Preparation and characterisation of spherical Co/SiO2 model catalysts with well-defined nano-sized cobalt crystallites and a comparison of their stability against oxidation with water

The oxidation of nanosized metallic cobalt to cobalt oxide during Fischer–Tropsch synthesis has long been postulated as a major deactivation mechanism apparently related to cobalt crystallite size. To establish a connection between cobalt crystallite size and oxidation behaviour, well-defined spherical Co/SiO2 model catalysts with average cobalt crystallites sizes of 4, 13, and 28 nm were synthesised. The crystallite size distribution of the spherical Co/SiO2 model catalysts was characterised with high-resolution transmission electron microscopy and in situ X-ray diffraction. The oxidation behaviour of the reduced spherical Co/SiO2 model catalysts of differing cobalt crystallite size was studied using in situ X-ray absorption fine structure under model oxidation conditions (H2O/He, PH2O=0.04 barPH2O=0.04 bar). Surprisingly, it was found that the spherical Co/SiO2 model catalyst with small cobalt crystallites (i.e., 4 nm) did not show oxidation under H2O/He mixtures (PH2O=0.04–0.3 barPH2O=0.04–0.3 bar) up to 400 °C, which is against bulk thermodynamic calculations for the oxidation of cobalt metal to cobalt oxide. This was attributed to the encapsulation of the cobalt crystallites with silica after reduction at 500 °C in hydrogen. The encapsulation was verified with high-resolution transmission electron microscopy. The spherical Co/SiO2 model catalysts with medium-sized cobalt crystallites (i.e., 13 nm) did oxidize at 100 °C and reached a maximum oxidation of 30% at 300 °C (H2O/He; PH2O=0.04 barPH2O=0.04 bar). The spherical Co/SiO2 model catalysts with large cobalt crystallites (i.e., 28 nm) was found to undergo very little oxidation, <2% at 300 °C under a H2O/He (PH2O=0.04 barPH2O=0.04 bar) environment. In general, it could be concluded that the oxidation of spherical Co/SiO2 model catalysts with water is difficult and is size-dependent.