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

Topics Covered

How Co-Mo-S Structures Behave during Hydrotreating Catalysis (HDS)

Using Surface Science Techniques has Increased Knowledge of Catalyst Model Systems

Changing the Shape of MoS2 Nanoclusters with Cobalt Promoter Atoms

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

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

How the Cobalt Edge Atoms Enhance the Electronic Density of Nearby Sulfur Atoms

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

How Co-Mo-S Structures Behave during Hydrotreating Catalysis (HDS)

It is generally accepted that the activity in hydrodesulfurization (HDS) catalysis is related to the presence of the so-called Co-Mo-S structures that consist of small MoS2 or WS2 clusters with promoter atoms, Co or Ni, located somewhere at the edges. However, the origin of the promoting role and in particular the atomic-scale location of the promoter atoms in Co-Mo-S nanoclusters is still the subject of intense debate. This is related to the fact that the spectroscopic techniques, which have provided insight into the nature of the Co-Mo-S structures, are not able to unequivocally map the real space atomic structure.

Using Surface Science Techniques has Increased Knowledge of Catalyst Model Systems

To aid the understanding of the industrial catalyst, new insight has been gained from studies of catalyst model systems applying surface science techniques. Recently, we have successfully used Scanning Tunneling Microscopy (STM) to study the real space structure of MoS2 nanoclusters synthesized on an inert Au(111) substrate as a model system for HDS catalysts. Atomically resolved STM images of the nanoclusters have revealed a hitherto unprecedented view of the atomic details. It has now been possible to obtain the first atomic-scale images of the promoted Co-Mo-S structure present in hydrodesulfurization (HDS) catalysts.

AZoNano, Nanotechnology - This Scanning Tunnelling Microscope image shows Co-Mo-S nanoclusters on Au (111). The inset image shows the predominant hexagonally truncated structure. The change in morphology from the triangular nanocluster to the hexagonally truncated Co-MoS nanocluster appears to be driven by a preference for Co to be located at the S-edge of MoS2.

Figure 1. Scanning Tunnelling Microscope image showing Co-Mo-S nanoclusters on Au(111). The inset image shows the predominant hexagonally truncated structure. The change in morphology from the triangular nanocluster to the hexagonally-truncated Co-MoS nanocluster, appears to be driven by a preference for Co to be located at the S-edge of MoS2.

Changing the Shape of MoS2 Nanoclusters with Cobalt Promoter Atoms

Information on the catalytically important edge structures has been obtained by synthesizing single layer Co-Mo-S nanoclusters using the Au(111) herringbone reconstruction as a template. It is observed that the presence of the Co promoter atoms causes the shape of the MoS2 nanoclusters to change from triangular to hexagonally-truncated. The observed morphology of the nanoclusters (the hexagons) is therefore attributed to the incorporation of cobalt in the MoS2 structure, i.e. the formation of the Co-Mo-S phase.

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

Atomically resolved STM-images (see figure 2) of the Co-Mo-S nanoclusters reveal that the arrangement of protrusions on the basal plane, corresponds closely to that of single layer MoS2 (see figure 3). This is expected since Co-Mo-S is known to have the same interior structure as MoS2.

AZoNano, Nanotechnology - This graphic is a Scanning Tunnelling Microscope (STM)  image of a single layer Co-Mo-S nanocluster.

Figure 2. A Scanning Tunnelling Microscope (STM) image of a single layer Co-Mo-S nanocluster.

AZoNano, Nanotechnology - This graphic shows a triangular single layer MoS2 nanocluster. In figures 2 and 3, the small white dots illustrate the position of the protrusions.

Figure 3. A triangular single layer MoS2 nanocluster. In figures 2 and 3, the small white dots illustrate the position of the protrusions.

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 implies that two different edge terminations, i.e. S- and Mo-edges, must be present. The identity of the two edges is inferred from the high resolution STM images of the Co-Mo-S edges. We find that the longer edges are identical to those observed for the triangular MoS2 nanoclusters, i.e. Mo-edges (see figure 5, below). From the symmetry of the crystal (see figure 4, below), the short edges are consequently attributed to S-edges.

AZoNano, Nanotechnology - This graphic shows a ball model (top view) of a hypothetical bulk truncated MoS2 hexagon exposing both the Mo- and S-edge.

Figure 4. A ball model (top view) of a hypothetical bulk truncated MoS2 hexagon exposing both the Mo- and S-edge.

AZoNano, Nanotechnology - This graphic shows a triangular MoS2 cluster exposing Mo-edges with edge sulfur atoms located out of registry with the basal plane.

Figure 5. A triangular MoS2 cluster exposing Mo-edges with edge sulfur atoms located out of registry with the basal plane.

How the Cobalt Edge Atoms Enhance the Electronic Density of Nearby Sulfur Atoms

Focusing on the short S-edges, the atomically-resolved image in figure 3 shows that the brim structure behind the outermost row of sulfur atoms appears to be imaged more brightly, relative to the S atoms on the basal plane. We suggest that this change is associated with the Co atoms present at the S-edges. The Co edge atoms appear to induce an enhanced electronic density at the nearby 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, we have proposed a structural model for Co-Mo-S, where the cobalt is substituted into Mo positions at the S-edge, as depicted in the ball model in figure 6. A tetrahedral environment of the Co is produced, if the outermost S atoms are assumed to be bridge-bonded monomer sulfur atoms, located in the plane of the Mo atoms. This new insight may lead to an improved understanding of the promoting role of Co and Ni in HDS catalysts.

AZoNano, Nanotechnology - This graphic shows a ball model of the proposed hexagonally truncated Co-Mo-S structures. The colours are as follows: Mo(blue);S(yellow);Co(red).

Figure 6. A ball model of the proposed hexagonally truncated Co-Mo-S structures. The colours are as follows: Mo(blue);S(yellow);Co(red).

Source: CAMP Aarhus: The Scanning Tunneling Microscopy Group, Department of Physics and Astronomy, University of Aarhus.

For more information on this source please visit CAMP Aarhus: The Scanning Tunneling Microscopy Group.

 

Date Added: Jun 28, 2005 | Updated: Jun 11, 2013
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