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The role of interfaces in ceria-based multi-phase membranes for membrane reactors

Subject Area Synthesis and Properties of Functional Materials
Term from 2017 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 387282673
 
Final Report Year 2023

Final Report Abstract

The use of catalytically coated, O2-permeable membrane reactors offers an energy-efficient alternative to commercial chemical synthesis processes, such as the partial oxidation of hydrocarbons. In such reactors, the oxygen required for the reaction is transported as O2--ions directly from the air through a gas-tight, thin ceramic layer to a catalyst. Investigations of typical materials for membrane reactors have so far concentrated mainly on temperatures in the range of >700 °C in order to achieve the required permeation rates. However, most relevant conversion reactions require lower temperatures (300-600 °C). In the project, therefore, a sufficient permeation performance of ceramic membranes at temperatures ≤600 °C was to be achieved by combining a good oxygen ion conducting component (doped cerium dioxide) and a good electron conducting component (transition metal oxides with spinel or perovskite structure). In addition, one focus was on the targeted tailoring of the grain boundaries in the membranes, as these often restrict the ion transport particularly strongly. CeO2 with 20 mol% Gd doping as the ion-conducting component and the Co-rich spinel FeCo2O4 were chosen as the starting system, as this composition had already achieved very good transport properties in previous investigations. In this benchmark system, an electron-conducting third phase with a perovskite structure (GeFeO3) is formed, which has better thermal stability and is reduced more slowly than the desired spinel phase. This is responsible for the good permeability of the material at low nominal spinel contents. The formation of the complex phase composition was elucidated in the project. By varying the dopants in the cerium dioxide, it was found that 20 mol% Sm-doped cerium dioxide also forms a perovskite third phase, while lower Gd concentrations, in contrast to the originally chosen system, do not lead to the formation of a third phase. Using microscopic measurement methods, it was also possible to prove that the variation of the doping concentration as well as the composition of the electron-conducting phase has a considerable influence on the composition and defect concentration at the interphase and intraphase grain boundaries and that these can be specifically influenced. Mechanically stable thin-film membranes to determine permeance even at temperatures below 500 °C have not yet been realized. This is probably due to phase transformations in the spinel system. The newly developed composite system of Gd-doped cerium dioxide and (Ce,Gd)(Fe,Co)O3 offers a high potential to advance into this range, since no phase transformations occur and it has an increased stability with almost the same permeation performance.

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