CaFeO2.5 analogous Brownmillerites prospecting substitution of ceria-based materials for heterogeneous Catalysis – CaFeCAT

Different perovskite and Brownmillerite materials were obtained from classic lab scale synthesis (complexation route) and industrial synthesis (electric arc fusion synthesis). All materials are characterized using x-ray diffraction, nitrogen physisorption and microscopy techniques. The oxygen mobility of all materials can be traced using 18O-exchange combined with thermogravimetric analysis and mass spectrometry. The impact of oxygen mobility on the catalytic reaction is studied using 18O-enriched material as starting material in the catalytic reaction. We here compare the onset of oxygen mobility and catalytic conversion. We also study the impact of the synthesis method on the stability. The impact of defects (in the form of different length of 1D(FeO4)? chains) and dopants (Cu, Ti) on the catalytic performance is studied using x-ray absorption spectroscopy, neutron scattering and pair-distribution function modelling.

Despite the low surface area, materials synthesized by electric arc fusion synthesis can be interesting candidates as heterogeneous catalysts, in particular due to their high stability compared to materials synthesized by lab scale methods. Neutron scattering shows that these materials show surprisingly small grain sizes (30-90 nm), despite their high synthesis temperature above 1200°C. Oxygen mobility studies show that 1D(FeO4)? chain length and oxygen mobility directly influence the catalytic performance: for SrFeO2.5, oxygen mobility and catalytic conversion of CO start simultaneously, while for CaFeO2.5, these two are separated.

One of the most interesting results for the catalysis community is the observation that structural defects in CaFeO2.5 enhance the oxygen mobility and that this translates into a better catalytic performance. Optimization of catalytic performance via the introduction of defects in the support material is a novel concept in catalysis and goes beyond general strategies such as increasing the surface area or dispersion of the active phase.

1. “In situ generated catalyst: copper(II) oxide and copper (I) supported on Ca2Fe2O5 for CO oxidation” B. Penkala, S. Gatla, D. Aubert, M. Ceretti, C. Tardivat, W. Paulus, H. Kaper, Catal. Sci. Technol., 2018, 8, 5236.
2. FR3086282 A1 « PRODUIT FONDU POLYCRISTALLIN A BASE DE BROWNMILLERITE » published on 2020-03-27

The present project is dedicated to industrial research for the development of innovative catalytic systems for air purification, such as those used for the control of road vehicle emission (three way converter, TWC). In the context of Europe’s dependency on imports of some critical elements currently used as catalyst support (e.g. cerium oxide), we focus on more available elements such as Ca, Fe, Mn, Sr, Cu… by keeping the well-understood mechanisms governing the catalytic activity of cerium oxide in mind. As such, we choose oxygen ion conductors of the Brownmillerite family as support material, because it has been reported that lattice oxygen atoms have a beneficial impact on the catalytic activity of oxidation reactions. Next to the pure support material, also the interaction of a noble metal with the oxygen ion conductive support for the efficient removal of gas phase pollutants will be studied. In terms of catalytic reactions, the oxidation of CO, and the storage and reduction of NOx will be the primary metrics.
In this project, oxygen ion conductors of the Brownmillerite family are chosen as support material. Brownmillerites can be regarded as oxygen-deficient perovskite type oxides. The Brownmillerite type structure is anisotropic with 1D-oxygen vacancy channels providing a catalytically enhanced surface/interface structure. Brownmillerites are known to reveal oxygen ion mobility down to ambient temperature. The presence of extended defects as anti-phase boundaries can significantly decrease the activation energy for oxygen diffusion. Defect-rich CaFeO2.5, which is traditionally known to be a stoichiometric line-phase, can be oxidized under mild conditions to CaFeO3, while the oxidation of ordinary CaFeO2.5 usually requires extreme reaction conditions, i.e. 1100°C and several GPa oxygen partial pressure. Thus, introducing a high concentration of defects seems to be a promising concept to transform even traditionally known stoichiometric line-phases to become a kind of oxygen sponge and behave as oxygen storage/buffer compound at very moderate temperatures. This mechanism is thus comparable to the oxygen storage capacity of doped cerium oxide, and offers a true potential for application in catalysis. Consequently, the Brownmillerite CaFeO2.5 will be a first candidate to study due to its known oxygen ion conductivity properties, however, also doping with other elements (e.g. Cu, Mn, W) and other compositions (e.g. SrFeO2.5) will be investigated.
For the support material, we will attempt to achieve (i)- a high degree of dispersion of the noble metal into the matrix, (ii)- a high oxygen mobility at moderate temperatures (e.g. by introducing defects) and (iii)- a high surface area, which we anticipate to be key aspects for achieving high catalytic activity. To date, it is still a challenge to achieve these goals simultaneously for Brownmillerites. As a result, in this project, several synthesis routes are foreseen. More straightforward synthesis routes, such as citrate-EDTA gel methods and spray pyrolysis, will be investigated alongside with more advanced synthetic approaches such and hard-templating routes. This multitude of possibilities allows for an easy adaption of a synthesis route to the material under study.
A major part of the project will be dedicated to the detailed characterization of the materials involving large scale facilities for structure analysis and spectroscopy (in-situ studies), including oxygen isotope exchange reactions to trace the oxygen ion mobility. These studies will allow for a detailed understanding of the materials properties in relation to its catalytic activity.
The most promising materials will be synthesized on a pilot-scale using electrofusion. This technique is well-established by the industrial partner and is extremely suitable for the synthesis of reduced powders, such as CaFeO2.5.