Supported cobalt nanocrystals as model catalysts for the Fischer-Tropsch synthesis

Abstract

Heterogeneous catalysis is an essential technology for the production of fuels and chemicals. The demand for fossil fuels, and in particular liquid fuels for transportation, is expected to grow in the next two decades while limitations on CO2 emissions are likely to become a major constraint for the production. Gas-to-liquids technology has the potential to reduce the overall CO2 footprint of liquid fuels and to produce ultra clean fuels as an additional benefit. The Fischer-Tropsch (FT) synthesis, i.e. the catalytic conversion of CO and H2 to long-chain hydrocarbons, is an important component of the gas-to-liquids process. Nanosized cobalt particles on a support material are often used to catalyze FT and the structure of the nanoparticles can strongly affect their catalytic performance. For example, Co nanoparticle size and crystal structure effects have already been reported. Further studies of cobalt structure performance relationships in FT could be facilitated with well-defined model catalysts based on cobalt nanocrystals (Co-NC), prepared using colloidal techniques. This provides, in principle, control over the size, shape, and composition of the Co-NC. However, subsequent attachment of the Co-NC to a support and activation are equally important for catalytic applications, but these are relatively unexplored research fields for non-noble metals. The aim of the research described in this thesis is to link the structure of supported cobalt catalysts to their performance in the Fischer-Tropsch synthesis using advanced model catalysts prepared through colloidal synthesis techniques. Through this approach, this work further aims to investigate and expand the applicability of nanocrystal-based catalysts for fundamental studies. Chapter 2 describes how to attach the NC to a support and how to mildly activate them in order to obtain highly active FT catalysts. In Chapter 3, the cause for the difference in FT performance between pristine and surface-oxidized carbon nanotubes (CNT) is investigated. Well-defined Co-NC/CNT catalysts and advanced characterization techniques (STEM-EDX mapping, in situ XRD) are applied to isolate these effects. In Chapter 4, the preparation of a range of Co-NC of different sizes (3, 6, 9 and 12 nm) on TiO2 and SiO2 supports is reported. Using these catalysts, the effect of particle size and support on the growth of Co-NC during activation and FT is investigated. Chapter 5 describes the synthesis of disk-shaped Co-NC supported on SiO2. Their FT performance is compared to that of spherical Co-NC and related to the structure of the catalysts. In Chapter 6, recent literature on the interaction between metal nanoparticles and supports is reviewed in with a focus on strategies to tune metal-support interactions to enhance catalytic performance. Chapter 7 focuses on cobalt-support interactions that occur specifically on reducible supports such as TiO2 and Nb2O5. A straightforward reduction-oxidation-reduction treatment is reported that doubles the accessible metallic surface area and cobalt-weight-based activity, while maintaining constant intrinsic activity. Finally, in Chapter 8, a summary and concluding remarks are provided.