Reaction pathways have been explored with periodic DFT calculations in order to understand the origin of the high selectivity for the hydrogenation of 1,3-butadiene on the Pt2Sn/Pt(1 1 1)-(root 3 x root 3)R30 degrees surface alloy. The adsorption structures of butadiene, 1-and 2-butenes and all the intermediate species have been studied. Compared to the reference catalyst Pt(1 1 1), there is a change both in geometries and in relative energies: for instance, the best adsorption mode of butadiene is cis 1,4-di-sigma-2,3-pi. while it is trans 1,2,3,4-tetra-sigma on Pt(1 1 1). On the alloy, all the adsorption energies are reduced compared to pure platinum, and the adsorption structures implying many Pt C bonds are more destabilized. The different pathways leading to partial hydrogenation products (1-butene, 2-butene) or other intermediate surface species (1,3- and 1,4-metallacycles) have been explored. The first hydrogenation step is clearly preferred at a terminal carbon, and the further hydrogenation to 1-butene has by far the lowest barrier. Other pathways exhibit larger activation barriers, particularly those leading to metallacycles, which is the key to explain the high selectivity to butene, in contrast to Pt(1 1 1). The role of tin is dual: a role of site blocking that forces unselective pathways to adopt distorted, high-energy transition states and a role of ligand that weakens the molecular adsorption and allows decoordination of double bonds prior to the hydrogenation, which decreases the energy barriers for the selective pathway to butene. (C) 2010 Elsevier Inc. All rights reserved.