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

Abstract

Nanocrystalline (4–8nm) 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 (111)∥CeO2 (111) 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 (<1nm) 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 (∼2nm) formed during reduction at higher temperatures.