Catalyst supports for polymer electrolyte fuel cells.

A major challenge in obtaining long-term durability in fuel cells is to discover catalyst supports that do not corrode, or corrode much more slowly than the current carbon blacks used in today's polymer electrolyte membrane fuel cells. Such materials must be sufficiently stable at low pH (acidic conditions) and high potential, in contact with the polymer membrane and under exposure to hydrogen gas and oxygen at temperatures up to perhaps 120 degrees C. Here, we report the initial discovery of a promising class of doped oxide materials for this purpose: Ti(1-x)M(x)O(2), where M=a variety of transition metals. Specifically, we show that Ti(0.7)W(0.3)O(2) is electrochemically inert over the appropriate potential range. Although the process is not yet optimized, when Pt nanoparticles are deposited on this oxide, electrochemical experiments show that hydrogen is oxidized and oxygen reduced at rates comparable to those seen using a commercial Pt on carbon black support.

Methanol electrooxidation on PtRu bulk alloys and carbon-supported PtRu nanoparticle catalysts: a quantitative DEMS study.

Methanol electrooxidation on smooth Pt and PtRu bulk alloys and carbon-supported Pt and PtRu nanoparticle catalysts has been studied using cyclic voltammetry and potential step chronoamperometry combined with differential electrochemical mass spectrometry (DEMS). The current efficiencies for generated CO2 and methyl formate were calculated from Faradaic current (coulometric charge) and mass spectrometric currents (charges) at m/z=44 and m/z=60. The effects of Ru content in PtRu catalysts, catalyst loading/roughness, and the concentration of sulfuric acid as supporting electrolyte on the reaction kinetics and product distribution during methanol electrooxidation have been investigated. The results indicate that Pt-rich PtRu alloys and carbon-supported PtRu catalysts with ca. 20 atom % Ru content exhibit the highest catalytic activity for methanol electrooxidation, that is, the highest Faradaic current and the highest current efficiency for CO2 generation at low applied potentials. As the catalyst loading/roughness increases, the current efficiency for CO2 formation increases due to the further oxidation of soluble intermediates (formaldehyde and formic acid). At high concentrations of sulfuric acid, the electrooxidation of methanol was suppressed; both the oxidative current and the current efficiency of CO2 decreased, likely due to sulfate/bisulfate adsorption.

Electrocatalytic mechanism and kinetics of SOMs oxidation on ordered PtPb and PtBi intermetallic compounds: DEMS and FTIRS study.

The electrocatalytic activities and mechanisms of PtPb and PtBi ordered intermetallic phases towards formic acid, formaldehyde and methanol oxidation have been studied by DEMS and FTIRS, and the results compared to those for a pure polycrystalline platinum electrode. While PtPb exhibits an enhanced electrocatalytic activity for the oxidation of all three organic molecules when compared to a Pt electrode, PtBi exhibits an enhanced catalytic activity towards formic acid and formaldehyde oxidation, but not methanol. FTIRS data indicate that adsorbed CO does not form on PtPb or PtBi intermetallic compounds during the oxidation of formic acid, formaldehyde and methanol, and therefore their oxidation on both PtPb and PtBi intermetallic compounds proceeds via a non-CO(ads) pathway. Quantitative DEMS measurements indicate that only CO(2) was detected as a final product during formic acid oxidation on Pt, PtPb and PtBi electrodes. At a smooth polycrystalline platinum electrode, the oxidation of formaldehyde and methanol produces mainly intermediates (formaldehyde and formic acid), while CO(2) is a minor product. In contrast, CO(2) is the major product for formaldehyde and methanol oxidation at a PtPb electrode. The high current efficiency of CO(2) formation for methanol and formaldehyde oxidation at a PtPb electrode can be ascribed to the complete dehydrogenation of formaldehyde and formic acid due to electronic effects. The low onset potential, high current density and high CO(2) yield make PtPb one of the most promising electrocatalysts for fuel cell applications using small organic molecules as fuels.

Synthesis and characterization of oxidized W(6)S(8)L(6) clusters.

Cationic [W(6)S(8)L(6)]PF(6) (L = PEt(3) (3), 4-tert-butylpyridine (4)) clusters were successfully synthesized and isolated for the first time by reacting the corresponding neutral W(6)S(8)L(6) (L = PEt(3) (1), 4-tert-butylpyridine (2)) clusters with [Cp(2)Fe]PF(6) as the oxidant. The products 3 and 4 were characterized by NMR spectroscopy, mass spectroscopy, and X-ray crystallography (only for 3) and shown to be the desired oxidized W(6)S(8) clusters with a metal electron count of 19. Magnetic property studies showed that they are paramagnetic compounds with S = (1)/(2). Their chemical properties and stability are also reported. Crystal data for 3.2 THF: space group, R3 (No. 148); a = 13.91170(10) A; c = 32.4106(2) A; Z = 3.

Synthesis, characterization, and ligand exchange studies of W(6)S(8)L(6) cluster compounds.

Eleven organic Lewis bases were investigated as potential ligands (L) on W(6)S(8)L'(6) clusters by exploring ligand exchange reactions to form W(6)S(8)L(6) clusters. Six new homoleptic W(6)S(8)L(6) cluster complexes were prepared and characterized with L = tri-n-butylphosphine (P(n)Bu(3)), triphenylphosphine (PPh(3)), tert-butylisocyanide ((t)BuNC), morpholine, methylamine (MeNH(2)), and tert-butylamine ((t)BuNH(2)). While partial replacement of ligands occurred with diethylamine (Et(2)NH) and dibutylamine (Bu(2)NH), homoleptic clusters could not be prepared by these exchange reactions. When aniline, tribenzylamine, and tri-tert-butylphosphine were the potential ligands, no exchange was observed. From ligand exchange studies of these ligands and others previously studied, a thermodynamic series of binding free energies for ligands on W(6)S(8)L(6) clusters was established as the following: non-Lewis base solvents, aniline, P(t)()Bu(3), etc. << Et(2)NH, Bu(2)NH < (t)BuNH(2) < morpholine, piperidine < or = (n)BuNH(2), MeNH(2) < or = 4-tert-butylpyridine, pyridine < (t)BuNC < tricyclohexylphosphine (PCy(3)) < PPh(3), P(n)Bu(3) < or = triethylphosphine (PEt(3)). Structures of the new cluster complexes were determined by X-ray crystallography. The new compounds were also characterized by NMR spectroscopy and thermogravimetric analyses (TGA). The W-L bond orders and TGA data qualitatively agree with the thermodynamic series above.

Novel octahedral tungsten sulfidocyanide cluster anion [W6S8(CN)6]6-.

Novel tungsten octahedral sulfidocyanide cluster compounds Na6[W6S8(CN)6].18DMSO 1 and K6[W6S8(CN)6] 2 have been synthesized and characterized by X-ray crystallography and NMR spectroscopy.

New calcium germanium nitrides: Ca2GeN2, Ca4GeN4, and Ca5Ge2N6.

We report three new calcium germanium nitrides synthesized as crystals from the elements in sealed niobium tubes at 760 degrees C using liquid sodium as a growth medium. Black Ca2GeN2 is isostructural with the previously reported strontium analogue. It is tetragonal P4(2)/mbc (no. 135) with a = 11.2004(8) A, c = 5.0482(6) A, and Z = 8. It contains GeN2(4-) units which have 18 valence electrons, and consequently are bent, like the isoelectronic molecule SO2. In contrast, clear, orange Ca4GeN4 with fully oxidized germanium contains isolated GeN4(8-) tetrahedra and is monoclinic P2(1)/c (no. 14) with a = 9.2823(8) A, b = 6.0429(5) A, c = 11.1612(9) A, beta = 116.498(6) degrees, and Z = 4. Clear, colorless Ca5Ge2N6, also with fully oxidized germanium, contains infinite chains, 1 infinity[GeN2N2/2(5-)], of corner-sharing tetrahedra similar to those found in pyroxenes. However, the precise structure of this latter phase has not yet been determined because of twinning problems.

Ligand substitution reactions of W6S8L6 with tricyclohexylphosphine (L = 4-tert-butylpyridine or n-butylamine): 31P NMR and structural studies of W6S8(PCy3)n(4-tert-butylpyridine)6-n (0 < n < or = 6) complexes.

The substitution reactions by bulky tricyclohexylphosphine (PCy3) ligands on W6S8L6 (L = 4-tert-butylpyridine or n-butylamine) clusters were investigated to prepare clusters with mixed axial ligands for low-dimensional cluster linking. When 4-6 equiv of PCy3 are used to react with W6S8(4-tert-butylpyridine)6 (4) in THF, cis-W6S8(PCy3)4(4-tert-butylpyridine)2 (1) is preferentially formed. But when starting with W6S8(n-butylamine)6 (2), only W6S8(PCy3)6 (3) is produced with 6 equiv of PCy3. Other conditions with fewer equivalents of PCy3 led to mixtures of partially substituted complexes in the W6S8L6-n(PCy3)n (0 < or = n < or = 6, L = 4-tert-butylpyridine or n-butylamine) series. A significantly distorted structure for 1 helps to explain its preferential formation. 1H NMR spectra were collected for clusters 1 and 2 and 31P NMR spectra for 1 and W6S8(4-tert-butylpyridine)6-n(PCy3)n complexes. P-P coupling through P-W-W-P is reported for the first time in octahedral metal clusters and shown to be very useful in identifying nearly all the W6S8L6-n(PR3)n complexes and their stereoisomers in the mixtures even before individual species are isolated.

Synthesis and Structure of One-, Two-, and Three-Dimensional Alkaline Earth Metal Gallium Nitrides: Sr(3)Ga(2)N(4), Ca(3)Ga(2)N(4), and Sr(3)Ga(3)N(5).

We report the structures of three new alkaline earth metal gallium nitrides synthesized as crystals from the elements in sealed Nb tubes at 760 degrees C using Na/Sr and Na/Ca melts as growth media. The materials are all transparent insulators. Yellow Sr(3)Ga(2)N(4) is isostructural with the previously reported Ba(3)Ga(2)N(4) and Sr(3)Al(2)N(4). It crystallizes in Pnna (No. 52) with a = 5.9552(6) Å, b = 10.2753(8) Å, c = 9.5595(9) Å, and Z = 4. It contains infinite chains, [GaN(4/2)(3)(-)], of trans-edge-shared tetrahedra extending along the b axis. Colorless Ca(3)Ga(2)N(4) is isostructural with Ca(3)Al(2)As(4) and gamma-Ca(3)Al(2)N(4). It crystallizes in C2/c (No. 15) with a = 10.6901(11) Å, b = 8.3655(7) Å, c = 5.5701(4) Å, beta = 91.194(6) degrees, and Z = 4. It contains infinite sheets, [GaN(4/2)(3)(-)], of edge- and corner-sharing tetrahedra in the bc plane. Orange-yellow Sr(3)Ga(3)N(5) has a new structure; it crystallizes in P&onemacr; (No. 2) with a = 5.9358(6) Å, b = 7.2383(8) Å, c = 8.6853(12) Å, alpha = 108.332(10) degrees, beta = 103.783(9) degrees, gamma = 95.326(8) degrees, and Z = 2. It is one of the few materials prepared from Na melts which has an infinite three-dimensional framework structure. It contains a [Ga(3)N(5)(6)(-)] framework of edge- and corner-sharing GaN(4) tetrahedra.

Synthesis and Structure of Two New Strontium Germanium Nitrides: Sr(3)Ge(2)N(2) and Sr(2)GeN(2).

We report the structures of two new strontium germanium nitrides synthesized as crystals from the elements in sealed Nb tubes at 750 degrees C using liquid Na as a growth medium. Sr(3)Ge(2)N(2) is isostructural with the previously reported Ba analogue. It crystallizes in P2(1)/m (No. 11), with a = 9.032(2) Å, b = 3.883(1) Å, c = 9.648(2) Å and beta = 112.42(3) degrees, and has two formula units per unit cell. It contains GeN(2)(4)(-) units and additionally |Ge(2)(-) zigzag chains. Sr(2)GeN(2) crystallizes in P4(2)/mbc (No. 135) with a= b = 11.773(2) Å and c= 5.409(1) Å and has Z = 8. It also contains GeN(2)(4)(-) units which have 18 valence electrons and, consequently are bent, like the isoelectronic molecule SO(2).

Solid-state chemistry: a a rediscovered chemical frontier.

Chemical bonding in solids is not completely understood, mainly because of the wide variation in the chemical properties of the elements. Many difficult challenges remain in predicting the composition, structure, and the properties of new materials. Consequently, the synthesis of novel solids is as much an art as a science. Discoveries of new compounds and structure types highlight the versatility that nature has allowed with the relatively small number of elements. This article explores the long-term challenges in solid-state chemistry and then focuses on efforts at Cornell to prepare new solids.

Intercalation complexes of lewis bases and layered sulfides: a large class of new superconductors.

Exploration of the generality of the recently discovered reaction whereby certain organic molecules can be inserted between the metallic layers of the superconductors tantalum disulfide and niobium disulfide revealed that a large variety of organic and inorganic molecules can penetrate between the crystalline layers of a number of transition metal dichalcogenides and that the resulting complexes are superconducting if the layered chalcogenide from which they are formed is superconducting. The critical temperatures of the 50 new superconductors we report depend on the nature of the intercalate but are insensitive to a separation of the superconducting planes of up to 57 angstroms.

Superconductivity in layered structure organometallic crystals.

Superconductivity persists in several, layered, transition metal dichalcogenide superconductors when the layers are spread apart to accommodate organic molecules between them. These materials are of interest not only because of their two-dimensional character but also because they may provide a means for examining hypotheses regarding organic molecules and superconductivity.