MICROSTRUCTURE OF PD CEO2 CATALYST - EFFECT OF HIGH-TEMPERATURE REDUCTION IN HYDROGEN/

The effect of high temperature reduction (HTR) in hydrogen (up to 1180 K) on the microstructure of 9 wt-% Pd/CeO2 catalyst was studied by HR TEM and XRD methods. Reduction of the catalyst at or above 973 K cause d severe recrystallization of CeO2 and Pd with simultaneous strong int eraction between the two components appearing as three phenomena: epit axial growth of small Pd particles on CeO2 (most frequently with [111] Pd\[111]CeO2); decoration of large Pd particles with ordered CeO2 over layer and expansion of the lattice parameter of Pd (by 2.1%). The orig in of the Pd lattice expansion is discussed and diffusion of Ce specie s into the Pd lattice seems to be the most probable one. HTR caused al so phase transformations in the ceria support. At 973 K and 1100 K, wh ole CeO2 was transformed into oxygen deficient CeOx phase exhibiting t he same or similar structure but with expanded lattice parameter (by 2 .8%). At 1180 K most ceria was transformed into hexagonal A-Ce2O3. The CeOx phase appeared to be stable in hydrogen and in vacuum at room te mperature, but upon exposure to air at room temperature it rapidly reo xidised to CeO2. Ce2O3 also reoxidised to CeO2 but much slower. Anothe r consequence of HTR at or above 773 K was formation of pits in CeO2 c rystallites, mainly on (112)-type crystal faces. The pits (1-10 nm) ex hibited well defined walls parallel to CeO2 lattice fringes and they c ould possibly constitute nucleation sites for strongly bonded, epitaxi al oriented Pd particles.