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Amit Sahu

PhD Défense

Sep 27, 2021 à 01:30 PM


Amphitheater IFPEN Solaize AIG 8

Molecular modeling of the genesis of the active phase of a supported catalyst

MoS3/MoS2 based nanomaterials exhibit a wide range of applications as heterogeneous catalysts for clean fuels production, or in the field of new energies as photo-/electro-photocatalysts or electrode materials components. To improve their resulting properties, it remains crucial to better understand the activation steps from the molybdenum oxide precursors.

For industrially used MoS2 based catalysts, this activation step involves a sulfo-reduction process of the oxide precursor deposited on the γ-alumina support. The complex mechanisms and various trisulfides or oxysulfides intermediates are still poorly rationalized at the molecular scale. Hence, this thesis investigates key mechanisms and intermediates involved in the transformation of γ-alumina supported Mo-oxide oligomers into Mo-sulfided ones by means of state-of-the-art density functional theory (DFT) simulation.

In the first part, we determine the sulfidation mechanisms and free energy profiles of the transformation of (100) γ-alumina supported Mo3O9 oligomers (cyclic and chain conformers) into Mo3S9. We unveil the activation energies for various O/S exchanges under H2S as a function of O sites of Mo3OxSy intermediates. The structural and spectroscopic features of these intermediates are compared to available experimental data. We quantify the reduction paths of Mo3O3S6 oxysulfides and Mo3S9 trisulfides oligomers into Mo3S6 disulfides under H2. We analyze the effects of the cluster’s nuclearity as well as reconstruction.

In the second part, we focus on the non-supported MoS3 intermediate involved in the activation process. We simulate the energetic, structural, and spectroscopic features of 0D-, 1D- and 2D-MoS3 polymorphs and revisit the interpretation of their IR spectrum. The growth energy evolution and the computed IR spectra suggest the coexistence of various polymorphs (chain or triangular) as a function of their size. Molecular dynamics reveals how small triangular oligomers reconstruct into MoS3patches resembling embryos of the 2D 1T’-MoS2 phase. We propose plausible transformation paths from one polymorph to another. We finally discuss the possible role of the support on the stabilization of chain and triangular conformers.

This thesis provides an atomic-scale understanding of the Mo sulfides activation crucial for optimizing the resulting catalytic properties.

Cette thèse s’est déroulée dans le cadre de la chaire « RatiOnAl Design for CATalysis (ROAD4CAT) », partenariat ENS Lyon – UDL – IFPEN, projet IDEXLYON soutenu par l’Agence Nationale de la Recherche (ANR-16-IDEX-0005) et le Commissariat Général de l’Investissement (CGI).