Alkali-Metal Promotion of Mass-Selected Metal Sulfide Nanoclusters on Au(111) for CO₂ Activation

Molybdenum sulfide promoted by alkali (K, Cs) and/or transition (Ni, Co) metals is known to catalyze syngas (CO+H₂) to alcohols. The hydrogenation of CO₂ to oxygenates on metal sulfides is less well studied, but is of current interest for recycling CO₂ into liquid fuels. Recent density functional calculations suggest that alkali-modified Mo₆S₈ clusters (unsupported) are active for CO₂ hydrogenation to methanol. In this work, we are using mass-selected cluster deposition to study the activation of CO₂ on K-modified, metal sulfide nanoclusters supported on Au(111). The clusters are prepared by reactive sputtering (3% H₂S in Ar) of a metal target (Mo, W), mass-selected by a quadrupole filter and then soft-landed onto an Au(111) crystal. Thermal desorption measurements show that CO₂ exposure of the K/Mo₆S₈/Au(111) surface leads to two distinct CO₂ desorption peaks (350 K, 450 K), which are not present for surfaces with only K-atoms or Mo₆S₈ clusters. Similar experiments were carried out for other Mo and W sulfide clusters. The experimental results are compared with DFT calculations of CO₂ binding on the K/Mo₆S₈/Au(111) surface.