Stabilisation of metal nanoparticles by confinement on curved supports
Supported metal nanoparticles present unique chemical and physical properties compared to their bulk counterparts. Their high surface energy provides outstanding catalytic activities, opening the door not only to improved catalytic systems but also to new catalytic routes. However, their high surface energy and liquid-like properties are responsible for their instability, usually leading to agglomeration under reaction conditions. This thesis seeks to investigate a novel nanoparticle stabilisation approach by physical confinement on curved supports. Specifically, the project focusses on the stabilisation of cobalt and gold nanoparticles on nanostructured -Al2O3 supports, motivated by the industrial interest of Sasol UK. The research hypothesis is validated by detailed characterisation and catalytic testing of a range of catalysts using different metal loadings and support morphologies. To enable this study, the mechanism of the hydrothermal synthesis of a series of nanostructured -Al2O3 supports with either flat or curved surfaces and differing degrees of curvature has been elucidated, leading to the development of a semi-continuous manufacturing process. Varying the method for loading cobalt onto -Al2O3 supports highlights the implications of method selection on the particle size, reducibility, composition and the tendency to form irreducible cobalt oxides, all of which affect the catalytic activity. The ability to obtain and stabilise small nanoparticles with low loading (1 wt% Co) without the formation of irreducible cobalt oxides exposes the beneficial effect of the support curvature. Specifically, the stabilisation effect is theorised to be effective under the condition where the ratio of the diameter of the nanoparticle (P) and the nanorod (R) is less than one, P:R < 1. In several cases, after cobalt or gold reduction, elongation of the nanoparticles, as opposed to agglomeration, is observed by electron microscopy confirming that the particles are physical confined by the curved surface in all directions except along the nanorod axis. In these cases, highly active Co/-Al2O3 and Au/-Al2O3 catalysts are reported for NH3 decomposition and CO oxidation respectively. For higher metal loadings (> 5 wt% Co), where the particles are the same size or larger than the diameter of the nanorod cross-section, no noticeable stabilisation effect is reported. The results of this thesis are scientifically and industrially important. If applied correctly, this novel nanoparticle stabilisation strategy could be used to design catalysts with improved activity and stability, resulting in lower operational costs and improved resource efficiency.