SPECTROSCOPIC STUDY OF ACTIVE PHASE-SUPPORT INTERACTIONS ON A RHOX CEO2 CATALYST - EVIDENCE FOR ELECTRONIC INTERACTIONS/

The effects of thermal treatments under vacuum, used as a way to gener ate reduced centers on Rh2O3 and RhOx/CeO2, have been studied by ESR a nd FTIR, using respectively oxygen and carbon monoxide as probe molecu les. The results obtained for the outgassed samples reveal the presenc e of ceria-rhodia interactions favoring the stabilization of paramagne tic Rh2+ cations in rhodium oxide clusters on the ceria surface. Subse quent O-2 adsorption leads to the formation of different oxygen-relate d paramagnetic species located on ceria, on rhodium oxide clusters and at the boundary between both oxides; their contribution to the spectr a depends on outgassing conditions and O-2 adsorption temperature, The unexpected absence of O-2(-)-Ce4+ species after O-2 contact at 77 K w ith RhOx/CeO2 outgassed above 573 K evidences the existence of electro nic interactions between the RhOx and CeO2 phases, being explained on the basis of electron transfer to the mixed valence RhOx phase from th e surface-reduced ceria, leading to electron depletion of the latter. This effect is inhibited by CO adsorption, showing the dependence betw een the electron-accepting properties of the rhodia clusters and the p resence of vacant coordination sites at the surface Rh ions, An effect of similar kind may be responsible for shifts observed in the IR band s of rhodium dicarbonyls formed in the RhOx/CeO2 system. The latter re sults suggest the possibility that thermal enhancement of surface reac tions in complex systems could depend on electron transfer between adj acent phases and that adsorption on one phase may influence the surfac e reactivity of another phase by affecting to the electron transfer be tween them. (C) 1997 Academic Press.