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

This contribution addresses some of the molecular-level aspects of classica l supported metal catalysts preparation procedures. While the problems sele cted here have been chosen for their fundamental and practical relevance, i mportant 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 act ivity in the field. We first briefly discuss the state of current knowledge on the surface spec ies exposed at oxide surfaces, based on the particular case of gamma -alumi na. While considerable uncertainty remains as to the precise interpretation s of spectroscopic data (especially in the presence of an aqueous phase), a concerted research effort might allow a molecular-level mapping of adsorpt ion sites as a function of operating conditions. We then present the main mechanisms that may be operative in the establishm ent of metal-support interaction at the oxide-water interface, again with e mphasis on the molecular interpretations. The distinction between strong an d weak interactions is found not to be very helpful, while distinctions bet ween 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 impor tant 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 int eraction (established during the deposition step) has a demonstrated, lasti ng influence on all further steps of catalyst synthesis, suggesting the pra ctical interest of these studies for fine-tuning of catalysts properties. Altogether, it appears that the molecular-level characterization of support ed metal catalysts preparation is a realistic research program in the middl e term. (C) 2000 Elsevier Science B.V. All rights reserved.