X-ray diffractometer

亚琛大学结晶研究所的各种研究课题主要集中在短程有序、结晶过程、纳米材料或多相催化剂等方面。对于此类材料的高级结构表征，使用例如Rietveld或全粉模式模型对倒易空间中的布拉格峰进行分析是不够的。相反，对分布函数(PDF)技术正好实现了对短程有序的这种洞察，因为PDF是样品中原子间距离的直方图，因此特别适合于检测局部与平均晶体结构的偏差。虽然几十年来一直在同步加速器和中子辐射设施中测量PDF，但我们Zobel工作组为优化这项技术的实验室访问做出了重大贡献。我们关于减少空气散射和优化光束停止定位的仪器改进建议得到了领先的衍射仪制造商的实施。例如，我们可以证明，仅从实验室数据就可以计算出多相催化剂的所谓差异PDF。为此，从催化剂(=载体上的纳米颗粒)减去空载体材料的相对较大的散射信号，以实现对纳米颗粒结构的直接访问。为了通过PDF以高仪器分辨率对纳米材料进行结构表征，需要使用单色化的银X射线和大约20反Angströms的高动量转移矢量Qmax的透射式几何衍射仪。对于多相催化领域的研究问题--进一步由联合提出者技术化学研究所代表--我们需要现场的X射线衍射/PDF数据，以便跟踪催化剂在反应气体气氛中的活化、催化、高温和高压下降解/烧结过程中的变化。为了解决可持续化学领域中的应用相关主题，例如Power-to-X过程中的甲烷化反应，需要一个样品环境，该环境可承受高达450°C的温度和高达20bar的压力，以及氮气、氢气、一氧化碳、二氧化碳的气体(混合气体)。此外，对反应产物的同时分析对于推导结构活性关联是必不可少的。为此，必须将质谱仪与衍射计相耦合。钼X射线为催化实验提供了更高的时间分辨率，特别是在使用Kalpha1，2波长混合物时，为各种材料提供了足够的仪器分辨率。测量时间最多只能根据不同的要求进行调整和优化(银的高仪器分辨率，钼的高通量)。

Various research topics of the Institute of Crystallography of RWTH Aachen University focus on short-range order, crystallization processes, nanomaterials or heterogeneous catalysts. For advanced structure characterization of such materials, an analysis of Bragg peaks in reciprocal space using, for instance, Rietveld or Whole Powder Pattern Modelling is not sufficient. On the contrary, the pair distribution function (PDF) technique achieves exactly this insight into the short-range order, because the PDF is a histogram of interatomic distances in the sample and is, hence, particularly well suited to detect deviations of the local from the average crystal structure. While for decades PDFs have been measured at synchrotron and neutron radiation facilities, we – the workgroup Zobel – contributed significantly to optimize laboratory-based access to this technique. Our suggestions for instrumental improvements regarding reduction of air scattering and optimisation of the beamstop positioning were implemented by leading diffractometer manufacturers. For instance, we could show that it is possible to calculate so-called difference-PDFs of heterogeneous catalysts from laboratory data only. For this, the comparably large scattering signal of the empty support material is subtracted from the catalyst (= nanoparticle on support) to achieve direct access to the nanoparticle structure. To perform structure characterization for nanomaterials via PDF with high instrumental resolution, a diffractometer in transmission geometry is required using monochromatized silver X-rays and high momentum transfer vector Qmax of ca. 20 inverse Angströms. For research questions in the field of heterogeneous catalysis - further represented by the co-proposer Institute of Technical Chemistry – we require in-situ XRD/PDF data in order to track changes of the catalysts during activation, catalysis, degradation / sintering at high temperatures and pressures in reactive gas atmosphere. In order to address applicational relevant topics in the field of sustainable chemistry, for instance for methanation reactions in power-to-X processes, a sample environment is required, which withstands temperatures up to 450 °C, and pressures up to 20 bar with gases (gas mixtures) of N2, H2, CO, CO2. Further, a simultaneous analysis of the reaction products is essential to derive structure activity correlations. For this a mass spectrometer has to be coupled to the diffractometer. Mo X-rays provide higher time resolution for catalysis experiments, in particular when using Kalpha1,2 wavelength mixtures, providing sufficient instrumental resolution for various materials. The measurement time shall be at best adjustable and to be optimized for the different requirements (high instrumental resolution with Ag, high flux with Mo).