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.

The production of ammonia via a two-step cyclic process is considered, consisting of an endothermic carbo-reduction of Al2O3 in a N2 atmosphere to form AlN, followed by exothermic steam-hydrolysis of AlN to produce NH3 and to reform Al2O3. The chemical kinetics of both steps were experimentally investigated by thermogravimetry and gas chromatography, without added catalysts and under external supply of process heat. Rate laws and the corresponding Arrhenius kinetic parameters were determined by applying the unreacted core model for the AlN production and the shrinking core model for the AlN hydrolysis.

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