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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.