AVS 53rd International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM9
New Atomic-Scale Insights Into Promoter and Support Effects on MoS@sub 2@ Nanoclusters from STM Studies of Hydrotreating Model Catalysts

Tuesday, November 14, 2006, 10:40 am, Room 2002

Session: Reactivity of Metal Clusters
Presenter: J. Kibsgaard, University of Aarhus, Denmark
Authors: J. Kibsgaard, University of Aarhus, Denmark
J.V. Lauritsen, University of Aarhus, Denmark
H. Topsoe, Haldor Topsoe A/S, Denmark
B.S. Clausen, Haldor Topsoe A/S, Denmark
F. Besenbacher, University of Aarhus, Denmark
Correspondent: Click to Email
To meet the stringent requirements for upgrade and clean-up of transport fuels by hydrotreating there is currently an urgent need to understand and improve the MoS@sub 2@-based catalyst applied for this service. The active phase of this catalyst is composed of 2-3nm wide MoS@sub 2@-like nanoclusters, typically promoted by Co or Ni, forming the so-called CoMoS and NiMoS nanostructures. Despite intense research efforts, the morphology of CoMoS and NiMoS and the detailed location and coordination of the promoter atoms have not been resolved. Consequently, the role of the Co and Ni is poorly understood, and existing models are not able to account for the difference in the specific selectivity of Co or Ni promoted catalysts with respect to hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrogenation (HYD). In this study, we present atom-resolved scanning tunneling microscopy (STM) of CoMoS and NiMoS nanoclusters supported on Au(111). In the STM images we observe a promoter dependent morphology, which is explained by the different affinities for Co and Ni to replace Mo at the edge sites of MoS@sub 2@ nanoclusters. This finding shows that the location and coordination of the promoter atoms are different in CoMoS and NiMoS, which may explain the different selectivity observed in the industrial catalysts. In order to include the effect of a different support, we also synthesized MoS@sub 2@ nanoclusters on highly oriented pyrolytic graphite (HOPG). HOPG as a substrate is indeed very interesting since carbon-supported hydrotreating catalysts have previously been found to exhibit very high HDS reactivity. STM images reveal that the morphology of the MoS@sub 2@ nanoclusters is very dependent on the exact preparation temperature. Furthermore, images of the top facet of the clusters provide new atomic-scale information on the MoS@sub 2@-HOPG topotaxy, and we find good correspondence between the edge structure and previous studies of MoS@sub 2@ on an Au substrate.