Hydride-containing 2-Electron Pd/Cu Superatoms as Catalysts for Efficient Electrochemical Hydrogen Evolution.

The first hydride-containing 2-electron palladium/copper alloys, [PdHCu 11 {S 2 P(O i Pr) 2 } 6 (C≡CPh) 4 ] (PdHCu 11 ) and [PdHCu 12 {S 2 P(O i Pr) 2 } 5 {S 2 PO(O i Pr)} (C≡CPh) 4 ] (PdHCu 12 ), are synthesized from the reaction of [PdH 2 Cu 14 {S 2 P(O i Pr) 2 } 6 (C≡CPh) 6 ] (PdH 2 Cu 14 ) with trifluoroacetic acid (TFA). X-ray diffraction reveals that the PdHCu 11 and PdHCu 12 kernels consist of a central PdH unit encapsulated within a vertex-missing Cu 11 cuboctahedron and complete Cu 12 cuboctahedron, respectively. DFT calculations indicate that both PdHCu 11 and PdHCu 12 can be considered as axially-distorted 2-electron superatoms. PdHCu 11 shows excellent HER activity, unprecedented within metal nanoclusters, with an onset potential of -0.05 V (at 10 mA cm -2 ), a Tafel slope of 40 mV dec -1 , and consistent HER activity during 1000 cycles in 0.5 M H 2 SO 4 . Our study suggests that the accessible central Pd site is the key to HER activity and may provide guidelines for correlating catalyst structures and HER activity.