Facet-dependent catalytic activity of shape-controllable palladium nanocrystals with clean surface in acetophenone selective hydrogenation

Shape-controlled metal nanocrystals have wide applications in catalysis. However, the capping agents playing significant roles in the shape-controlled synthesis often remain on the nanocrystal surfaces and are difficult to be removed completely, which not only causes the decrease in activity but also hinders the establishement of the correlation between various facets and intrinsic activity. In our present work, surface-clean Pd nanocrystals enclosed by different facets without capping agents were successfully synthesized by simply tuning the reduction kinetics in ascorbic acid mediated reduction of H2PdCl4 in water system. It was found that the turnover frequency (TOF) in acetophenone selective hydrogenation decreased with the decrease of the percentage of Pd{1 0 0} facet exposed in the Pd nanocrystals. The TOF value on Pd{1 0 0} facet was found 10 times higher than that on Pd {111} facet, while Pd{1 1 0} facet was found not favorable for acetophenone hydrogenation. Density functional theory (DFT) calculations reveal the more negative adsorption energy of acetophenone, the relatively shorter O-Pd distance between carbonyl group and the Pd surface, along with the much lower energy barrier in the acetophenone hydrogenation reaction, which leads to the Pd{1 0 0} surface exhibiting much higher TOF value than the other two surfaces. As for the Pd{1 1 0} surface, the long O-Pd distance, together with the large energy barrier makes it not favorable for the acetophenone hydrogenation.