The actinide(IV) hexanuclear [M6(μ3‐O)4(μ3‐OH)4(HCOO)12(LT)6] complexes were prepared (LT = H2O or CH3OH). Their structures were investigated by single‐crystal X‐ray analysis and XAFS spectroscopy. HCOO– acts as a bridging ligand, which prevents the formation of polynuclear hydrolysis species like UIV hydrous oxide colloids at least up to pH = 3.25, and stabilizes the nanosized clusters in solution. The charge of the hexamer is balanced by the O/OH ratio of the μ3‐bridges.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The U(VI) complex with a pentadentate Schiff base ligand (N,N'-disalicylidenediethylenetriaminate = saldien(2-)) was prepared as a starting material of a potentially stable U(V) complex without any possibility of U(V)O(2)(+)...U(V)O(2)(+) cation-cation interaction and was found in three different crystal phases. Two of them had the same composition of U(VI)O(2)(saldien) x DMSO in orthorhombic and monoclinic systems (DMSO = dimethyl sulfoxide, 1a and 1c, respectively). The DMSO molecule in both 1a and 1c does not show any coordination to U(VI)O(2)(saldien), but it is just present as a solvent in the crystal structures. The other isolated crystals consisted only of U(VI)O(2)(saldien) without incorporation of solvent molecules (1b, orthorhombic). A different conformation of the coordinated saldien(2-) in 1c from those in 1a and 1b was observed. The conformers exchange each other in a solution through a flipping motion of the phenyl rings. The pentagonal equatorial coordination of U(VI)O(2)(saldien) remains unchanged even in strongly Lewis-basic solvents, DMSO and N,N-dimethylformamide. Cyclic voltammetry of U(VI)O(2)(saldien) in DMSO showed a quasireversible redox reaction without any successive reactions. The electron stoichiometry determined by the UV-vis-NIR spectroelectrochemical technique is close to 1, indicating that the reduction product of U(VI)O(2)(saldien) is [U(V)O(2)(saldien)](-), which is stable in DMSO. The standard redox potential of [U(V)O(2)(saldien)](-)/U(VI)O(2)(saldien) in DMSO is -1.584 V vs Fc/Fc(+). This U(V) complex shows the characteristic absorption bands due to f-f transitions in its 5f(1) configuration and charge-transfer from the axial oxygen to U(5+).

Simultaneous improvements in oxygen reduction reaction (ORR) activity and long-term durability of Ptbased cathode catalysts are indispensable for the development of next-generation polymer electrolyte fuel cells but are still a major dilemma. We present a robust octahedral core−shell PtNi x /C electrocatalyst with high ORR performance (mass activity and surface specific activity 6.8−16.9 and 20.3−24.0 times larger than those of Pt/C, respectively) and durability (negligible loss after 10000 accelerated durability test (ADT) cycles). The key factors of the robust octahedral nanostructure (core−shell Pt 73 Ni 27 /C) responsible for the remarkable activity and durability were found to be three continuous Pt skin layers with 2.0−3.6% compressive strain, concave facet arrangements (concave defects and high coordination), a symmetric Pt/Ni distribution, and a Pt 67 Ni 33 intermetallic core, as found by STEM-EDS, in situ XAFS, XPS, etc. The robust core−shell Pt 73 Ni 27 /C was produced by the partial release of the stress, Pt/Ni rearrangement, and dimension reduction of an as-synthesized octahedral Pt 50 Ni 50 /C with 3.6−6.7% compressive Pt skin layers by Ni leaching during the activation process. The present results on the tailored synthesis of the PtNi x structure and composition and the better control of the robust catalytic architecture renew the current knowledge and viewpoint for instability of octahedral PtNi x /C samples to provide a new insight into the development of next-generation PEFC cathode catalysts. KEYWORDS: robust octahedral core−shell PtNi x /C electrocatalyst, polymer electrolyte fuel cell, high performance and durability, continuous, compressive and concave Pt skin layers, structural and electronic property, in situ XAFS/STEM-EDS/XPS/ICP-AES

Complexation of Np(IV) with several carboxylates (RCOO(-); R = H, CH(3), or CHR'NH(2); R' = H, CH(3), or CH(2)SH) in moderately acidic aqueous solutions was studied by using UV-vis-NIR and X-ray absorption spectroscopy. As the pH increased, all investigated carboxylates initiated formation of water-soluble hexanuclear complexes, Np(6)(μ-RCOO)(12)(μ(3)-O)(4)(μ(3)-OH)(4), in which the neighboring Np atoms are connected by RCOO(-)syn-syn bridges and the triangular faces of the Np(6) octahedron are capped with μ(3)-O(2-)/μ(3)-OH(-). The structure information of Np(6)(μ-RCOO)(12)(μ(3)-O)(4)(μ(3)-OH)(4) in aqueous solution was extracted from the extended X-ray absorption fine structure data: Np-O(2-) = 2.22-2.23 Å (coordination number N = 1.9-2.2), Np-O(RCOO(-)) and Np-OH(-) = 2.42-2.43 Å (N = 5.6-6.7 in total), Np···C(RCOO(-)) = 3.43 Å (N = 3.3-3.9), Np···Np(neighbor) = 3.80-3.82 Å (N = 3.6-4.0), and Np···Np(terminal) = 5.39-5.41 Å (N = 1.0-1.2). For the simpler carboxylates, the gross stability constants of Np(6)(μ-RCOO)(12)(μ(3)-O)(4)(μ(3)-OH)(4) and related monomers, Np(RCOO)(OH)(2)(+), were determined from the UV-vis-NIR titration data: when R = H, log β(6,12,-12) = 42.7 ± 1.2 and log β(1,1,-2) = 2.51 ± 0.05 at I = 0.62 M and 295 K; when R = CH(3), log β(6,12,-12) = 52.0 ± 0.7 and log β(1,1,-2) = 3.86 ± 0.03 at I = 0.66 M and 295 K.