Hydrogen scrambling over Rh/Ce0.68Zr0.32O2 and Rh/Al2O3catalysts: Effects of support, metal precursor and redox aging

Abstract

The effects of noble metal precursor, nature of the support and pre-treatment on hydrogen isotopic exchange were investigated over Rh/Al₂O₃ and Rh/Ce_0.68Zr_0.32O₂ catalysts. It is observed that deuterium exchange occurs rather efficiently over both Rh-loaded Al₂O₃ and Ce_0.68Zr_0.32O₂, mainly at sub-ambient temperatures in the case of Rh/Ce_0.68Zr_0.32O₂, provided that no high temperature oxidation is employed as a pre-treatment. In fact, after this pre-treatment, H₂/D₂ scrambling is strongly hindered, the exchange being completed at temperatures which are higher by about 140–200 °C. Interestingly, the unfavorable effect of high temperature oxidation appears related to kinetic aspects of the H₂/D₂ reaction since, irrespective of the pre-treatment, comparable amounts of H₂ spilt over the support were detected by static H₂ chemisorption. Using a chloride-containing Rh precursor, H₂/D₂ is partially blocked compared to using a nitrate-containing Rh precursor, suggesting effective blockage of hydrogen migration over the support by some retained Cl-species.