Eur. Phys. J. D 24, 41–44 (2003) DOI: 10.1140/epjd/e2003-00124-7 T HE EUROPEAN P HYSICAL JOURNAL D Structural properties and reactivity of bimetallic silver-gold clusters R. Mitri´ c 1 , C. B¨ urgel 1 , J. Burda 1, a , V. Bonaˇ ci´ c-Kouteck´ y 1, b , and P. Fantucci 2 1 Humboldt-Universit¨at zu Berlin, Institut f¨ ur Chemie, Brook-Taylor-Strasse 2, 12489 Berlin, Germany 2 Dipartmento di Biotechnologie e Bioscienze Universita degli Studi di Milano Bicocca, Piazza della Scienza 2, 20126 Milano, Italy Received 10 September 2002 Published online 3 July 2003 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2003 Abstract. Bimetallic silver-gold clusters are well suited to study changes in metallic versus “ionic” proper- ties involving charge transfer as a function of the size and the composition. We present structures, ioniza- tion potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic AunAgm (2 ≤ (n + m) ≤ 5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m =1 - 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n = m =5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au8Ag12 cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O2 and CO molecules. As a starting point we consider reactivity towards CO and O2 molecules on the example of AgAu - dimer. The results show that the catalytic cycle can be fullfilled. PACS. 31.15.Qg Molecular dynamics and other numerical methods – 31.15.Ar Ab initio calculations 1 Introduction In contrast to extensively investigated structural and elec- tronic properties of gold [1–5] and silver clusters [6–9], considerably less information is available about bimetallic silver-gold clusters [10,11]. The role of the hetero-polar bonding versus homonuclear metallic bonding in small bimetallic clusters is of considerable interest, since the former can promote charge transfer from the Ag to the Au atoms. In the context of the reactivity, it is impor- tant to find the structural patterns favorable for charge transfer in bimetallic clusters. Therefore the investigation of “ionic” versus metallic properties in these clusters as a function of their size and composition provides an infor- mation about their reactive properties. In this regard it is also useful to compare the structural and reactivity prop- erties of gold-silver clusters with those of gold [12–15] and silver one-component systems. According to our density functional results the role of d-electrons in determining structural properties drastically increases from silver and bimetallic clusters towards gold clusters due to a decrease a Present Address: Department of Chemical Physics and Optics Charles University, 12116 Praha, KeKarlovu 3, Czech Republic. b e-mail: vbk@chemie.hu-berlin.de in the s-d energy separation [11]. Consequently silver and bimetallic clusters assume related structures in contrast to gold clusters which tend to remain planar for larger sizes [5,16]. In this contribution we present the ground state properties of small neutral and charged bimetallic Au n Ag m clusters (2 ≤ (n + m) ≤ 5) in which the influ- ence of increasing Ag-component on their reactivity has been considered. IP values for selected bimetallic clusters with equal number of hetero atoms (m = n =1-5) are then compared with those obtained for pure gold clusters. Characteristic features of Au 10 Ag 10 and Au 8 Ag 12 clusters are discussed. Finally we address the reactivity of AgAu - towards CO and O 2 molecules. 2 Computational methods We use gradient corrected density functional theory (DFT) combined with relativistic effective core potentials. Previous studies of the structural properties of the charged [8,16] and neutral [11] gold clusters have shown that ex- plicit treatment of the d-electrons is mandatory for proper description of the structural properties of pure gold clus- ters. Therefore, we determined them using the Stuttgart group [17] relativistic effective core potentials with