Oxidation Kinetics of Hercynite Spinels for Solar Thermochemical Fuel Production

Abstract The development of an economically viable solar thermochemical fuel production process relies largely on identifying redox active materials with optimized thermodynamic and kinetic properties. Iron aluminate (FeAl2O4, hercynite) and cobalt-iron aluminate (CoxFe1-xAl2O4) have both been demonstrated as viable redox-active materials for this process. However, doping with cobalt creates a tradeoff between the thermodynamics and kinetics of H2 production mediated by hercynite in which the kinetics are improved at the expense of lowering the yield. In this work, we evaluate four spinel aluminate materials with varying cobalt contents (FeAl2O4, Co0.05Fe0.95Al2O4, Co0.25Fe0.75Al2O4, and Co0.40Fe0.60Al2O4) to better understand the role of cobalt in the redox mediating properties of these materials and to quantify its effect on the thermodynamic and kinetic properties for CO2 reduction. A solid-state kinetic analysis was performed on each sample to model its CO2 reduction kinetics at temperatures ranging from 1200°C to 1350°C. An F1 model representative of first-order reaction kinetics was found to most accurately represent the experimental data for all materials evaluated. The computed rate constants, activation energies, and pre-exponential factors all increase with increasing cobalt content. High temperature in-situ XPS was utilized to characterize the spinel surfaces and indicated the presence of metallic states of the reduced cobalt-iron spinel, which are not present in un-doped hercynite. These species provide a new site for the CO2 reduction reaction and enhance its rate through an increased pre-exponential factor.