Electron Microscopy Studies of Structure and Dynamics in MoS2-based Hydrodesulfurization Catalysts

Lars P. Hansen, Erik Johnson, Quentin M. Ramasse, Christian Kisielowski, Michael Brorson, Stig Helveg
2014 Microscopy and Microanalysis  
New environmental legislation for clean fuels demands an enhanced removal of sulfur and other impurities from mineral oil. Production of ultra-low sulfur transport fuels at oil refineries requires very efficient catalytic hydrodesulfurization processes, and attention is currently being devoted to understand the catalysts' structure-activity relationships. This requires a detailed characterization of the catalyst's nanostructure and in particular the nature of the atomic-scale active sites. For
more » ... ndustrial hydrodesulfurization at oil refineries, the catalysts are based on MoS 2 nanostructures [1]. Basically, the MoS 2 structure consists of two-dimensional S-Mo-S slabs which can be stacked to varying degrees by van der Waals interactions. In each layer, hexagonally arranged Mo atoms are trigonal-prismatically coordinated to S atoms. Such two-dimensional S-Mo-S structures have been found to form polymorphs including fullerene structures, nano-tubes/wires and platelets with physicochemical properties significantly different from bulk MoS 2 [2]. It is well-known that the catalytic reactivity of the MoS 2 slabs is associated with their exposed edges and relies on the size, morphology and the degree of slab stacking [3] . Consequently, the ability to synthesize MoS 2 structures with controlled morphology and specific size is of particular interest. However, although MoS 2 nanostructures are synthesized at a very large scale in the refining industry, insight into the structure and dynamics of the MoS 2 nanostructures has remained a challenge to unveil. Recent advancements have made transmission electron microscopy (TEM) a powerful technique for studying individual (supported) nanoparticles at the atomic-level [4, 5] . In this presentation, we demonstrate the application of such advancements for single atom sensitivity and in situ electron microscopy of MoS 2 -based hydrotreating catalysts. By means of time-resolved TEM imaging, the growth of MoS 2 nanocrystals is monitored in situ during the sulfidation reaction that transforms a molybdenum oxide precursor material into highly dispersed MoS 2 nanocrystals (Figure 1a-b) . Specifically, the time-resolved image series provide new information about the evolution of MoS2 nanocrystals with different size, morphology and stacking and thus uncover the nucleation and growth of the MoS 2 nanocrystals. The in situ observations are beneficially combined with single-atom sensitive imaging by (S)TEM, because such microscopy techniques enable detection of the catalytic active edge structures at the levels of a single atom (Figure 1c-e) . Thus the combined use of in situ and single-atom sensitive (S)TEM provides unprecedented new insight into the formation of MoS 2 nanocrystals with specific distribution of active sites. In combination with information about structure-sensitive catalytic functionality, obtained from scanning tunneling or density functional theory calculations, the electron microscopy observations provide information that can help establish new, improved structurefunctionality relationships of the technological relevant hydrodesulfurization catalysts. 1566
doi:10.1017/s1431927614009568 fatcat:idherplcoff57etosqavpshpye