Size and structure effects on the oxygen reduction reaction on PtN clusters with N = 12-13 atoms have been investigated using periodic density functional theory calculations with the generalized gradient approximation. To describe the catalytic activity, we calculated the O and OH adsorption energies on the cluster surface. The oxygen binding on the 3-fold hollow sites on stable Pt12-13 cluster models resulted more favorable for the reaction with O, compared with the Pt13(Ih) and Pt55(Ih) icosahedral particles, in which O binds strongly. However, the rate-limiting step resulted in the removal of the OH species due to strong adsorptions on the vertex sites, reducing the utility of the catalyst surface. On the other hand, the active sites of Pt12-13 clusters have been localized on the edge sites. In particular, the OH adsorption on a bilayer Pt12 cluster is the closest to the optimal target; with 0.0-0.2 eV weaker than the Pt(111) surface. However, more progress is necessary to activate the vertex sites of the clusters. The d-band center of PtN clusters shows that the structural dependence plays a decisive factor in the cluster reactivity.

Structural properties of Ag n Rh (n ≤ 15) clusters are investigated using a successive growth algorithm coupled with density functional theory computations. The structures of the clusters are revisited, including a detailed discussion of their electronic properties. In contrast to these previous contributions, the lowest energy structures of the clusters are planar for n = 3−6, while threedimensional for n = 7 onward. Our present searches identify new lowest energy structures for n = 3−6 and 9−13. The most stable isomers are selected to study the adsorption of NO. The size-dependent reactivity of the clusters indicates that Rh atom acts as a more effective adsorption site for NO than Ag. Since the transition from Rh-exposed to Rh-encapsulated structures occurs at n = 9, the reactivity toward NO for Ag n Rh clusters with n ≤ 8 is considerably higher than that for the larger homologues. Further, the results show that doping Ag n clusters with Rh increases the reactivity toward NO adsorption.

Structural, magnetic and adsorption properties of Ni n (n = 2−16, 21, 55) clusters have been investigated based on density funciontal theory (DFT) with the spin polarized generalized gradient approximation, using the Perdew−Burke−Ernzerhof functional. The most stable isomers have been selected to study the adsorption of methane CH 4 and methyl CH 3 . It is found that the CH 4 molecule adsorbs on the top site for all clusters considered. The most selective Ni n clusters are the tetrahedron (n = 4) and icosahedral clusters due to highcoordinated edge atoms (n = 13, 21, and 55). For CH 3 , stronger adsorption tendencies were found with similar patterns. Our results show that clusters with n = 6, 10, and 13 with complete atomic shells are relatively more stable. Besides, they perform the lowest adsorption for CH 3 , indicating that they possess such a desirable property of a higher carbon poisoning resistance, than for the rest of the clusters. This result can be understood in terms of the electronic stability and localization of the frontier molecular orbitals. ■ INTRODUCTIONSmall clusters and nanoparticles have been widely studied in the past few years due to their high surface to volume ratio and enhanced reactivity properties compared with their bulk counterparts. 1 For example, gold nanoparticles are active catalyts although gold in the bulk is practically inert. 2,3 In catalytic reactions, they usually increase the selectivity and bond dissociation steps since they have more edge sites and lowcoordinated atoms. 4 Although in some reactions the presence of edge sites on the surface of nanoparticles can reduce the specific catalytic activity, 5 they are useful for different reactions, such as decomposition of hydrocarbons and alcohols, which can serve as important hydrogen sources. 6,7 Recently, much attention has been paid to the hydrogen production relating to the fuel cell technology. 8 In practice, hydrogen is produced from natural gas via steam reforming of methane, a process highly endothermic and very expensive, due to the high heat demand. 9 Among a wide range of heterogeneous catalyst, noble metal catalysts show higher activity and stability for this reaction, but the prohibitive cost and scarce resources makes their use very limited. On the other hand, Ni-based are promising catalysts in terms of cost and outstanding activity compared with noble-metals. However, the major problem of Ni-based systems is the strongly bonded carbon deposition on the catalyst surface which can deactivate the catalyst reducing its stability. 10 In a previous work, 11 we have shown that small Rh clusters possess an excellent selectivity to catalytically adsorb and dissociate the acid rain precursor N 2 O molecule. Furthermore, in the limit of going to arbitrary large clusters, it was interesting to compare this effect with that of a flat (111) slab of same metal, which happened to be provided with a much more smaller selectivity as the corresponding to small clusters. The problem, however, appears when we begin to consider cl...

The development of well-defined atomically precise heteronuclear nanoclusters passivated by protecting ligands is presently a booming area, owing to the fact that doping well-known homonuclear nanostructures allows fine-tuning of their properties. Here, we explore by means of density functional theory calculations the possibility of doping the central gold atom in the classical [Au13(dppe)5Cl2]3+cluster (1) by Os. Although both [Au13(dppe)5Cl2]3+ and [Os@Au12(dppe)5Cl2] have the same total number of electrons, we show that they are not isoelectronic within the formalism of the superatom model, being respectively an 8- and an 18-electron species. It results that they exhibit similar structures but present significantly different optical behaviors (ultraviolet/visible and circular dichroism). Similar results are obtained for the Ru and Fe relatives. Emission properties indicate some redshift of the T1→S1 decay with respect to [Au13(dppe)5Cl2]3+, involving an equatorial distortion of the Au12Cl2 core in the T1 state, rather than the axial distortion afforded by 1. The sizable highest occupied molecular orbital–lowest unoccupied molecular orbital gaps found for the three doped species suggest that further experimental exploration of different stable doped species derived from the ligand-protected Au12Cl2 core should be encouraged.

Structures and electronic properties of TinV (n = 1-16) clusters have been investigated using density functional theory with the generalized gradient approximatio. The calculations have shown that the TinV clusters favor compact spherical structures having similar conformations to the pure Tin clusters. The results show that the vanadium atom remains on the surface when n ⩽ 8 and n = 16, while for n = 9-15, it occupies the endohedral position. The Ti6V, Ti12V, and Ti14V clusters are found to be more stable than their neighbors, consistent with pure Tin clusters that have the same size. Additionally Ti4V has been found to be also magic, consistent with recent reports of B- and Al-doped Ti clusters. Small TinV (n ⩽ 4) clusters exhibit a transition from metallic-like to semimetallic electronic structure, while for n = 5 onward, no significant changes are observed compared with pure Tin clusters.