C. R. Acad. Sci. Paris, Se ´rie IIc, Chimie / Chemistry 3 (2000) 163–174 © 2000 Acade ´mie des sciences / E ´ ditions scientifiques et me ´dicales Elsevier SAS. Tous droits re ´serve ´ 1387 1609(00) 00138 9/FLA Surface chemistry and catalysis / Chimie des surfaces et catalyse Catalytic chemistry of transition metal ions on oxide surfaces. A molecular approach using EPR techniques Zbigniew Sojka a,b , Michel Che b,c, * a Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Cracow, Poland b Laboratoire de re ´activite ´ de surface, UMR 7609–CNRS, universite ´ Pierre-et-Marie-Curie, 4, place Jussieu, 75252 Paris cedex 05, France c Institut universitaire de France, Paris, France Received 18 December 1999, accepted 10 May 2000 Communicated by Franc ¸ois Mathey Abstract – The applications of EPR spectroscopy to the study of catalysis and surface chemistry of oxides are reviewed. The specific features of the EPR spectra of catalytic materials are described and the means employed to enhance the amount of information that can be extracted from the spectra are discussed. Selected recent examples illustrate the possibilities offered by EPR techniques to the understanding of catalyst preparation, of the nature of active sites and reaction intermediates as well as of catalytic reaction mechanisms. One of the advantages of EPR is the possibility to investigate electron transfer phenomena (metal to ligand, ligand to metal or ligand to ligand) occurring between the surface transition metal ions and the adsorbed molecules. © 2000 Acade ´mie des sciences / E ´ ditions scientifiques et me ´dicales Elsevier SAS EPR / catalysis / oxides / adsorption / reaction mechanisms / surface / electron transfer Re ´sume ´– La pre ´sente revue concerne les applications de la RPE a ` l’e ´tude des phe ´nome ´nes de catalyse et de chimie de surface des oxydes. Elle de ´crit les parame `tres spe ´cifiques des spectres RPE des mate ´riaux catalytiques et les moyens qui sont employe ´s pour augmenter la quantite ´ d’information qui peut e ˆtre tire ´e des spectre RPE. Quelques examples typiques illustrent les possibilite ´s apporte ´es par les techniques RPE pour l’e ´tude de la pre ´paration des catalyseurs, de la nature des sites actifs, des interme ´diaires et des me ´canismes re ´actionnels. L’un des avantages de la RPE est la possibilite ´ qu’elle offre d’e ´tudier les phe ´nome `nes de transfert d’e ´lectron (me ´tal–ligande, ligande–me ´tal ou ligande–ligande) entre les ions de transition superfi- ciels et les mole ´cules adsorbe ´es. © 2000 Acade ´mie des sciences / E ´ ditions scientifiques et me ´dicales Elsevier SAS RPE / catalyse / oxydes / adsorption / me ´canisme / re ´action / surface / transfert d’e ´lectron 1. Introduction A catalyst is the substance that transforms reac- tants into products through a perpetual cycle of elementary steps in which the catalyst participates being, however, returned into its original state at the end of each cycle [1]. A catalytic reaction begins with the adsorption of the reactants on surface active sites during which intra-molecular bonds are weakened or broken. The adsorbed intermediate species react on the surface often following a complex multi-step mechanism. Eventually the products formed desorb from the surface, making the active sites available for another cycle. The principal function of the catalyst * Correspondence and reprints: che@ccr.jussieu.fr 163 MISE AU POINT / ACCOUNT