Showing papers on "Platinum published in 1979"

Electrochemical polymerization of pyrrole

Abstract: The electrochemical polymerization of pyrrole on platinum produces a strongly adhered, durable film with enhanced conductivity and good electrode properties.

Elementary Steps in the Catalytic Oxidation of Carbon Monoxide on Platinum Metals

Abstract: Publisher Summary Catalytic oxidation of carbon monoxide over catalysts from the platinum group metals has been investigated. Apart from its enormous practical importance, this reaction is considered to proceed through a relatively simple mechanism because only diatomic molecules are involved and product formation occurs presumably only over a very few steps. This chapter discusses the adsorptive properties of the reactants, their mutual interaction, and the mechanism and kinetics of product formation as well as the investigations with well defined single-crystal surfaces. The activity of a catalyst for a particular reaction is strongly dependent on the surface structure. Directive investigation of the influence of the surface structure on the catalytic activity was performed by using a platinum single crystal whose surface was curved in such a way that not only the plane but also vicinals with varying step density of two different crystallographic directions were present.

Quantitative analysis of XPS intensities for supported catalysts

Abstract: A quantitative analysis of XPS (ESCA) intensities for supported catalysts was based on a model of cubic catalyst crystallites between sheets of support and the assumption that the relative ESCA intensities of the support and catalyst depend on the photoelectron cross section, the atomic ratio of catalyst and support, the escape depths of the electrons, the support surface area, the catalyst weight fraction, the atomic densities of support and catalyst, and the detector efficiency. The model predicted accurately the densities of new ESCA measurements of alumina-supported rhenium oxide and available data for silica-supported platinum and fluorine-contining aluminas at low fluorine contents; low data points for alumina containing Vertical Bar3: 10Vertical Bar3< fluorine were possibly due to the formation of aluminum hydroxyfluoride and aluminum fluoride which were partly ''invisible'' to ESCA. A method for estimating crystallite size is outlined.

Preparation of Colloidal Transition Metals in Polymers by Reduction with Alcohols or Ethers

Abstract: Colloidal dispersions of rhodium, palladium, osmium, iridium, and platinum are prepared by refluxing the methanol-water solutions of rhodium(III) chloride, palladium(II) chloride, osmium(VIII) oxide, sodium chloroiridate, and chloroplatinic acid, respectively, in the presence of poly(vinyl alcohol) as a protective colloid. The preparations of colloidal dispersions of rhodium are successful in the presence of vinyl polymer with polar group such as poly(vinyl alcohol), polyvinylpyrrolidone, or poly(methyl vinyl ether). Polyethyleneimine, gelatin, polyethylene glycol), and dextran are ineffective as the protective colloid. Water-soluble primary alcohols such as methanol and ethanol, water-soluble secondary alcohols such as 2-propanol, and water-soluble diethers such as 1,4-dioxane are available as reductants for preparation of the colloidal dispersion of rhodium. The average diameters of metal particles in the colloidal dispersions of palladium, rhodium, platinum, iridium, and osmium in poly(vinyl a...

Effect of chemical environment on magnitude of x‐ray absorption resonance at LIII edges. Studies on metallic elements, compounds, and catalysts

Abstract: We report spectra isolating the LIII x‐ray absorption threshold resonance of the elements iridium, platinum, and gold in the pure metallic state and in a variety of compounds. When normalized spectra obtained on the metals are subtracted from those obtained on the compounds, the resulting difference spectra are related to differences in the electronic structure of the absorber atom in the two types of environment. The change in area of a threshold resonance line obtained from such a difference spectrum can be related to the ionicity of the bonds of the absorber atom in its compounds. Measurements on supported platinum and iridium catalysts provide information on electronic changes in the metal due to the small size of the metal clusters or to interaction with the support material. Information on electronic changes due to interaction of the catalysts with gas molecules may also be obtained.

Hydrogen evolution from water induced by visible light mediated by redox catalysis

Abstract: Noble metal dispersions are suitable for mediating light-induced hydrogen1–5 and oxygen6–8 evolution from water. We report here a dramatic improvement of the hydrogen production rate when very finely dispersed platinum is used as a mediator in reaction (1). The first observations of the dynamics of intervention of the Pt particles in the redox events are presented. A centrifuged colloidal Pt catalyst stabilised by polyvinyl alcohol showed exceptionally high activity in promoting hydrogen evolution from water via 2MV+ + H2O→PtH2 + 2OH− + 2MV2+ (1) where MV+ stands for reduced methylviologen. The latter is produced photochemically in aqueous solution containing Ru(bipy)32+ as a sensitiser and EDTA as an electron donor. At 10−3 M Pt the reoxidation of MV+ requires only 15 µs and leads to quantitative formation of H2.

Cathodic and Anodic Polarization Phenomena at Platinum Electrodes with Doped CeO2 as Electrolyte I . Steady‐State Overpotential

Abstract: Electrode polarization is studied by means of a current interruption method with the aid of a reference electrode. In this way anodic and cathodic overpotentials, η, can be measured separately, and the electrolyte polarization (IR drop) can be distinguished from the electrode overpotentials. Platinum paste electrodes are used with several doped ceria electrolytes in oxygen‐argon gas atmospheres. At high , the curves obey the Butler‐Volmer equation, with transfer coefficients of unity, both for anodic and cathodic polarizations. This result implies a charge‐transfer (or activation) mechanism. Study of the exchange current, , as a function of and temperature shows that oxygen for the charge‐transfer process is supplied through adsorbed adatoms on the Pt electrode surface, obeying the Langmuir isotherm. An activation energy of 1.75 eV is obtained for the charge‐transfer step. At low , limiting‐current behavior is obtained for the cathodic polarization, characteristic of a concentration polarization. A scanning electron microscope study of the electrode structure helps to understand the deterioration of electrical properties which occurs when samples are subjected to high temperatures and/or current densities.

Extended x-ray absorption fine structure (EXAFS) of dispersed metal catalysts

Abstract: Extended x‐ray absorption fine structure (EXAFS) studies were made in a series of highly dispersed metal catalysts to obtain structural information on the metal clusters present. Clusters of osmium, iridium, and platinum dispersed on silica or alumina were investigated. The metal clusters in the catalysts constituted 1.0 wt.% of the total mass. Chemisorption measurements indicated that the metal dispersions approached unity, where dispersion is defined as the ratio of surface metal atoms to total metal atoms in the clusters. When EXAFS data on the metal clusters are compared with data on the corresponding bulk metals, the lower average coordination number of the metal atoms in a cluster is evident. The decrease in coordination number is accompanied by an increase in the root mean square deviation of interatomic distance about the equilibrium value.

Oxygen Reduction on Platinum in 85% Orthophosphoric Acid

Abstract: Oxygen reduction on Pt in 85% has been studied using a rotating disk electrode at various temperatures. At a prereduced Pt electrode the Tafel slope is ~ −0.12 V/decade. Kinetic studies of reduction on Pt at room temperature using rotating ring‐disk technique have shown that in purified 85% the parallel mechanism is operative. The direct reduction to water is the predominant path, and only about 10% of the reduction proceeds via the production of . oxidation and reduction on Pt have also been studied at room temperature using the rotating disk electrode. The oxidation of is one‐half order with respect to while the reduction is first order.

Ternary rme2x2 alloys of the rare earths with the precious metals and silicon (or germanium)

Abstract: The structural characteristics of about 30 compounds with the composition RMe2X2 have been studied for R = rare earth (or uranium), Me = rhodium, palladium, silver, iridium, platinum or gold and X = silicon or germanium Their structures proved to be related to the Al4Ba (ThCu2Si2) types The average atomic volumes of these phases are compared with those of similar groups of alloys and are briefly discussed Special reference is made to the cerium and ytterbium compounds and the behaviour of these elements is compared with that shown in their binary alloys