Enhanced intermediate-temperature CO2 splitting using nonstoichiometric ceria and ceria–zirconia

Abstract

CO₂ splitting via thermo-chemical or reactive redox has emerged as a novel and promising carbon-neutral energy solution. Its performance depends critically on the properties of the oxygen carriers (OC). Ceria is recognized as one of the most promising OC candidates, because of its fast chemistry, high ionic diffusivity, and large oxygen storage capacity. The fundamental surface ion-incorporation pathways, along with the role of surface defects and the adsorbates remain largely unknown. This study presents a detailed kinetics study of CO₂ splitting using CeO₂ and Ce_0.5Zr_0.5O₂ (CZO) in the temperature range 600–900 °C. Given our interest in fuel-assisted reduction, we limit our study to relatively lower temperatures to avoid excessive sintering and the need for high temperature heat. Compared to what has been reported previously, we observe higher splitting kinetics, resulting from the utilization of fine particles and well-controlled experiments which ensure a surface-limited-process. The peak rates with CZO are 85.9 μmole g⁻¹ s⁻¹ at 900 °C and 61.2 μmole g⁻¹ s⁻¹ at 700 °C, and those of CeO₂ are 70.6 μmole g⁻¹ s⁻¹ and 28.9 μmole g⁻¹ s⁻¹. Kinetic models are developed to describe the ion incorporation dynamics, with consideration of CO₂ activation and the charge transfer reactions. CO₂ activation energy is found to be −120 kJ mole⁻¹ for CZO, half of that for CeO₂, while CO desorption energetics is analogous between the two samples with a value of ∼160 kJ mole⁻¹. The charge-transfer process is found to be the rate-limiting step for CO₂ splitting. The evolution of CO₃²⁻ with surface Ce³⁺ is examined based on the modeled kinetics. We show that the concentration of CO₃²⁻ varies with Ce³⁺ in a linear-flattened-decay pattern, resulting from a mismatch between the kinetics of the two reactions. Our study provides new insights into the significant role of surface defects and adsorbates in determining the splitting kinetics.