Showing papers on "Platinum published in 1998"

Platinum Catalysts for the High-Yield Oxidation of Methane to a Methanol Derivative

Abstract: Platinum catalysts are reported for the direct, low-temperature, oxidative conversion of methane to a methanol derivative at greater than 70 percent one-pass yield based on methane. The catalysts are platinum complexes derived from the bidiazine ligand family that are stable, active, and selective for the oxidation of a carbon-hydrogen bond of methane to produce methyl esters. Mechanistic studies show that platinum(II) is the most active oxidation state of platinum for reaction with methane, and are consistent with reaction proceeding through carbon-hydrogen bond activation of methane to generate a platinum-methyl intermediate that is oxidized to generate the methyl ester product.

Combinatorial Electrochemistry: A Highly Parallel, Optical Screening Method for Discovery of Better Electrocatalysts

Abstract: Combinatorial screening of electrochemical catalysts by current-voltage methods can be unwieldy for large sample sizes. By converting the ions generated in an electrochemical half-cell reaction to a fluorescence signal, the most active compositions in a large electrode array have been identified. A fluorescent acid-base indicator was used to image high concentrations of hydrogen ions, which were generated in the electrooxidation of methanol. A 645-member electrode array containing five elements (platinum, ruthenium, osmium, iridium, and rhodium), 80 binary, 280 ternary, and 280 quaternary combinations was screened to identify the most active regions of phase space. Subsequent “zoom” screens pinpointed several very active compositions, some in ternary and quaternary regions that were bounded by rather inactive binaries. The best catalyst, platinum(44)/ruthenium(41)/osmium(10)/iridium(5) (numbers in parentheses are atomic percent), was significantly more active than platinum(50)/ruthenium(50) in a direct methanol fuel cell operating at 60°C, even though the latter catalyst had about twice the surface area of the former.

Kinetically Controlled Growth and Shape Formation Mechanism of Platinum Nanoparticles

Abstract: Recently, we have been able to synthesize platinum colloidal nanoparticles of different shapes (Science, 1996, 272, 1924). In this report, we present transmission electron microscopic (TEM) results on the time-dependent shape distribution of platinum nanoparticles during the growth period and its dependence on the concentration of the capping polymer as well as the pH of the solution. The results suggest a shape-controlled growth mechanism in which the difference between the rate of the catalytic reduction process of Pt2+ on the {111} and {100} faces, the competition between the Pt2+ reduction and the capping process on the different nanoparticle surfaces, and the concentration-dependent buffer action of the polymer itself all control the final distribution of the different shapes observed.

Structural and electrochemical characterization of binary, ternary, and quaternary platinum alloy catalysts for methanol electro-oxidation'

Abstract: The bifunctional model for methanol electro-oxidation suggests that competent catalysts should contain at least two types of surface elements: those that bind methanol and activate its C−H bonds and those that adsorb and activate water. Our previous work considered phase equilibria and relative Pt−C and M−O (M = Ru, Os) bond strengths in predicting improved activity among single-phase Pt−Ru−Os ternary alloys. By addition of a correlation with M−C bond strengths (M = Pt, Ir), it is possible to rationalize the recent combinatorial discovery of further improved Pt−Ru−Os−Ir quaternaries. X-ray diffraction experiments show that these quaternary catalysts are composed primarily of a nanocrystalline face-centered cubic (fcc) phase, in combination with an amorphous minor component. For catalysts of relatively high Ru content, the lattice parameter deviates positively from that of the corresponding arc-melted fcc alloy, suggesting that the nanocrystalline fcc phase is Pt-rich. Anode catalyst polarization curves i...

Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media

Abstract: The influence of deposited silver upon the photocatalytic activity of titania in the rutile and anatase allotropic forms has been studied in three different reactions. The common feature found for these three reactions was an electron transfer from illuminated TiO 2 to silver particles. In the first reaction (platinum photodeposition). it has been shown that the initial presence of metallic silver orientates the localization of subsequent Pt photodeposits with, in particular, a 100% selectivity to Pt deposition on top of silver particles or agglomerates previously deposited on anatase. In the second reaction (2-propanol oxidation), Ag deposit was found beneficial for the activity of rutile and detrimental for that of anatase. For rutile which is less active, silver helps for the electron-hole pair dissociation. By contrast, for anatase, the negatively charged Ag particles preferentially attract photoholes and become recombination centers, thus decreasing the photocatalytic activity. For the third reaction (dehydrogenation of 2-propanol), Ag/TiO 2 catalysts were found very poorly active, as expected for a group I-B metal. However, in the additional presence of platinum deposits, Pt/Ag/TiO 2 catalysts are as active as their Pt/TiO 2 homologues, thus confirming that Pt is deposited on top of silver with good electronic contacts between both metals. The three photocatalytic reactions have common electronic processes based on the photoelectron generation on titania and the subsequent electron transfer to the metal (s).

Nox gas sensing characteristics of wo3 thin films activated by noble metals (pd, pt, au) layers

Abstract: WO3-based gas sensor devices have been fabricated by reactive rf sputtering on glass substrates. Palladium (Pd), platinum (Pt), gold (Au) noble metals (100–600 A) were evaporated as activator layers onto WO3 thin films and Al layers (1500 A) were sputtered on top of them as upper electrodes for sensor output. The described sensing element was found to possess excellent sensitivity towards NO and NO2 gases, at low temperatures (100–300°C). The optimum operating temperature was 150 and 200°C for Pt, Pd and Au, respectively. The resistance of the thin-film gas sensor increases reversibly in the presence of low concentrations of NO (0–440 ppm) and NO2 (0–10 ppm) gases in air. The activator layers have an promotional effect on the speed of response to NOx at low temperatures and on selectivity enhanced with respect to other reducing gases (CO, CH4, H2, SO2, H2S, NH3). The temperature dependence of response and recovery times has been examined. The influence of the thickness of the noble metals on NOx gases sensitivity has been investigated. The NO and NO2 gases calibration curves have been recorded for the optimum performances of each fabricated sensor-device.

CO2 reforming of methane on Ni catalysts: Effects of the support phase and preparation technique

Abstract: The effects of the support phase and catalyst preparation methods on catalytic activity and carbon deposition were systematically investigated over nickel catalysts supported on Al2O3, SiO2 and MgO for the reforming reaction of methane with carbon dioxide. It is found that the pore structure of the support and metal-support interaction significantly affected the catalytic activity and coking resistance. Catalyst with well-developed porosity exhibited higher catalytic activity. Strong interaction between metal and the support made the catalyst more resistant to sintering and coking, thus resulting in a longer time of catalyst stability. (C) 1998 Elsevier Science B.V.

Electrochemical Preparation of Platinum Nanocrystallites with Size Selectivity on Basal Plane Oriented Graphite Surfaces

Abstract: Platinum nanocrystals were deposited on basal plane oriented graphite surfaces from dilute (1.0 mM) PtCl62--containing electrolytes using a pulsed potentiostatic method. The deposition of platinum nanocrystals occurred via an instantaneous nucleation and diffusion-limited growth mechanism which resulted in narrow particle size distributions (relative standard deviation <35%) for mean crystallite diameters smaller than 40 A. The number of particles per unit area on these surfaces was 109−1010 cm-2. Noncontact atomic force microscopy images reveal that platinum nanocrystals nucleated both at defect sitessuch as step edgesand on apparently defect-free regions of the atomically smooth graphite basal plane. Using electron transparent graphite surfaces, selected area electron diffraction analyses revealed that the structure of deposited platinum nanocrystals was fcc with a lattice constant that was indistinguishable from bulk fcc platinum. Platinum nanocrystals were not epitaxially oriented on the graphite basa...

Surface Structure Effects in Platinum/Ruthenium Methanol Oxidation Electrocatalysis

Abstract: Catalytic activity of low-index platinum single-crystal faces, Pt(111), Pt(100), and Pt(110), and polycrystalline platinum toward methanol electrooxidation was enhanced by controlled amounts of electrodeposited ruthenium. Platinum surface structure affects all factors involved in the oxidation process: (i) ruthenium coverage corresponding to the highest methanol oxidation rate, (ii) rate of oxidation current decay, (iii) Tafel slopes and reaction turnovers, and (iv) apparent activation energy. We found a unique methanol oxidation reactivity at the Pt(111) surface covered by 0.2 monolayer of ruthenium. The turnover number from such a surface at 80 °C is approximately 1 order of magnitude higher than that from the industrial platinum/ruthenium catalyst. Therefore, the Pt(111)/Ru electrode is the best laboratory scale fuel cell anode for methanol oxidation. We conclude that crystallographic variables should be exploited in syntheses of novel metal-alloy catalysts for fuel cell use.

The Electrocatalytic Oxidation of Methanol at Finely Dispersed Platinum Nanoparticles in Polypyrrole Films

Abstract: A new method of the formation of composite polypyrrole films containing a highly dispersed three-dimensional array of platinum catalyst particles is presented. PtCl{sub 4}{sup 2{minus}} anions were trapped inside the polypyrrole matrix during the electropolymerization of pyrrole. In the next step followed by solution exchange, PtCl{sub 4}{sup 2{minus}} anions were reduced to Pt{sup 0} particles with an average size of 10 nm. Metallic particles were incorporated in electrically conducting polypyrrole films in order to achieve multielectron-transfer processes in a three-dimensional matrix. These films were characterized using the electrochemical quartz crystal microbalance technique. The use of this technique allowed evaluation of the Pt{sup 0} loading inside the polymer film. The electropolymerization process was controlled by measuring frequency changes of piezoelectrodes. The presence of Pt{sup 0} particles in composite polypyrrole films and their uniform distribution were confirmed by energy-dispersive x-ray spectroscopy and x-ray diffraction. The size of the Pt{sup 0} particles was evaluated from transmission electron microscopy experiments. The electrocatalytic effect toward the methanol oxidation was observed. Larger surface area and higher catalytic activity were found for electrodes with dispersed Pt{sup 0} nanoparticles in the polymer evaluation of matrix than electrodes with electrodeposited Pt{sup 0} on the surface of the conductive polymer.

Promotion effect of potassium on the catalytic property of CuFe2O4 for the simultaneous removal of NOx and diesel soot particulate

Abstract: Modification of CuFe2O4 catalyst for the simultaneous NOx-soot removal has been studied by doping alkali metals (Li, Na, K, Cs), vanadium and platinum. In pre-doped catalysts, designated as Cu1−xAxFe2O4 (A : dopant), potassium was superior to the other alkali metals and vanadium with respect to enhancing the activity and selectivity to NOx reduction into N2, and the optimum doping level of potassium was x=0.05. Such a promotion effect of potassium was also observed with the impregnated 5 mol% K/CuFe2O4 catalyst. The impregnation of platinum to CuFe2O4 and Cu0.95K0.05Fe2O4 resulted in a decrease in the selectivity to N2 formation with the activity being almost unchanged. The high catalytic performance, especially high selectivity, was realized only when the proper amount of potassium is doped in the Cu–Fe spinel catalyst. XPS analysis showed the segregation (enrichment) of potassium on the surface, and too much doping of potassium might cause the covering of active sites of the Cu–Fe spinel oxide.

Improved Preparation Process of Very‐Low‐Platinum‐Loading Electrodes for Polymer Electrolyte Fuel Cells

Abstract: The authors have previously reported a preparation process for the catalyst layer of polymer electrolyte fuel cells (PEFCs) using perfluorosulfonate ionomer (PFSI) colloid. In this paper they report recent improvements in this method of preparation. In the present method, a carbon-supported platinum catalyst (Pt-C) was mixed with poor solvent for PFSI, and PFSI solution was added to this mixture. The colloidal PFSI formed was immediately adsorbed on Pt-C without coagulation. As a result, the reaction area was increased, the internal resistance was decreased, and the gaseous reactants were more readily supplied to the reaction area. The authors also studied the influence of the carbon support on the microstructure of the catalyst layer. They have found that the performance of a PEFC prepared by this process with optimum carbon support and with very low platinum loading (0.1 mg/cm{sup 2}) was superior to that prepared by the previous process with conventional carbon support and with a 0.5 mg/cm{sup 2} platinum loading.

Monte Carlo simulations of a simple model for the electrocatalytic CO oxidation on platinum

Abstract: A simple lattice-gas model for the electrocatalytic carbon monoxide oxidation on a platinum electrode is studied by dynamic Monte Carlo simulations. The CO oxidation takes place through a Langmuir–Hinshelwood reaction between adsorbed CO and an adsorbed OH radical resulting from the dissociative adsorption of water. The model enables the investigation of the role of CO surface mobility on the macroscopic electrochemical response such as linear sweep voltammetry and potential step chronoamperometry. Our results show that the mean-field approximation, the traditional but often tacitly made assumption in electrochemistry, breaks down severely in the limit of vanishing CO surface mobility. Comparison of the simulated and experimental voltammetry suggests that on platinum CO oxidation is the intrinsically fastest reaction on the surface and that CO has a high surface mobility. However, under the same conditions, the model predicts some interesting deviations from the potential step current transients derived f...

Splitting of water by electrochemical combination of two photocatalytic reactions on TiO2 particles

Abstract: Photochemical splitting of water was achieved by combining two photocatalytic reactions on suspended titanium dioxide particles, namely, the reduction of water to hydrogen using bromide ions, which were oxidized to bromine and the oxidation of water to oxygen using FeIII ions, which were reduced to FeII ions. These two reactions were carried out in separate compartments and combined via platinum electrodes and cation-exchange membranes. At the electrodes, FeII ions were oxidized by bromine, and protons were transported through the membranes to maintain the electric neutrality and pH of the solutions in the two compartments. As a result, water was continuously split into hydrogen and oxygen under photoirradiation. Reversible reactions on photocatalysts often suffer from the effects of back reactions, unless the products are removed. In the present system the problem is largely prevented, because the concentrations of the products in solution are automatically maintained at a low level.

Synthesis, Electrochemistry, and Spectroscopy of Blue Platinum(II) Polyynes and Diynes.

Abstract: The smallest band gap observed so far (1.77 eV) for an organometallic polymer is exhibited by the blue, rigid-rod polymer 2, which is prepared by the reaction of trans-[PtCl2(PnBu3)2] with one equivalent of 1.

Amorphous microporous titania modified with platinum(iv) chloride-a new type of hybrid photocatalyst for visible light detoxification

Abstract: Amorphous microporous metal oxides of titanium (AMM-Ti) modified with a few percent of a platinum(IV) halide have been prepared by a sol−gel procedure and characterized by various surface analytical methods. By these methods the metal salt is homogeneously distributed in an almost exclusively amorphous powder of high specific surface area (160−200 m2/g) and a pore size of 0.8 nm. From the coordination number of 4.3 and a Pt−Cl distance of 2.28 A, as calculated from extended X-ray absorption fine structure results, we conclude that the platinum salt is present as PtCl4. The hybrid sample (Pt(IV)/AMM-Ti) catalyzes the photodegradation of 4-chlorophenol with visible light. Apparent disappearance quantum yields decrease from 8.6 × 10-3 at 335 nm to 4.5 × 10-3 at 366 nm and 2.8 × 10-3 at 400 nm; lower values of 1.6 × 10-3 and 1.3 × 10-3 are found at 436 and 546 nm, respectively. The quantum yield at both 366 and 436 nm decreases linearly with the square root of the incident photon flux, suggesting increasing r...

Mechanistic Investigation of Catalytic Carbon−Carbon Bond Activation and Formation by Platinum and Palladium Phosphine Complexes

Abstract: The complexes Pt(PEt3)3 and Pd(PEt3)3 cleave the C−C bond of biphenylene to give (PEt3)2Pt(2,2‘-biphenyl), 1, and (PEt3)2Pd(2,2‘-biphenyl), respectively. Heating (PEt3)2Pt(2,2‘-biphenyl) in the presence of biphenylene leads to C−C cleavage of a second biphenylene to give (PEt3)2Pt(2,2‘-tetraphenyl), 2, via a Pt(IV) intermediate. 2 reductively eliminates tetraphenylene at 115 °C. At 120 °C the reaction is catalytic; Pt(PEt3)3 or 1 converts biphenylene to tetraphenylene. The intermediates in the catalytic cycle have been identified, and 1 and 2 have been characterized by X-ray analysis. Under catalytic conditions 1 and 2 approach steady-state concentrations. Kinetic analysis reveals that the steady-state concentration ratio, resting state species, and overall rate of catalysis, kobs, depend on the ratio of biphenylene to PEt3. This observation is consistent with loss of PEt3 from 1, resulting in the 14-electron species (PEt3)Pt(2,2‘-biphenyl), I. At 130 °C, I coordinates to PEt3 approximately 130 times fast...

Enhancement in methanol oxidation by spontaneously deposited ruthenium on low-index platinum electrodes

Abstract: Using Auger electron spectroscopy and electrochemistry we confirm that ruthenium is spontaneously deposited on platinum single-crystal faces from ruthenium(III) chloride solutions in perchloric acid The deposit is catalytically active towards methanol present in electrolytic media On the Pt(111) surface - the most active catalyst for the oxidation process - the data show that the packing density of ruthenium is 99±15% and increases with ruthenium concentration in the electrolytic bath, confirming the previous electrochemical coverage estimate In the most favorable instances the ruthenium enhancement factor, the current ratio due to methanol oxidation with and without ruthenium on the surface, is equal to15 (at 0490 V vs RHE reference on Pt(111)) We believe this study is an important addition to our long-range efforts in investigating surface structure effects in platinum-ruthenium heterogeneous electrocatalysis

Methanol Tolerant Oxygen Reduction Catalysts Based on Transition Metal Sulfides

Abstract: The oxygen reduction activity and methanol tolerance of a range of transition metal sulfide electrocatalysts have been evaluated in half-cell experiments and in a liquid-feed solid polymer electrolyte direct methanol fuel cell. These catalysts were prepared in high surface area form by direct synthesis onto various surface-functionalized carbon blacks. Of the materials tested, mixed-metal catalysts based on ReRuS and MoRuS were observed to give the best oxygen reduction activities. In addition, significant increases in performance were observed when employing sulfur-functionalized carbon black, which were attributed to the preferential deposition of active Ru sites in the catalyst-preparation process. Although the intrinsic activity of the best material tested, namely, Mo{sub 2}Ru{sub 5}S{sub 5} on sulfur-treated XC-72, was lower than Pt (by ca. 1545 mV throughout the entire polarization curve), its activity relative to Pt increased significantly in methanol-contaminated electrolytes. This was due to methanol oxidation side reactions reducing the net activity of the Pt, especially at low overpotentials.