Characterisation of the Metal Phase in NM/Ce_0.68Zr_0.32O₂ (NM = Pt and Pd) Catalysts by Hydrogen Chemisorption and HRTEM Microscopy: A Comparative Study

Pt and Pd particles supported on a texturally stable Ce0.68Zr0.32O2 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 degreesC, the chemisorption isotherms recorded at -80 degreesC provide reliable dispersion data. For catalysts reduced at, or above, 350 degreesC, some platinum deactivation occurred. However, H-2 chemisorption capability was recovered by reoxidation at 427 degreesC and further reduction at 150 degreesC. For catalysts reduced at 700 degreesC or 900 degreesC, the recovery of the platinum chemisorptive capability was only partial, even if a more severe reoxidation treatment (700 degreesC) was applied. To prevent hydride formation, H-2 chemisorption on the Pd/Ce0.68Zr0.32O2 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.