Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

The goal of this thesis is to perform a complete development cycle of 4H-Ag nanoparticles. This silver nanoparticles adopt an unusual crystal phase different from that typically encountered in bulk metallic silver, i.e., hexagonal close packed instead of cubic close packed. A complete development cycle means fine-tuning of the synthesis, characterization of the synthesized nanoparticles and applying them as catalysts in various catalytic reactions. Electrodeposition and colloidal synthesis are two pathways discussed in literature to produce samples of which at least part of the Ag nanoparticles adopt the 4H phase. The characterization is executed with Scanning Electron Microscopy (SEM) and Powder X-Ray Diffraction (PXRD). Electrodeposition and colloidal synthesis are both used to synthesize silver nanoparticles. The synthesis via electrodeposition partially results in the formation of the 4H phase and still needs to be further investigated. With the colloidal synthesis, nanoparticles that partially exist out of the 4H phase have been obtained as well. To stabilize nanoparticles, capping agents have been shown to play an important role not only to minimize aggregation, but also to minimize the surface energy. After the syntheses, PXRD patterns are recorded to determine the crystal phases. When using polyvinylpolypyrrolidone (PVPP) as a capping agent mixed phase particles, which are partly in the 3C and partly in the 4H phase, are obtained. Substituting this PVPP with oleic acid further increases the relative intensity of the 4H-Ag reflections, compared to those of 3C-Ag in PXRD. To assess the catalytic properties of the synthesized Ag nanoparticles, the chemoselective hydrogenation of 4-nitrostyrene and cinnamaldehyde was studied. In the hydrogenation of 4-nitrostyrene all of them showed a selectivity towards the 4-aminostyrene, going from 49 – 95%. However, at a rather low conversion (1.3 – 13.6% of 0.35 mmole 4-nitrostyrene). This catalytic performance cannot be fully attributed to the presence of the 4H phase, as the reference 3C-Ag nanoparticles showed the same selectivity. Nevertheless, commercially available Ag nanoparticles showed selectivity 4-ethyl nitrobenzene. With Pt, 4-aminostyrene is formed at first, but when the reaction is continued, 4-ethylaniline is formed. The hydrogenation of cinnamaldehyde gave low conversions as well (2.3 – 2.6% of 2.30 mmole cinnamaldehyde). This can probably be attributed to the bulky phenyl group present in the molecule. All tested catalysts showed a selectivity towards 3-phenyl propanal. The 4H phase of Ag can be considered as stable under ambient conditions. Some samples have been stored for seven months and showed similar XRD patterns after storage. Unfortunately, the conditions used for catalysis do not suit the catalysts. Catalysis was executed at 110°C for 4-nitrostyrene and 140°C for cinnamaldehyde under 20 bar and 40 bar H2. The 4H-Ag is stable until 70°C, but phase conversion to 3C-Ag occurs at 90°C and after 1 hour at 120°C the characteristic peaks are no longer found in PXRD. High pressure also seems to cause a phase transition from the 4H- to the 3C-phase. This is shown by pressuring the mixed 4H/3C-Ag catalyst for 4 hours at 20 bar H2 at 30°C. After this test, only a very small 4H peak could be retained, while under 40 bar it completely disappeared. These findings indicate that during the catalytic testing, all catalysts were present mainly in the ‘normal’ 3C crystal phase.

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Metal-organic frameworks are a class of often crystalline, potentially porous materials constructed from metal ions connected through organic linkers. The materials excel in modularity, metal concentration and well-defined porosity rendering them suitable candidates for various applications like catalysis, gas sorption and separation. The often limited stability of MOFs with low-valent cations drove the focus towards M4+-based materials, such as Zr and Ti MOFs, resulting in a large number of Zr MOFs. Ti MOFs feature additional redox-and photoactivity, but only a few structures are reported due to their challenging synthesis procedures: Ti salts are prone to uncontrolled hydrolysis leading to ill-defined oxohydroxides instead of the desired MOF structures. In this PhD thesis, we reported the synthesis of a new layered Ti MOF, COK-47, from the hydrolytically more stable titanocene dichloride precursor. The material could be synthesized as an inherently defective, nanoparticulate MOF, denoted COK-47S, and is the first Ti MOF with known defects. Bridging methoxides were observed on the missing-linker defects, which could be converted to open sites with neighboring terminal methoxides upon activation. The open sites imbue the material with catalytic activity, as demonstrated by the oxidative desulfurization of thiophenes. Furthermore, the photoactivity of COK-47 was demonstrated by the degradation of Rhodamine 6G. In the second part of this thesis, another type of redox-active M4+-MOFs was studied: Ce MOFs. First, we tackled the challenging synthesis of Ce MOFs with reactive tetracarboxylate linkers, which could not be performed conventionally due to the highly oxidative nature of the Ce4+ salt. A widely applicable synthesis method was developed by adding a preformed molecular Ce6 cluster to the synthesis mixture as redox-stable Ce precursor, thereby circumventing any reactivity issues. The redox activity of the benchmark Ce MOF, Ce-UiO-66, was first demonstrated by the TEMPO-mediated alcohol oxidation, but the exact role of the metal sites remained unclear. We therefore performed an X-ray absorption spectroscopy investigation, revealing that only one Ce4+ ion per cluster could be reduced, which explained the need for a redox mediator. Ce MOFs would thus be more suitable for one-electron oxidations, which was tested on the industrially relevant selective catalytic reduction (SCR) of NO by NH3.Ce-UiO-66 outperformed CeO2 as SCR catalyst by virtue of its larger Ce accessibility, but both catalysts suffered from a limited acidity and consequently low ammonia adsorption and activity. Therefore, a new Cex/Zr-CAU-24 series was developed because the CAU-24 materials exhibit a larger number of accessible sites and the stronger Zr acid could improve the NH3 adsorption. Ce10/Zr-CAU-24 contains open Ce sites surrounded by two Zr ions that can adsorb NH3, which resulted in a much higher SCR activity.

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Technological advancements are paramount for solving the climate change predicament. Climate change is multi-faceted, requiring local and global solutions. Energy production and consumption are both important topics to address to reduce the load of our society and economy on the ecosystem. Advancements in solar energy production, as well as reducing the energy demand of lighting applications are on the horizon due to the advent of metal halide perovskites (MHP). In less than a decade, MHPs have taken its place in solid-state technology, as a suitable candidate material for photovoltaic (PV) cells and light-emitting diode (LED) lighting as a semi-conductor material. CsPbI3, one of the many perovskite materials, has the material properties worthy for commercialization as a perovskite material, due to its good thermal stability and low fabrication costs. Its small band gap (1.73 eV) is able to capture light from the entire visible spectrum. CsPbI3 has different polymorphs, where the perovskite forms (α-, β- or γ-phase) are only stable at higher temperatures. At ambient temperature, the perovskite structure is instable and quickly transforms to its non-perovskite form (δ-phase). The instability issue is the main hurdle to overcome. Direct laser writing (DLW) has proven to enhance the ambient stability of CsPbI3 perovskite, as shown by Steele et al. (2017). This work investigates the stabilizing effect of DLW on CsPbI3 thin films. Two parameters in particular, the size of the laser written grid and the laser power density, were varied to reveal their influence in the DLW stabilization of α-phase CsPbI3 thin films. It was shown that higher power densities will stabilize the α-phase more effectively. The stabilizing effect of DLW is inversely dependent on the grid size. Furthermore, many characterization methods were used to unveil the structure and elemental composition of the laser written material. No crystallographic or elemental analysis was able to distinguish the laser written material from the as-grown thin film CsPbI3, but SEM imaging revealed that the nano-crystal are no more present. Photoluminescence spectroscopy of the laser written material points to the formation PbO, or possibly highly stabilized γ-phase CsPbI3. Applying the DLW method in LED fabrication yielded no successful devices, however this is not necessarily caused by the DLW treatment, but has more likely to do with the difficulty of the fabrication process as a whole. The enhanced stability shown in this work is an encouragement to further develop the DLW technique, as well to further investigate the microstructural stability enhancements achieved at the interfaces of perovskites. These developments would improve the feasibility of commercial application of CsPbI3 and other MHP materials.