The stability of nanosized materials differs significantly from the stability of bulk materials. In this study a thermodynamic analysis on the simultaneous oxidation and re-reduction of small metallic cobalt crystallites in the presence of water and hydrogen as a function of the crystallite diameter was performed as a model for catalyst deactivation in the Fischer-Tropsch synthesis. It is shown that spherical cobalt crystallites with a diameter less than 4.4 nm are likely to be oxidized under realistic Fischer-Tropsch synthesis conditions (p(H)(2)(O)/p(H)(2) < 1.5, T = 493 K).

The Fischer-Tropsch synthesis is at the heart of the Biomass-to-Liquids (BTL) process. Feasibility studies published in open literature typically consider cobaltbased catalysts for the Fischer-Tropsch synthesis. Here, we present an overview on the history and development up until the present for both cobalt-and ironbased Fischer-Tropsch catalysts. The role of the support material and various other additives to the catalyst formulation are discussed in detail with regard to activity, catalyst deactivation, and selectivity. Tentative explanations for e.g. the observed size dependency in cobalt-based catalysts and phase transformations in iron-based Fischer-Tropsch catalysts are offered. The productivity of cobalt-based catalysts at high conversion level is currently higher than that of iron-based catalysts. Nevertheless, it is argued that iron-based catalysts may be an attractive option for the BTL-process, since it is much cheaper, impacting on the cost of the process due to inevitable process set-ups in industrial operation. Improvement of current iron-based catalysts is however desired.

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Sintering of supported cobalt nanoparticles is one of the main deactivation mechanisms in the Fischer–Tropsch synthesis. In this study, crystallite growth was studied with an alumina-supported catalyst in real time and as a function of process conditions using a novel in situ magnetometer. It could be shown that sintering with this catalyst occurred via a combination of high CO and high water partial pressures. It is proposed that particle growth proceeds via cobalt subcarbonyl migration over the hydroxylated support surface.

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