Structural studies of catalytically stabilized model and industrial-supported hydrodesulfurization catalysts

The high anisotropic character and inherent disorder in the structure of supported MoS₂-based catalysts that are used extensively to perform hydrotreating reactions for the removal of heteroatoms (S, N, and O), aromatics, and metals make characterization of the active catalyst a difficult challenge. XAS (X-ray absorption spectroscopy), XRD (X-ray diffraction), and HRTEM (high-resolution transmission electron microscopy) have been widely used in an attempt to understand the structure and origin of the active phase in these catalysts. However, all these techniques have limitations in determining the structure of the active MoS₂ phase and the associated Co promoter when used individually. Current techniques are not able to provide information of both lateral dimensions along the basal direction and of stacking height of MoS₂ slabs without ambiguity. We report here the use of a synchrotron source for X-ray-scattering measurements of supported MoS₂ and cobalt-promoted MoS₂ catalysts. This synchrotron source strongly increases the signal to noise ratio resulting in the detection of diffraction features providing information on the dispersion of the active phase. Synchrotron X-ray measurements in combination with HRTEM can then give a more complete picture of catalyst structure and of the active phases present. Furthermore, supported industrial catalysts that have operated under refinery conditions for more than four years have been studied to better understand the stabilized catalytic phase under these conditions. Industrial hydrotreating conditions induce a "destacking" process resulting in the stabilization of single-layered MoS₂-like nanoparticles. This effect has been confirmed on a freshly sulfided model CoMo/Al₂O₃ catalyst that underwent substantial morphological change leading to the formation of single slabs under HDS conditions. Other structural effects are also reported. This study emphasizes the importance of determining the catalytically stabilized phases under operating hydrotreating conditions as a basis for understanding the activity and selectivity of this class of catalysts.