Atomic-Scale Structure of Co-Mo-S Nanoclusters in Hydrodesulfurization Catalysis (HDS)

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A commonly accepted activity in hydrodesulfurization (HDS) catalysis is connected to the presence of the so-called Co-Mo-S structures that comprise small WS₂ or MoS₂ clusters with promoter atoms, Ni or Co, placed somewhere at the edges.

But, the basis of the promoting role and specifically the atomic-scale location of the promoter atoms in Co-Mo-S nanoclusters is still a debatable topic. This is because the spectroscopic methods, which have offered insight into the nature of the Co-Mo-S structures, are unable to explicitly map the true space atomic structure.

Using Surface Science Methods has Increased Knowledge of Catalyst Model Systems

In understanding the industrial catalyst, new insight has been gained from the investigations of catalyst model systems applying surface science methods. Recently, scanning tunneling microscopy (STM) has been successfully used to explore the real space structure of MoS₂ nanoclusters produced on an inert Au (111) substrate as a model system for HDS catalysts. Atomically resolved STM images of the nanoclusters have exposed a hitherto unparalleled view of the atomic details. It is currently possible to acquire the first atomic-scale images of the promoted Co-Mo-S structure present in hydrodesulfurization (HDS) catalysts.

Changing the Shape of MoS₂ Nanoclusters with Cobalt Promoter Atoms

Data on the catalytically vital edge structures has been acquired by synthesizing single layer Co-Mo-S nanoclusters using the Au (111) herringbone reconstruction as a template. It is seen that the presence of the Co-promoter atoms renders the shape of the MoS₂ nanoclusters to alter from triangular to hexagonally-truncated.

The observed morphology of the nanoclusters (the hexagons) is thus credited to the integration of cobalt in the MoS₂ structure, that is, the formation of the Co-Mo-S phase.

What Scanning Tunneling Microscope (STM) Images Reveal About Co-Mo-S Nanoclusters

Atomically resolved STM-images of the Co-Mo-S nanoclusters affirm that the organization of protrusions on the basal plane closely matches that of single layer MoS₂. This is anticipated as Co-Mo-S is known to possess the same interior structure as MoS₂.

Identifying the S-and Mo-edges of Co-Mo-S Nanoclusters from its Hexagonal Shape

The observed hexagonal truncated shape of the Co-Mo-S nanoclusters suggests that two varied edge terminations, that is, S-and Mo-edges, must exist. The identity of the two edges is deduced from the high-resolution STM images of the Co-Mo-S edges. It is seen that the longer edges are similar to those seen in the triangular MoS₂ nanoclusters, that is, Mo-edges. From the crystal’s symmetry, the short edges are consequently credited to S-edges.

How the Cobalt Edge Atoms Improve the Electronic Density of Neighboring Sulfur Atoms

Concentrating on the short S-edges, the atomically-resolved image illustrates that the brim structure behind the outermost row of sulfur atoms seems to be imaged more brightly, relative to the S atoms on the basal plane. This change may be related to the Co atoms present at the S-edges. The Co edge atoms seem to trigger an improved electronic density at the adjacent S atoms, which consequently are imaged brighter.

New Insights that May Increase Knowledge About Using Cobalt and Nickel in Hydrotreating Catalysis (HDS)

Based on these STM observations, a structural model for Co-Mo-S has been proposed, where cobalt is replaced in Mo positions at the S-edge. A tetrahedral environment of the Co is created, if the outermost S atoms are believed to be bridge-bonded monomer sulfur atoms, situated in the plane of the Mo atoms. This new learning may result in better understanding of the promoting role of Ni and Co in HDS catalysts.