Colloidal Pt nanoparticles synthesized by a 1 : 5 concentration ratio of K2PtCl4 to polyacrylate were loaded on nanoporous alumina using the impregnation method at room temperature. The deposited Pt particles, present on the external surfaces of the support, were characterized by transmission electron microscopy, which indicated predominantly truncated octahedral (TO) shapes with a mean diameter of 10 nm. Their catalytic performance in the hydrogenation of propene at 30-80 degreesC was studied as a test reaction. The initial rate, reaction order, rate constant, activation energy, and turnover frequency were determined. The activation energy was found to be 8.4 +/-0.2 kcal/mol, which is slightly lower than results reported for other platinum systems (10-14 kcal/mol). The TO platinum nanoparticles have atom-high surface steps, ledges, and kinks, and these atomic-scale fine structures are expected to decrease the activation energy. The reactivity of the surface atoms in this nanoparticle is so high that above 50 degreesC side reactions leading to complete surface poisoning take place within a few minutes. The effect of the polymer concentration of the polyacrylate-capped TO Pt/Al2O3 on the hydrogenation catalytic activity was also investigated. (C) 2003 Elsevier Science (USA). All rights reserved.