”

“…It is commonly agreed upon that Co or Ni promoter ions are located at the edges of the two-dimensional MoS 2 platelets in what is known as the “Co–Mo–S” model. − These promoted sites are believed to be active sites in HDS catalysis, and it is of crucial importance to understand their role in the catalytic cycle in order to establish design criteria for optimized HDS catalyst performance. Despite considerable efforts, no consensus on the precise location and coordination of the cobalt promoter in alumina-supported HDS catalysts was reached. − …”