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

MS2 morphology is strongly influenced by several parameters including the addition of a chelating agent and sulfidation temperature. In this work, we report the use of citric acid as chelating agent in order to prepare a series of WS2/Al2O3 catalysts that were submitted to sulfidation at several temperatures. The effect of these two parameters in the morphology of the slabs was explored by means of CO adsorption at low temperature followed by IR spectroscopy (IR/CO) and later confirmed by High-Resolution Scanning Transmission Electron Microscopy coupled with High Angular Annular Dark Field detector (HR STEM - HAADF). This allowed to depict the morphology of WS2 slabs by means of calculating the M-edge/S-edge site ratio. The use of citric acid in the preparation stage favors the increase of S-edge site concentration whereas it keeps that of M-edge sites: according to IR/CO, with an increasing amount of citric acid, the WS2 morphology progressively changes from a slightly truncated triangle exhibiting predominantly M edges to a hexagon with both M edge and S edge. In addition, HR STEM-HAADF demonstrated that the addition of citric acid in the impregnation step of W catalysts considerably reduces the size of WS2 nanoparticles increasing their dispersion degree. The morphology of the WS2 plates on the activated WS2/Al2O3 catalyst with a typical sulfidation temperature range (573–673 K) was detected to be a truncated triangle exposing both the M-edge and the S-edge. Furthermore, the IR/CO results indicate that the degree of truncation (ratio of S-edge/M-edge) of WS2 slabs gradually rises with the increasing sulfidation temperature. However, the most determining factor for a modification of the morphology of the slabs turns out to be the presence of citric acid as a chelating agent and not the sulfidation temperature. This change in morphology (i.e., change of S-edge/M-edge ratio) is a key factor for catalytic performance, since the M-edge and the S-edge show different reactivity in hydrodesulfurization (HDS) reactions. Notably, it was also found that the addition of citric acid not only improves the catalytic activity but also the stability of the catalysts, giving the best performance in concentrations higher than (CA/W = 1).

High-resolution STEM-HAADF microscopy on a γ-Al2O3 supported MoS2 catalyst-proof of the changes in dispersion and morphology of the slabs with the addition of citric acid

Atomic-scale images of MoS₂ slabs supported on γ-Al₂O₃ were obtained by high-resolution scanning transmission electron microscopy equipped with a high angular annular dark field detector (HR STEM-HAADF). These observations, obtained for sulfide catalysts prepared with or without citric acid as a chelating agent, evidenced variations in morphology (shape) and size of the MoS₂ nanoslabs, as detected indirectly by the adsorption of CO followed by infrared spectroscopy. Quantitative dispersion values and a morphology index (S-edge/M-edge ratio) were determined from the slabs observed. In this way, HR STEM-HAADF underlines that the addition of citric acid to Mo catalysts decreases the size of the particles and modifies the shape of the MoS₂ nanoslabs from slightly truncated triangles to particles with a higher ratio of S-edge/M-edge. These finding are of great significance since tayloring the morphology of the slabs is a way to increase their catalytic activity and selectivity. Furthermore, this work demonstrated that the IR/CO method is a relevant approach to describe the MoS₂ morphology of supported catalysts used in hydrotreatment processes for clean fuel production.

Controlled growth of MoS2 via surface-energy alterations

Monolayer MoS₂ in triangular configurations with rich edges or high-quality uniform films are either catalytically active for the hydrogen evolution reaction or flexible for functional electronic and optoelectronic devices. Here, we have experimentally discovered that these two types of MoS₂ products can be selectively synthesized on graphene or sapphire substrates, which are associated with both different adsorption energy and diffusion-energy barrier for vapor precursors during growth. Our study not only provides insights into the on-surface synthesis of high-quality MoS₂ monolayers, but also can be applied to the growth of vertically-stacked and large-scale in-plane lateral MoS₂-graphene heterostructures.

AC. Characterisation and performance of hydrotalcite-derived CoMo sulphide catalysts for selective HDS in the presence of olefin

Infrared spectroscopy of adsorbed CO showed that, in hydrotalcite-derived CoMoMgAl catalysts, Co-promoted sites catalyse HDS, and un-promoted sites, hydrogenation.

Unusual reactivity of MoS2 nanosheets

The reactivity of the 2D nanosheets of MoS2 with silver ions in solution, leading to their spontaneous morphological and chemical transformations, is reported. This unique reactivity of the nanoscale form of MoS2 was in stark contrast to its bulk counterpart. While the gradual morphological transformation involving several steps has been captured with an electron microscope, precise chemical identification of the species involved was achieved by electron spectroscopy and mass spectrometry. The energetics of the system investigated supports the observed chemical transformation. The reaction with mercury and gold ions shows similar and dissimilar reaction products, respectively and points to the stability of the metal-sulphur bond in determining the chemical compositions of the final products.