On the catalytic activity of palladium clusters generated with the electrochemical scanning tunnelling microscope C.G. Saanchez a, * , E.P.M. Leiva b , W. Schmickler c a Atomistic Simulation Group, School of Mathematics and Physics, QueenÕs University Belfast, BT7 1NN Belfast, UK b Universidad Nacional de C oordoba, Unidad de Matem aatica y F ıısica, Facultad de Ciencias Qu ıımicas, 5000 C oordoba, Argentina c Abteilung Elektrochemie, Universit€aat Ulm, D-89069 Ulm, Germany Received 9 April 2003; received in revised form 27 May 2003; accepted 27 May 2003 Published online: 16 June 2003 Abstract Recent experiments have shown that small palladium clusters, deposited on Au(1 1 1) with a scanning tunnelling micr better catalysts for the hydrogen evolution reaction than larger clusters, and may even be more active than bulk palladium. We suggest that the larger clusters contain an admixture of gold, which lowers the density of states at the Fermi level. In ad is charge transfer from the gold to the palladium, which induces a high electric field at the surfaces of small clusters, wh hydrogen evolution. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Catalysis; Palladium; Gold; Clusters Recently,Meier et al.[1]have investigated the cata- lytic activity of palladium clusters for the electrochemi- cal hydrogen-evolution reaction and found a significant dependence of the reaction rate on the cluster size: small clusters were found to be much more active than larger ones, the current densities differing by up to two orders of magnitude. There is also some indication that the rates on small clusters are significantly larger than on bulk palladium, though this comparison is more difficult to make.In this note,we want to discuss possible rea- sons for this enhanced activity and suggest, that it is due to a combination of several effects rather than to a single cause. In the experiments, the palladium clusters were gen- erated with a scanning tunnelling microscope (STM) by a method originally developed by Kolb et al. [2].In a first step, palladium atomswere deposited electro- chemically on the tip of the STM; subsequently, the tip was brought into contact with a Au(1 1 1) electrode. On withdrawalof the tip,a palladium cluster was left be- hind on the surface. The clusters were larger and more stable,if the tip was moved further towards the gold surface in the generation process [3]. There is good ev- idencethat the larger clusterscontain a significant amount of gold: 1. Computersimulationsof the deposition process, based on the technique of Landman and Luedtke [4], resulted in palladium–gold clusters, the gold con- tent increasing with the size of the clusters. 2. The large clusters are unusually stable, which again can be attributed to alloy formation, since Au–Pd al- loys are more stable than pure palladium. 3. After dissolution of the smaller clusters, smallholes were often found in the Au(1 1 1) substrate, which also suggests that gold atoms had been incorporated into the clusters. 4. The situation appears similar to the formation of cop- per clusters on Au(1 1 1), where again there is evidenc for Cu–Au alloy formation [5]. In fact, in all cases where stable clusters have been formed by this meth- od, the alloy is more stable than the pure cluster [6]. The catalytic activity of a metal can often be correlated with the energy of the centre of the d band and with its density of states [7]. In pure palladium, the density of Electrochemistry Communications 5 (2003) 584–586 www.elsevier.com/locate/elecom * Correspondingauthor. Tel.: +44-28-90273557; fax: +44-28- 90241958. E-mail address: c.sanchez@qub.ac.uk (C.G. S aanchez). 1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00134-6