Iron Oxide Nanoparticles on Support Materials

Abstract

Synthesizing catalysts with well-defined size and shape of metal nanoparticles is of vital importance to obtain a better understanding of fundamental processes taking place in catalysis. The catalytic reaction takes place on the surface of active metal nanoparticles, hence typically as high as possible specific metal surface area is required to improve the efficiency of the catalysts. However, in the harsh environment of a catalytic reaction, such as high temperatures or pressures, these particles can grow and the active surface area will decrease as a consequence. Therefore, a support material is often used to anchor the nanoparticles to prevent growth during the reaction. Maintaining the particle size and shape during catalytic reactions is one of the key challenges to ensure high activity and selectivity of catalysts, and thereby catalyst stability.

The synthesis of colloidal metal particles can be used to allow more control over the particle size and shape. In this method, the metal or metal oxide nanoparticle synthesis occurs in the absence of a support. This control can give more insight on catalytically active surfaces and sites, which is currently a subject of intense investigation. These colloidal particles can be synthesized with different metals, such as cobalt, nickel and iron. Iron can be used to catalyze the Fischer-Tropsch to olefins (FTO) reaction, which converts synthesis gas (CO and H2) to lower olefins. The FTO reaction is sensitive to iron particle size, morphology, distribution and support interactions, making iron oxide colloidal nanoparticles attached to support materials relevant model catalysts to investigate fundamental processes during this reaction.

In this thesis, a variety of studies are presented which focus on the use of colloidal iron oxide nanoparticles attached to several supports for the Fischer-Tropsch to olefins catalytic reaction. The main interest was to gain insight in the process of deposition of colloidal iron oxide on the support materials and its dependence on support properties and synthesis conditions. Furthermore, these studies focused on understanding the underlying causes for particle growth as well as the impact of the use of inorganic ligands as promoters on catalytic performance in FTO reactions. Insights gathered in these studies can help designing catalysts with enhanced control over nanoparticle deposition, particle stability and activity, ultimately leading to new and improved catalysts for more sustainable processes.

This thesis has shown that colloidal particles can be successfully used to investigate different fundamental questions related to catalysis. When the methodology is developed and monitored using specific conditions in which the system is stable, these systems can also be used in combination with in situ electron microscopy. Even though this thesis is focused mostly on fundamental questions related to particle-support interactions for catalysis, it was also shown that the colloidal catalytical systems perform admirably in the Fischer-Tropsch to olefins reaction, with activities, selectivities and stabilities that compare or are even superior to conventional, impregnated catalysts. If such colloid-based catalysts could be effectively scaled up, the enhanced stability, activity and selectivity compared to conventional catalysts are envisaged, leading to ultimately superior catalytic performance.