Structure evolution of nanocrystalline Ce1−xPdxO2−y mixed oxide in oxidizing and reducing atmosphere: Reduction-induced activity in low-temperature CO oxidation

Nanocrystalline (4–8 nm) Ce1−xPdxO2−y mixed oxide (0 < x < 0.3), active in low temperature CO oxidation, has been prepared by microemulsion method. Thorough XRD, TEM, SEM-EDS, BET and FT Raman studies revealed that the oxide with x < 0.2 is a homogeneous solid solution structurally stable up to 800 °C in oxidizing atmosphere. In hydrogen, already at 500 °C segregation of Pd particles occurs, which exhibit preferential Pd (1 1 1)∥CeO2 (1 1 1) orientation, preserved even after reduction at 800 °C. Ce0.89Pd0.11O2−y oxide showed reversible extraction–dissolution of Pd upon successive reduction–oxidation cycles at 500 °C, which is an example of “self-regenerative” property important for potential catalytic applications. Doping with Pd strongly hinders the sintering of ceria at high temperatures and enhances its reducibility at low temperatures (below 500 °C).“As prepared”, oxidized Ce0.89Pd0.11O2−y sample demonstrates moderate activity in CO oxidation (reaching 85% conversion at ∼250 °C) similar to that of 3% Pd/CeO2 prepared by impregnation. The activity improves dramatically (measurable CO conversion below room temperature and 100% conversion at ∼120 °C) after pre-reduction at 400 °C in H2. It appears that partially reduced Pd species, or extremely small particles (<1 nm) at the surface of ceria are responsible for the low temperature activity in CO oxidation. Such Pd species could be strongly bonded to the surface, e.g., exhibiting a special epitaxial orientation observed for larger Pd crystallites (∼2 nm) formed during reduction at higher temperatures.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Thermally stable, nanocrystalline Ce1−xPdxO2−y oxide with x up to 0.2 was obtained by microemulsion method. ► Reversible extraction–dissolution of Pd occurs during successive reduction–oxidation cycles at 500 °C. ► Reduction treatment in H2 at 400 °C enhances activity of Ce1−xPdxO2−y in low-temperature CO oxidation.