Characterization of Rh particles and Li-promoted Rh particles in Y zeolite during CO2 hydrogenation -: A new mechanism for catalysis controlled by the dynamic structure of Rh particles and the Li additive effect

The structure transformations of Rh particles in Rh ion-exchanged zeolite catalysts (RhY) and in Li-doped RhY catalysts (Li/RhY) were studied by means of EXAFS (extended X-ray absorption fine structure) in order to reveal the origin of the different CO2 hydrogenation catalyses of the two catalysts. CH4 was mainly produced from CO2 + H-2 on RhY, while CO was found on Li-doped RhY under similar reaction conditions. The geometric and electronic structure of Rh species in Y zeolite had little difference between the two catalysts before the reaction. However, EXAFS revealed that the structures of Rh species under the reaction conditions were quite different. During the reaction, we found disassembly of Rh particles in RhY, while Rh particles were stably present in Li/RhY. We discuss the different reaction mechanisms and catalytic properties of RhY and Li-doped RhY catalysts on the basis of their different dynamic structures which might be controlled by Li additive. (C) 2000 Academic Press.