Hydrogen of high purity can be produced by chemical looping using iron oxide as an oxygen carrier and making use of the reaction between steam and either iron or FeO. However, this process is viable only if the iron oxide can be cycled between the fully-oxidised and fully-reduced states many times. This can be achieved if the iron oxide is supported on refractory oxides such as alumina. Unfortunately, the interaction between alumina and oxides of iron to form FeAl₂O₄ hinders the kinetics of the reactions essential to the production of hydrogen, viz. the reduction of Fe(II) to metallic iron by a mixture of CO and CO₂ prior to the oxidation by steam. Here, oxygen carriers containing Fe₂O₃ and Al₂O₃ were doped with Na₂O and, or, MgO, in order to inhibit the formation of FeAl₂O₄ by forming NaAlO₂ or MgAl₂O₄, respectively. The performance of the modified oxygen carriers for producing hydrogen, i.e. cyclic transitions between Fe₂O₃ (or Fe₃O₄) and metallic Fe at 1123 K were investigated. It was found that the interaction between FeO and Al₂O₃ was successfully mitigated in an oxygen carrier containing Mg, with an Al: Mg ratio of 2, resulting in consistently stable and high capacity for producing hydrogen by chemical looping, whether or not the material was oxidised fully in air in each cycle. However, the oxygen carrier without Mg only remained active when a step to oxidise the sample in air was included in each cycle. Otherwise it progressively deactivated with cycling, showing substantial interaction between Al₂O₃ and oxides of Fe.

The reversible decomposition of CuO into Cu₂O and oxygen at high temperature, typically between 850 and 1000 °C, provides a means of separating pure oxygen from air. In such a process, the oxide generally has to be supported on a refractory oxide, e.g. alumina, to maintain its capacity when cycled many times between CuO and Cu₂O. One problem is that if the CuO reacts with the alumina to form CuAl₂O₄, the latter releases oxygen too slowly to be of practical use so that the capacity for oxygen release of such a carrier falls progressively as more aluminate is formed. However, the reported temperatures at which CuAl₂O₄ forms are inconsistent so far. This work sets out to investigate the interaction between CuO and different aluminas (and precursors), which are commonly used as support materials, at temperatures between 700 and 1000 °C, as well as some chemical properties of the resulting copper aluminates. It was found that the formation of CuAl₂O₄ occurred at around 700 °C, 800 °C and 950 °C, when amorphous aluminium hydroxide, γ-alumina, and α-alumina were used as the source of alumina support, respectively. The decomposition of CuAl₂O₄ in an oxygen-lean environment can lead to the formation of α-alumina as well as γ-alumina, depending on the partial pressure of oxygen. Given that the α-form does not react with CuO around 900 °C, the typical operating temperature for the CuO/Cu₂O couple, this observation can be used to partially regenerate CuO from CuAl₂O₄, for oxygen storage and production at this temperature by decomposing the spinel in a controlled atmosphere to form only α-alumina. However, during the decomposition of CuAl₂O₄, delafossite CuAlO₂ is also formed, limiting the amount of Cu that could be recovered as CuO in a single process cycle.

Chemical looping combustion (CLC) has been proposed as an efficient carbon capture process for power generation. Oxygen stored within a solid metal oxide is used to combust the fuel, either by releasing the oxygen into the gas phase, or by direct contact with the fuel; this oxyfuel combustion produces flue gases which are not diluted by N₂. These materials can also be used to perform air-separation to produce a stream of oxygen mixed with CO₂, which can subsequently be used in the conventional oxyfuel combustion process to produce sequesterable CO₂. The temperature and oxygen partial pressures under which various oxide materials will react in this way are controlled by their thermodynamic equilibria with respect to reduction and oxidation. While many materials have been proposed for use in chemical looping, many suffer from poor kinetics or irreversible capacity loss due to carbonation, and therefore applying large scale in silico screening methods to this process is a promising way to obtain new candidate materials. In this study we report the first such large scale screening of oxide materials for oxyfuel combustion, utilising the Materials Project database of theoretically determined structures and ground state energies. From this screening several promising candidates were selected due to their predicted thermodynamic properties and subjected to initial experimental thermodynamic testing, with SrFeO_3−δ emerging as a promising material for use in CLC. SrFeO_3−δ was further shown to have excellent cycling stability and resistance to carbonation over the temperatures of operation. This work further advances how in silico screening methods can be implemented as an efficient way to sample a large compositional space in order to find novel functional materials.

The data files are named in accordance with the sections in which they are relevant in the publication: Hu W., Donat F., Scott S. A. and Dennis J. S. 2015. Kinetics of oxygen uncoupling of a copper based oxygen carrier. Applied Energy doi:10.1016/j.apenergy.2015.10.006. Data files from thermogravimetric analyser (TGA/DSC1, Mettler Toledo) are space separated txt files containing the following main entries: (each field separated by "/") Index/time stamp/sample temperature/reference temperature/sample mass. Data files for fluidised bed measurement are comma separated txt files in the format "time stamp, voltage". The detailed processing of the data is described in the file named "Note on data processing.txt".This work was supported by the EPSRC [grant number EP/I010912/1]