Interfacial oxidized Pd species dominate catalytic hydrogenation of polar unsaturated bonds

The determination of catalytically active sites is crucial for the design of efficient and stable catalysts toward desired reactions. However, the complexity of supported noble metal catalysts has led to controversy over the locations of catalytically active sites (e.g., metal, support, and metal/support interface). Here we develop a structurally controllable catalyst system (Pd/SBA-15) to reveal the catalytic active sites for the selective hydrogenation of ketones to alcohol using acetophenone hydrogenation as model reaction. Systematic investigations demonstrated that unsupported Pd nanocrystals have no catalytic activity for acetophenone hydrogenation. However, oxidized Pd species were catalytically highly active for acetophenone hydrogenation. The catalytic activity decreased with the decreased oxidation state of Pd. This work provides insights into the hydrogenation mechanism of ketones but also other unsaturated compounds containing polar bonds, e.g., nitrobenzene, N-benzylidene-benzylamine, and carbon dioxide.