OOH dissociation on Pt-2, Pt-3, Pt-4, Pt-7 and Pt-12 clusters is studied using density functional theory. The activation energy varies widely with the size and geometry of the clusters, from 2.23 kcal/mol for Pt-2 to 13.9 kcal/mol for Pt-12. Calculations on a single-layer Pt-12 used to emulate surface, edge, and corners of Pt(111) nanoparticles reveal that the energy barrier depends significantly on the adsorbate position on the cluster. The edge adsorption sites exhibit the highest catalytic activity. A correlation between the activation energy and the Pt-Pt distance suggests that the dissociation reaction can be facilitated by increasing the Pt-Pt distance. (C) 2007 Elsevier B.V. All rights reserved.