In situ studies of mechanochemical reactions in catalysis

Abstract Mechanochemistry has found increasing interest since the turn of the millennium. The paramount motivation is striving for greener and more sustainable syntheses and production pathways. Nevertheless, a fundamental understanding of the underlying phenomena of mechanochemistry is a goal whose achievement is pending. Methods allowing the ongoing processes in situ analysis may increase the comprehension of these phenomena. Applying, expanding, and optimizing such methods is a continuous challenge. This thesis focuses on extending and using the available possibilities for in situ analyzing mechanosyntheses of oxide ceramics with possible applications in heterogeneous catalysis. First, it presents the development of a suitable setup to perform in situ X-ray diffraction experiments with a shaker mill involving hard materials. The synthesis of corundum from a boehmite precursor serves as a benchmark reaction for equipment tests and validation experiments. Optimizing an existing setup led to equipment that allowed for monitoring this reaction via in situ X-ray diffraction for the first time. The optimization was possible by considering carefully how modifications to the applied mill affect its mechanical performance parameters and using steel vessels with a polymer and a metal layer in the X-ray transparent sections. Second, the investigation focuses on the effects of different possible starting materials and milling conditions on a perovskite synthesis by ball milling. The example perovskite in this part is LaAlO3. Oxides and (oxy-) hydroxides are among the investigated starting materials and the varied conditions include the ball size and the frequency. Replacing one oxide precursor with a corresponding (oxy-) hydroxide significantly alters the synthesis process concerning aspects like reaction pathways, statistical factors of the mechanosynthesis, and product formation according to kinetic studies resting on in situ diffraction data. Moreover, the replacement also affects important product characteristics like the microstructure, the specific surface area, or the abrasion content resulting from the milling tools. One investigated responsible factor is the water that originates from the reacting (oxy-) hydroxides and affects the surface properties of the milled powder. Furthermore, the hygroscopic properties of La2O3 are important for the synthesis process if it is among the precursors, even if only small amounts of environmental water are present in the milling vessel. While changing the frequency primarily affects the reaction kinetics, in and ex situ diffraction data revealed that the ball size significantly affects the behavior of crystal seed formation and growth and thus the material's microstructure. Third, the last part of this thesis focuses primarily on atmosphere and additive effects. The syntheses examined are those of LaCoO3 and La2CoO4. While in situ mass spectrometry data shows continuous oxygen consumption by the LaCoO3 synthesis from La2O3 and Co3O4 that depends on the oxygen amount supplied, complementary in situ diffraction data revealed that the mechanical conditions of the milling process have a more significant impact on the synthesis process. Furthermore, it was possible to synthesize La2CoO4 in a shaker mill in two different ways. In the first case, the cobalt source is CoO. In the second case, Co3O4 is the cobalt source, while adding activated carbon results in the required reductive conditions. A third attempt using hydrogen to reduce present Co(III)-ions did not result in the product targeted. For the first case, the diffraction data obtained in situ indicated a strong dependency of the achievable synthesis outcome on the mixing quality.