Preparation of Cobalt Nanocrystals Supported on Metal Oxides To Study Particle Growth in Fischer-Tropsch Catalysts.

Colloidal synthesis of nanocrystals (NC) followed by their attachment to a support and activation is a promising route to prepare model catalysts for research on structure-performance relationships. Here, we investigated the suitability of this method to prepare well-defined Co/TiO and Co/SiO catalysts for the Fischer-Tropsch (FT) synthesis with high control over the cobalt particle size. To this end, Co-NC of 3, 6, 9, and 12 nm with narrow size distributions were synthesized and attached uniformly on either TiO or SiO supports with comparable morphology and Co loadings of 2-10 wt %. After activation in H, the FT activity of the TiO-supported 6 and 12 nm Co-NC was similar to that of a Co/TiO catalyst prepared by impregnation, showing that full activation was achieved and relevant catalysts had been obtained; however, 3 nm Co-NC on TiO were less active than anticipated. Analysis after FT revealed that all Co-NC on TiO as well as 3 nm Co-NC on SiO had grown to ∼13 nm, while the sizes of the 6 and 9 nm Co-NC on SiO had remained stable. It was found that the 3 nm Co-NC on TiO already grew to 10 nm during activation in H. Furthermore, substantial amounts of Co (up to 60%) migrated from the Co-NC to the support during activation on TiO against only 15% on SiO. We showed that the stronger interaction between cobalt and TiO leads to enhanced catalyst restructuring as compared to SiO. These findings demonstrate the potential of the NC-based method to produce relevant model catalysts to investigate phenomena that could not be studied using conventionally synthesized catalysts.