Strategies for Cobalt Nanoparticle Synthesis Using Surface Organometallic Chemistry

This project explores a route for controlling the size of nanoparticles without capping agents with the aim of targeting sub-nanosized particles. A cobalt organometallic compound is tethered to a dehydroxylated silica support to produce a supported cobalt precursor. Reduction of this precursor leads to silica-supported cobalt nanoparticles without the use of capping agents. The silica support is prepared for the tethering of the cobalt organometallic compound through calcination. The hydroxyl density of three silica supports (Aeroperl 300/30, SBA-15, and KIT-6) are determined at different calcination temperatures through 29Si and 1H solid-state NMR spectroscopy and chemical titration of the OH surface species. The surface area of the silica supports is also probed through BET analysis to determine if any pore structure collapse occurs during the calcination. Studies showed Aeroperl 300/30 calcined at 600 °C to be the most appropriate potential nanoparticle support for this project due to its relatively low hydroxyl density (0.8 mmol g-1) and stable pore structure. Chapter 3 describes attempts to tether various cobalt organometallic and amide species to the SiO2-600 support. Here, [Co(N(TMS)2)2THF] was found to demonstrate significant promise as a precursor for SiO2-600-supported cobalt nanoparticle formation. The composition of the cobalt-decorated silica material obtained following reaction of [Co(N(TMS)2)2THF] with SiO2-600 was probed using SS-NMR spectroscopy, ICP-OES, BET, and CHN analyses. The surface species of [Co(N(TMS)2)2(THF)]/SiO2 was found to be a monografted -Si-O-Co-N(TMS)2.THF species. In Chapter 4, with a view to the generation of silica-supported cobalt nanoparticles, the thermal treatment and/or reduction under hydrogen at elevated temperature of the silica-grafted cobalt species was explored. The analysis of the magnetic properties of the thermally treated-/reduced-cobalt materials with a Guoy balance was inconclusive, the cobalt surface species could not be determined as either Co2+ or Co0. TEM imaging suggests the presence of cobalt particles, and it was concluded that heating [Co(N(TMS)2)2(THF)]/SiO2 under hydrogen likely afforded cobalt nanoparticles (0.5 – 3 nm), while heating [Co(N(TMS)2)2(THF)]/SiO2 under nitrogen produced cobalt nanoparticles (2 - 3 nm). However, the exact nature of the cobalt surface species has been difficult to characterise due to their air-sensitivity. Following synthesis of these small cobalt/cobalt-oxide particles, the use of these materials in catalytic hydrogenation of cinnamaldehyde was examined. While the catalysts containing cobalt nanoparticles were found to be mostly selective towards hydrocinnamaldehyde (11 – 24%), the formation of side- products, including (1E,2E)-3-Phenyl-N-(trimethylsilyl)-2-propen-1-imine (6%), was also observed during the reaction. It was determined that the cause of the formation of the unwanted side-product was due to traces of H-N(TMS)2 physisorbed to the silica surface.