Insight into the Microstructure and Deactivation Effects on Commercial NiMo/γ-Al2O3 Catalyst through Aberration-Corrected Scanning Transmission Electron Microscopy

Abstract

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.

Thermocatalytic Hydrodeoxygenation and Depolymerization of Waste Lignin to Oxygenates and Biofuels in a Continuous Flow Reactor at Atmospheric Pressure

“…It has been reported that compounds containing single oxygenated groups such as phenol tend to produce less coke when compared to guaiacol or veratrole due to condensation reactions . He et al also reported that coke deposited on the metal site of the NiMo/γ–Al 2 O 3 catalyst can be removed by the self-cleaning process by the active H 2 spillover . Furthermore, the coke deposited over the metal sulfide active sites can be partially removed by H 2 during reduction, whereas the coke deposited on carbon support pore channels are difficult to eliminate .…”

“…50 He et al also reported that coke deposited on the metal site of the NiMo/γ−Al 2 O 3 catalyst can be removed by the self-cleaning process by the active H 2 spillover. 51 Furthermore, the coke deposited over the metal sulfide active sites can be partially removed by H 2 during reduction, whereas the coke deposited on carbon support pore channels are difficult to eliminate. 52 In our case, it is noted that the percentage of coke deposited on the catalyst at the end of each cycle decreased after hydrogen reduction before being recycled in the next cycle, probably due to the partial hydrogenation of the poorly organized carbon formed close to the metal sites due to hydrogen spillover (Figure 5).…”

Techniques for the characterization of single atom catalysts

“…As a result, EDX has been widely applied for the determination of SACs together with HAADF-STEM. [72][73][74] 2.5 Electron energy loss spectroscopy EELS is utilized to investigate the chemical composition, stoichiometry, energy levels and electronic structure of catalysts. 75,76 EELS requires electrons to strike the catalyst surface, so it is conducted on TEM or STEM.…”

“…As a result, EDX has been widely applied for the determination of SACs together with HAADF-STEM. 72–74 …”

Diffusivities of Hydrocarbons in the Fresh and Aged Hydrotreating Catalysts Studied by Zero-Length Column Method

“…The development of microscopy technology contributes to analyzing the surface property of the deactivated catalysts. The major active phases of hydrotreating catalyst are MoS 2 , NiMoS, and CoMoS phases, which have been investigated by scanning tunneling microscopy (STM). − atom-resolved microstructure variations, and the deactivation effects on NiMo/Al 2 O 3 catalyst have been represented by aberration-corrected scanning transmission electron microscope (Cs-STEM) . It revealed that MoS 2 nanoslabs got longer and thinner under mild to harsh reaction conditions, which led to a reduction in the number of active sites and a significant decrease in activity.…”