Structural, electronic, and impurity-doping effects in nanoscale chemistry: supported gold nanoclusters.

Metal clusters exhibit unique size-dependent physical[1] and chemical properties[2] that differ from those of bulk materials. While inert as a bulk material, gold nanoparticles and clusters have attracted considerable interest lately as active catalysts for a number of industrially relevant reactions.[3–6] Unlike supported particles of larger size or extended solid surfaces,[7–10] size-selected small metal clusters adsorbed at specific sites of a support material (e.g. oxygen vacancies in the case of a MgO(100) surface) exhibit unique properties that originate from the highly reduced dimensions of the individual metal aggregates. These properties underlie the remarkable, newly found catalytic activity of small gold clusters, and they include: 1) dynamic structural fluxionality that exhibits itself through the propensity of small clusters to transform, in the course of chemical reactions, between various energetically accessible structural isomers, thus enhancing the rates for overcoming reaction barriers, 2) quantum size effects that are reflected in size-dependent characteristics of the electronic spectra of small gold clusters, and in charge transfer from the support to the clusters, 3) impurity-doping effects that allow modification and control of the electronic structure, and consequently the chemical reactivity, of small supported clusters, through incorporation of judiciously chosen impurity atoms in otherwise inert clusters. Herein, we focus on gaining fundamental insights into the above size-dependent “nanocatalytic factors”, and illustrate through experimental and theoretical investigations the manner in which such fundamental understanding may guide the design and atomic-scale modifications of nanocatalysts. Recently, a set of model catalysts have been prepared by soft-landing[11] of mass-selected Aun and AunSr cluster ions onto well-characterized MgO(100) thin films. These substrate films contained a low concentration (typically 5 6 1013 cm 1) of oxygen vacancies (surface F-centers, FC), that act as strong trapping sites for the clusters at low temperatures.[12–14] Temperature-programmed reaction (TPR) measurements of CO oxidation (CO+ =2O2!CO2) have shown that the smallest gold cluster that catalyzes the reaction is Au8. Furthermore, it has been found that while Au4 is catalytically inert, the doped cluster Au3Sr is active. These findings, in conjunction with ab initio calculations, have revealed that underlying the aforementioned remarkable chemical size-sensitivity is the nature of bonding and the activation of molecular oxygen by these nanocluster catalysts. The measured chemical activity is summarized in Figure 1, which shows typical TPR spectra for selected samples (a–e). The total CO2 yield per cluster obtained in a one-cycle heating experiment for Aun and AunSr with 1 n 9 is shown in the inset. The active model systems (n 8 for the pure Aun and