Improvement in the Reduction Behavior of Novel ZrO2-CeO2 Solid Solutions with a Tubular Nanostructure by Incorporation of Pd

In this work, 1 wt % Pd/ZrO2-CeO2 mixed oxide nanotubes with 90 mol % CeO2 were synthesized following a very simple, high-yield procedure and their properties were characterized by synchrotron radiation X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES), and scanning and high-resolution transmission electron microscopy (SEM and HRTEM). In situ XANES experiments were carried out under reducing conditions to investigate the reduction behavior of these novel nanotube materials. The Pd/CeO2-based nanotubes exhibited the cubic phase (Fm3m space group). The nanotube walls were composed of nanoparticles with an average crystallite size of about 7 nm, and the nanotubes exhibited a large specific surface area (85 m(2).g(-1)). SEM and HRTEM studies showed that individual nanotubes were composed of a curved sheet of these nanoparticles. Elemental analysis showed that the Ce:Zr:Pd ratios appeared to be approximately constant across space, suggesting compositional homogeneity in the samples. XANES results indicated that the extent of reduction of these materials is low and that the Ce4+ state is in the majority over the reduced Ce3+ state. The results suggest that Pd cations-most likely Pd2+-form a Pd-Ce-Zr oxide solid solution and that the Pd2+ is stabilized against reduction in this phase. However, incorporation of the Pd (1 wt %) into the crystal lattice of the nanotubes also appeared to destabilize Ce4+ against reduction to Ce3+ and caused a significant increase in its reducibility.