The molecular approach to supported catalysts synthesis: state of the art and future challenges

This contribution addresses some of the molecular-level aspects of classical supported metal catalysts preparation procedures. While the problems selected here have been chosen for their fundamental and practical relevance, important studies due to Knozinger and coworkers will be quoted in each case, and this short review may thus be regarded as a homage to their early activity in the field. We first briefly discuss the state of current knowledge on the surface species exposed at oxide surfaces, based on the particular case of gamma -alumina. While considerable uncertainty remains as to the precise interpretations of spectroscopic data (especially in the presence of an aqueous phase), a concerted research effort might allow a molecular-level mapping of adsorption sites as a function of operating conditions. We then present the main mechanisms that may be operative in the establishment of metal-support interaction at the oxide-water interface, again with emphasis on the molecular interpretations. The distinction between strong and weak interactions is found not to be very helpful, while distinctions between selective and non-selective, or reversible/irreversible interactions, are more justified at the molecular level. Although our knowledge of these interactions is still lacunary, recent developments raise the hope of important progress in the near future. Molecular characterization during the later steps of catalyst preparation (thermal activation, calcination, reduction) is much less advanced. However, examples are presented in which the mechanism of initial metal-support interaction (established during the deposition step) has a demonstrated, lasting influence on all further steps of catalyst synthesis, suggesting the practical interest of these studies for fine-tuning of catalysts properties. Altogether, it appears that the molecular-level characterization of supported metal catalysts preparation is a realistic research program in the middle term. (C) 2000 Elsevier Science B.V. All rights reserved.