Exact analytical formulae for mean coordination numbers in clusters

Fritsche, H.‐G.; Benfield, Robert E.

doi:10.1007/bf01425603

Cited by 77 publications

(64 citation statements)

References 16 publications

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“…In order to confirm the conclusion further, we study the melting behavior of Ni55 clusters. We find that the overall melting The overall T, and surface T, melting temperatures for the 13and 55-Ni atom clusters obtained by our simulation, T*, and TZ given by (5) and the modified equation ( 5 ) , and C,,, and C, taken from [22]. The bulk melting temperature [24] temperature T, of Nis5 clusters is higher than that of Nil3 in agreement with the experimental results.…”

Section: Resultssupporting

confidence: 84%

Surface thermal stability of nickel clusters

Chen¹,

Zhao²,

Sun³

et al. 1996

Physica Status Solidi (b)

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Overall and surface melting of NiN clusters have been studied using Monte Carlo simulation and effective interaction model potential. Physical quantities such as internal energy per atom, specific heat, and bond length fluctuations are calculated as a function of temperature to illustrate overall and surface thermal softening of these clusters. It is found that the surface melting temperature is lower than the overall melting temperature for a Ni cluster of certain size. The results show that the surface and core atoms play different roles in the melting process. The recent experimental observations about metal cluster melting might be explained by using surface melting arising from the surface atom disorder first appearing due to an increase of temperature.

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Section: Resultssupporting

confidence: 84%

Surface thermal stability of nickel clusters

Chen¹,

Zhao²,

Sun³

et al. 1996

Physica Status Solidi (b)

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“…8,26,69 To achieve close-packing of the surface atoms in a 13-atom icosahedron, the bond distances between atoms on the surface of the cluster must be 5% longer than the distances between the surface atoms and the central atom. 69,70 Taking this 5% strain into consideration, the SS3 path length expected for the icosahedral structure would be 4.65 Å. This is shorter than the 4.75 Å measured path length.…”

Section: Resultsmentioning

confidence: 90%

Metal Core Bonding Motifs of Monodisperse Icosahedral Au₁₃ and Larger Au Monolayer-Protected Clusters As Revealed by X-ray Absorption Spectroscopy and Transmission Electron Microscopy

Ménard

Gao

et al. 2006

J. Phys. Chem. B

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The atomic metal core structures of the subnanometer clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) were characterized using advanced methods of electron microscopy and X-ray absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 +/- 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180[S(CH2)11CH3]40. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer.

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“…This is based on a cubo-octahedron cluster model described by algorithms of Fritsche and Benfield. 38 This prior report clearly showed Pt-Mo alloy formation on the basis of fits in K-space (k 3 weighted͒, where unique solutions to two-shell fits ͑Pt-Pt and Pt-Mo͒ were reported.…”

Section: Resultsmentioning

confidence: 94%

Electrocatalysis of CO Tolerance by Carbon-Supported PtMo Electrocatalysts in PEMFCs

Mukerjee

Urian

Lee

et al. 2004

J. Electrochem. Soc.

122

104

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This paper is a full version of an earlier short communication, where significantly higher ͑up to threefold͒ CO tolerance was reported for PtMo/C ͑atomic ratio, Pt:Mo, 3:1͒ relative to the current state-of-the-art PtRu/C ͑1:1͒ in a proton exchange membrane fuel cell ͑PEMFC͒ under standard operating conditions ͑85°C, 100% humidification, with H 2 ϩ 100 pm CO//O 2 ). We report significantly different behavior for PtMo/C in contrast to PtRu/C, wherein there is negligible variation in CO tolerance ͑100 ppm CO in H 2 ) with variations in alloying compositions ͑Pt:Mo, 1:1 to 5:1͒. Further, in contrast to Pt/C and PtRu/C, significantly lower variations in overpotential losses is observed for PtMo/C as a function of temperature ͑55-115°C͒ and CO concentrations ͑5-100 ppm, balance H 2 ). In addition, excellent long-term stability is reported for PtMo/C ͑1:1͒ under steady-state conditions ͑constant potential conditions at 0.6 V͒ for a total duration of 1500 h, with anode gas composition varied between pure H 2 and those with 100 ppm CO, with or without the presence of other reformate gases ͑primarily CO 2 and N 2 ). These are discussed in the context of detailed physicochemical characterization of the nanoparticles using a combination of X-ray diffraction, transmission electron microscopy, and in situ synchrotron X-ray absorption spectroscopy.CO tolerance in reformer-based low-and medium-temperature proton exchange membrane fuel cells ͑PEMFCs͒ is crucial for the viability of this technology for transportation and portable power applications. The choice of an appropriate anode electrocatalyst with low susceptibility to CO poisoning and a high kinetic rate for hydrogen oxidation is therefore paramount. Despite its superior activity for anodic, hydrogen oxidation, and interfacial stability under acidic pH conditions and the operating temperatures of an actual PEMFC, electrocatalysis by Pt/C suffers from the problem of high polarization losses due to CO poisoning. This is especially true for temperatures below 115°C.The large affinity for CO chemisorption at potentials lower than ϳ0.65 V on Pt/C necessitates the search for other nanophase Ptbased electrocatalysts capable of initiating CO oxidation, preferably close to the hydrogen oxidation potential. This need manifested in the ''bifunctional'' 1,2 approach, where a second, more oxidizable element, present either as an admetal or as an alloying element, initiates the CO oxidation at lower potentials. The result is enough bare surface on Pt crystallites to efficiently oxidize hydrogen at lower overpotentials.Prior literature, involving several decades of research, is replete with investigations of alloys such as PtSn, 3,4 PtRh, 5 PtRu, 6-8 and Pt with adsorbing adatoms such as Ge, Sb, and Sn, 2,9 etc. In recent years nanophase PtRu electrocatalysts have received renewed attention as promising candidates for CO oxidation in PEMFCs. 10-12 A recent report by Oetjen et al., 13 using steady-state polarization data, indicates a fourfold performance enhancement with highly dispersed ...

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Exact analytical formulae for mean coordination numbers in clusters

Cited by 77 publications

References 16 publications

Surface thermal stability of nickel clusters

Surface thermal stability of nickel clusters

Metal Core Bonding Motifs of Monodisperse Icosahedral Au₁₃ and Larger Au Monolayer-Protected Clusters As Revealed by X-ray Absorption Spectroscopy and Transmission Electron Microscopy

Electrocatalysis of CO Tolerance by Carbon-Supported PtMo Electrocatalysts in PEMFCs

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Exact analytical formulae for mean coordination numbers in clusters

Cited by 77 publications

References 16 publications

Surface thermal stability of nickel clusters

Surface thermal stability of nickel clusters

Metal Core Bonding Motifs of Monodisperse Icosahedral Au13 and Larger Au Monolayer-Protected Clusters As Revealed by X-ray Absorption Spectroscopy and Transmission Electron Microscopy

Electrocatalysis of CO Tolerance by Carbon-Supported PtMo Electrocatalysts in PEMFCs

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Metal Core Bonding Motifs of Monodisperse Icosahedral Au₁₃ and Larger Au Monolayer-Protected Clusters As Revealed by X-ray Absorption Spectroscopy and Transmission Electron Microscopy