Showing papers on "Platinum published in 1994"

Carbon monoxide electrooxidation on well-characterized platinum-ruthenium alloys

Abstract: The electrocatalytic activity of well-characterized Pt-Ru alloy electrodes toward the electrooxidation of CO in acidic electrolyte at room temperature was measured on alloy surfaces prepared in UHV (ultrahigh vacuum). Clearly defined surface composition was determined via LEIS (low-energy ion scattering). Electrocatalytic activities were measured by CO stripping voltammetry as well as by potentiostatic oxidation of adsorbed CO. It was found that the property of Ru atoms to nucleate oxygen-containing species at low potentials produced a strong enhancement in the catalytic activity of sputter-cleaned Pt-Ru alloy electrodes compared to pure Pt, thereby supporting the concept of the bifunctional character of the oxidation process of these alloys

The Chemistry of Metal Cvd

Abstract: CVD of aluminium tungsten copper from Cu(II) percursors copper from Cu(I) precursors gold and silver platinum, pallaium, and nickel assorted metals.

Electrochemistry of Methanol at Low Index Crystal Planes of Platinum: An Integrated Voltammetric and Chronoamperometric Study

Abstract: We have studied a catalytic decomposition of methanol on low Miller index platinum surfaces, Pt(111), Pt(110), and Pt(100) in perchloric, sulfuric, and phosphoric acids at room temperature. The instantaneous methanol oxidation current is unaffected by the methanolic CO formation (surface poisoning) and depends on platinum surface structure and composition of supporting electrolyte with respect to the anions. The highest oxidation current, 156 mA-cm[sup [minus]2], is observed with the Pt(110) electrode in perchloric acid solution at 0.200 V vs Ag/AgCl reference. In terms of turnover, this current translates to 163 molecules-(Pt site)[sup [minus]1][center dot]s[sup [minus]1], a high rate exceeding previous expectations in methanol electrode kinetics. Overall, the oxidation current changes by 3 orders of magnitude between the extreme cases examined in this study. Breaking up the total effect into individual components shows that the surface geometry and anionic effects are roughly comparable. Therefore, we have an evidence that anion-platinum interactions are as important in determining the methanol oxidation rate as is the surface geometry of the Pt catalyst. 52 refs., 13 figs., 4 tabs.

Mechanism of the selective reduction of nitrogen monoxide on platinum-based catalysts in the presence of excess oxygen

Abstract: A range of alumina-supported platinum catalysts have been prepared and investigated for the selective reduction of nitrogen monoxide in the presence of a large excess of oxygen. Steady-state microreactor experiments have demonstrated that these catalysts are very active and selective for the reduction of nitrogen monoxide by propene at temperatures as low as 200°C. There does not appear to be a simple correlation between the activity for nitrogen monoxide reduction and the platinum surface area. Instead it is found that there is a very good inverse correlation between the maximum nitrogen monoxide reduction activity and the temperature. The most active catalysts for selective nitrogen monoxide reduction are those that generate activity at the lowest temperature. The technique of temporal analysis of products (TAP) has been used to obtain detailed mechanistic data about the selective nitrogen monoxide reduction reaction on an alumina-supported platinum catalyst. Using carbon monoxide, hydrogen or propene as reductant it has been demonstrated that the predominant mechanism for selective nitrogen monoxide reduction involves the decomposition of nitrogen monoxide on reduced platinum metal sites, followed by the regeneration of the active platinum sites by the reductant. In the decomposition step it has been shown that oxygen from nitrogen monoxide is retained on the surface of the platinum and blocks the surface for further adsorption/reaction of nitrogen monoxide; it has been observed that oxidised platinum catalysts are not active for the nitrogen monoxide reduction reaction. Under typical operating conditions, propene is a far more efficient reductant than either carbon monoxide or hydrogen. The greater efficiency of propene as a reductant is explained on the basis of the additional reducing power of the propene molecule, which can react with as many as nine adsorbed oxygen atoms, ensuring that 'patches' of reduced platinum are available for nitrogen monoxide adsorption/reaction. A small additional activity of reduced platinum in the presence of propene, which is not observed when carbon monoxide or hydrogen is used as reductant, has been explained on the basis of a second mechanism involving the carbon-assisted decomposition of nitrogen monoxide at sites on the reduced platinum adjacent to adsorbed carbon-containing moieties, believed to be fragments from adsorbed propene molecules. A model for the selective reduction of nitrogen monoxide on alumina-supported platinum catalysts is presented which is capable of explaining all the results obtained in this work and in the published literature on this subject.

Semiconductor, semiconductor device, and method for fabricating the same

Abstract: Method of fabricating semiconductor devices such as thin-film transistors by annealing a substantially amorphous silicon film at a temperature either lower than normal crystallization temperature of amorphous silicon or lower than the glass transition point of the substrate so as to crystallize the silicon film. Islands, stripes, lines, or dots of nickel, iron, cobalt, or platinum, silicide, acetate, or nitrate of nickel, iron, cobalt, or platinum, film containing various salts, particles, or clusters containing at least one of nickel, iron, cobalt, and platinum are used as starting materials for crystallization. These materials are formed on or under the amorphous silicon film.

Method for producing an unsaturated carboxylic acid

Abstract: A method for producing an unsaturated carboxylic acid, which comprises subjecting an alkane to a vapor phase catalytic oxidation reaction in the presence of a catalyst containing a mixed metal oxide comprising, as essential components, Mo, V, Te, O and X wherein X is at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron, indium and cerium, wherein the proportions of the respective essential components, based on the total amount of the essential components exclusive of oxygen, satisfy the following formulas: 0.25

Investigation of Pt/Al2O3 and Pd/Al2O3 catalysts for the combustion of methane at low concentrations

Abstract: Pt/Al 2 O 3 and Pd/Al 2 O 3 catalysts have been prepared from chlorine-free precursors and investigated for the combustion of methane under lean, stoichiometric, and rich conditions using dilute mixtures. It has been found that under lean conditions, and at low conversions under stoichiometric or rich conditions, Pd/Al 2 O 3 is a more effective catalyst. However, at higher conversions with stoichiometric or rich mixtures Pt/Al 2 O 3 is a more active catalyst. This change over between Pd/Al 2 O 3 and Pt/Al 2 O 3 is associated with a ‘light-off’ effect observed with Pt/Al 2 O 3 catalysts. Various possible explanations for these effects are discussed. With the Pt/Al 2 O 3 catalysts there is no evidence of a particle size effect, which is in contrast to reports in the literature that the methane combustion reaction is structure sensitive. Reasons for these variations, including the possible inhibition of activity by chlorine used in many catalyst preparations, are discussed. It is concluded that platinum can be a more effective catalyst than palladium for methane combustion under real conditions and that, in consequence, platinum may play an important role in multimetallic catalysts for emission control for natural gas vehicles.

Zeolite-containing oxidation catalyst and method of use

Abstract: Oxidation catalyst compositions for treating diesel exhaust include ceria and, optionally, alumina, each having a surface area of at least about 10 m 2 /g, and a zeolite, e.g., Beta zeolite. Optionally, platinum may be included in the catalytic material, preferably in amounts which are sufficient to promote some gas-phase oxidation of carbon monoxide (“CO”) and hydrocarbons (“HC”) but which are limited to preclude excessive oxidation of SO 2 to SO 3 . Alternatively, palladium in any desired amount may be included in the catalytic material. The zeolite is optionally doped, e.g., ion-exchanged, with one or more of hydrogen, a platinum group metal or other catalytic metals. The catalyst compositions may be used in a method to treat diesel engine exhaust by contacting the hot exhaust with the catalyst composition to promote the oxidation of gas-phase CO and HC and of the volatile organic fraction component of particulates in the exhaust.

Propane and propene oxidation over platinum and palladium on alumina: Effects of chloride and water

Abstract: The oxidation of propane and propene was investigated on palladium and platinum supported catalysts. Catalyst intrinsic activities, evaluated by light-off temperatures in slightly oxidizing reactant mixture (5% excess oxygen), show an optimum particle size which maximizes the catalytic activity for a given metal loading. On catalysts prepared from chloride containing precursor salts, chloride poisons the metallic activity whatever the particle size. Moreover, reaction isotherms under varying oxygen levels point out that the effect of chloride is more detrimental under oxidizing conditions. After successive oxidation cycles, this poisoning effect disappears as a consequence of the removal of chloride from the catalyst surface by water produced during propane and propene combustion. On the other hand, addition of relatively large quantities of water (equivalent to the content of the exhaust gas) inhibits the oxidation of hydrocarbon. Poisoning effects of chloride and water are explained by a decreasing active surface for the reactions under consideration.

Zerovalent palladium and platinum complexes containing rigid bidentate nitrogen ligands and alkenes: synthesis, characterization, alkene rotation and substitution reactions. X-ray crystal structure of [Bis((2,6-diisopropylphenyl)imino)acenaphthene](maleic

Abstract: A number of zerovalent palladium and platinum M(NN)(alkene) complexes, containing the rigid bidentate nitrogen ligands bis(arylimino)acenaphthene (Ar-BIAN) and bis(phenylimino)camphane (Ph-BIC) have been synthetized and characterized. Stable complexes were obtained with electron poor alkenes, such as dimethyl fumarate, fumaronitrile, maleic anhydride, and tetracyanoethylene. Complexes bearing asymmetric Ar-BIAN or Ph-BIC ligands occurred as mixtures of isomers. From IR, UV, and 1 H and 13 C NMR spectroscopy and from substitution reactions it was concluded that the back-donation of electron density from the metal to the alkene is the major factor determining the stability of the complexes

Effects of boundaries on pattern formation: Catalytic oxidation of CO on platinum

Abstract: The effect of boundaries on pattern formation was studied for the catalytic oxidation of carbon monoxide on platinum surfaces. Photolithography was used to create microscopic reacting domains on polycrystalline foils and single-crystal platinum (110) surfaces with inert titanium overlayers. Certain domain geometries give rise to patterns that have not been observed on the untreated catalyst and bring to light surface mechanisms that have no analog in homogeneous reaction-diffusion systems.

Electrochemical enhancement of a catalytic reaction in aqueous solution

Abstract: THE rates of many heterogeneous catalytic reactions between gaseous adsorbates on metal surfaces supported on solid electrolytes can be increased by applying a potential to the metal1–8 This phenomenon, which has been reported previously at temperatures of 250–750°C, has been shown9 to be due to the electrochemically induced spillover of ions from the support onto the catalyst surface; the ions then act as promoters for the catalytic reaction Here we report that a similar effect can be observed in aqueous solution at ambient temperatures We studied the oxidation of H2 on a platinum/graphite electrode immersed in aqueous KOH Application of a positive potential of 1–2 V to the Pt electrode increased the rate of H2 oxidation by up to 500% We deduce that hydroxide ions are acting as promoters, and find that each ion supplied to the catalyst causes the oxidation of up to 20 hydrogen atoms This kind of rate enhancement for heterogeneous catalytic reactions in solution may be of considerable technological value, for example in the electrochemical treatment of toxic organics10 or the generation of useful industrial chemicals4

Optical spectroscopy of platinum and palladium containing poly‐ynes

Abstract: We present a comparative study of two isostructural platinum and palladium poly‐ynes, poly[trans(bis(tributylphosphine))M(1,4) phenylenediethynylene], where M represents either platinum or palladium. Optical excitations are associated with the π‐molecular orbitals, and with strong spin–orbit coupling provided by the transition metal, both singlet and triplet excitations can be measured. The platinum and palladium polymers show similar low‐temperature photoluminescence and photoinduced absorption spectra, with a blue‐shift for the palladium polymer of around 5% for electronic transitions with respect to the platinum polymer. For Pt (Pd) we see strong singlet absorption peaks at 3.26 eV (3.49 eV), and weaker triplet absorption peaks at 2.43 eV (≊2.5 eV) at room temperature. Strong photoluminescence from triplet to ground state peaks at 2.38 eV (2.5 eV), and weak photoluminescence from the excited singlet peaks at 3.06 eV (3.2 eV) at low temperatures. Photoinduced absorption that peaks at 1.49 eV (1.72 eV) i...

Method of making semiconductor device/circuit having at least partially crystallized semiconductor layer

Abstract: Amorphous silicon in impurity regions (source and drain regions or N-type or p-type regions) of TFT and TFD are crystallized and activated to lower electric resistance, by depositing film having a catalyst element such as nickel (Ni), iron (Fe), cobalt (Co) or platinum (Pt) on or beneath an amorphous silicon film, or introducing such a catalyst element into the amorphous silicon film by ion implantation and subsequently crystallizing the same by applying heat annealing at an appropriate temperature.