Showing papers on "Platinum published in 2006"

Effect of surface composition on electronic structure, stability, and electrocatalytic properties of Pt-transition metal alloys: Pt-skin versus Pt-skeleton surfaces.

Abstract: The surface properties of PtM (M = Co, Ni, Fe) polycrystalline alloys are studied by utilizing Auger electron spectroscopy, low energy ion scattering spectroscopy, and ultraviolet photoemission spectroscopy. For each alloy initial surface characterization was done in an ultrahigh vacuum (UHV) system, and depending on preparation procedure it was possible to form surfaces with two different compositions. Due to surface segregation thermodynamics, annealed alloy surfaces form the outermost Pt-skin surface layer, which consists only platinum atoms, while the sputtered surfaces have the bulk ratio of alloying components. The measured valence band density of state spectra clearly shows the differences in electronic structures between Pt-skin and sputtered surfaces. Well-defined surfaces were hereafter transferred out from UHV and exposed to the acidic (electro)chemical environment. The electrochemical and post-electrochemical UHV surface characterizations revealed that Pt-skin surfaces are stable during and af...

Morphological Control of Catalytically Active Platinum Nanocrystals

Abstract: Colloidal metallic nanocrystals have been explored for catalytic applications, including fine chemicals synthesis, fuel-cell technology, hydrogen production, and gas sensing. The catalytic activity of a metallic catalyst depends strongly on its surface properties. For instance, hexagonal (111) Pt surfaces are 3–7-times more active than cubic (100) surfaces for aromatization reactions. The reactivity and selectivity of nanoparticles can therefore be tuned by controlling the morphology because the exposed surfaces of the particles have distinct crystallographic planes depending on the shape. A variety of Pt nanostructures, including polyhedra, wires, tubes, dendritic structures, and multipods have been synthesized by regulating growth at specific surfaces or by templating methods. Amphiphilic polymers or surfactants typically stabilize high-energy surfaces of nanoparticles. However, chemical reactions can only occur effectively on catalytically “clean” nanoparticles when the reactants adsorb more strongly to the particle surface than the surfacestabilizing agents do. When the interaction between the stabilizing agent and metal surface is too strong, the catalytic activity is greatly reduced. For instance, the carbonyl group of polyvinylpyrrolidone (PVP) or polyacrylate, which are the most widely used surface-regulating polymers in shapednanoparticle synthesis, interacts strongly with the platinum surface and thus blocks a significant number of active sites. On the other hand, alkylammonium ions have been widely used in synthesizing Au nanoparticles, and their interactions with Pt surfaces are considerably weaker than that of the carbonyl group. Therefore, this class of molecules could serve as ideal surface-stabilizing agents that can regulate the shape of nanoparticles while preserving catalytically active sites.

Heat-treated Fe/N/C catalysts for O2 electroreduction: are active sites hosted in micropores?

Abstract: Limited availability of platinum is a potential threat to fuel cell commercialization. Since the 1970s, alternative catalysts to the electrochemical reduction of oxygen have been obtained from heat treatment at T > 600 degrees C of carbon with a non-noble metal and a source of nitrogen atoms. However, the process by which the heat treatment activates these materials remains an open question. Here, we report that the activation process of carbon black and iron acetate heat-treated in NH(3) comprises three consecutive steps: (i) incorporation of nitrogen atoms in the carbon, (ii) micropore formation through reaction between carbon and ammonia, and (iii) completion of active sites in the micropores by reaction of iron with ammonia. Step (ii) is the slowest. Moreover, the microporous surface per mass of catalyst controls the macroscopic activity when enough nitrogen atoms are incorporated in the structure of the carbon support. These facts should help in determining the structure of the active sites and in identifying methods to increase the site density of such catalysts.

Synthesis and Characterization of the Nitrides of Platinum and Iridium

Abstract: Transition metal nitrides are of great technological and fundamental importance because of their strength and durability and because of their useful optical, electronic, and magnetic properties. We have evaluated a recently synthesized platinum nitride (PtN) that was shown to have a large bulk modulus, and we propose a structure that is isostructural with pyrite and has the stoichiometry PtN2. We have also synthesized a recoverable nitride of iridium under nearly the same conditions of pressure and temperature as PtN2. Although it has the same stoichiometry, it exhibits much lower structural symmetry. Preliminary results suggest that the bulk modulus of this material is also very large.

The methanol oxidation reaction on platinum alloys with the first row transition metals The case of Pt-Co and -Ni alloy electrocatalysts for DMFCs: A short review

Abstract: In recent years there has been much activity in examining Pt alloys with first row transition metals as catalysts materials for DMFCs In this work, the electrochemical oxidation of methanol on Pt–Co and –Ni alloy electrocatalysts is reviewed The effect of the transition metal on the electrocatalytic activity of Pt–Co and –Ni for the methanol oxidation reaction (MOR) has been investigated both in half-cell and in direct methanol fuel cells Conflicting results regarding the effect of the presence of Co(Ni) on the MOR are examined and the primary importance of the amount of non-precious metal in the catalyst is remarked For low base metal contents, an enhancement of the onset potential for the MOR with increasing Co(Ni) amount in the catalyst is observed, whereas for high contents of the base metal, a drop of the MOR onset potential with increasing Co(Ni) is found As well as the base metal content, an important role on the MOR activity of these catalysts has to be ascribed to the degree of alloying

Room temperature ferroelectric properties of Mn-substituted BiFeO3 thin films deposited on Pt electrodes using chemical solution deposition

Abstract: Mn-substituted BiFeO3 (BFO) thin films were fabricated by chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) structures. X-ray diffraction analysis revealed that the BFO lattice was somewhat distorted when 5% of Fe atoms were substituted with Mn atoms, but no secondary phase appeared by Mn substitution up to 20%. The leakage current density at higher electric field than 0.6MV∕cm decreased by Mn substitution of 3%–5%, compared with a pure BFO film. Because of the low leakage current density in the high electric field region, well saturated polarization hysteresis loops with remanent polarization of 100μC∕cm2 were observed in the 5% Mn-substituted BFO films at a measurement frequency of 1kHz.

Platinum dissolution and deposition in the polymer electrolyte membrane of a PEM fuel cell as studied by potential cycling

Abstract: The behavior of platinum dissolution and deposition in the polymer electrolyte membrane of a membrane-electrode-assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using potential cycling experiment and high-resolution transmission electron microscopy (HRTEM). The electrochemically active surface area decreased depending on the cycle number and the upper potential limit. Platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. Platinum deposition was accelerated by the presence of hydrogen transported through the membrane from an anode compartment. Platinum was transported across the membrane and deposited on the anode layer in the absence of hydrogen in the anode compartment. This deposition was also affected by the presence of oxygen in the cathode compartment.

Light-emitting device

Abstract: The present invention provides a novel light-emitting device being capable of application in various areas and having excellent characters and efficiency of light-emitting The invention relates to a light-emitting device containing at least one platinum complex represented by the following general formula (1): (wherein, ring A, ring B, ring C, every two independently, represent optionally substituted nitrogen-containing aromatic heterocyclic groups by coordination of nitrogen atom and platinum atom, and the remaining ring represents optionally substituted aryl group or optionally substituted heteroaryl group; Y represents halogen atom, or a direct bond or bonding via oxygen atom (-O-) or sulfur atom (-S-) to an optionally substituted aryl group or optionally substituted heteroaryl group (exception: adjacent two rings are nitrogen-containing aromatic heterocyclic group, Y is not a chlorine atom; non-adjacent two rings are nitrogen-containing aromatic heterocyclic group, Y is halogen atom)

Direct formation of highly porous gas-sensing films by in situ thermophoretic deposition of flame-made Pt/SnO2 nanoparticles

Abstract: Flame spray pyrolysis (FSP) was used to make pure and Pt-doped tin dioxide nanoparticles in one-step. The aerosol generated by the dry FSP method was directly, in situ thermophoretically deposited onto interdigitated Pt-electrodes to form a porous, thick film of controlled thickness within the active sensor area. Tin oxide grain size (10 nm) and a high film porosity (98%) were preserved for all film thicknesses from 9 to 40 μm using different deposition times. The dependence of the film thickness on deposition time was theoretically estimated to enable precise control of the deposition process. Platinum doping did not affect the SnO2 grain size, crystallinity, or the porous film structure. These sensors exhibited high carbon monoxide (CO) sensor signals (8 for 50 ppm CO in dry air at 350 °C), good reproducibility, high analytical sensitivity and a remarkably low detection limit (1 ppm CO in dry air at 350 °C). The in situ platinum doping enhanced the overall sensor performance. Increasing the film thickness increased the sensor resistance and can be used to tune sensor performance.

Effect of voltage on platinum dissolution : Relevance to polymer electrolyte fuel cells

Abstract: One of the processes responsible for performance degradation of a polymer electrolyte fuel cell (PEFC) is the loss of the electrochemically active surface area of the platinum-based electrocatalysts, due in part to platinum dissolution. The long-term dissolution behavior of polycrystalline platinum and high-surface-area carbon-supported platinum particles was studied under potentiostatic conditions relevant to PEFC cathode conditions. The equilibrium concentration of dissolved Pt was found to increase monotonically from 0.65 to 1.1 V (vs SHE) and decrease at potentials >1.1 V. Dissolution rates measured at 0.9 V were comparable for the two types of electrodes (1.4 and 1.7 × 10 -14 g/cm 2 s).

Single-wall carbon nanotubes supported platinum nanoparticles with improved electrocatalytic activity for oxygen reduction reaction.

Abstract: Significant enhancement in the electrocatalytic activity of Pt particles toward oxygen reduction reaction (ORR) has been achieved by depositing them on a single wall carbon nanotubes (SWCNT) support. Compared to a commercial Pt/carbon black catalyst, Pt/SWCNT films cast on a rotating disk electrode exhibit a lower onset potential and a higher electron-transfer rate constant for oxygen reduction. Improved stability of the SWCNT support is also confirmed from the minimal change in the oxygen reduction current during repeated cycling over a period of 36 h. These studies open up ways to utilize SWCNT/Pt electrocatalyst as a cathode in the proton-exchange-membrane-based hydrogen and methanol fuel cells.

Superoxide anion is the intermediate in the oxygen reduction reaction on platinum electrodes

Abstract: We identified the superoxide anion as the intermediate in the oxygen (O2) reduction reaction on a platinum (Pt) electrode in alkaline solution (pH = 11) using a surface-enhanced infrared spectroscopy technique with an attenuated total reflection mode. Spectral and voltammetry data, together with the vibrational frequencies calculated using the density functional theory, provide evidence for the formation of O2-. The supporting evidence includes similar spectra that we obtained for O2 reduction on Pt in acetonitrile solutions and a lack of spectra in the absence of O2 or its reduction. The appearance of O2- means that the series reaction pathway operates during O2 reduction on Pt electrodes in alkaline solutions and very likely also in acid solutions. This finding opens up the possibility of formulating a detailed reaction mechanism on surfaces supporting a four-electron reduction, which is critical in completely understanding the kinetics of O2 reduction, thus resolving dilemmas in the theoretical treatment of its kinetics and the design of new electrocatalysts.

Non-metal Catalysts for Dioxygen Reduction in an Acidic Electrolyte

Abstract: Active non-metal catalysts for the Oxygen Reduction Reaction (ORR) were prepared by decomposition of acetonitrile vapor at 900°C over a pure alumina support, and supports containing 2 wt% Fe or 2 wt% Ni on alumina. The exposed alumina and metal in the samples were subsequently washed away with HF acid to purify the solid carbon material. The sample prepared with iron was the most active sample for the ORR, with only 100 mV greater overpotential than a commercial 20 wt% Pt / Vulcan Carbon catalyst. However, nitrogen-containing carbon deposited on pure alumina (which contained less than 1 ppm metal contamination) was also quite active, demonstrating that platinum or iron is not required for ORR activity. Characterization by XPS and TEM revealed that the more active samples had nanostructured carbon with more edge plane exposure than the less active tube structures formed from the nickel sample.

Photocatalytic properties of TiO2 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution

Abstract: The commercially available TiO2-catalyst (Degussa P25) was modified with nanosized platinum and silver particles by the photoreduction method to obtain different noble metal loading (0.5 and 1 wt.%). The characterization of the synthesized catalysts was carried out by the BET method, XPS, TEM and the adsorption of the model pollutant. The degradation of oxalic acid has been studied in aqueous solution photocatalyzed by band-gap-irradiated TiO2, modified with nanosized platinum or silver particles. The photocatalytic activity of TiO2, modified with noble metal, is approximately double that of the semiconducting support. The adsorption properties of the catalysts, as well as the noble metal content on the surface of the support, influence the efficiency of the photocatalytic process. The reaction rate of photocatalytic degradation of the oxalic acid follows a zero kinetic order according to the Langmuir–Hinshelwood model. The increase of the quantum yield of the photodestruction reaction of the studied model pollutant is due to the formation of Schottky barriers on the metal–semiconductor interface, which serve as efficient electron traps, preventing the electron–hole recombination.

Enhancement of Pt Utilization in Electrocatalysts by Using Gold Nanoparticles

Abstract: Onewaytolowercostsistomakethemostefficientuseofthenoblemetalssothateverymetalatomisusedinthecatalyticprocess.Becausecatalysisattheelectrodeofafuelcellisasurfacephenomenon,theutilizationofPtinelectrocatalystscanbedefinedasthedispersionorexposedpercentageofPtatoms in the catalyst, which is approximately equal to thereciprocalofthePtparticlesizeinnanometers.

Thermal decomposition behaviors of PVP coated on platinum nanoparticles

Abstract: TG-DTA, TEM, and IR were used to investigate the thermal decomposition behavior of poly(N-vinyl-2-pyrrolidone) (PVP). The TG-DTA results show that the thermal decomposition behavior of PVP on platinum (Pt) is quite different from that of pure PVP. For pure PVP, 95.25% is decomposed when the temperature is increased up to 500°C; while under the same experimental condition, PVP coated on the Pt nanoparticles is only 66.7% decomposed. This is further supported by IR measurement. TEM results exhibited that the partially decomposed PVP still plays a role in stabilizing Pt nanoparticles: after heating treatment at 500°C for half an hour, the platinum nanoparticles did not aggregate heavily. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 23–26, 2006

Synthesis and Characterization of Pt(0) Nanoparticles in Imidazolium Ionic Liquids

Abstract: The controlled decomposition of Pt2(dba)3 (dba = dibenzylideneacetone) dispersed in 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4) and hexafluorophosphate (BMI.PF6) ionic liquids in the presence of cyclohexene by molecular hydrogen produces Pt(0) nanoparticles. The formation of these nanoparticles follows the two-step [A → B, A + B → 2B (k1, k2)] autocatalytic mechanism. The catalytic activity in the hydrogenation of cyclohexene is influenced by the nature of the anion rather than the mean-diameter of the nanoparticles. Thus, higher catalytic activity was obtained with Pt(0) dispersed in BMI.BF4 containing the less coordinating anion although these nanoparticles possess a larger mean diameter (3.4 nm) than those obtained in BMI.PF6 (2.3 nm). Similar mean diameter values were estimated from in situ XRD and SAXS. XPS analyses clearly show the interactions of the ionic liquid with the metal surface demonstrating the formation of an ionic liquid protective layer surrounding the platinum nanoparticl...

Platinum Catalysts Prepared with Functional Carbon Nanotube Defects and Its Improved Catalytic Performance for Methanol Oxidation

Abstract: Carbon nanotube (CNT) supported Pt nanoparticle catalysts have been prepared by spontaneous reduction of PtCl6(2-) ion as a result of direct redox reactions between PtCl6(2-) and oxygen-containing functional groups at defect sites of CNTs, which were introduced by chemical and electrochemical oxidation treatment of CNTs. The electrocatalytic properties of as-prepared Pt-CNT catalysts for methanol oxidation were investigated by chronopotentiometry and cyclic voltammetry. Compared with Pt catalysts prepared by hydrogen reduction and electrochemical deposition methods, Pt catalysts synthesized by functional CNT defects show excellent antipoisoning ability and long-term cycle stability.