Showing papers on "Platinum published in 2003"

Active Nonmetallic Au and Pt Species on Ceria-Based Water-Gas Shift Catalysts

Abstract: Traditional analysis of reactions catalyzed by supported metals involves the structure of the metallic particles. However, we report here that for the class of nanostructured gold- or platinum-cerium oxide catalysts, which are active for the water-gas shift reaction, metal nanoparticles do not participate in the reaction. Nonmetallic gold or platinum species strongly associated with surface cerium-oxygen groups are responsible for the activity.

Preparation and Characterization of Multiwalled Carbon Nanotube-Supported Platinum for Cathode Catalysts of Direct Methanol Fuel Cells

Abstract: Multiwalled carbon nanotube-supported Pt (Pt/MWNT) nanocomposites were prepared by both the aqueous solution reduction of a Pt salt (HCHO reduction) and the reduction of a Pt ion salt in ethylene glycol solution. For comparison, a Pt/XC-72 nanocomposite was also prepared by the EG method. The Pt/MWNT catalyst prepared by the EG method has a high and homogeneous dispersion of spherical Pt metal particles with a narrow particle-size distribution. TEM images show that the Pt particle size is in the range of 2-5 nm with a peak at 2.6 nm, which is consistent with 2.5 nm obtained from the XRD broadening calculation. Surface chemical modifications of MWNTs and water content in EG solvent are found to be the key factors in depositing Pt particles on MWNTs. In the case of the direct methanol fuel cell (DMFC) test, the Pt/MWNT catalyst prepared by EG reduction is slightly superior to the catalyst prepared by aqueous reduction and displays significantly higher performance than the Pt/XC-72 catalyst. These differences in catalytic performance between the MWNT-supported or the carbon black XC-72-supported catalysts are attributed to a greater dispersion of the supported Pt particles when the EG method is used, in contrast to aqueous HCHO reduction and to possible unique structural and higher electrical properties when contrasting MWNTs to carbon black XC-72 as a support.

Raney Ni-Sn Catalyst for H2 Production from Biomass-Derived Hydrocarbons

Abstract: Hydrogen (H2) was produced by aqueous-phase reforming of biomass-derived oxygenated hydrocarbons at temperatures near 500 kelvin over a tin-promoted Raney-nickel catalyst. The performance of this non-precious metal catalyst compares favorably with that of platinum-based catalysts for production of hydrogen from ethylene glycol, glycerol, and sorbitol. The addition of tin to nickel decreases the rate of methane formation from C-O bond cleavage while maintaining the high rates of C-C bond cleavage required for hydrogen formation.

Kinetic Model of Platinum Dissolution in PEMFCs

Abstract: This paper presents a mathematical model of oxidation and dissolution of supported platinum catalysts in polymer electrolyte membrane fuel cells (PEMFCs). Kinetic expressions for the oxidation and dissolution reactions are developed and compared to available experimental data. The model is used to investigate the influences of electrode potential and particle size on catalyst stability. © 2003 The Electrochemical Society. All rights reserved.

Nonenzymatic glucose detection using mesoporous platinum.

Abstract: Roughness of nanoscopic dimensions can be used to selectively enhance the faradaic current of a sluggish reaction. Using this principle, we constructed mesoporous structures on the surfaces of pure platinum electrodes responding even more sensitively to glucose than to common interfering species, such as l-ascorbic acid and 4-acetamidophenol. Good sensitivities, as high as 9.6 μA cm-2 mM-1, were reproducibly observed in the presence of high concentration of chloride ion. The selectivities, sensitivities, and stabilities determined experimentally have demonstrated the potential of mesoporous platinum as a novel candidate for nonenzymatic glucose sensors.

Catalysis and Electrocatalysis at Nanoparticle Surfaces

Abstract: THEORY OF NANOPARTICLE CATALYSIS AND ELECTROCATALYSIS Theory and modelling of catalytic and electrocatalytic reactions - some selected examples Simulations of the reaction kinetics on nm supported catalyst particles Electronic structure and chemisorption properties of supported metal clusters - model calculations MODEL SYSTEMS - FROM SINGLE CRYSTALS TO NANOPARTICLES State-of-the-art characterization of single crystal surfaces Single crystal surfaces as model platinum-based fuell cell electrocatalysts Electrochemical nanostructuring of surfaces Adsorption and reaction at supported model catalysts Size-dependent electronic, structural, and catalytic properties of metal clusters supported on ultra-thin oxide films Physical and electrochemical characterization of bimetallic nanoparticle electrodes. SYNTHETIC APPROACHES IN NANOPARTICLE CATALYSIS AND ELECTROCATALYSIS Nanomaterials as precursors for electrocatalysts preparation, characterization, and properties of bimetallic nanoparticles Physicochemical aspects of preparation of carbon supported Nobel metal catalysis. ADVANCED EXPERIMENTAL CONCEPTS NMR investigation of supported metal catalysts In situ X-ray adsorption spectroscopy investigations of the carbon-supported pt electrocatalysts Stm and infrared spectroscopy in studies of fuel cell model catalysts. PARTICLE SIZE, SUPPORT, AND PROMOTIONAL EFFECTS Electrochemical and chemical promotion on metal films and nanoparticles Metal-supported interaction in low temperature fuel cell electrocatalysis Effects of nanoparticle size, structure, and metal-support interactions promotion, electrochemical promotion and metal-support interactions Support effects on catalytic performance of nanoparticles abnormal infrared effects of nanometer scale thins film material of platinum group metals and alloys at electrode/electrolyte interfaces Design of electrocatalysts for fuel cells effect of particle size and support on some catalytic properties of metallic and bimetallic catalysts. ADVANCED ELECTROCATALYTIC MATERIALS Catalyst nanoparticles on synthetic diamond surfaces Electrocatalysis with electron conduction polymers modified by platinum metal nanoparticles Novel nanostructured material based on transition metal compounds for electrocatalysis.

Atomic Layer Deposition of Platinum Thin Films

Abstract: Platinum thin films were grown at 300 °C by atomic layer deposition (ALD) using (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3) and oxygen as precursors. The films had excellent uniformity, low resistivity, and low-impurity contents. Structural studies by X-ray diffraction showed that the films were strongly (111) oriented. Growth rates of 0.45 A cycle-1 were obtained with 4 s total cycle times. The film thickness was found to linearly depend on the number of the reaction cycles. Also, the possible reaction mechanism is discussed.

Platinum-Maghemite Core−Shell Nanoparticles Using a Sequential Synthesis

Abstract: Pt@Fe2O3 core−shell nanoparticles have been made using a sequential synthetic method. Platinum nanoparticles were synthesized via reduction of platinum acetylacetonate in octyl ether, and layers of iron oxide were subsequently deposited on the surface of Pt nanoparticles through thermal decomposition of iron pentacarbonyl. The core−shell nanoparticles were characterized by powder X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoemission spectroscopy. Thickness of the shell can be controlled by changing concentrations of the reactants and the reaction conditions. These Pt@Fe2O3 core−shell nanoparticles could have potential applications in catalysis and as precursors for making property-tunable magnetic nanoparticles, thin films, and nanocomposites.

An Alternative Route to Highly Luminescent Platinum(II) Complexes: Cyclometalation with N∧C∧N-Coordinating Dipyridylbenzene Ligands

Abstract: The remarkable luminescence properties of the platinum(II) complex of 1,3-di(2-pyridyl)benzene, acting as a terdentate N∧C∧N-coordinating ligand cyclometalated at C2 of the benzene ring ([PtL1Cl]), have been investigated, together with those of two new 5-substituted analogues [PtL2Cl] and [PtL3Cl] {HL2 = methyl-3,5-di(2-pyridyl)benzoate; HL3 = 3,5-di(2-pyridyl)toluene}. All three complexes are intense emitters in degassed solution at 298 K (λmax 480−580 nm; φlum = 0.60, 0.58, and 0.68 in CH2Cl2), displaying highly structured emission spectra in dilute solution, with lifetimes in the microsecond range (7.2, 8.0, and 7.8 μs). On the basis of the very small Stokes shift, the highly structured profiles, and the relatively long lifetimes, the emission is attributed to an excited state of primarily 3π−π* character. At concentrations >1 × 10-5 M, structureless excimer emission centered at ca. 700 nm is observed. The X-ray crystal structure of [PtL2Cl] is also reported.

Formation, microstructural characteristics and stability of carbon supported platinum catalysts for low temperature fuel cells

Abstract: Supported platinum electrocatalysts are generally used in low temperature fuel cells to enhance the rates of the hydrogen oxidation and oxygen reduction reactions. In such catalysts, the high surface to volume ratios of the platinum particles maximize the area of the surfaces available for reaction. It is the structure and proper dispersal of these platinum particles that make low-loading catalysts feasible for fuel cell operation, lowering the cost of the system. If the platinum particles cannot maintain their structure over the lifetime of the fuel cell, change in the morphology of the catalyst layer from the initial state will result in a loss of electrochemical activity. This loss of activity in the platinum/carbon catalysts due to the agglomeration of platinum particles is considered to be a major cause of the decrease in cell performance, especially in the case of the cathode. In the light of the latest advances on this field, this paper reviews the preparation methods of these catalysts, their microstructural characteristic and their effect on both thermal and in cell conditions stability.

Characterization of platinum nanoparticle-embedded carbon film electrode and its detection of hydrogen peroxide.

Abstract: A method for the highly sensitive determination of acetylcholine (ACh) and choline (Ch) that employs a graphite-like carbon film electrode containing 6.5% platinum (Pt) nanoparticles was developed for use as a detector in microbore liquid chromatography (LC) with a postcolumn enzyme reactor. The film electrode was prepared by RF cosputtering carbon and Pt, which requires only a one-step formation process. This method can control the Pt content of the film at a relatively low deposition temperature (below 200 degrees C). The average size of the Pt nanoparticles was 2.5 nm. The film electrode showed excellent electrocatalytic activity, high sensitivity, and negligible baseline drift when detecting hydrogen peroxide. The electrode was modified with glucose oxidase and responded rapidly to glucose with a much more stable baseline current than at a Pt bulk electrode based sensor. Therefore, it is appropriate to employ the electrode to detect trace amounts of biomolecules, such as neurotransmitters and hormones combined with various oxidase enzymes. We used the electrode as a detector for microbore LC and observed a low detection limit of 2.5 and 2.3 fmol (10-microL injection) for ACh and Ch, respectively, which is approximately 1 order of magnitude lower than that of a Pt bulk electrode.

Ionic dispersion of Pt over CeO2 by the combustion method: Structural investigation by XRD, TEM, XPS, and EXAFS

Abstract: The structure and chemical nature of Pt in combustion-synthesized $Pt/CeO_2$ catalysts have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and temperature-programmed reaction (TPR) Catalytic oxidation of CO over $Pt/CeO_2$ is correlated with its structure High-resolution XRD studies show that the structure could be refined for the composition of $Ce_1_-_xPt_xO_2_-_\delta $ in the fluorite structure with 6% oxide ion vacancy TEM images show very few Pt particles on the $CeO_2$ crystallite surface in as-prepared samples and a decrease in the density of Pt metal particles is observed on heating XPS studies demonstrate that Pt is dispersed mostly in +2(72%) and +4(21%) oxidation states on $CeO_2$, whereas only 7% is present as Pt metal particles On heat treatment, $Pt^2^+$ species increase at the cost of $Pt^4^+$ ions EXAFS studies show the average coordination number of 13 around the platinum ion in the first shell of 1% $Pt/CeO_2$ at a distance of 198 A, indicating oxide ion vacancy around the platinum ion On heating, the average oxygen coordination of Pt and oxygen increases to 23 The second shell at 297 A is due to Pt-Pt coordination, which is absent in $PtO_2$ and PtO The third shell at 328 A is not observed either in Pt metal or any of the platinum oxides, which could be attributed to $Pt^2^+-Ce^4^+$ correlation Thus, $Pt/CeO_2$ forms a $Ce_1_-_xPt_xO_2_-_\delta $ type of solid solution having $-0-Pt^2^+-O-Ce^4^+-$ kinds of linkages

Aqueous-Phase Reforming of Ethylene Glycol Over Supported Platinum Catalysts

Abstract: Aqueous-phase reforming of 10 wt% ethylene glycol solutions was studied at temperatures of 483 and 498 K over Pt-black and Pt supported on TiO2, Al2O3, carbon, SiO2, SiO2-Al2O3, ZrO2, CeO2, and ZnO. High activity for the production of H2 by aqueous-phase reforming was observed over Pt-black and over Pt supported on TiO2, carbon, and Al2O3 (i.e., turnover frequencies near 8-15 min-1 at 498 K); moderate catalytic activity for the production of hydrogen is demonstrated by Pt supported on SiO2-Al2O3 and ZrO2 (turnover frequencies near 5 min-1); and lower catalytic activity is exhibited by Pt supported on CeO2, ZnO, and SiO2 (H2 turnover frequencies lower than about 2 min-1). Pt supported on Al2O3, and to a lesser extent ZrO2, exhibits high selectivity for production of H2 and CO2 from aqueous-phase reforming of ethylene glycol. In contrast, Pt supported on carbon, TiO2, SiO2-Al2O3 and Pt-black produce measurable amounts of gaseous alkanes and liquid-phase compounds that would lead to alkanes at higher conversions (e.g., ethanol, acetic acid, acetaldehyde). The total rate of formation of these byproducts is about 1-3 min-1 at 498 K. An important bifunctional route for the formation of liquid-phase alkane-precursor compounds over less selective catalysts involves dehydration reactions on the catalyst support (or in the aqueous reforming solution) followed by hydrogenation reactions on Pt.

Bimetallic palladium-platinum dendrimer-encapsulated catalysts.

Abstract: We report the synthesis, characterization, and catalytic activity of bimetallic palladium−platinum dendrimer-encapsulated catalysts (DECs). These materials are prepared by co-complexation of different ratios of palladium and platinum salts to the interior tertiary amines of fourth-generation, hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers. Chemical reduction of these composites yields stable, fairly monodisperse, water-soluble bimetallic DECs having sizes on the order of 1.9 ± 0.4 nm. Evidence that these nanoparticles are bimetallic comes from single-particle X-ray energy dispersive spectroscopy (EDS) and catalysis experiments. The latter indicate that the hydrogenation rate of allyl alcohol is enhanced in the presence of the bimetallic nanoparticles compared to DECs containing only platinum or only palladium nanoparticles. EDS results indicate that the percentage composition of the bimetallics is reflected by the percentage of metal salts initially complexed with the dendrimer.

Dendrimer-encapsulated nanoparticle precursors to supported platinum catalysts.

Abstract: In this contribution, we report the successful preparation of supported metal catalysts using dendrimer-encapsulated Pt nanoparticles as metal precursors. Polyamidoamine (PAMAM) dendrimers were first used to template and stabilize Pt nanoparticles prepared in solution. These dendrimer-encapsulated nanoparticles were then deposited onto a commercial high surface area silica support and thermally activated to remove the organic dendrimer. The resulting materials are active oxidation and hydrogenation catalysts. The effects of catalyst preparation and activation on activity for toluene hydrogenation and CO oxidation catalysis are discussed.

Formation of nanoporous platinum by selective dissolution of Cu from Cu0.75Pt0.25

Abstract: This paper gives results demonstrating the production of nanoporous platinum through the de-alloying of Cu075Pt025 alloy in 1 M H2SO4 Both field emission scanning electron microscopy and small angle neutron scattering confirm the presence of porosity with a diameter of approximately 34 nm This is the smallest porosity quantitatively reported from a de-alloying process to date The small size is attributed to the extremely small values of surface diffusivity expected for Pt at room temperature, effectively eliminating room-temperature coarsening processes The results also show that larger length scales can be achieved through coarsening at elevated temperatures The ease of production of porous platinum makes it attractive for possible applications, such as high surface area electrodes for biomedical devices or as catalyst materials

Hollow nanostructures of platinum with controllable dimensions can be synthesized by templating against selenium nanowires and colloids.

Abstract: Hollow nanostructures of platinum have been synthesized by reducing PtCl2 with alcohol in the presence of selenium nanowires or colloids. The Se template could be removed by soaking the resultant Se@Pt nanostructures in hydrazine or by heating them to 200−250 °C. The size and wall thickness of the polycrystalline hollow nanostructures could be controlled by varying the template, reaction time, and the concentration of PtCl2.

Effect of CeO2 loading on the surface and catalytic behaviors of CeO2-Al2O3-supported Pt catalysts

Abstract: Pt catalysts supported on mixed CeO2-Al2O3 carriers with different CeO2 loading (0.5–10.3 wt.%) were prepared by wetness impregnation method. The catalysts were characterized by SBET, X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and thermogravimetric analysis (TG). It was shown that pretreatment temperature and the concentration of CeO2 in the support influences significantly on the morphology of Pt. XRD showed the formation of nanocrystallites of Pt on the surface of alumina and low-loaded CeO2 (≤6 wt.%) samples at higher temperature of calcination (1073 K). Amorphous Pt was observed in all reduced samples. XPS spectra showed the presence of interaction between Pt and Ce, which leads to easy surface reduction of both, ceria and platinum, as revealed by TPR patterns. The effect of CeO2 loading on the catalytic behavior of supported Pt catalysts in the reaction of CO2 reforming of CH4 was determined. Addition of cerium oxide results in improvement of catalytic performance for the reforming of methane with CO2. Pt catalyst with 1 wt.% of CeO2 exhibited the highest specific activity and stability, due to the increase in the metal–support interface area, caused by the higher Pt dispersion.

Intramolecular reactions of alkynes with furans and electron rich arenes catalyzed by PtCl2: the role of platinum carbenes as intermediates.

Abstract: 5-(2-Furyl)-1-alkynes react, with PtCl2 as catalyst, to give phenols. On the basis of DFT calculations, a cyclopropyl platinacarbene complex was found as the key intermediate in the process. The cyclopropane and dihydrofuran rings of this intermediate open to form a carbonyl compound, which reacts with the platinum carbene to form an oxepin, which is in equilibrium with an arene oxide. When the reaction is carried out in the presence of water, dicarbonyl compounds are obtained, which support the proposed mechanism. Other cyclizations of alkynes with furans or electron-rich arenes give products of apparent Friedel−Crafts-type reactions, although these processes could also proceed by pathways involving the formation of cyclopropyl platinum carbenes.

Reaction Mechanism Studies on Atomic Layer Deposition of Ruthenium and Platinum

Abstract: Reaction mechanisms in atomic layer deposition (ALD) of ruthenium from bis(cyclopentadienyl)ruthenium (RuCp 2 ) and oxygen were studied in situ with a quadruple mass spectrometer (QMS) and a quartz crystal microbalance (QCM). In addition, QMS was used to study ALD of platinum from (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe 3 ) and oxygen. The QMS studies showed that the reaction by-products H 2 O and CO 2 were released during both the oxygen and the metal precursor pulses. Adsorbed oxygen layer on the metal surface thus oxidizes part of the ligands during the metal precursor pulse. The remaining ligand species become oxidized and a new layer of adsorbed oxygen forms on the surface during the following oxygen pulse. The QCM analysis of the ruthenium process showed a mass decrease during the RuCp 2 pulse and a mass increase during the oxygen pulse, which further supports the proposed mechanism.

Preparation of gold, platinum, palladium and silver nanoparticles by the reduction of their salts with a weak reductant–potassium bitartrate

Abstract: Gold and platinum nanoparticles were synthesized by the reduction of their salts with potassium bitartrate as the reductant and poly(N-vinyl-2-pyrrolidone) (PVP), polyethylene glycol (PEG) (for Pt nanoparticles) or 3,3′-thiodipropionic acid (TDPC) as the protective agent. In the presence of PVP or TDPC, the preparation of palladium and silver nanoparticles were also realized by the reduction of their salts with potassium bitartrate under alkaline conditions. The effect of the concentration of the protective agents on the particle size and morphology were investigated in detail. At a constant concentration of Au and Pt ions, the average diameter of Au or Pt nanoparticles tends to decrease with an increase in the concentration ratios of the protective agents to AuCl4− or PtCl62−. Except for PVP passivated Au nanoparticles, smaller Au and Pt particles with a sharp size distribution could be obtained at higher concentration ratios of the protective agents to the metal ions, as determined by TEM measurements. All the noble metal colloidal particles have very high stability.

Oxygen Is Electroreduced to Water on a "Wired" Enzyme Electrode at a Lesser Overpotential than on Platinum

Abstract: The first enzyme-based catalyst that is superior to platinum in the four-electron electroreduction of oxygen to water is reported. The smooth Pt cathode reached half and 90% of the mass transport-limited current density at respective overpotentials of −0.4 and −0.58 V in 0.5 M sulfuric acid, and only at even higher overpotentials in pH 7.2 phosphate buffer. In contrast, the smooth “wired” bilirubin oxidase cathode reached half and 90% of the mass transport-limited current density at respective overpotentials as low as −0.2 and −0.25 V. The mass transport-limited current density for the smooth “wired” enzyme cathode in PBS was twice that with smooth Pt in 0.5 M sulfuric acid. Under 1 atm O2 pressure, O2 was electroreduced to water on a polished carbon cathode, coated with the “wired” BOD film, in pH 7.2 saline buffer (PBS) at an overpotential of −0.31 V at a current density of 9.5 mA cm-2. At the same overpotential, the current density of the polished platinum cathode in 0.5 M H2SO4 was 16-fold lower, only...