CO₂Splitting via Two-Step Solar Thermochemical Cycles with Zn/ZnO and FeO/Fe₃O₄Redox Reactions II: Kinetic Analysis

Abstract

Two-step thermochemical cycles for splitting CO 2 with Zn/ZnO and FeO/Fe 3 O 4 redox pairs using concentrated solar energy are considered. Thermogravimetric-based kinetic analyses were performed for the reduction of CO 2 to CO with Zn and FeO. Both reactions are characterized by an initial fast interface-controlled regime followed by a slow diffusion-controlled regime, which are described using a shell−core kinetic model. In the interface-controlled regime, a power rate law is applied with apparent activation energies 113.7 and 73.4 kJ mol −1, and corresponding reaction orders 0.339 and 0.792, for the Zn/CO 2 and FeO/CO 2 systems, respectively. In the diffusion-controlled regime, limited by the ion mobility through the oxide shells, the apparent activation energies are 162.3 kJ mol −1 for Zn/CO 2 and 106.4 kJ mol −1 for FeO/CO 2 . Additional reaction mechanisms above the Zn melting point for Zn/CO 2 reactions are postulated.