Catalysts involved in hydrotreatment are based on transition metal sulphides, usually CoMoS phase supported on alumina for selective hydrodesulfurization (HDS) of FCC gasoline. The aim of this work is to relate the catalytic performances of HDS catalysts to specific features of the CoMoS active phase and its associated support.
The CoMoS phase was dispersed on three supports: g-Al2O3, dq-Al2O3, SiO2. Two parameters were studied to understand the catalytic performances of HDS catalysts: the nature of the support and the molybdenum surface density. HDS activity and selectivity were measured by performing high pressure catalytic tests. In parallel, characterizations of the CoMoS phase were performed. XPS, TEM (slab stacking and length) and HR HAADF-STEM analysis combined with DFT were used to develop a 2D morphological model of the CoMoS slab by considering the Co decoration at the M/S edges. CoMoS/SiO2 exposes low cobalt promotion compared with CoMoS/g-Al2O3. NO-FTIR in situ experiments were used to describe surface site speciation and support effects on the CoMoS phase. Different metal support interactions were observed. In addition, surface sites were described by a chemometric approach.
Simultaneously, an innovative experimental set up for in situ DRIFT/ATR analysis was built on purpose for HDS catalysts. It allowed the investigation of the interactions of model reactants contacting the CoMoS phase and its supports in close to real conditions. Adsorption/desorption of 3-methylthiophene (3MT) over CoMoS/(g-Al2O3 or SiO2) were shown to be non-reversible and reversible, respectively. g-Al2O3 exhibits a specific surface reactivity towards 3MT that does not occur on CoMoS/g-Al2O3 or on the SiO2 support. Moreover, adding H2 was shown to modify the adsorption modes of 3MT over CoMoS catalysts and the supports. As a consequence, distinct interactions of the 3MT reactant with the active phase and its supports were highlighted which may be related to catalytic reactivity.