hide
Free keywords:
-
Abstract:
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 gcat-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.
