Controlling Hydrogenation Selectivity by Size: 3-Hexyne on Supported Pt Clusters

Size-selected metal clusters supported on metal oxides have recently gained significant scientific attention because of their potential to investigate hydrogenation reactions on a fundamental level. To expand previous studies on ethylene hydrogenation, we report the selective hydrogenation of 3-hexyne using the same model systems of Pt clusters supported on MgO, but introducing the additional parameter of reaction selectivity. Isotopically labeled temperature-programmed reaction experiments show that the surface chemistry of 3-hexyne is dependent on cluster size and characterized by desorption of several reaction products. The latter include formation of molecules involving dehydrogenation as well as hydrogenation steps. By comparison between hydrogenation of hexyne and ethylene, an atomic window is found for Pt9, where activation barriers favor triple- over double-bond hydrogenation, effectively leading to enhanced selectivity. The favored hydrogenation of the triple bond is caused by cluster morphology and correlated adsorption sites available for the alkyne. The interplay between the cluster and adsorbed 3-hexyne leads to enhanced activation of hydrogen not observed for the bare metal clusters. This is the first experimental evidence of the potential use of supported, size-selected metal clusters for selective hydrogenation reactions.