Characterisation of Co/SiO 2 catalysts prepared from Co(acac) 3 by gas phase deposition

Abstract

Silica supported cobalt catalysts prepared by a chemical vapour deposition technique, atomic layer epitaxy (ALE), were characterised in order to study the interaction between cobalt species and silica. The catalysts were prepared by chemisorption of cobalt(III)acetylacetonate from the gas phase onto silica. The metal loading on the catalysts varied from 5.7 to 19.5 wt.%. Concentration profiles obtained with SEM/EDS showed that the cobalt species were evenly distributed through the catalyst particles. The dispersion was estimated by hydrogen chemisorption. The metallic cobalt was well dispersed on samples containing less than 6 wt.% cobalt but the dispersion decreased with increasing cobalt loading. XRD showed only weak reflections even for a sample containing 19.5 wt.%, which indicated weakly ordered cobalt species. The degree of reduction estimated by XPS was less than 30% on all samples even after reduction at 550°C for 7 h. The low reducibility could be explained by the formation of cobalt silicate during air calcination. The presence of silicates was also indicated by XRD and XPS. The catalysts were tested for gas phase toluene hydrogenation in a microreactor system. The reaction rate per gram sample increased with cobalt loading but the turn over frequency remained essentially constant on all samples. This indicated that the surface area of metallic cobalt is the main factor in determining the overall activity of the catalysts in this reaction.