A new method was developed to fabricate ultrafine cobalt nanoparticles with an average diameter of 2.8 nm from large precursory cobalt nanoparticles (around 20 nm). The obtained ultrafine nanoparticles have a narrow size distribution and a much higher Co/B ratio than that of precursory cobalt nanoparticles (almost 100 times higher). The 2.8 nm cobalt nanoparticles were used to catalyze hydroformylation of 1-hexene and syngas (CO:H-2 = 1) at 100 degrees C and 2.4 MPa, resulting in an average TOF (turnover frequency) of 130 h(-1). Through the mercury poisoning experiments, it was observed that the presence of Hg (0) can effectively suppress the catalytic activity of the 2.8 nm Co nanoparticle catalyst system, demonstrating that a heterogeneous catalysis mechanism was followed in the ultrafine catalyst system. (C) 2010 Elsevier B.V. All rights reserved.