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 behavior influences catalytic performances and should thus be taken carefully 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 surfaces under oxidation conditions dramatically dictate performances. It is concluded that the crucial key to maximize performances is to stabilize catalytic surfaces in slightly reduced suboxidic states. Such states allow catalytic oxidation cycles to proceed with high frequencies and favor reconstruction 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 in the feed, adjustments of the stoichiometries and uses of the remote control 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 phenomena at the surface of oxides are discussed. (C) 2000 Elsevier Science B.V. All rights reserved