Mechanochemical Activation in Heterogeneous Catalysis

In recent years the field of mechanochemical activation of solids for the enhancement of heterogeneously catalyzed reactions has grown substantially. Mechanochemical activation has shown to change the overall reactivity of solids, allowing catalytic reactions to run through new pathways at nearly ambient conditions. Additionally, mechanochemical reactions allow greener approaches to e.g. synthesis of solids by preventing the use of toxic solvents and reagents. Thus, this thesis commences by a detailed study of a top-down synthesis approach to support Au nanoparticles on metal oxide supports starting from macroscopic Au powder. The research focused on investigating the reproducibility, scalability between different milling vials, and versatility of this novel synthesis approach. It was found that by ball milling macroscopic Au powder with a respective metal oxide support, Au nanoparticles with mean diameters of 3 – 4 nm could be homogeneously distributed on the support. These catalysts showed high activity towards CO oxidation, resulting in temperature of half conversion for 1 wt% Au on TiO₂ as low as 60 °C at a weight-hourly-space velocity of 80000 ml h^-1 g_cat^-1, showcasing the potential of this mechanochemical synthesis method. Furthermore, this synthesis method could be rescaled to different milling vials, allowing the deposition of further metals (Ag, Pt, Cu, Ni) and the use of carbon as a support material, underlining the versatility of this mechanochemical approach. The second part follows up on the investigation of the phase transformation observed for γ-Al₂O₃ upon ball milling. Ball milling of γ-Al₂O₃ with initial surface areas of 84 m² g^-1 resulted in α-Al₂O₃ with unusually high BET surface areas of approximately 70 m²g^-1. This phase transformation occurred after only 2 h milling, resulting in lower abrasion from the milling vial than previously reported in literature, making these materials interesting as potential catalyst support materials. In the third part of this thesis, in-situ solid catalyst mechanical activation of Cr₂O₃ and CeO₂ for propene combustion was studied. During in-situ ball milling of the respective metal oxides under propene oxidation conditions, oscillations in CO and CO₂ formation were observed. Abrasion from the steel milling vial could be eliminated as the sole cause for the oscillations through substitution by a tungsten carbide milling vial. The intensity and frequency of oscillations is shown to be dependent on the propene to oxygen ratio, the milling frequency, milling ball size, and metal oxide used. Overall, Cr₂O₃ shows higher activity for oscillatory propene combustion under in-situ mechanical activation. In the final part, the source of mechanical activation is changed from a ball mill to the tip of a scanning tunneling microscope. This served to improve the understanding of the molecular chemistry of 1,4-β-glucans which is essential for designing new approaches to the conversion of cellulosic biomass into platform chemicals and biofuels. Catalytic hydrolysis of the 1,4-β-glycosidic bond is known to proceed under mechanical forces in a ball mill using strong acids, however the understanding on a mechanistic level is limited. In order to investigate the chemical behavior of 1,4-β-glucans towards different acids on a molecular level, cellobiose was imaged on single-crystal metal surfaces using lowtemperature scanning tunneling microscopy in the presence of different acids. By mechanical manipulation of the cellobiose-acid assembly structures, it was possible to directly investigate the impact in the differences of acid strength on its interaction with cellobiose, which reflects the differences in basicity of the cellobiose OH-groups with respect to the glycosidic oxygen atom and verifies the expected H-bonding between acid and cellobiose molecules