Showing papers on "Platinum published in 1992"

Thin-film catalyst layers for polymer electrolyte fuel cell electrodes

Abstract: New structures for the Pt/C catalyst layer of polymer electrolyte fuel cell electrodes have been developed that substantially increase the utilization efficiency of the catalyst. Fabricating the catalyst layers and gas diffusion backings separately makes it possible to formulate each structure with the properties that are most suitable for its function. In the case of the catalyst layer, the optimal properties are hydrophilicity, thinness, uniformity, and the proper ratio of ionomer and supported catalyst. The catalyst layers are cast from solution as thin films that utilize the ionomer itself as a binder. The thin films are hot pressed directly onto the ionomer membranes, and the hydrophobic gas diffusion backings are inserted when the cells are assembled. The performances of fuel cells based on the thin film catalyst layers are comparable with those of gas diffusion electrode designs that utilize several times as much platinum, thus the specific activities of the Pt catalysts in the new structures are significantly higher.

Temperature Dependence of the Electrode Kinetics of Oxygen Reduction at the Platinum/Nafion® Interface—A Microelectrode Investigation

Abstract: Results of a study of the temperature dependence of the oxygen reduction kinetics at the Pt/Nafion interface are presented. This study was carried out in the temperature range of 30-80 C and at 5 atm of oxygen pressure. The results showed a linear increase of the Tafel slope with temperature in the low current density region, but the Tafel slope was found to be independent of temperature in the high current density region. The values of the activation energy for oxygen reduction at the platinum/Nafion interface are nearly the same as those obtained at the platinum/trifluoromethane sulfonic acid interface but less than values obtained at the Pt/H3PO4 and Pt/HClO4 interfaces. The diffusion coefficient of oxygen in Nafion increases with temperature while its solubility decreases with temperature. These temperatures also depend on the water content of the membrane.

All-solid-state sodium-selective electrode based on a calixarene ionophore in a poly(vinyl chloride) membrane with a polypyrrole solid contact

Abstract: Sodium-selective coated wire electrodes (CWE) and solid contact (SC) electrodes have been constructed and investigated. The CWE is based on the application of a poly(vinyl chloride) (PVC) membrane incorporating the sodium-selective ionophore tetraethyl ester of p-tert-butylicalix[4]arene onto the surface of a platinum disk. The SC electrode is based on the use of a conducting polymer, polypyrrole (PPy), doped with NaBF 4 as the mediating layer between platinum and the same PVC membrane as above

Methanol oxidation in acid on ordered NiTi

Abstract: A 50–50 ordered NiTi alloy is shown to form a passivating layer that makes it electrochemically stable in acid solution and to be an electrocatalyst for the methanol oxidation reaction To elucidate the reaction pathways, the electrochemical properties of Ti and Ni electrodes were compared with those of NiTi and Pt electrodes Alloying Ti with Ni suppresses the corrosion reaction on Ni electrodes in acid by shifting the potential for Ni2+ion dissolution to a value above that at which the Ni2+ ions at the surface are converted to Ni3+ ions NiTi electrodes, like Ni and Ti electrodes, form oxyhydroxide films at anodic potentials, and the thickness of the film increases with applied anodic voltage Methanol oxidation occurs on the oxyhydroxide films Dissociative chemisorption requires a potential sufficiently anodic of the flat-band potential of the film composition at the surface, or threshold potential Eth for the creation of surface Ni3+ ions, for electron transfer to take place via tunnelling across a Schottky barrier to either a conduction band, in the case of Ti, or an Ni3+/2+ redox band in the case of Ni In the case of NiTi, the former mechanism appears to be operative at lower potentials, the latter at higher potentials The reaction is completed with an attack by water on a surface carboxy group, which also requires electron tunnelling across a depletion layer Methanol oxidation on NiTi is more active than on Ti or Ni, and it is completed at a potential E < 123 V A distinction was found between p-type and n-type oxyhydroxide layers The n-type layers, formed only occasionally, gave cyclic voltammograms which resembled those obtained on elemental platinum

Metal complexes of buckminsterfullerene (C60)

Abstract: This account describes the author's work on the attachment of metal complexes directly to the exterior of the C[sub 60] framework via solution chemistry forming metal-C[sub 60] bonds. Ruthenium, platinum, palladium, and nickel complexes are discussed.

Membrane catalyst layer for fuel cells

Abstract: A gas reaction fuel cell (10) incorporates a thin catalyst layer (22) between a solid polymer electrolyte (SPE) membrane (26) and a porous electrode backing (18). The catalyst layer is preferably less than about 10 νm in thickness with a carbon supported platinum catalyst loading less than about 0.35 mgPt/cm2. The layer (22) is formed as an ink that is spread and cured on a film release blank. The cured film is then transferred to the SPE membrane (26) and hot pressed into the surface to form a catalyst layer (22) having a controlled thickness and catalyst distribution. Alternatively, the catalyst layer (22) is formed by applying an Na+ form of a perfluorosulfonate ionomer directly to the membrane (26), drying the film at a high temperature, and then converting the film back to the protonated form of the ionomer. The layer (22) has adequate gas permeability so that cell performance is not affected and has a density and particle distribution effective to optimize proton access to the catalyst and electronic continuity for electron flow from the half-cell reaction occurring at the catalyst.

Unwinding of supercoiled DNA by platinum-ethidium and related complexes

Abstract: A gel electrophoretic mobility shift assay has been used to determine the unwinding of closed circular, supercoiled pUC 19 plasmid DNA induced by a variety of platinum complexes that differ in their coordination mode to the double helix. Included are the anticancer drug cisplatin and several of its analogues, compounds in which the organic intercalator ethidium is coordinated to platinum through an exocyclic amino group, and molecules in which two platinum centers are tethered by a polymethylene chain

Catalyst for catalytic reduction of nitrogen oxides

Abstract: A novel catalyst for catalytic reduction of nitrogen oxide using a hydrocarbon as a reducing agent is provided, which comprises (a) at least an element of the platinum group selected from the group consisting of platinum, iridium, rhodium and ruthenium and (b) gold, and which may further comprises (c) at least a metallic oxide selected from the group consisting of cerium oxide, lanthanum oxide, neodymium oxide, germanium oxide and gallium oxide.

Pressure Dependence of the Oxygen Reduction Reaction at the Platinum Microelectrode/Nafion Interface: Electrode Kinetics and Mass Transport

Abstract: The investigation of oxygen reduction kinetics at the platinum/Nafion interface is of great importance in the advancement of proton-exchange-membrane (PEM) fuel-cell technology. This study focuses on the dependence of the oxygen reduction kinetics on oxygen pressure. Conventional Tafel analysis of the data shows that the reaction order with respect to oxygen is unity at both high and low current densities. Chronoamperometric measurements of the transport parameters for oxygen in Nafion show that oxygen dissolution follows Henry's isotherm. The diffusion coefficient of oxygen is invariant with pressure; however, the diffusion coefficient for oxygen is lower when air is used as the equilibrating gas as compared to when oxygen is used for equilibration. These results are of value in understanding the influence of O2 partial pressure on the performance of PEM fuel cells and also in elucidating the mechanism of oxygen reduction at the platinum/Nafion interface.

Platinum complexes and use thereof

Abstract: Method for preparing a platinum-organosiloxane complex which comprises reacting a platinous halide with an organo-siloxane having from 2 to 4 silicon-bonded organic groups containing terminal olefinic unsaturation, the reaction being carried out in the presence of a polar organic liquid, for example dimethylformamide or butan-2-one, which is at least a partial solvent for the platinous halide. The acomplexes are useful as catalysts for hydrosilylation reactions.

Identification of two types of oxidized palladium on γ-alumina by X-ray photoelectron spectroscopy

Abstract: It was demonstrated by X-ray photoelectron spectroscopy (XPS) that oxidized palladium supported on γ-alumina can exist in two chemically different entities, distinguishable by their observed electron binding energies and by selective reduction. Larger oxidized particles are characterized by a binding energy attributed to PdO. This oxide is reduced to metallic palladium at room temperature in hydrogen, as would be expected for bulk PdO. On the other hand, at low palladium loadings of 0.5 wt.-% and less, the majority of the palladium is present at a binding energy shifted approximately 1.6 eV higher than that observed for PdO. The shift appears to be associated with a metal-support interaction at concentrations where the palladium is highly dispersed. Contrary to that observed at loadings greater than 0.5 wt.-%, this palladium remains mostly oxidized when exposed to hydrogen at room temperature. The results are compared with literature data reported for platinum/γ-alumina.

Preparation of High‐Platinum‐Utilization Gas Diffusion Electrodes for Proton‐Exchange‐Membrane Fuel Cells

Abstract: We evaluated an electrochemical catalyzation technique for producing gas-diffusion electrodes for proton-exchange-membrane fuel cells (PEMFC). The electrochemical technique deposits platinum catalyst particles in regions of the electrode that are in ionic contact with the proton-exchange membrane and in electronic contact with the carbon support. Since ionic and electronic contact the amount of platinum required for PEMFC. We present data for oxygen reduction and hydrogen oxidation with gas-diffusion electrodes containing 0.05 mg Pt/cm 2

Synthesis of palladium and platinum nanoclusters within microphase-separated diblock copolymers

Abstract: Small palladium and platinum clusters have been synthetized within a purely hydrocarbon polymer matrix. The procedure consists of ring-opening metathesis polymerisation of norbornene-derived organometallic complexes, Pd(Cp N )(PA) and Pt(Cp N )Me 3 (Cp N = endo-2-(cyclopentadienylmethyl)norbon-5-ene and PA = η 3 -1-phenylallyl), using W or Mo alkylidene initiators, followed by static casting of films and subsequent reduction of the organometallic complexes under mild conditions using molecular hydrogen

Charge transfer photochemistry of coordination compounds

Abstract: Photochemical laws photochemistry and photophysics spectra and photochemistry of co-ordination compounds copper, silver, and gold magnesium, zinc, cadmium, and mercury aluminum, gallium, indium, thallium, yttrium, lanthanides, and actinides silicon, germanium, tin, lead, and titanium antimony, bismuthy, vanadium, niobium, and tantalum tellurium, chromium, molybdenum, and tungsten manganese, technetium, and rhenium nickel, palladium, and platinum.

A comparison of electrochemical and gas-phase decomposition of methanol on platinum surfaces

Abstract: By using electrochemical and ultrahigh-vacuum (UHV) techniques, combined with an isotope substitution method, it is found that the mechanism of methanol decomposition on platinum in the electrochemical environment is different than that in the UHV. In the UHV, the first step in the decomposition process is the scission of an O-H bond to yield a methoxy intermediate, whereas in the electrochemical environment, the first step is the scission of a C-H bond. The difference in the decomposition mechanism is discussed in terms of differences in the local electric field at the surface and in terms of methanol hydrophobic/hydrophilic interactions in solution. The latter affect methanol-water near-surface conformation and predetermine the destiny of the individual methanolic bonds in the catalytic splitting. 51 refs., 11 figs., 4 tabs.

Structure and catalytic activity of alumina supported platinum-cobalt bimetallic catalysts. 3. Effect of treatment on the interface layer

Abstract: The structure of the interface between metal and alumina in a series of Pt 1-x Co x /Al 2 O 3 bimetallic samples has been investigated by X-ray photoelectron spectroscopy (XPS) after calcination in oxygen at 770 K with subsequent in situ reduction in hydrogen in H 2 at 770 K. On all calcined samples platinum was found int he Pt(4+) valence state as an oxide species. Subsequent treatment in H 2 at 770 K resulted only in a partial reduction of ionic platinium into the zerovalent state

Gold and Platinum Diffusion: The Key to the Understanding of Intrinsic Point Defect Behavior in Silicon

Abstract: The study of gold and platinum diffusion is found to allow the separate observation of the intrinsic point defects, i.e., of silicon self-interstitials and of vacancies. The diffusion of gold in float zone (FZ) silicon is found to be dominated by the kick-out mechanism for temperatures of 800° C and higher. The diffusion of platinum in FZ silicon is described by the kick-out mechanism for temperatures above approximately 900° C, whereas for temperatures below approximately 850° C the dissociative mechanism governs platinum diffusion. As a result of numerical simulations, we suggest a complete and consistent set of parameters which describes the diffusion of platinum in silicon in the temperature range from 700° C to 950° C and the diffusion of gold in the temperature range from 800° C to 1100° C. The generation or recombination of self-interstitials and vacancies is found to be ineffective at least below 850° C. The concentration of substitutional platinum is determined by the initial concentration of vacancies at diffusion temperatures below 850° C. Platinum diffusion below 850°C can be used to measure vacancy distributions in silicon quantitatively.

Toward artificial metallopeptidases: mechanisms by which platinum(II) and palladium(II) complexes promote selective, fast hydrolysis of unactivated amide bonds in peptides

Abstract: Certain platinum(II) and palladium(II) complexes attached to the sulfur atom of cysteine, S-methylcysteinc, and methionine in peptides and in other amino-acid derivatives promote, under relatively mild conditions, selective hydrolysis of the unactivated amide hond involving the carhoxylic group of the amino acid anchoring the metal complex.

In Situ Electrode Formation on a Nafion Membrane by Chemical Platinization

Abstract: Three chemical techniques to platinize the surface of a Nafion polymer electrolyte membrane (PEM) to form a Pt/PEM electrode have been studied. The resulting Pt/PEM composites were characterized visually by transmission electron microscopy and scanning electron microscopy and electrochemically by (i) hydrogen adsorption to determine surface area, and (ii) the polarization characteristics for the oxidation of hydrogen. The Takenaka‐Torikai method was used in which a reductant diffuses through the membrane to contact and reduce platinic acid at the opposite face, and two variations of an impregnation‐reduction procedure were employed in which a cationic salt is first impregnated into the Nafion and subsequently the membrane is exposed to a reductant. A previously reported equilibrium impregnation method was modified such that the impregnation step was not carried to equilibrium and the concentration of the anionic reductant was significantly lowered. Upon judicious choice of the fabrication conditions, platinum deposited by the nonequilibrium impregnation ‐reduction procedure is concentrated in a thin (submicron), porous layer located predominantly within the membrane but near the surface where it is accessible to both gaseous reactants and a current collector. The metal distribution forms an electrode with a larger active surface area per unit weight of platinum and a better platinum utilization (mA/mg Pt for the hydrogen oxidation reaction) compared to Pt/PEM electrode prepared by the equilibrium impregnation‐reduction process or the Takenaka‐Torikai method. The chemical and physical processes which occur to affect the metal film formed in all three techniques are discussed.