Characterization and reactivity of the Mo₄S₆ ⁺ cluster deposited on Au(111)

Mass-selected cluster deposition was used to investigate the chemical and thermal properties of the Mo₄S₆ cluster deposited onto a Au(111) substrate. Auger spectroscopy and ¹³CO thermal desorption measurements demonstrate that the clusters behave independently up to coverages of ∼0.15 ML, while at higher coverages, cluster crowding or island formation results in no net increase in Mo-atom adsorption sites. DFT calculations show that CO binding on the Mo-atom top site is strongly preferred over the side sites, with scaled binding energies in reasonable agreement with the experimentally derived binding energy of 0.7 eV. DFT calculations predict that the total adsorption energy for sequential addition of two CO molecules (top and side site) is nearly additive, whereas the addition of a third CO to another empty Mo side site is less stable. The latter is attributed to repulsive intercluster interactions and is consistent with the experimentally estimated sticking coefficient of 0.4 ±0.1. In contrast to CO, we were unable to detect any adsorption of NH₃ onto the deposited cluster. The DFT calculations confirm these observations by predicting a very small NH ₃ adsorption energy to the Mo₄S₆2/Au(111) supported cluster. The difference in adsorbate binding (CO, NH₃) between the gas-phase and supported cluster highlights the role of the Au(111) substrate in modifying the electronic structure and chemical behavior of the supported cluster. Annealing of the Mo₄S₆/Au(111) surface above ∼500 K was found to significantly reduce the CO uptake of the supported clusters. These data are consistent with diffusion of intact clusters along the Au surface and the formation of 2D islands. Because of the unique stoichiometry of the as-deposited Mo₄S₆ clusters, aggregates formed by cluster diffusion are expected to exhibit distinctly different chemical behavior compared to near-stoichiomerric MoS_x (x ≈ 2) platelet nanoclusters or amorphous thin films.