Highly loaded Co/SiO2 catalysts were prepared by a co-precipitation technique with an n-butanol drying process for Fischer-Tropsch synthesis (FTS). With the increase in cobalt loading from 20% to 80%, the FTS activity of the catalysts increased greatly. The addition of ZrO2 improved the dispersion of cobalt and promoted the adsorption of H-2 and CO on cobalt. The preadsorbed H atoms significantly enhanced the adsorption of CO and C2H4, especially on cobalt promoted by ZrO2. While the adsorption of C2H4 onto clean cobalt led to the formation of ethylidyne, pi- and di-sigma-bonded C2H4 might form on the CO-preadsorbed 80%Co/SiO2 and 80%Co-8%ZrO2/SiO2, respectively. The presence of ZrO2 strengthened the bonding of molecularly adsorbed ethylene and increased its uptake on CO-preadsorbed cobalt, which might be why the 80%Co-8%ZrO2/SiO2 exhibited such high activity for the hydrogenation of CO to heavy hydrocarbons. (C) 2011 Elsevier Inc. All rights reserved.