(685c) Applicability of Perovskite-Based Oxides As Redox Materials for Thermochemical CO2 Dissociation

Two-step thermochemical cycles utilizing metal oxide based intermediates are capable of dissociating CO₂ to CO using concentrated solar energy as the source of high-temperature process heat. The first step consists of a high-temperature, endothermic reduction of a metal oxide and the subsequent evolution of oxygen. The second step consists of a lower temperature exothermic oxidation reaction, in which CO₂ is reduced to CO and oxygen is re-incorporated into the metal oxide.  Because the metal oxide is recycled, the net reaction is simply the dissociation of CO₂, whose products can be further processed into liquid hydrocarbons via Fischer-Tropsch and other catalytic processes. In contrast to the direct thermolysis, CO and O₂ are derived in different steps, thereby eliminating the need for high-temperature separation.

Ceria-based (CeO_2-δ) materials are attractive as metal oxide intermediates because of their high-temperature morphological and crystallographic phase stability [1]. Rapid kinetics are also achievable compared to other systems because of relatively large ambipolar diffusion coefficients. There are, however, many other nonstoichiometric oxides which have yet to be evaluated as redox splitting materials but exhibit many of the advantages of ceria-based materials. One such class of materials is known as perovskites, which have the crystallographic form ABO_3-δ. A and B can represent a wide range of cations and have a large influence the degree of reduction as a function of both O₂ partial pressure (pO₂) and temperature.

A large quantity of thermodynamic data exists for many perovskites which describe the degree of nonstoichiometry (δ) as a function of pO₂ and temperature. Based on this data we have identified a number of materials which may be suitable as metal oxide redox intermediates. In short, we have modeled oxygen nonstoichiometry and extrapolated the data to high temperatures Subsequently, relevant thermodynamic parameters were computed and equilibrium CO concentrations determined as a function of reduction conditions (T, pO₂) and ensuing oxidation temperature. We have coupled thermodynamic modeling with experimental demonstration (thermogravimetric analysis) of select materials in order to further identify their suitability as redox materials. Data presented here will focus primarily on thermodynamic modeling and experimental screening of various perovskite-based oxides in relation to thermochemical CO₂ splitting. A comparison of these systems with CeO₂-based materials from a thermodynamic and kinetic perspective will be highlighted.

References

1.         Chueh, W.C. and S.M. Haile, A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO₂ mitigation. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences, 2010. 368(1923): p. 3269-3294.