In-situ XAFS Analysis of Y Zeolite-Supported Rh Catalysts during High-Pressure Hydrogenation of CO2

Abstract

In-situ XAFS (X-ray absorption fine structure) analysis under high-pressure, high-temperature reaction conditions was done to clarify the effect of Li doping on the structures of Rh species in Rh ion-exchanged NaY zeolite catalysts (RhY). The analysis showed that Rh species in RhY were reduced at a lower temperature than those in Li-doped RhY (Li/RhY). The reduction of Rh in RhY at a lower temperature was related to the formation of amorphous Rh oxide species during the initial stage of reduction, which was not observed in Li/RhY catalysts. The average particle size of metallic Rh species formed under the reaction conditions was 1.3 nm for RhY catalysts and 0.8 nm for Li/RhY catalysts. During exposure to either CO or air, the metallic Rh species in RhY were more easily subject to structural changes than the metallic Rh species in Li/RhY. These differences in the properties of Rh species between the RhY and Li/RhY catalysts resulted from the replacement of hydroxyl groups on the zeolite surface by O–Li compounds, and caused different catalytic activities during CO2 hydrogenation.