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Bridging the gap between spectroscopic and catalytic properties of supported CoMoS catalysts

Abstract : Catalysts used in the selective gasoline hydrodesulphurization (HDS) process are mainly cobalt promoted MoS2 active phase (CoMoS) supported on alumina. The aim of this work is to explore correlation between specific features of those HDS catalysts and their catalytic performances. Three parameters are studied to understand the catalytic performances of HDS catalysts: the nature of the support (γ-Al2O3, δθ-Al2O3, SiO2), the molybdenum surface density and the cobalt/molybdenum ratio. HDS activity and selectivity are measured by performing high pressure catalytic tests on model molecules. In parallel, characterizations of the CoMoS phase were performed by XPS and HRTEM analysis to determine the chemical speciation and structure (size and stacking) of the active phase. Additionally, these techniques are combined with previous ab initio quantum calculations to develop a 2D morphology model of the CoMoS slab by considering the location of cobalt atoms at the edges of the slab. For the three supports, a correlation is found between the HDS activity and the number of Co atoms at the slab edges. The normalization by stacking appears to be necessary to obtain this correlation. Moreover, HRSTEM analysis on CoMoS on δθ-Al2O3 and γ-Al2O3 reveals the impact of the facets of dq-alumina crystallites and the possible role of corner and edges of g-Al2O3 crystallites on the size of the CoMoS slabs. The FTIR spectroscopic study of the adsorption of the NO probe molecule combined with ab initio calculations enables the identification of the nature of the active sites of the CoMoS phase as a function of the support. The spectroscopic features revealed for the silica supported catalyst are in line with its lower HDS activity. Simultaneously, an innovative experimental set up for in situ GC-DRIFT and GC-ATR analysis has been built on-purpose for studying the catalysts. It allows the investigation of the interactions of model reactants contacting the CoMoS phase and its supports close to HDS reaction conditions. This technique shows distinct behaviors of the adsorption and desorption process (non-reversible and reversible) of 3-methylthiophene (3MT) on γ-Al2O3 and SiO2 supported catalysts due to specific surface reactivity towards 3MT depending on the support and the presence of the CoMoS phase. Moreover, the presence of H2 modified the vibrational adsorption modes of 3MT linked to its adsorption and/or reactivity.
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Fabien Caron. Bridging the gap between spectroscopic and catalytic properties of supported CoMoS catalysts. Catalysis. Université de Lyon, 2017. English. ⟨NNT : 2017LYSEN053⟩. ⟨tel-01982797⟩

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