Direct imaging and identification of individual dopant atoms in MoS ₂ and WS₂ catalysts by aberration corrected scanning transmission electron microscopy

In the case of the hydrodesulfurization (HDS) processes one of the best catalysts currently available is that based on MoS₂ and WS ₂. A very significant increase in their activity can be achieved by adding Co or Ni as a promoter. In the present report we have used probe aberration corrected STEM (scanning transmission electron microscopy) for the first time to characterize Co doped MoS₂/WS₂ nanowire catalysts (supported on Al₂O₃ substrates). The high-resolution imaging reveals clearly the location of Co in the individual catalysts. This has not been possible to date with other experimental techniques because of the insufficient image contrast and/or resolution. On the basis of the HAADF-STEM images, we built two models for the Co-Mo-S and Co-W-S catalysts to illustrate the different morphologies found in the catalysts. With this study it is now possible to better locate, identify, and understand the role of promoters in the design and functioning of catalysts.