Quantitative Two-Dimensional (2D) Morphology-Selectivity Relationship of CoMoS Nanolayers: A Combined High-Resolution High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HR HAADF-STEM) and Density Functional Theory (DFT) Study
Résumé
Cobalt-promoted and nonpromoted MoS2 nanolayers supported on alumina are prepared and activated under various sulfidation (temperature/pressure (T, P)) conditions which induce the formation of nanolayers with two-dimensional (2D) morphology of MoS2 tuned by the presence of the promoter and by the sulfidation conditions. An unprecedented high selectivity is found for the CoMoS nanolayers. The origin of this selectivity is explained by 2D morphology effects quantified by
high-resolution scanning transmission electron microscopy in high-angle annular dark field mode (HR HAADF-STEM) and density functional theory (DFT) calculations. A quantitative structure selectivity relationship is identified between the 2D shape index of CoMoS nanolayers and their selectivity performances. This 2D shape index is determined by
statistical analysis of the CoMoS nanolayers identified after principal component analysis processing of HR HAADF-STEM images. It is shown that this shape index, reflecting the isotropic/anisotropic degree of the nanolayers' morphology, is directly linked to the nature of active M- and S-edges exposed by the CoMoS nanolayers, as proposed by DFT
calculations. This 2D shape index may thus serve as a key descriptor for the selectivity of the CoMoS nanolayers. The correlation is
rationalized by a simple kinetic modeling where hydrodesulfurization (HDS) and hydrogenation (HYD) rate constants are parametrized as a function of the S-edge/M-edge sites by using DFT-calculated descriptors. HR HAADF-STEM also highlights the existence of nonequilibrium CoMoS layers with more irregular 2D shapes, which can also be correlated to selectivity through a specific shape descriptor. More generally, this study reveals that the HDS/HYD selectivity can be controlled by the 2D shape driven by the activation sulfidation steps of the catalyst. It provides a new approach for establishing a reliable methodology for the rational design of highly selective nanocatalysts.