Surface Oxygen Atom as a Cooperative Ligand in Pd Nanoparticle Catalysis for Selective Hydration of Nitriles to Amides in Water: Experimental and Theoretical Studies

On the basis of an insight in surface science that Pd surfaces partially covered with oxygen adatoms (O-ad) show higher reactivity for water dissociation than clean Pd surfaces, we studied the effect of O-ad on the activity of carbon-supported palladium metal nanoparticle catalysts (Pd/C) for the selective hydration of nitriles to amides in water. A series of Pd/C with the same Pd loading (5 wt %) and a similar particle size (5.3-6.5 nm) but with different surface coverage of O-ad were prepared and characterized by various spectroscopic methods. The freshly H-2-reduced Pd/C shows no catalytic activity for hydration of acetonitrile, indicating that clean Pd metal surfaces are inactive. Air exposure of this catalyst under ambient conditions results in the formation of Pd metal NPs partially covered with O-ad, which act as effective and recyclable heterogeneous catalysts for selective hydration of various nitriles to the corresponding amides. Theoretical studies based on density functional theory calculations clarified a cooperative mechanism between metallic Pd and O-ad, in which O-ad as a Bronsted base site plays an important role in the dissociation of water via hydrogen bonding, and the mechanism is verified by kinetic results (activation energy, H2O/D2O kinetic isotope effect, Hammett slope). The mechanistic finding demonstrates a new design strategy of metal nanoparticle catalysts based on a molecular-level understanding of catalysis on oxygen-adsorbed metal surfaces.