Invited Paper TF2-FrM1
Preparation and Characterization of Supported Metal Sulfide Nanoclusters Using Size-Selected Deposition

Friday, October 19, 2007, 8:00 am, Room 613/614

The layered (S-M-S) structure of the bulk metal sulfides leads to unusual structures at the nanoscale (disks, fullerenes, tube) that exhibit a size-dependent band gap and/or unique catalytic properties. For example, MoS₂ is widely used as a commercial catalyst for hydrogenation and hydrodesulfurization, where it is dispersed as nanoparticles (10-30 Å) on a high surface area support. It is generally accepted that the active sites for reaction are associated with Mo atoms (with Ni/Co "promoter" atoms) located at defects or edges of the nanoparticle, however, many questions remain about the inter-relationship between nanocluster size, structure (atomic and electronic) and metal-sulfur composition, and how these physical properties influence catalytic activity. In this work, we are using mass (size) selected cluster ion beams to explore the size dependent reactivity of small nanoclusters of the early transition metal sulfides (Mo, W). Specifically, we use reactive sputtering in a magnetron source to generate gas-phase cluster cations which are then mass-analyzed to yield a beam of clusters with a precisely defined mass (size) and metal-sulfur composition. The mass-selected clusters are deposited at low kinetic energies ("soft landing") onto a solid support, e.g., Au(111) and Al₂O₃, for preparing model nanocatalysts whose properties can be studied by conventional surface science techniques. To date, we have explored the structure and stability of several prominent Mo_xS_y⁺and W_xS_y⁺ cluster species (x/y = 2/6, 3/7, 4/6, 5/7, 6,/8, 7/10, 8/12) in the gas phase and deposited on surfaces. These clusters are substoichiometric relative to the bulk material (sulfur to metal ratio < 2), and generally have exposed metal atoms (edge sites) that are available for adsorbate binding and reaction. Using thermal desorption (TPD) and electron spectroscopy (Auger, XPS), we have studied the reactivity and thermal stability of molybenum sulfide nanoclusters deposited on a Au(111) surface. This work was motivated by recent theoretical calculations that predict that the highly symmetric and stable Mo₄S₆ and Mo₆S₈ clusters bind to the Au(111) surface via relatively strong Au-S linkages, but with little distortion of the cluster structure. Experiments indicate that the clusters can be deposited intact, with adsorbate binding energies that depend on cluster size and atomic composition, and the presence of the Au substrate.