Gold octamers (Au8) bound to oxygen-vacancy F-center defects on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to CO2, whereas 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. Infrared measurements of the stretch vibration of CO adsorbed on mass-selected gold octamers soft-landed on MgO(001) with coadsorbed O2 show a red shift on an F-center-rich surface with respect to the perfect surface. The experiments agree with quantum ab initio calculations that predict that a red shift of the C-O vibration should arise via electron back-donation to the CO antibonding orbital.

https://science.sciencemag.org/content/sci/307/5708/403.full.pdf

Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO

Gold nanoclusters (phi = 1.3 nm) stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP NCs) readily oxidize benzylic alcohols to the corresponding aldehydes and/or carboxylic acids under ambient temperature in water. Kinetic measurement revealed that smaller Au:PVP NCs exhibit higher catalytic activity than larger (9.5 nm) homologues and, more surprisingly, than Pd:PVP NCs of comparable size (1.5 and 2.2 nm). On the basis of the marked difference in the kinetic isotope effect and activation energy between Au:PVP and Pd:PVP NCs, a reaction mechanism for alcohol oxidation catalyzed by Au:PVP NCs is proposed in which a superoxo-like molecular oxygen species adsorbed on the surface of the small Au NCs abstracts a hydrogen atom from the alkoxide.

Size-specific catalytic activity of polymer-stabilized gold nanoclusters for aerobic alcohol oxidation in water.

/pdf/surface-chemistry-of-catalysis-by-gold-37hj5cal3q.pdf

Surface chemistry of catalysis by gold

Gas-phase experiments with state-of-the-art techniques of mass spectrometry provide detailed insights into numerous elementary processes. The focus of this Review is on elementary reactions of ions that achieve complete catalytic cycles under thermal conditions. The examples chosen cover aspects of catalysis pertinent to areas as diverse as atmospheric chemistry and surface chemistry. We describe how transfer of oxygen atoms, bond activation, and coupling of fragments can be mediated by atomic or cluster metal ions. In some cases truly unexpected analogies of the idealized gas-phase ion catalysis can be drawn with related chemical transformations in solution or the solid state, and so improve our understanding of the intrinsic operation of a practical catalyst at a strictly molecular level.

Gas-phase catalysis by atomic and cluster metal ions: the ultimate single-site catalysts.

Au clusters smaller than 1.5 nm and stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) showed higher activity for aerobic oxidation of alcohol than those of larger size or stabilized by poly(allylamine) (PAA). X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy of adsorbed CO, and X-ray absorption near edge structure measurements revealed that the catalytically active Au clusters are negatively charged by electron donation from PVP, and the catalytic activity is enhanced with increasing electron density on the Au core. Based on similar observations of Au cluster anions in the gas phase, we propose that electron transfer from the anionic Au cores of Au:PVP into the LUMO (pi*) of O(2) generates superoxo- or peroxo-like species, which plays a key role in the oxidation of alcohol. On the basis of these results, a simple principle is presented for the synthesis of Au oxidation catalysts stabilized by organic molecules.

Effect of electronic structures of Au clusters stabilized by poly(N-vinyl-2-pyrrolidone) on aerobic oxidation catalysis.

Temperature-dependent rf-ion trap mass spectrometry and first-principles simulations reveal the detailed reaction mechanism of the catalytic gas-phase oxidation of CO by free Au2- ions in the presence of O2. A metastable intermediate with a mass of Au2CO3- was observed at low temperatures. Two alternative structures corresponding to digold carbonate or peroxyformate are predicted for this intermediate. Both structures are characterized by low activation barriers for the formation of CO2. These combined experimental and theoretical investigations provide a comprehensive understanding of the kinetics, energetics, and atomic arrangements along the reaction path, thus allowing a formulation of the catalytic cycle for the oxidation reaction.

Catalytic CO oxidation by free Au2-: experiment and theory.

Catalytic CO Oxidation by Free Au2-: Experiment and Theory.

Whether one can learn something about the condensed phase from clusters strongly depends on the property under consideration. For a chem. reaction for instance subtle changes in the electronic and geometric structure when going from the atom to the bulk are detg. for the outcome of the reaction. In our exptl. studies on the chem. properties of cluster-assembled materials, metal atoms and small metal clusters are formed in the gas phase, size-selected and then deposited on thin MgO and TiO2 films grown on metal surfaces. Various chem. reactions on the obtained cluster-assembled materials are then investigated under UHV conditions by means of thermal desorption and IR spectroscopies. Oxidn. and polymn. reactions are strongly dependent on cluster size and on the cluster-support interaction, and not only the no. of product mols. per cluster is changed, but also the branching ratio of the certain reactions. In many cases the measured reactivities are different from the ones obtained for the corresponding bulk systems. In addn., first-principle calcns. are used for studying some of the chem. reactions on oxide supported metal clusters. Detailed reaction mechanisms are proposed, which describe many of the exptl. observations. Comparing the proposed reaction mechanisms with the ones for the bulk analogs can elucidate distinct differencies in the chem. behavior of clusters and condensed matter. [on SciFinder(R)]

Ulrich Heiz

Papers

Catalytic CO oxidation by free Au2-: experiment and theory.

Catalytic CO Oxidation by Free Au2-: Experiment and Theory.

How different are mechanisms of chemical reactions on atoms and clusters compared to their bulk analogs