We propose to introduce intrinsic dynamical properties as a criterion for promoting reactivity of small size noble metal reactive centers relevant for heterogeneous catalysis. To illustrate the concept, collisions between Ag 6 ؊ or Au 6 ؊ clusters and molecular oxygen have been investigated with direct ab initio molecular dynamics by using DFT. We show that different nature and efficiency of internal vibrational energy redistribution (IVR) during reaction dynamics is responsible for significantly different sticking probabilities of O2 to gold and to silver clusters.
Small nanoparticles and clusters exhibit surprising new chemical and physical properties because of size and structure selective quantum confinement. In such a nonscalable size regime in which ''each atom counts,'' by adding or removing a single atom, the properties of clusters can be tuned to achieve desired functionality. A prominent example is the unexpected finding that small, nanosized gold particles can catalyze oxidation reactions occurring in, e.g., combustion (1-5). Also, silver is widely used as a catalyst in important industrial processes such as epoxidation (5). In both cases, mechanisms responsible for oxidation reactions are required for the rational design of efficient nanocatalysts, but they remain to be unravelled.Until now, studies (3, 4, 6-13) of the size-dependent reactivity of small gold and silver clusters in the gas phase or on supports have been focused on establishing relationships between structure, charge, and reactivity based on energetics, which is a very important aspect (6). However, dynamical processes such as nature and timescale of the internal vibrational energy redistribution (IVR) between interacting subunits during the reaction have not yet been considered.Here, we wish to introduce the dynamical aspect as a reactivity promoting criterion. We will show that resonant IVR between reactants promotes reactivity toward adsorbates in contrast to dissipative IVR, which prevents reactions. This will be illustrated with anionic gold versus silver clusters reacting with molecular oxygen as examples. Although the influence of an interface plays an important role, the gas phase study allows a clear conceptual framework for examination of dynamical aspects. We first briefly describe previous findings and then address new aspects involving dynamical properties.From the electronic and structural size-dependent properties of anionic gold and silver clusters, an understanding of the binding toward molecular oxygen has been gained (4,7,10,13,14). An even/odd alternation of binding energies has been found for adsorption of O 2 onto small anionic gold (9, 11, 15) and silver clusters (9, 13). Clusters with an odd number of electrons strongly bind O 2 , which is one-electron acceptor. Also, electron affinities of coinage metals exhibit an even/odd alternation (9). Therefore, the anionic noble metal clusters with an even number of electrons (odd number of atoms) are inert toward O 2 . Because O 2 acts as a one-electron acceptor, these...