Michele L. Kimble

Influence of Charge State on the Mechanism of CO Oxidation on Gold Clusters

We present results from our joint experimental and theoretical study of the reactivity of anionic and cationic gold oxide clusters toward CO, focusing on the role of atomic oxygen, different charge states, and mechanisms for oxidation. We show that anionic clusters react by an Eley-Rideal-like mechanism involving the preferential attack of CO on oxygen rather than gold. In contrast, the oxidation of CO on cationic gold oxide clusters can occur by both an Eley-Rideal-like and a Langmuir-Hinshelwood-like mechanism at multiple collision conditions as a result of the high adsorption energy of two CO molecules. This large energy of CO adsorption on cationic gold oxide clusters is the driving force for the CO oxidation. Therefore, in the presence of cationic gold species at high pressures of CO, the oxidation reaction is self-promoting (i.e., the oxidation of one CO molecule is promoted by the binding of a second CO). Our findings provide new insight into the role of charge state in gold-cluster-based nanocatalysis.

Joint experimental and theoretical investigations of the reactivity of Au2On− and Au3On− (n=1–5) with carbon monoxide

The interactions between small gold oxide cluster anions, Au(2,3)O-(n) (n=1-5), and CO were investigated in a fast-flow reactor mass spectrometer, and experimental results were verified with a guided ion beam mass spectrometer. Density functional calculations along with molecular dynamics simulations were also utilized to explain the experimental findings. From these studies, we show that, for the interactions between Au(m)O-(n) and CO, each atom counts. With the addition of a single gold atom, it is observed that association of CO and replacement of O(2) by CO become the dominant reaction channels as opposed to CO oxidation. We also present results that show that the oxidation of CO takes place only in the presence of a peripheral oxygen atom. However, this condition is not always sufficient. Furthermore, the association of CO onto Au(m)O-(n) follows a general qualitative rule based on the relationship between the energy of the cluster lowest unoccupied molecular orbital and the binding energy of CO.