Formation and stability of CoMoS nanoclusters by the addition of citric acid: A study by high resolution STEM-HAADF microscopy

Abstract

In this work, we present atomic-scale images of Co promoted MoS2 slabs supported on γ-Al2O3 obtained by High Resolution Scanning Transmission Electron Microscopy equipped with High Angular Annular Dark Field detector (HR STEM-HAADF). These images allow to study the effect of citric acid (CA) addition during preparation, on the slab length and its stability during thiophene hydrodesulfurization (HDS) reaction.Thus, the observations, obtained for sulfide catalysts prepared without or with citric acid as chelating agent, evidenced strong decrease in size of the CoMoS nano-slabs as detected indirectly by the adsorption of CO followed by Infrared spectroscopy (IR/CO). Quantitative dispersion values were calculated from the images taking into account the true shape of the particles instead of the classical hexagonal shape 2 hypothesis. Characterization of the used catalyst shows that these nanoclusters are stable under model thiophene HDS reaction.. Such observations allow a better understanding of the effect of chelating agent addition on promoted MoS2 samples in order to explain their catalytic activity.

Ultradispersed (Co)Mo catalysts with high hydrodesulfurization activity

“…For example, the evolution of HDS activity vs. the preparation temperature could not be explained by only slight variations of slabs size and shape as observed by conventional TEM [36]. In several recent papers studying highly dispersed MoS2-based catalysts, clusters of several atoms or even single heavy atoms could be clearly seen in the STEM-HAADF images [31,33,37]. Such ultra-small species coexist as a minor component with extended (several nm size) MoS2 slabs, and their role in the catalytic activity was never assessed.…”

“…To improve the catalytic performance of MoS2 systems, the amount of exposed edges should be maximized and therefore highly dispersed MoS2 slabs should be prepared. This goal was achieved via different approaches such as special preparation techniques [30], engineering of support properties [31], or use of organic additives [32,33]. In such catalysts, along with typical hexagonal/triangular structures, very small clusters of several atoms or even single Mo atoms might be present.…”

Infrared Spectroscopic Evidence of WS2 Morphology Change With Citric Acid Addition and Sulfidation Temperature

“…We have detected a similar phenomenon in MoS 2 /Al 2 O 3 and CoMoS/Al 2 O 3 prepared with CA as chelating agent. (Zavala-Sanchez et al, 2019b;Zavala-Sanchez et al, 2021). Such clusters and single atoms were also observed in the images of sulfided W(CA/W 0)/Al 2 O 3 sample but in much lesser extent.…”

“…The addition of chelating agents has an effective role in increasing the degree of dispersion of the active phases of TMS. (Cattaneo et al, 1999;Al-Dalama and Stanislaus, 2006;Zavala-Sanchez et al, 2021). This, by interacting with the metallic precursor ions and the OH groups present in the support surface.…”

Direct Evidence of the Role of Co or Pt, Co Single-Atom Promoters on the Performance of MoS2 Nanoclusters for the Hydrogen Evolution Reaction

“…The image treatment was performed using the commercial software GMS3 from GATAN Inc. DigitalMicrograph. The average length ( L̅ ) was calculated according to the previous methodology described in refs and .…”