Olefin hydrogenation catalysis of platinum nanocrystals with different shapes

Abstract

We have investigated the catalysis of Pt nanocrystals for olefin hydrogenation in liquid phase by using several kinds of Pt nanocrystals with different shapes, whose results suggest that Pt cube shows the highest catalytic activity without significant shape change after the repeated hydrogenation. We prepared four nanocrystals according to our original methods: four kinds of Pt cubes (6.7, 8.2, 9.5, and 10.1 nm), Pt tetrahedron (4.6 nm), Pt nanowire (2.0 nm), and Pt cuboctahedron (7.5 nm) with high shape selectivity and narrow size distributions. To avoid the effect of different surface protecting agent on hydrogenation, we use the same polymer (polyacrylic acid) to stabilize four kinds of Pt nanocrystals. We adopted olefins with different structural characteristics, such as hexene, cyclohexene, cis-, and trans-stilbene. Turnover frequency (TOF) values for the hydrogenation of both cis- and trans-stilbene exhibit the following order: Pt cube > Pt cuboctahedron > Pt tetrahedron ≈ Pt nanowire, in accord with the ratio of Pt(100) on the surface of Pt nanocrystals. Raman spectroscopy of chemically adsorbed trans-stilbene on Pt cube and Pt tetrahedron indicates that the C–C double bonds of olefin group and phenyl group prefer to interact with Pt(100) facet, coinciding with its highest activity for olefin hydrogenation. The hydrogenation of cyclohexene as well as 1-hexene over Pt cubes with different sizes gave similar TOF values, suggesting that the hydrogenation takes place preferentially on the planes of Pt(100) facet in liquid phase.