METAL-SUPPORT BOUNDARY SITES IN CATALYSIS

In highly-dispersed metal catalysts, the supporting oxide stabilises t he metal particles, but may also promote individual reaction steps. In particular, many reactions are believed to proceed, preferentially or exclusively, at the metal-oxide interface which contains sites with d istances between metal atoms and support in the range of atomic dimens ions. In this paper, the physicochemical characterisation of these so- called adlineation sites and their possible catalytic effects are revi ewed. It will be shown in various examples that they may have a partic ular activity and can be poisoned selectively. They may contribute to several particle size effects observed in hydrocarbon reactions, e.g. to ring opening and isomerization, but they may also be responsible fo r >C=O activation on noble metals supported by reducible oxides. Furth ermore, they may play a role in selectively adsorbing chlorine which i s an important component in catalyst restructuring. It will be attempt ed to detect and to relate common features of the observed phase bound ary effects and to seek for a common explanation. The paper is, howeve r, not intended as a review of strong metal-support interaction phenom ena. The ring opening of methylcyclopentane on platinum is particularl y sensitive to phase-boundary effects. Starting out from the authors' results it will be shown how this reaction is influenced by modificati ons of the support, by changes of hydrogen pressure and reduction stat e, and by selectively poisoning the interface. Further on, the CO hydr ogenation on supported Ph and the CH4-CO2 reforming reaction on Pt/ZrO 2 serve as examples for >C=O activation, and the influence of Lewis ac id sites at the boundary on the catalytic activity is discussed in mor e detail. The last chapter is dedicated to the role of adsorbed chlori ne and to the partly controversial results concerning the effect of ha logen additives on the properties of supported catalysts, including th e possible existence of 'soluble platinum'. (C) 1997 Elsevier Science B.V.