X-ray diffractometer

• Subject Area: Chemical Solid State and Surface Research
• Term: Funded in 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524943955

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).

• DFG Programme: Major Research Instrumentation
• Major Instrumentation: Röntgendiffraktometer
• Instrumentation Group: 4010 Einkristall-Diffraktometer
• Applicant Institution: Rheinisch-Westfälische Technische Hochschule Aachen
• Leader: Professorin Dr. Mirijam Zobel