A relationship between the size of metal particles and their catalytic activity has been established over a nanometer scale (2-10 nm). However, application on a subnanometer scale (0.5-2 nm) is difficult, a possible reason being that the activity no longer relies on the size but rather the geometric structure as a cluster (or superatomic) compound. We now report that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13). The synthesis with an atomic-level precision was successfully achieved by using a dendrimer ligand as the macromolecular template strictly defining the number of metal atoms. It was quite surprising that the Pt12 cluster exhibited more than 2-fold catalytic activity compared with that of the Pt13 cluster. ESI-TOF-mass and EXAFS analyses provided information about the structures. These analyses suggested that the Pt12 has a deformed coordination, while the Pt13 has a well-known icosahedral atomic coordination as part of the stable cluster series. Theoretical analyses based on density functional theory (DFT) also supported this idea. The present results suggest potential activity of the metastable clusters although they have been "missing" species in conventional statistical synthesis.

On a subnanometer scale, an only one-atom difference in a metal cluster may cause significant transitions in the catalytic activity due to the electronic and geometric configurations. We now report the atomicity-specific catalytic activity of platinum clusters with significantly small atomicity, especially below 20. The atomic coordination structure is completely different from that of the larger face-centered cubic (fcc) nanocrystals. Here, an electrochemical study on such small clusters, in which the atomicity ranged between 12 and 20, revealed Pt19 as the most catalytically active species. In combination with a theoretical study, a common structure that leads to a high catalytic performance is proposed.

4-(Fluoro­phenyl)­thiolate-protected Ag44 clusters (Ph4P)4[Ag44(SC6H4F)30] were calcined on mesoporous carbon (MPC) under vacuum at 300–500 °C for 2 h. X-ray absorption spectroscopy, transmission electron microscopy, and thermal-desorption mass spectrometry revealed that sulfur-free Ag44 clusters were successfully produced by the calcination of [Ag44(SC6H4F)30]4– at 300 °C, in sharp contrast to the formation of silver sulfide nanoparticles by the calcination of dodecanethiolate-protected Ag nanoparticles (3.0 ± 0.6 nm). Ag44/MPC was applied in the catalytic dehydrogenation of ammonia–borane (NH3BH3) as a test reaction. It turned out that the Ag44/MPC catalysts produced 1 equiv of H2 from NH3BH3 but only in the presence of O2 (turnover frequency 1.9 × 103 h–1 Ag atom–1). Given that nanoparticles of other metals (Pt, Pd, Rh, Ni, or Ru) produced 3 equiv of H2 under an inert atmosphere, this result indicates that the Ag44/MPC-catalyzed dehydrogenation of NH3BH3 proceeds by a different mechanism from that on other nanoparticles.

Gas-phase photoelectron spectroscopy( PES) was conducted on [XAg 24 (SPhMe 2 ) 18 ] À (X = Ag, Au) and [YAg 24 -(SPhMe 2 ) 18 ] 2À (Y = Pd, Pt), which have aformal superatomic core (X@Ag 12 ) 5+ or (Y@Ag 12 ) 4+ with icosahedral symmetry. PES results show that superatomic orbitals in the (Au@Ag 12 ) 5+ core remain unshifted with respect to those in the (Ag@Ag 12 ) 5+ core,whereas the orbitals in the (Y@Ag 12 ) 4+ (Y = Pd, Pt) core shift up in energy by about 1.4 eV.T he remarkable doping effect of asingle Yatom (Y = Pd, Pt) on the electronic structure of the chemically modified (Ag@Ag 12 ) 5+ superatom was reproduced by theoretical calculations on simplified model systems and was ascribed to 1) the weaker binding of valence electrons in Y@(Ag + ) 12 compared to Ag + @(Ag + ) 12 due to the reduction in formal charge of the core potential, and 2) the upward shift of the apparent vacuum level due to the presence of ar epulsive Coulomb barrier between [YAg 24 (SPhMe 2 ) 18 ] À and electron.Thiolate (RS)-or dithiolate (RS 2 )-protected silver clusters, such as [Ag 25 (SR) 18 ] À , [1] [Ag 29 (S 2 R) 12 ] 3À , [2] and [Ag 44 -(SR) 30 ] 4À , [3][4][5] are an emerging class of nanomaterials.S inglecrystal X-ray diffraction (SCXRD) analysis showed that [Ag 25 (SR) 18 ] À and [Ag 29 (S 2 R) 12 ] 3À have an icosahedral Ag 13 core (Scheme 1), [1,2] whereas [Ag 44 (SR) 30 ] 4À has at wo-shell Keplerate Ag 32 core. [5] TheA g 13 and Ag 32 cores form the closed-shell electronic configurations (1S) 2 (1P) 6 and (1S) 2 -(1P) 6 (1D) 10 ,r espectively:1 S, 1P,a nd 1D represent superatomic orbitals with angular momenta of 0, 1, and 2, respectively. [6] Structural similarities to gold analogues indicate that the thiolate-protected Ag clusters represent another family of chemically modified superatoms. [7][8][9] Thiolate-protected Ag clusters have attracted researchers due to specific properties such as photoluminescence [10] although they are generally less stable than the gold analogues.Doping with heteroatoms is apromising approach to enhance the stability and further improve the properties of the Ag clusters.S tate-of-the-art synthesis based on coreduction [11,12] and galvanic replacement [13,14] allowed us to precisely define the number, element, and location of the heteroatom(s) introduced into the Ag clusters.F or example, as ingle Ma tom (M = Au,P d, Pt) can be integrated exclusively at the central position of an icosahedral Ag 13 core of [Ag 25 (SPhMe 2 ) 18 ] À[1] to form M@Ag 12 cores in [AuAg 24 (SPhMe 2 ) 18 ] À[13] and [MAg 24 (SPhCl 2 ) 18 ] 2À (M = Pd, Pt) [11] (Scheme 1). Both the undoped Ag 13 and doped M@Ag 12 cores form ac losed electron configuration, (1S) 2 -(1P) 6 .These atomically defined bimetallic clusters provide an ideal platform to study the effect of single-atom doping on their properties.O ptical spectroscopy (Figure 1a)a nd voltammetry [15] showed that the doping slightly modulates the HOMO-LUMO gap of [Ag 25 (SPhMe 2 ) 18 ] À .T he stability [13,15] and photoluminescence quantum yield (PLQY) [10] of [A...