Impact of lattice properties on the CO2 splitting kinetics of lanthanide-doped cerium oxides for chemical looping syngas production

Cerium oxide (CeO2) is the state-of-the-art material for syngas production from CO2 and H2O splitting via (methane assisted-) solar-driven thermochemical cycles. This technology consists of two separate processes: (1) a partial reduction of the oxide with methane at -900 degrees C that produces syngas and generates oxygen vacancies in the oxide lattice; and (2) oxidation of the reduced oxide by reaction with H2O and/or CO2 to form H2 and/or CO, respectively, which incorporates oxygen into the oxygen vacancy lattice sites. Doping the cerium oxide with other cations enables to increase the oxygen-vacancy concentration or to induce changes in the crystal lattice, aiming at increasing the fuel yield and/or boosting the redox oxygen-exchange kinetics. Traditionally, CeO2 has been doped with lanthanides to improve the redox behavior and enhance ionic conduction. For lanthanide-doped cerium oxides, a correlation between the dopant's ionic radius and the material's ionic conductivity has been established. Here, we examine the impact of the trivalent lanthanide dopants on the methane partial oxidation and CO2 splitting. Our results reveal a correlation between the dopant ionic radius, ionic conductivity, oxygen exchange kinetics, and CO2 splitting kinetics. We could identify the optimum ionic radius for enhanced CO2 splitting kinetics based on our observations. These findings are an essential framework for designing more advanced redox materials for chemical looping syngas production.