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 11and PdHCu 12 can be considered as axially-distorted 2electron superatoms. PdHCu 11 shows excellent HER activity, unprecedented within metal nanoclusters, with an onset potential of À 0.05 V (at 10 mA cm À 2 ), a Tafel slope of 40 mV dec À 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.

Electrocatalytic CO 2 reduction reaction (CO 2 RR) is greatly facilitated by Au surfaces. However, large fractions of underlying Au atoms are generally unused during the catalytic reaction, which limits mass activity. Herein, we report a strategy for preparing efficient electrocatalysts with high mass activities by the atomic-level transplantation of Au active sites into a Ni 4 nanocluster (NC). While the Ni 4 NC exclusively produces H 2 , the Au-transplanted NC selectively produces CO over H 2 . The origin of the contrasting selectivity observed for this NC is investigated by combining operando and theoretical studies, which reveal that while the Ni sites are almost completely blocked by the CO intermediate in both NCs, the Au sites act as active sites for CO 2 -to-CO electroreduction. The Au-transplanted NC exhibits a remarkable turnover frequency and mass activity for CO production (206 mol CO /mol NC /s and 25,228 A/g Au , respectively, at an overpotential of 0.32 V) and high durability toward the CO 2 RR over 25 h.

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 11and PdHCu 12 can be considered as axially-distorted 2electron superatoms. PdHCu 11 shows excellent HER activity, unprecedented within metal nanoclusters, with an onset potential of À 0.05 V (at 10 mA cm À 2 ), a Tafel slope of 40 mV dec À 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.

Alloy catalysts are widely used in electrocatalytic hydrogen production because of the synergistic effect produced by constituent metals. However, an atomiclevel understanding of the alloy effect remains elusive because of their polydispersity. In this study, we investigated the effects of an atomically precise Ag 25 nanocluster catalyst doped with a single Ni atom in an alkaline electrolyte on the hydrogen evolution reaction (HER). The synthesized NiAg 24 nanoclusters exhibited significantly altered electronic structures and reduction potentials, leading to substantially reduced overpotentials and enhanced HER activity. Tafel analysis revealed that Ni doping drastically improved the sluggish Volmer step. Density functional theory studies further revealed that the hydrogen adsorption energy was substantially reduced upon Ni doping, which accounted for the enhanced HER activity observed for the NiAg 24 nanoclusters.

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