NANOPHASE FLUORITE STRUCTURED CEO2-ZRO2 CATALYSTS PREPARED BY HIGH-ENERGY MECHANICAL MILLING. ANALYSIS OF LOW TEMPERATURE REDOX ACTIVITY AND OXYGEN STORAGE CAPACITY

The utilization of mechanical milling for the preparation of catalysts based on ceria structurally modified with zirconia is presented. It is shown that room-temperature high-energy ball milling is an effective tool for the synthesis of nanophase CeO2-ZrO2 solid solution in a wide composition range. The use of combined X-ray diffraction analysis, Raman spectroscopy and electron microscopy indicate that the milling process induces the formation of true solid solutions with a contraction of the cell parameter for cubic ceria following the introduction of Zr into the lattice. This, in turn, remarkably enhances the oxygen storage/transport and redox capacity compared to pure ceria and zirconia or to a simple mixture thereof. An unusual resistance to high-temperature cycling was also evidenced. These features were analyzed by the study of the reduction profile of doped ceria carried out by temperature-programmed reduction at different milling times. The oxygen storage capacity (OSC) of the catalysts was also evaluated; both the total and the kinetic accessible OSC indicated that the best composition is CexZr1-xO2 with x > 0.5. This was correlated to the structural features and to the presence of a high concentration of ions with redox character (i.e., Ce4+ ions) which favor oxygen mobility.