Control and impact of the nanoscale distribution of supported cobalt particles used in Fischer-Tropsch catalysis.

The proximity of nanoparticles may affect the performance, in particular the stability, of supported metal catalysts. Short interparticle distances often arise during catalyst preparation by formation of aggregates. The cause of aggregation of cobalt nanoparticles during the synthesis of highly loaded silica-supported catalysts was found to originate from the drying process after impregnation of the silica grains with an aqueous cobalt nitrate precursor. Maximal spacing of the Co3O4 nanoparticles was obtained by fluid-bed drying at 100 °C in a N2 flow. Below this temperature, redistribution of liquid occurred before and during precipitation of a solid phase, leading to aggregation of the cobalt particles. At higher temperatures, nucleation and growth of Co3O4 occurred during the drying process also giving rise to aggregation. Fischer-Tropsch catalysis performed under industrially relevant conditions for unpromoted and Pt-promoted cobalt catalysts revealed that the size of aggregates (13-80 nm) of Co particles (size ~9 nm) had little effect on activity. Large aggregates exhibited higher selectivities to long chain alkanes, possibly related to higher olefin formation with subsequent readsorption and secondary chain growth. Most importantly, larger aggregates of Co particles gave rise to extensive migration of cobalt (up to 75%) to the external surface of the macroscopic catalyst grains (38-75 μm). Although particle size did not increase inside the silica support grains, migration of cobalt to the external surface partly led to particle growth, thus causing a loss of activity. This cobalt migration over macroscopic length scales was suppressed by maximizing the distance between nanoparticles over the support. Clearly, the nanoscale distribution of particles is an important design parameter of supported catalysts in particular and functional nanomaterials in general.