The genesis of industrially used MoS2 based catalysts involves a crucial activation step where the sulfo-reduction of a Mo-oxide precursor occurs on the g-alumina support. However, the atomic scale description of chemical intermediates involved in this step remains a challenge. This thesis investigates key mechanisms and intermediates involved in the transformation of g-alumina supported Mo-oxide oligomers into Mo-sulfided ones by means of state-of-the-art density functional theory (DFT) simulation.
First, we determine the sulfidation mechanisms and free energy profiles of the transformation of (100) g-alumina supported Mo3O9 clusters (cyclic and chain conformers) into Mo3S9. We unveil the activation energies for various O/S exchanges under H2S based on the type of O sites on Mo3OxSy intermediates. We highlight how Mo3S9 trisulfides clusters are then reduced into Mo3S6 disulphides under H2. We analyse the effect of cluster’s nuclearity and compare our results with available experimental data.
Then, we focus on the 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 IR spectrum. The growth energy evolution and the computed IR spectra suggest the coexistence of various polymorphs as a function of their size. Molecular dynamics reveals how small triangular oligomers reconstruct into MoS3 patches resembling embryos of the 2D 1T’-MoS2 phase. We propose plausible transformation paths from one polymorph to another.
This thesis provides an atomic scale understanding of the Mo sulfides activation crucial for controlling catalytic properties.