Redox kinetics of Zr+4 and Sm+3 doped ceria used for thermochemical hydrogen production

Abstract

The kinetic of Zr+4 and Sm+3 doped CeO2, synthesized via citric acid combustion and Pechini method, have been investigated in view of their applicability as redox material in solar thermochemical cycles. Characterization by XRD indicates that single phase solid solutions with fluorite structure were formed. The microstructure and the molar ratio were analyzed by SEM and EDS. Thermo gravimetric measurements (TG) were performed on dense and porous samples between 1543 and 1753K at various oxygen partial pressures. The TG data suggest that doping with Zr+4 enhances the reduction of CeO2. A high porosity lowers the characteristic time constant of the mass change during redox reaction. For the water splitting reaction the porosity of the sample and O2 impurities of carrier gas and steam are found to be crucial. Moreover the hydrogen production rate of dense pellets is significantly lower as compared to powder samples. The XRD investigations indicate a structural evolution of CeO2-δ at high reduction state leading to the formation of a second phase that might hinder the following oxidation. Sm+3 doping prevents this evolution and hence the re-oxidation is facilitated. Furthermore a calculation of kinetic parameters based on the TG data, SIMS profile and dilatometry suggest that the reduction as well as oxidation is controlled by surface reactions.