Case Studies: Aberration Corrected High-Angle Annular Dark-Field (AC-HAADF) Microscopy

As demonstrated in the previous chapter, scanning transmission electron microscopy-high-angle annular dark field (STEM-HAADF) imaging is widely applied to reveal the structural features of catalyst specimens at the atomic scale. This chapter will continue the topic of STEM-HAADF imaging by focusing on the application in α-MoC-supported metal catalysts, categorizing the catalysts by their supported metals. Owing to the robust atomic number (Z)-dependent contrast in STEM-HAADF imaging, the structure and distribution of the heavy noble metal species supported on the α-MoC support can be directly imaged. The collaboration of atomic-scale STEM-HAADF imaging with X-ray spectroscopies and theoretical calculations provided invaluable insights into the origins of the outstanding catalytic performance of these catalysts. In cases of α-MoC-supported light transition metal catalysts where STEM-HAADF imaging cannot provide direct visualization of the surface metal species, we show that the spectroscopy methods available in STEM are good supplements for imaging. It is noteworthy that the above discussion is based on results obtained from ex situ STEM experiments. It is necessary to extend the studies by employing in situ (S)TEM. Additionally, we point out that the newly emerging 4D-STEM techniques are promising to make a breakthrough in revealing the electronic structure at the level of individual catalytically active sites.