Pd 1 /MgO(100): a model system in nanocatalysis S. Abbet a, * , A.M. Ferrari b , L. Giordano c , G. Pacchioni c ,H.H€ akkinen d , U. Landman d ,U.Heiz e a Institut de Physique de la Mati  ere Condens ee, Universit e de Lausanne, CH-1015 Lausanne, Switzerland b Dipartimento di Chimica IFM, Universit  a di Torino, via P. Giuria 5, I-10125 Torino, Italy c Dipartimento di Scienza dei Materiali, Istituto Nazionale per la Fisica della Materia, Universit  a di Milano-Bicocca, via R. Cozzi 53, I-20125 Milano, Italy d School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430, USA e Abteilung f€ ur Oberfl€ achenchemie und Katalyse, Universit€ at Ulm, D-89069 Ulm, Germany Received 26 September 2001; accepted for publication 1 March 2002 Abstract Nanocatalystsconsistofsmallsize-selectedclustersadsorbedonuniformsitesofasupportmaterial.Here,wefocus on a simple model system, which is fabricated by soft-landing atomic Pd ions on oxygen vacancies (F-centers) of a MgO(100)surface(Pd 1 /MgO(F s )).Weusedthermaldesorptionandinfraredspectroscopies(TDS,FTIR)tostudythe acetylene polymerization and the CO oxidation catalyzed by this system. In one-heating-cycle experiments, only the formationofbenzeneisobservedduringthepolymerizationreactionandthecombustionofCOleadstotheformation of CO 2 detected at 260 and 500 K. Experimental results in combination with ab initio calculations reveal the mech- anisms of these reactions and demonstrate the role of surface defects in nanocatalysis. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Thermal desorption spectroscopy; Catalysis; Surface chemical reaction; Palladium; Magnesium oxides; Alkynes; Carbon monoxide 1. Introduction Molecular-scaleunderstandingoftheenergetics and mechanisms of catalytic reactions could open newavenuestothedesignofcatalystswithspecific functions [1,2]. In this respect nanocatalysts con- sisting of small size-selected clusters on uniform adsorptionsitesmaybeofspecialinterestassmall clusters reveal distinct quantum size effects mani- fested, e.g., in the strong size-dependent chemical reactivity of gas-phase clusters [3–7]. In addition, these small clusters reveal a distinct interaction withthesubstrateandarehighlyfluxional.Recent experimental and theoretical joint studies revealed e.g. the reaction mechanism of the oxidation of CO on Au 8 clusters [8] and Pd atoms [9] and the polymerization of acetylene on Pd n clusters [10]. Inthispaperwefocusonthechemicalreactivity ofpalladiumatomsadsorbedonoxygenvacancies of a MgO(100) thin film and we summarize and compare the reaction mechanisms of the acetylene Surface Science 514 (2002) 249–255 www.elsevier.com/locate/susc * Correspondingauthor.Tel.:+41-21-692-36-53/87;fax:+41- 21-692-36-35. E-mail address: stephane.abbet@ipmc.unil.ch (S. Abbet). 0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0039-6028(02)01637-0