Christian Bürgel

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.

Probing the Electronic Structure and Chemical Bonding of Gold Oxides and Sulfides in AuOn− and AuSn− (n = 1, 2)

The Au-O and Au-S interactions are essential in nanogold catalysis and nanotechnology, for which monogold oxide and sulfide clusters serve as the simplest molecular models. We report a combined photoelectron spectroscopy and ab initio study on AuO (-) and AuO 2 (-) and their valent isoelectronic AuS (-) and AuS 2 (-) species to probe their electronic structure and to elucidate the Au-O and Au-S chemical bonding. Vibrationally resolved spectra were obtained at different photon energies, providing a wealth of electronic structure information for each species. Similar spectra were observed for AuO (-) and AuS (-) and for the linear OAuO (-) and SAuS (-) species. A bent isomer was also observed as Au(S 2) (-) in the AuS 2 (-) spectra, whereas a similar Au(O 2) (-) complex was not observed in the case of AuO 2 (-). High-level ab initio calculations were conducted to aid spectral assignments and provide insight into the chemical bonding in the AuX (-) and AuX 2 (-) molecules. Excellent agreement is achieved between the calculated electronic excitations and the observed spectra. Configuration interactions and spin-orbit couplings were shown to be important and were necessary to achieve good agreement between theory and experiment. Strong covalent bonding was found in both the AuX (-) and the XAuX (-) species with multiple bonding characters. While Au(S 2) (-) was found to be a low-lying isomer with a significant binding energy, Au(O 2) (-) was shown to be unbound consistent with the experimental observation. The latter is understood in the context of the size-dependent reactivity of Au n (-) clusters with O 2.

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.