Characterization of the Metal Phase in NM/Ce0.68Zr0.32O₂ (NM:  Pt and Pd) Catalysts by Hydrogen Chemisorption and HRTEM Microscopy:  A Comparative Study

Pt and Pd particles supported on a texturally stable Ce 0.68 Zr 0.32 O 2 mixed oxide were investigated by means of H 2 chemisorption and high-resolution transmission electron microscopy (HRTEM). The comparison of the metal dispersion data as determined by HRTEM with those estimated from H 2 volumetric adsorption reveals that, in the case of Pt catalysts reduced at temperatures below 350 °C, the chemisorption isotherms recorded at −80 °C provide reliable dispersion data. For catalysts reduced at, or above, 350 °C, some platinum deactivation occurred. However, H 2 chemisorption capability was recovered by reoxidation at 427 °C and further reduction at 150 °C. For catalysts reduced at 700 °C or 900 °C, the recovery of the platinum chemisorptive capability was only partial, even if a more severe reoxidation treatment (700 °C) was applied. To prevent hydride formation, H 2 chemisorption on the Pd/Ce 0.68 Zr 0.32 O 2 was investigated at low H 2 pressures and at room temperature. Under these experimental conditions, which are often employed for characterizing Pd catalysts, hydrogen spillover cannot be blocked, thus preventing a reliable estimate of the metal dispersion in our ceria−zirconia -supported palladium catalysts.