Thermochemical CO2 splitting reaction with CexM1−xO2−δ (M = Ti4+, Sn4+, Hf4+, Zr4+, La3+, Y3+ and Sm3+) solid solutions

• Doping of tetravalent cations into CeO2 improves the O2 evolution activity.
• Doping of tetravalent cations into CeO2 improves the CO production.
• The estimated activation energies for the reduction step was calculated.
• The kinetic models for CO generation reaction was given.
• One step thermochemical CO2 splitting reaction was discovered.

This study deals with doping of CeO2 with different cations M (M = Ti4+, Sn4+, Hf4+, Zr4+, La3+, Y3+ and Sm3+) to improve the reaction activity in two step thermochemical CO2 splitting reaction. The results show that the addition of tetravalent cations M (M = Ti4+, Sn4+, Hf4+ and Zr4+) into CeO2 significantly enhances the O2 evolution activity. The O2 production at 1400 °C increases as follows: CeO2 (2.5 ml/g) < Ce0.75Zr0.25O2 (6.5 ml/g) < Ce0.75Hf0.25O2 (7.2 ml/g) < Ce0.8Sn0.2O2 (11.2 ml/g) < Ce0.8Ti0.2O2 (13.2 ml/g). The corresponding CO production is increased from 4.5 ml/g for CeO2 up to 10.6 ml/g for Ce0.75Zr0.25O2 synthesized by solution combustion method. For Ce0.75Hf0.25O2 and Ce0.75Zr0.25O2 synthesized by hydrothermal method, steady-state CO and O2 production with a ratio of near 2 occurs simultaneously when the temperature exceeds 1100 °C and we define the phenomena as “one step thermochemical CO2 splitting reaction”. Further investigations for one step thermochemical CO2 splitting with both CeO2 and Ce0.75Zr0.25O2 as catalysts and blank reaction tube as reference from 1200 °C to 1400 °C are also performed. 16.8 ml/h of CO was produced for 0.5 g of Ce0.75Zr0.25O2 at 1400 °C which indicates that one step reaction may be a promising way for CO2 reduction. For Ce0.8Ti0.2O2 and Ce0.8Sn0.2O2, although the O2 production is increased several times as compared to CeO2, the CO generation activity is still low due to the formation of Ce2Ti2O7 and Ce2Sn2O7 after the high temperature reduction reaction. The doping of trivalent cations M (M = La3+, Y3+ and Sm3+) into ceria has negative effect both on the O2 evolution activity and the CO production. The estimated activation energy for the reduction step of Ce0.75Zr0.25O2 is much lower than that of CeO2 and Ce0.85La0.15O2−δ. The CO generation reaction of CeO2, Ce0.85La0.15O2−δ and Ce0.75Zr0.25O2–C (synthesized by solid solution combustion method) is a surface limited reaction.