Abstract
•Reduction kinetics investigated for hercynite (iron aluminate) at up to 1500 °C.•Two different reaction mechanisms are identified by isoconversional kinetic analysis and XRD.•Redox reaction occurs by hercynite maintaining its spinel structure with a change in excess O.•No phase change during redox contributes to robustness of hercynite as an active material.
Solar thermochemical water splitting (STWS) using a hercynite redox cycle is a promising technology for producing renewable H2. Here, the reduction kinetics of hercynite (FeAl2O4) is evaluated using thermogravimetric and XRD analyses. The results indicate that as-prepared hercynite materials undergo reduction via two different reaction mechanisms. The reaction first proceeds by a nucleation and growth mechanism (AE1), followed by a third-order kinetic model (F3). XRD analyses show the occurrence of superstoichiometric oxygen in the spinel structure of FeAl2O4+δ in the second reaction mechanism, which indicates the formation of cationic vacancies. TGA and XRD analyses confirm that hercynite maintains its spinel structure when the materials are thermally reduced and oxidized with steam. Differential scanning calorimetry (DSC) performed on FeAl2O4 particles shows two heats of reaction in agreement with the two observed mechanisms. This is the first kinetics study to report on the thermal reduction of hercynite.