In ethylene hydroformylation over Co/activated carbon (AC) catalyst, in addition to propanal, 3-pentanone was formed for the first time as one of a main product over heterogeneous cobalt catalyst. It was interesting to find that the properties of the supports, including both micropore proportion and the carbonaceous materials, are key factors in the formation of ketones. Based on the characterization results, cobalt carbonyl species was confirmed to be related with the formation of ketones on the surface of heterogeneous Co based catalysts. The small particle of cobalt and the high ratio of chemisorbed CO/H-2 might favor the formation of cobalt carbonyl clusters on the activated carbon supported cobalt catalysts. (c) 2012 Elsevier B.V. All rights reserved.