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 produces a qualitative tradeoff between the thermodynamic and kinetic properties of hercynite by improving the reaction kinetics although, so far, at the expense of some drop in the overall activity per unit weight. 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 Classification: General Business Use 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.

A thermochemical study of iron aluminate-based materials: a preferred class for isothermal water splitting

“…Importantly, the improved performance was consistent, as the presence of cobalt has been suggested to impede the diffusion of iron and thus prevent deactivation by sintering; 21 a later study reported stable performance over 200 cycles. 22 In addition, cobalt has been shown, using in situ X-ray photoelectron spectroscopy, to promote reduction at the cobalt–iron aluminate surface, 23 thereby increasing the thermodynamic driving force for subsequent oxidation. As a result, samples with higher cobalt content, when reduced under equivalent conditions, exhibit faster kinetics for the dissociation of both H 2 O 24 and CO 2 .…”

“…As a result, samples with higher cobalt content, when reduced under equivalent conditions, exhibit faster kinetics for the dissociation of both H 2 O 24 and CO 2 . 23 However, a trade-off between more rapid kinetics and higher product yields was also observed, 23 such that the largest quantities of H 2 and/or CO are obtained for the oxidation of reduced iron aluminates. Slower kinetics are generally associated with the presence of hercynite (FeAl 2 O 4 ), 25,26 a spinel phase that is only predicted to exist in samples that are cobalt deficient.…”

“…Furthermore, the characteristic peak assigned to cobalt ferrite was not detected in either reducing or oxidizing atmospheres, thereby confirming that, although cobalt ferrite may exist at ambient conditions, it does not directly participate in the production of H 2 and/or CO. This understanding that – depending on the cobalt content – either hercynite or a mixed cobalt–iron aluminate compound is redox active has been adopted in later studies; 23,35–37 however, there remains debate on the mechanism, with a recent work 35 suggesting that hercynite actually accommodates cation, not oxygen, vacancies.…”

Thermodynamic guiding principles of high-capacity phase transformation materials for splitting H2O and CO2 by thermochemical looping

“…The main technological requirements for lowering the product cost in the thermochemical looping cycle include: (1) large thermodynamic equilibrium CO 2 /water splitting capacity (ref. 45; defined in Subsection 2.1) at feasible conditions, usually described as a large oxygen release from the oxide at a practical temperature and oxygen partial pressure ( p O 2 ), 46 and a high CO 2 /water conversion; (2) fast reaction kinetics that enables a short cycle time; 47–49 and (3) long-term cyclability of both the reactor and the redox material.…”

Magnetic mapping of hercynite produced by combustion synthesis

“…Spinel-type ferrites have attracted great attention due to its structural characteristics and potential applications, specially related to their magnetic properties. Spinel structured materials have FCC structure and general formula AB2O4 [1] and can be synthetized by several methods, including co-precipitation, sol-gel [2] and combustion reaction [3]. The latter is particularly convenient, even for nanoparticles, due the low cost, high reaction speed, that may take only a few minutes, and for being energetically efficient, because the reaction environment releases enough energy for the reaction to occur [4].…”