Insight into the microstructure and deactivation effects on commercial NiMo/γ-Al₂O₃ catalyst through aberration-corrected scanning transmission electron microscopy

Atom-resolved microstructure variations and deactivation effects on the commercial NiMo/γ-Al₂O₃ catalysts were revealed by aberration-corrected scanning transmission electron microscope (Cs-STEM) equipped with enhanced energy dispersive X-ray spectroscopy (EDS). Structural information parallel to and vertical to the electron beam provides definitive insight toward an understanding of structure-activity relations. Under the mild to harsher reaction conditions, “fragment” structures (like metal single atoms, metal clusters, and nanoparticles) of commercial NiMo/γ-Al₂O₃ catalysts, gradually reduces, while MoS₂ nanoslabs get longer and thinner. Such a result about active slabs leads to the reduction in the number of active sites, resulting in a significant decrease in activity. Likewise, the average atomic ratio of promoter Ni and Ni/(Mo + S) ratio of slabs decrease from 2.53% to 0.45% and from 0.0788 to 0.0326, respectively, by means of EDS under the same conditions stated above, reflecting the weakening of the promotional effect. XPS result confirms the existence of NixSy species in deactivated catalysts. This could be ascribed to the Ni segregation from active phase. Furthermore, statistical data give realistic coke behaviors associated with the active metals. With catalytic activity decreasing, the coke on the active metals regions tends to increase faster than that on the support regions. This highlights that the commercial NiMo/γ-Al₂O₃ catalyst during catalysis is prone to produce more coke on the active metal areas.