Probing of Charging and Reactivity of Supported Gold Nanoclusters: Bonding of CO to Gold Octamers Adsorbed on Magnesia

Gold octamers supported on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to carbon dioxide, while clusters deposited on close-to-perfect magnesia surfaces remain chemically inert. Charging of the supported clusters plays a key role in promoting their chemical activity. Direct evidence for enhanced electron transfer from the F-centers to the anchored clusters is provided here through infra-red measurements, under reaction conditions, of the stretch vibration of CO adsorbed on mass-selected gold octamers soft- landed on close-to-perfect or defect-rich MgO(001). The experiments agree with quantum ab-initio calculations that predict a red-shift of the C–O vibration when the molecules are adsorbed on gold octamers anchored to F-centers. This results from electron back-donation to the CO anti-bonding orbital that weakens the intra-molecular bond.