Revealing the Active Sites of Pd Nanocrystals for Propyne Semihydrogenation: From Theory to Experiment

The development of highly selective catalysts has been remarkably relying on the understanding of catalytic active sites. Pd-catalyzed semihydrogenation of propyne has been a focus of research with industrial applications toward the production of polymer-grade propylene. In this work, combining density functional theory (DFT) calculations and experimental observations, we propose that, different from the existing debates where the formation of palladium carbide (Pd–C) species or specific facets of Pd nanoparticles are critical, the apexes of Pd (111) octahedrons are the active sites for highly selective propyne semihydrogenation. The propylene selectivity on Pd octahedrons can be ascribed to site-selective propyne adsorption on the apexes prior to reactions and subsequent difficult to access intermediate states toward overhydrogenation. To reveal the active sites of Pd, propyne semihydrogenation was performed on shaped-Pd nanoparticles with designed exposed facets: e.g., (111) and (100) facets. Of practical importance, the propyne conversion and propylene selectivity exceed ∼94% and ∼96% on Pd octahedrons, respectively, at low temperature (35 °C) and atmospheric pressure. In addition, more control experiments have been performed to verify the effects of apexes of Pd octahedrons on propylene selectivity. It is shown that the propylene selectivity decreases to ∼50% when the apexes of Pd octahedrons are gradually removed. The experimental observations have further confirmed that the apexes of Pd octahedrons can be used as the active sites for propyne semihydrogenation, which is in good agreement with the results from theoretical calculations. This work may not only reveal the active sites of Pd nanoparticles for selective semihydrogenation of propyne but also open an avenue for designing highly active and selective catalysts in the chemical industry.