Hydrogen scrambling over Rh/Ce0.68Zr0.32O2 and Rh/Al2O3 catalysts: Effects of support, metal precursor and redox aging

The effects of noble metal precursor, nature of the support and pre-treatment on hydrogen isotopic exchange were investigated over Rh/Al2O3 and Rh/Ce0.68Zr0.32O2 catalysts. It is observed that deuterium exchange occurs rather efficiently over both Rh-loaded Al2O3 and Ce0.68Zr0.32O2, mainly at sub-ambient temperatures in the case of Rh/Ce0.68Zr0.32O2, provided that no high temperature oxidation is employed as a pre-treatment. In fact, after this pre-treatment, H-2/D-2 scrambling is strongly hindered, the exchange being completed at temperatures which are higher by about 140-200degreesC. Interestingly, the unfavorable effect of high temperature oxidation appears related to kinetic aspects of the H-2/D-2 reaction since, irrespective of the pre-treatment, comparable amounts of H-2 spilt over the support were detected by static H-2 chemisorption. Using a chloride-containing Rh precursor, H-2/D-2 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.