Effects of drying conditions on the synthesis of Co/SiO2 and Co/Al2O3 fischer-tropsch catalysts

Abstract

The nanoscale distribution of the supported metal phase is an important property for highly active, selective, and stable catalysts. Here, the nanoscale redistribution and aggregate formation of cobalt nitrate during the synthesis of supported cobalt catalysts were studied. Drying over a range of temperatures in stagnant air resulted in cobalt particles (8 nm) present in large aggregates (30-150 nm). However, drying in a N2 flow resulted in cobalt nanoparticles distributed either in aggregates or uniformly on various SiO2 and-Al2O3 supports, critically dependent on the drying temperature. The mechanism of aggregation was studied through chemical immobilization of the precursor on a silica support after drying in a N2 flow. The aggregation behavior upon drying in a gas flow at temperatures below 100 °C showed a remarkable similarity to distributions obtained upon the dewetting of colloidal films, suggesting a physical process. Alternatively, by inducing decomposition of the cobalt nitrate above 100 °C before drying was complete, aggregation was brought about through a chemical process that occurred both in stagnant and flowing gas. A-alumina support exhibited increased precursor-support interactions and displayed little cobalt aggregation upon drying in a gas flow but extensive aggregation upon drying in stagnant air. The aggregation behavior was further tested on silica supports with pore sizes between 3 and 15 nm and tested under industrially relevant Fischer-Tropsch conditions, which revealed that uniform cobalt nanoparticle distributions were up to 50% more active compared to aggregated systems. Thus, hydrodynamics and the temperature of the gas phase are critical parameters to control nanoscale distributions during drying of functional nanomaterials such as supported catalysts.