Dynamic phenomena and catalytic reactivities of oxide surfaces

Oxide catalysts undergo numerous dynamic phenomena at their surfaces under the conditions used for selective oxidation of hydrocarbons. This dynamic b ehavior influences catalytic performances and should thus be taken carefull y into account in the understanding of the exact state of catalysts during the reaction and of phenomena like deactivations, synergetic effects, etc. This contribution reports examples in which dynamic behaviors of oxide surf aces under oxidation conditions dramatically dictate performances. It is co ncluded that the crucial key to maximize performances is to stabilize catal ytic surfaces in slightly reduced suboxidic states. Such states allow catal ytic oxidation cycles to proceed with high frequencies and favor reconstruc tion of catalysts and creation of active sites. Contrary, dynamic behaviors leading catalysts to more reduced or more oxidized states result in lower performances. Next to the tuning of O-2 and hydrocarbon partial pressures i n the feed, adjustments of the stoichiometries and uses of the remote contr ol mechanism and spillover oxygen appear as efficient approaches to master the dynamism of catalysts 'at work' and thus to optimize their performances . Finally, implications and perspectives of the understanding of dynamic ph enomena at the surface of oxides are discussed. (C) 2000 Elsevier Science B .V. All rights reserved.