Low-Temperature Reducibility of MxCe1-xO2 (M = Zr, Hf) under Hydrogen Atmosphere

Redox cycles utilize the reversible oxygen release/uptake of cerium oxide in a variety of renewable energy applications such as fuel cells, water gas shift reactions, and solar thermochemical fuel production. For all applications the degree of reduction/oxidation determines the overall performance. In this study we report on the redox behavior of MxCe1xO2-(delta) (M = Zr, Hf; x = 0, 0.15, 0.2) solid solutions monitored by high-temperature in situ X-ray diffraction. During reduction in H2 at 600 degrees C and the successive formation of Ce3+ and oxygen vacancies, the lattice of ceria expands up to 0.3%. The lattice expansion of hafnium-doped ceria samples is 4 times larger than in zirconium-doped or undoped ceria, indicating drastically higher extents of oxygen vacancy formation. The same trends are validated using temperature-programmed reduction measurements. Complete reoxidation of the MxCe1xO2-delta solid solutions in air at 600 degrees C is reflecting the reversibility of the redox process. Scanning electron microscopy and X-ray diffraction analysis before and after redox cycling indicate phase and microstructural stability of all compositions during reduction.