How the charge transfer in high performance oxidation catalysts controls activity and selectivity: noncontact measurement of Hall charge carrier mobilities and concentrations under reaction conditions

我们的目标是发展一种基于微波的非接触光谱方法，以研究在反应条件下选定的氧化催化剂的动态电荷转移。微波霍尔效应的测量是为了确定密度，迁移率和类型的载流子（电子，空穴，离子）的催化剂位于固定床流动反应器。通过改变反应温度、气相组成和气体每小时空速（接触时间），研究了在磷酸钒和氧化物催化剂上丙烷选择性氧化成丙烯酸和正丁烷选择性氧化成马来酸酐等催化体系和反应中测量值与催化活性和选择性的关系。我们的目标是区分催化剂表面、次表面和整体上的电荷转移，区分载流子浓度和迁移率，并确定对催化性能的影响。为此，我们最近开发的用于非均相催化剂原位表征的微波腔微扰技术将扩展到测量静态磁场中双峰谐振腔中的微波霍尔效应。该装置将用合适的单晶和粉末进行校准。结果将通过原位光电子能谱和其他可用的原位和非原位光谱技术、x射线衍射和热分析进行比较和解释。此外，现有的9 GHz微波电导率测量将通过构建足够的谐振器扩展到1至20 GHz的频率范围，以探测晶界和粒子接触的影响，并能够将结果与低频和直流测量结果进行比较。

We are aiming at the development of a microwave-based noncontact spectroscopic method in order to study the dynamic charge transfer in selected oxidation catalysts under reaction conditions. The microwave Hall effect is to be measured to determine the density, mobility, and type of charge carriers (electron, hole, ion) in catalysts situated in a fixed bed flow-through reactor. By the variation of reaction temperature, gas phase composition, and gas hourly space velocity (contact time) the correlation of the measured values with the catalytic activity and selectivity is to be investigated for selected catalytic systems and reactions, i.e. the selective oxidation of propane to acrylic acid and n-butane to maleic anhydride on vanadium phosphate and oxide catalysts. Our goal is to discriminate between charge transfer on the catalyst surface, sub-surface, and in the bulk and to distinguish between charge carrier concentration and mobility and to identify the impact on the catalytic performance. For this purpose our recently developed microwave cavity perturbation technique for the in situ characterization of heterogeneous catalysts is to be extended to measure the microwave Hall effect in a bimodal resonator in a static magnetic field. The setup is to be calibrated with suitable single crystals and powders. The results are to be compared and explained by in situ photoelectron spectroscopy and alternative available in situ and ex situ spectroscopic techniques, X-ray diffractometry, and thermoanalysis. Moreover, the hitherto existing microwave conductivity measurement at 9 GHz is to be extended to the frequency range between 1 and 20 GHz by the construction of adequate resonators in order to probe the effect of grain boundaries and particle contacts and to be able to compare the results with low frequency and DC measurements.