Atomic-scale structure of Co-Mo-S nanoclusters

in hydrotreating catalysts

F. Besenbacher∗, J. V. Lauritsen∗, S. Helveg∗, E. Lægsgaard∗,I. Stensgaard∗, B. S. Clausen† and H. Topsøe†

∗ CAMP and Institute of Physics and Astronomy, University ofAarhus, DK-8000 Aarhus C, Denmark

† Haldor Topsøe Research Laboratories, Nymøllevej 55, DK-2800Lyngby, Denmark

ABSTRACT

The production of clean transport fuels by hydrotreating and especially deephydrodesulfurization (HDS) has recently attracted increased attention due to theintroduction of new environmental legislation regarding fuel specifications. Inorder to meet the specifications there is an increased need to understand and im-prove HDS catalysts. Supported sulfides of Mo or W promoted with Co or Ni areapplied widely for this service and they have been the subject of extensive inves-tigations. It is generally accepted that the HDS activity is related to the presenceof the so-called Co-Mo-S structures that consist of small MoS2 or WS2 clusterswith promoter atoms located somewhere at the edges. However, the origin ofthe promoting role of Co or Ni and in particular the atomic-scale location of thepromoter 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 providedinsight into the nature of the Co-Mo-S structures, are not able to unequivocallymap the real space atomic structure [1].

To aid the understanding of the industrial catalyst, new insight has beengained from studies of catalyst model systems applying surface science tech-niques. Recently, we have successfully used Scanning Tunneling Microscopy(STM) to study the real space structure of MoS2 nanoclusters synthesized onan inert Au(111) substrate as a model system for HDS catalysts [2]. Atomi-cally resolved STM images (Fig. 1b) of the nanoclusters have revealed a hithertounpresedented view of the atomic details. Contrary to expectations from bulkMoS2, the ∼ 30A wide MoS2 nanoclusters exhibit a triangular morphology. Thecatalytically important MoS2 edges are found to be reconstructed relative to theperfect MoS2 lattice, and by in situ treatment of the nanoclusters with atomichydrogen, unique images of the catalytically active sites in form of S vacancies atthe edges have been obtained .

The industrial HDS catalysts are, however, promoted with Co or Ni andfocus in the present study will thus be on the important promoted Co-Mo-S

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Figure 1: Atom-resolved STM images of (a) the promoted CoMoS structure and (b) theunpromoted MoS2 nanoclusters.

structures [3]. On the basis of atom-resolved STM images (Fig. 1a) it is found thatthe presence of the Co atoms has a dramatic influence on the morphology of theMoS2 structures. This is related to the observation that in Co-Mo-S, Co is onlypresent at one type of MoS2 edges, the so-called S-edge. Besides morphologicaland structural information, the results also provide direct information on changesin the local electronic environment of the sulfur atoms neighboring the Co edgeatoms. This insight may be important for understanding the promoting role.Finally, it is interesting that the STM results also reveal substitution of Co intothe bulk structure of MoS2 nanoclusters.

Preliminary results on the adsorption of thiophene at low temperatures revealinteresting information. Thiophene, a sulfur containing compound, is widelyused as a test-molecule in reactivity measurements of HDS catalysts, and it isobserved that the molecules decorate the cluster edges. This may provide valuableinformation on the desulfurization process.

References

[1] H. Topsøe, B. S. Clausen, and F. E. Massoth, in Hydrotreating Catalysis, Science andTechnology, vol. 11, edited by J. R. Anderson and M. Boudart (Springer Verlag, Berlin,1996).

[2] S. Helveg, J. V. Lauritsen, E. Lægsgaard, I. Stensgaard, J. K. Nørskov, B. S. Clausen, H.Topsøe, and F. Besenbacher, Phys. Rev. Lett. 84, 951 (2000).

[3] J. V. Lauritsen, S. Helveg, E. Lægsgaard, I. Stensgaard, B. S. Clausen, H. Topsøe, and F.Besenbacher, Accepted in J. Catal.

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