Activation of Co-Mo-S Hydrodesulfurization Catalysts Under Refinery Conditions-A Combined SAXS/XAS Study

The activation of heterogeneous catalysts is challenging to follow experimentally and poorly understood in many cases. We combine in situ and time-resolved X-ray absorption spectroscopy and small-angle X-ray scattering to study the activation of Co-Mo-S hydrotreating catalysts under conditions close to a refinery start-up, i. e., 20 bar H pressure and gaseous HS or liquid sulfiding agents. Formation of MoS nanoparticles involves three stages, namely aggregation (50-150 °C) and self-assembly (150-250 °C) of two distinct amorphous MoS intermediates, followed by a transition into layered structures that closely relate to the MoS structure. In stage III, these nuclei coalesce into the final MoS nanoparticles. Their growth distribution and stacking degree can be tailored by controlling the aggregation step, paving the way for the synthesis of better hydrotreating catalysts. Our experimental approach enables studying a broad range of supported amorphous nanomaterials at various length scales and operating conditions relevant to important catalytic processes.