(385d) Materials Design Strategies for Performing ‘Unmixed Reactions’ Using Non-Stoichiometric Solids As Oxygen Carriers

Many reactions require complex product separation steps. Such separation steps are often costly and lead to process inefficiencies. Here we discuss the thermodynamic possibilities and challenges of using ‘unmixed reactions’ where individual reactants are never mixed thus reducing or even eliminating the need for separation. Unmixed reactions require either the spatial separation of process streams (using e.g. membranes with permselectivity for a reaction intermediate) or the temporal separation of process streams using dynamic processes and materials that can periodically store (a carrier material) a reaction intermediate. After discussing general considerations for unmixed reactions this presentation will focus on materials design for periodic applications.

Chemical looping involves a sequence of cyclically repeated reactions and e.g. a solid carrier material to store and release a reaction intermediate. Theoretically, the carrier at the end of the cycle will remain unchanged whilst the products from the reduction and the oxidation steps are separated. For example, iron oxide can be used as an oxygen carrier material (OCM) for hydrogen production from the water-gas shift reaction. The OCM is reduced in carbon monoxide to produce carbon dioxide and iron or wustite and the iron or wustite are oxidised in water to produce hydrogen and magnetite. However, although iron oxide shows significant usable oxygen capacity for this reaction, it loses its activity rapidly due to sintering and agglomeration. Perovskites, such as La_0.7Sr_0.3FeO_3-δ, have been proposed as alternative oxygen carrier materials due to their high stability and stable productivity under long term redox cycles. They are also oxygen ion-conducting MIECs (mixed ionic and electronic conductors) allowing rapid oxygen permeation through their structure. We will discuss how the particular thermodynamic behaviour of non-stoichiometric oxides can be exploited to develop novel processes. However, the small oxygen capacity of perovskites results in less hydrogen production per cycle than with iron oxide. Here we show how composite OCMs can be designed to achieve high stability and capacity where iron oxide clusters are embedded in an La_0.7Sr_0.3FeO_3-δ perovskite matrix.

Parallels will also be drawn between membrane and chemical looping processes. We will show that any chemical looping process has a membrane analogue and vice versa.