Interaction of CO with PdCu surface alloys supported on Ru(0 0 0 1) T. Hager, H. Rauscher, R.J. Behm * Department of Surface Chemistry and Catalysis, University of Ulm, Ulm D-89069, Germany Received 12 January 2004; accepted for publication 25 March 2004 Available online 15 April 2004 Abstract The adsorption and desorption of CO on monolayer PdCu surface alloys on a Ru(0 0 0 1) substrate and, for com- parison, on Pd/Ru(0 0 0 1), and Cu/Ru(0 0 0 1) monolayer films, have been studied by temperature programmed desorption and infrared reflection absorption spectroscopy, aiming at a detailed understanding of the different effects controlling the chemical properties of bimetallic surfaces, geometric ensemble effects, electronic ligand effects and strain induced effects. The results show that all of these effects are present in these surfaces, acting in a cooperative, synergetic way. Strain effects lead to a destabilization of the Pd–CO and a stabilization of the Cu–CO interaction as compared to CO adsorption on the respective bulk substrates. Geometric ensemble effects are imposed by the highly disperse dis- tribution of the Cu and Pd atoms, reducing the number of otherwise favorable Pd 3 sites, and electronic ligand effects lead to an increasing stability of the Pd–CO bond with increasing Cu content. Ó 2004 Published by Elsevier B.V. Keywords: Metallic surfaces; Alloys; Adsorption kinetics; Surface structure, morphology, roughness, and topography; Surface electronic phenomena (work function, surface potential, surface states, etc.); Surface stress; Carbon monoxide 1. Introduction Surfaces of bimetallic alloys have attracted considerable interest in recent years as model sys- tems for bimetallic catalysts, which often exhibit significantly different catalytic properties com- pared to those of the respective individual com- ponents [1–3]. These modified chemical properties have been explained mainly by two effects, the geometric ensemble effect and the electronic ligand effect [2,4–6]. The first one describes the require- ment of a (minimum) ensemble of surface atoms of one species for adsorption or reaction of a specific adsorbate or reactant [2,5], while the latter one refers to changes of the electronic properties of a specific adsorption site induced by the nature of the respective neighboring surface atoms [4,6]. More recently, electronic modifications due to strain effects have been identified as a third con- tribution [7], which can even be the dominant ef- fect. Traditionally these effects have been investigated by following the adsorption/reaction kinetics on * Corresponding author. E-mail address: juergen.behm@chemie.uni-ulm.de (R.J. Behm). 0039-6028/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.susc.2004.04.001 Surface Science 558 (2004) 181–194 www.elsevier.com/locate/susc