Surface controlled reduction kinetics of nominally undoped polycrystalline CeO2

Area of Science:

• Materials science
• Chemical engineering
• Renewable energy

Background:

• Ceria is crucial for high-temperature redox applications, such as solar-thermal splitting of CO2 and H2O.
• Accurate data on oxygen surface exchange and diffusivity are vital for reactor design in solar-thermal fuel generation.

Purpose of the Study:

• To investigate the reduction reaction kinetics of pure ceria under solar-thermal conditions.
• To determine the chemical surface exchange coefficient and chemical diffusivity of oxygen in ceria.

Main Methods:

• Thermogravimetric relaxation experiments.
• Equilibrium oxygen isotope exchange experiments.
• Depth profiling analysis.

Main Results:

• The reduction of pure ceria is surface reaction controlled, even for dense samples.
• The chemical surface exchange coefficient shows a negative apparent activation energy (-64 kJ mol⁻¹).
• This behavior is consistent with literature data for the tracer surface exchange coefficient.

Conclusions:

• The reduction of ceria in solar-thermal applications is primarily limited by surface reactions.
• The negative apparent activation energy suggests unique surface kinetics for ceria.
• Doping ceria with lower valence cations may lead to a weak temperature dependence of the chemical surface exchange coefficient.