Cobalt Particle Size Effects in the Fischer–Tropsch Synthesis and in the Hydrogenation of CO2 Studied with Nanoparticle Model Catalysts on Silica

Abstract

We have investigated the effect of cobalt nanoparticle size in Fischer–Tropsch synthesis (CO/H2) and have compared it to data obtained for carbon dioxide hydrogenation (CO2/H2) using model catalysts produced by colloidal methods. Both reactions demonstrated size dependence, in which we observed an increase of the turnover frequency with increasing average particle size. In both case, a maximum activity was found for cobalt particles around 10–11 nm in size. Regarding the selectivity, no size-dependent effect has been observed for the CO2 hydrogenation, whereas CO hydrogenation selectivity depends both on the temperature and on the size of the particles. The hydrogenation of CO2 produces mainly methane and carbon monoxide for all sizes and temperatures. The Fischer–Tropsch reaction exhibited small changes in the selectivity at low temperature (below 250 °C) while at high temperatures we observed an increase in chain growth with the increase of the size of cobalt particles. At 250 °C, large crystallites exhibit a higher selectivity to olefin than to the paraffin equivalents, indicating a decrease in the hydrogenation activity.