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

在非均相催化领域，应用催化剂包覆的o2透膜反应器是替代商业应用的合成方法的一种节能方法。在这种处理过程中，转化所必需的氧气是从空气中提取的。氧离子通过一个气密的薄陶瓷层直接输送到催化剂上。它将空气分离和化学转化两个步骤结合在一个过程中。由于大多数转化发生在200至500°C之间，因此在该温度范围内，需要具有大约100万n cm-2 min-1的特定渗透速率的膜来确保显著的转化率。陶瓷氧透膜的渗透是通过固态扩散进行的，主要取决于膜的双极性电导率、膜的厚度和膜上的氧分压梯度。陶瓷复合材料具有一个主要的氧化离子导电性和一个主要的电子导电性，是这种应用的有希望的候选者，因为这些材料在反应条件下结合了高氧通量和高机械和化学稳定性。然而，迄今为止的研究工作主要集中在700°C以上的温度状态。为了在较低的温度范围内达到良好的渗透率，除了基本的成分适应外，主要的努力将是优化单一组分之间的界面。原因是在界面处发生了各种各样的反应，这些反应对氧通过膜的运输有根本性的影响。在本项目的过程中，该联盟计划系统地研究不同界面的化学和物理特性，或者更确切地说，是在以尖晶石相作为电子导电成分的受体掺杂的ceo2基复合材料的界面上发生的传输过程，其中FeCo2O4将是初始材料。更精确地理解晶界和三相界的过程是计划有目的地优化复合材料用于催化应用的关键，例如通过微观结构或不同相的化学成分的适应。所选择的复合材料计划作为一系列ceo2基多相复合材料的示例。从这些研究中获得的基本见解将用于在中等温度下达到技术上相关的渗透速率，从而为膜反应器的工作概念开辟更广阔的应用领域。

The application of catalyst-coated O2-permeable membrane reactors is an energy efficient alternative to the commercially applied methods of synthesis in the area of heterogeneous catalysis. During such treatments, the oxygen which is necessary for conversion is taken from air. O2- ions are transported straight to the catalyst through a gastight thin ceramic layer. This combines the two steps of air separation and chemical conversion in one process.As most of these conversions take place at temperatures between 200 and 500 °C, membranes with a specific permeation rate of roughly 1 mlN cm-2 min-1 in this temperature range are necessary to ensure a significant conversion rate. The permeation through a ceramic oxygen permeation membrane takes place by solid state diffusion and mainly depends on the ambipolar conductivity, the thickness of the membrane and the oxygen partial pressure gradient across the membrane.Ceramic composites with one mainly oxide ion conductive and one electron conductive constituent are promising candidates for this kind of application as these materials combine a high oxygen flux with high mechanical and chemical stability under reaction conditions. However, research efforts so far have been focused on the temperature regime above 700 °C. To reach a good permeation rate also in the lower temperature range, the main effort, apart from basic adaption of the composition, will be the optimization of the interfaces between the single constituents. The reason is that a variety of reactions occur at the interfaces, which have a fundamental impact on the oxygen transport through the membrane.In course of the present project the consortium plans to systematically investigate the chemical and physical characteristics of the different interfaces or rather of transport processes taking place at the interfaces of an acceptor doped CeO2-based composite material with a spinel phase as electron conductive constituent, where FeCo2O4 will be the initial material. A more precise comprehension of the processes at grain boundaries and triple phase boundaries is the key for the planned purposeful optimization of the composite material for catalytic applications, e.g. by micro structuration or adaption of the chemical composition of the different phases. The chosen composites are planned to serve as examples for a series of CeO2-based multiphase composites. The fundamental insights from these investigations will be used to reach technologically relevant permeation rates at intermediate temperatures and thus to open a broader field of application to the working concept of a membrane reactor.