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

Abstract

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.

Surface-dependent sulfidation and orientation of MoS2 slabs on alumina-supported model hydrodesulfurization catalysts

“…Sulfide model catalysts have also been analyzed by HR HAADF-STEM in order to confirm the orientation of MoS 2 slabs determined by GI-EXAFS. In fact, with STEM in the HAADF detection mode, intensity is proportional to the squared atomic number (Z²) of the observed atoms [54,55], making feasible the detection of MoS 2 slabs oriented perpendicular to the incident electron beam and thus parallel to the surface and possibly the determination of the associated 2D morphology [56][57][58][59]. The analysis was also performed on samples sulfided at 300 °C to compare HAADF-STEM images to GI-EXAFS results.…”

In situ observations of an active MoS2 model hydrodesulfurization catalyst

“…In most cases, the MoS 2 islands exhibit a high degree of crystallinity, resulting in island shapes close to the thermodynamically favored truncated triangle 4–8,15,16 . Consequently, two types of edge termination are mainly observed, usually referred to as the Mo edge and the S edge, with the Mo edge being dominant.…”

Twinned Tungsten Carbonitride Nanocrystals Boost Hydrogen Evolution Activity and Stability

“…The core–shell nanostructure was further analyzed using high‐angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging, and line‐scan energy dispersive spectroscopy (EDS) in the STEM mode. Due to a significantly higher sensitivity toward heavy elements, HAADF‐STEM image (Figure c) discloses an obvious boundary of the core and shell, where the carbon shell as a protect layer can be easily observed . Bright contrast twinned WCN can be readily distinguished from the dark background.…”