Les petites particules métalliques supportées
Small Supported Metallic Particles
Institut Français du Pétrole
Une étude critique des méthodes de préparation de catalyseurs monométalliques permet de voir que les techniques impliquant un échange (ionique ou chimique) conduisent à des catalyseurs à forte dispersion avec des moyens relativement simples. Au niveau de la caractérisation des catalyseurs, il ne semble pas exister de méthodes exhaustives permettant d'étudier les phases métalliques contenant des cristallites de taille inférieure à 1. 0 nm. Les techniques de microscopie électronique à haute résolution, la spectroscopie XPS et l'EXAFS semblent par contre les plus prometteuses. L'influence du support joue à plusieurs niveaux, en particulier sur la morphologie, la structure et sur les propriétés électroniques de la phase métallique. L'intérêt particulier d'étudier les effets de support résulte dans la création de phase métallique plus active et sélective.
The current aim of catalyst research is to develop more and more active and selective dispersed metallic phases for a given reaction as well as to improve their stability. Catalytic properties depend directly on the physicochemical characteristics of the metallic phase (size of crystallites, composition, electron structure, interactions with the support). This is why improving such catalysts requires mastering synthesis routes and the characterizing of small supported metallic particles. The aim of this critical study is to highlight both the preparation methods leading to well-controlled monometallic phases and the physical techniques suited for characterizing them. By considering the metal-support entity as a veritable catalytic space, we have examined the direct influence of the nature of the support on the end characteristics of the metallic particles (morphology, structure, electron properties). We have found that impregnation techniques with an interaction (physical or chemical) can lead to high-dispersion metallic phases associated with a narrow dispersion of crystallite size. Among other techniques used, note will be taken of ion implantation, vapodeposition and radiolytic synthesis. Among overall and controlled-atmosphere characterization techniques, XPS and EXAFS spectroscopy were used to characterize, respectively, the electron properties and the crystalline structure (chemical environment) of metallic particles. Small metallic particles (less than 1. 0 nm in size) are more difficult to characterize because of the limitations of the physical techniques normally used. High-resolution electron microscopy was used for the local characterization of these small particles. The present results mainly concern the size of these small aggregates, but this technique also gives information on their morphology (aggregates of atoms, individual atoms in the aggregate) and on their crystalline structure (microdiffraction). Likewise, there is an entire domain of indirectcharacterization that is widely used, in which especially the accessibility and degree of oxidation reduction are measured of the metals by means of reagents or probe molecules (H2, O2, CO). However, this very precious information in the field of catalysis must always be compared with the result of direct characterization (size, electron properties). Concerning the influence of the support, this review shows that the metal-support pair is a full-fledged entity and that it is almost impossible to characterize it from the individual properties of the metal and of the support. At present, the principal results have to do with semiempirical classifications concerning the stability of metallic phases during heat treatments and with the influence of the metal-support interaction force on the morphology and electron properties of particles (load transfer, modifications of the electron structure of the transition metal).
© IFP, 1991