Cobalt particle size effects in catalysis

Abstract

Aim of the work described in this thesis was first to investigate cobalt particle size effects in heterogeneous catalysis. The main focus was to provide a deeper understanding of the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis in which synthesis gas (H2/CO) is converted into hydrocarbons. It was shown earlier using among others cobalt on carbon nanofiber (CNF) catalysts that Co particles smaller than 6 nm display a decreased surface-specific activity (Turn-Over Frequency, TOF) and increased methane selectivity in FT experiments. In Chapter 2 Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to investigate the effect of the cobalt particle size on the surface residence times and coverages of FT reaction intermediates. It was concluded that the surface residence times of reversibly bonded CHx and OHx intermediates increased significantly for particles smaller than 6 nm, whereas a decrease was observed in the CO residence time. The surface coverages of the CHx, OHx and CO intermediates decreased for small particles and appeared constant for large particles. In contrast, an increase in the coverage of H and irreversibly bonded CO was observed for small (