(257b) Understanding the Oxidation Kinetics Mechanism of Ceria Based Materials in Solar Thermochemical Gas Splitting

Ceria is commonly doped with transition metal and rare earth, particularly Zr, Hf, Pr and Gd, to increase the product yield and or stabilize gain sizes during solar thermochemical water and carbon dioxide splitting. In these cycles the neat or doped ceria is heated via concentrated sunlight to over 1400°C, causing the material to reduce and release O₂; subsequently the material is exposed to steam or carbon dioxide to re-oxidize the ceria and generate the desirable H₂ or CO. The use of dopants including Hf and Zr have proven to be useful in increasing the overall cycle efficiency while Pr and Gd help to stabilize the phases. Here we use density functional theory, and kinetic Monte Carlo simulations to explore the effects of these dopants on the kinetics of re-oxidation. We find that the reactions are not diffusion limited but rather surface limited, being either sub-surface/surface O migration, or H2 formation limited, depending on the dopant. These results correspond with existing experimental results and provide an explanation for the surface limited reaction.