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

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• 作者：Ken-ichi Shimizu ; Takahiro Kubo ; Atsushi Satsuma ; Takashi Kamachi ; Kazunari Yoshizawa
• 刊名：ACS Catalysis
• 出版年：2012
• 出版时间：December 7, 2012
• 年：2012
• 卷：2
• 期：12
• 页码：2467-2474
• 全文大小：402K
• 年卷期：v.2,no.12(December 7, 2012)
• ISSN：2155-5435

文摘

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鈥?.5 nm) but with different surface coverage of O_ad were prepared and characterized by various spectroscopic methods. The freshly H₂-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 Br酶nsted 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.

Keywords:

bifunctional catalysis; density functional theory; hydration of nitriles; palladium