Thermodynamic assessment of Gd-doped CeO2 for microwave-assisted thermochemical reduction†

Abstract

Microwave-assisted hydrogen production is a promising technology with the capability to decompose H₂O into H₂ economically. The potential of this technology depends on the parameter f_r that measures the fraction of microwave energy directly contributing to the reduction reaction of metal oxides by extracting the lattice oxygen. We quantitatively examine f_r for Gd-doped ceria (CeO₂), a well-known benchmark material, using the Van't Hoff method. Our study reveals that approximately more than 1/2 of the reduction enthalpy is attributed to microwave energy, suggesting that f_r exceeds 0.5. Simultaneously, we introduce a defect equilibria model to identify the contribution of f_r and derive equilibrium constants for isolated defects and associated defects at T = 450–600 °C and P(O₂) = 2 × 10⁻⁴ to 2.1 × 10⁻¹ atm. The results advocate that microwave energy significantly contributes to defect formation under alleviated conditions (lower T and higher P(O₂)) with a shorter timescale compared to conventional thermal reduction. Our study reaffirms the importance of f_r in microwave-assisted reduction and provides a new thermodynamic insight into the interaction between defects and microwave fields in doped ceria.