Dynamic fluxionality and enhanced CO adsorption in the presence of coadsorbed H2O on free gold cluster cations

Abstract

This paper presents mass spectrometry measurements of the saturated adsorption of CO in the presence of coadsorbed H2O on gas phase gold cluster cations, Aun+, n=3–20, stored in a quadrupole ion trap. Initial mass spectra obtained at 150K for specific cluster ion sizes as a function of CO pressure and reaction time, indicate increased CO saturation levels correlated with the coadsorption of background H2O vapor. Subsequent to these low temperature experiments, measurements were made of CO and H2O coadsorbed on Au6+ as a function of reaction time at 300K. These mass spectra indicate that the reaction rate at constant CO pressure increases by an order of magnitude for a constant H2O pressure. First-principles density-functional theory calculations in conjunction with the above measurements allowed identification of energy barriers that control dynamic structural fluxionality between adsorption complexes that depends strongly on preadsorbed water. The calculations revealed that in the presence of H2O the energy barrier for the transition state between ground-state triangular and the incomplete hexagonal isomers of the [Au6(CO)3(H2O)2]+ complex is reduced to ∼0eV and the exothermicity is increased by 0.43eV. The theoretical results also identified kinetic pathways exhibiting a transition of the incomplete hexagonal isomer of [Au6(ih)(CO)3(H2O)2]+ to the final saturated complex, Au6(ih)(CO)4+. The energetics and kinetic pathway calculations are consistent with increased formation rates of Au6(CO)4+ as observed in mass spectra. The insights gained from these theoretical results not only explain measurements of the CO saturated adsorption on Au6+ in the presence of water, but also assist in rationalizing coadsorption results obtained over the broader range of cluster size at 150K.