Showing papers on "Platinum published in 1985"

Correlation between catalytic activity and bonding and coordination number of atoms and molecules on transition metal surfaces: Theory and experimental evidence.

Abstract: Correlation between catalytic activity and low-energy local electronic fluctuations in transition metals is proposed. A theory and calculations are presented which indicate that maximum electronic fluctuations take place at high-coordination metal sites. Either (i) atomically rough surfaces that expose to the reactant molecules atoms with large numbers of nonmagnetic or weakly magnetic neighbors in the first or second layer at the surface or (ii) stepped and kinked surfaces are the most active in carrying out structure-sensitive catalytic reactions. The synthesis of ammonia from N2 and H2 over iron and rhenium surfaces, 1H2/2H2 exchange over stepped platinum crystal surfaces at low pressures, and the hydrogenolysis (C—C bond breaking) of isobutane at kinked platinum crystal surfaces are presented as experimental evidence in support of the theory.

Gas Sensing Characteristics of Porous ZnO and Pt/ZnO Ceramics

Abstract: Changes in resistivity and chemical changes in reducing gases were measured for porous zinc oxide ceramics with and without a platinum catalyst at 300° and 400°C to examine the gas sensing mechanism and the effect of platinum additions. Reducing gases were oxidized to CO2 and Hz2 on the sensor surfaces. Platinum addition promoted the oxidation of reducing gases but did not lead to an increase in the resistivity change at 400°C. The reaction sequences for the gas sensing process are proposed, taking into account partially oxidized intermediates of hydrocarbons and oxidation on platinum without an electron transfer process.

Micron-spaced platinum interdigitated array electrode: Fabrication, theory and initial use

Abstract: Abstract : An interdigitated array (IDA) electrode composed of 40 pairs of 0.3 micrometers thick Pt fingers 3.5 micrometers wide and separated by 2.5 micrometer gaps of insulating borosilicate glass substrate has been fashioned by a modified microlithographic technique. The two IDA is applied to measurement of redox electron conduction through films of poly-Os(bpy)2 (vpy)2 (C1O4)2, poly-CRu(bpy)2 (vpy)2 (C104)2, and Prussian Blue that had been electrochemically deposited over the Pt fingers and in te insulating gaps. Appropriate theory for using the IDA for this purpose is developed. Electrochemically generated luminescence from solutions of Ru(bpy)3 (2+) is also observed with the IDA. (Author)

Electronic properties of inorganic quasi-one-dimensional compounds

Abstract: The close relationship between experimentalists and theorists - whether solid state chemists or physicists - has, in the last few years, inspired much research in the field of materials with quasi one-dimensional structures. Part I of this two-volume set, reviews the basic theories describing the physical properties of one-dimensional materials including their superconducting characteristics. This description is mainly based on the properties of transition metal trichalcogenides. The novel collective transport mechanism for electronic conduction, exhibited by some of the latter compounds - NbSe3 being considered as the prototype - is surveyed according to a classical theory and a theory including macroscopic quantum effects. In addition, the book contains a description of the properties of non-linear excitations, or solitons, in one-dimensional systems. Part II of this two-volume set deals with the experimental treatment of pseudo-one-dimensional conductors. Included are contributions on platinum chains, (SN)x and (SNBry)x, the optical properties of 1-D inorganic metals, CDW transport in transition metal chalcogenides, and a lattice dynamical study of transition metal trichalcogenides.

An Introduction to Chemisorption and Catalysis by Metals

Abstract: The fundamentals Primary process during collisions Adsorption and desorption Metallic structures and bonding Low energy electron diffraction Electron emission Surface potentials and vibrational spectra General background to heterogeneous catalysis and its application to the reaction o hydrogen isotopes The catalytic oxidation of carbon monoxide on palladium and platinum.

Ordered ionic layers formed on platinum(111) from aqueous solutions

Abstract: Mise en evidence de la formation de couches ordonnees d'adsorption lors de l'immersion d'une surface de Pt(111) dans des solutions ioniques aqueuses de KCN, KSCN, K 2 S, KBr, KI et d'hydroquinonesulfonate de K. Resultats d'etudes par diffraction d'electrons lents et par spectrometrie Auger

15N and 195Pt NMR spectra of platinum ammine complexes: trans- and cis-influence series based on 195Pt-15N coupling constants and 15N chemical shifts

Abstract: 195Pt and 15N NMR spectra have been obtained for the series of platinum(II) complexes Pt(15NH3)3Zn+, cis-Pt(15NH3)2Z2m+, and cis-Pt(15NH3)2YZm+ (Z, Y are neutral or anionic ligands) and for the platinum(IV) complexes derived from them by peroxide oxidation, mer-Pt(15NH3)3Z(OH)2 n+ and Pt(15NH3)2Z2(OH)2 m+. From the spectra of the triammineplatinum(II) complexes, the effects on δN and 1J(Pt-N) trans and cis to Z, and on δPt, of replacing one of the ammine ligands of Pt(15NH3)4 2+ by Z were measured. With the assumption that the effects of replacing two ammine ligands are additive, the NMR parameters for diammineplatinum(II) complexes were calculated. Agreement with experimental values was good overall. NMR data for the platinum(IV) complexes were treated similarly, again with good agreement between calculated and experimental values for the diammine complexes. The influence of different ligands Z on the NMR parameters for the two different oxidation states, while generally similar, did show some consistent differences in detail. The influences of the halide ions on δPt, and on δN and J(Pt-N) trans to Z, were greater for platinum(IV) relative to platinum(II). For the series Pt(15NH3)3Zn+, results are consistent with previous generalizations, that changes in δPt mainly depend on the influence of Z on Pt 5d orbitale (through involvement in the Pt-Z σ-bond and through splitting of the d-orbital energy levels) and that changes in J(Pt-N) trans to Z depend mainly on variations in the Pt 6s contribution to the Pt-N bond. δN trans to Z shows an inverse linear correlation with δPt. Cis influences on δN and J(Pt-N) are smaller than trans influences but are still significant.

Investigation of the mechanism of hydrogen evolution during photocatalytic water decomposition on metal-loaded semiconductor powders

Abstract: The active sites for photocatalytic hydrogen evolution are identified for some noble metals deposited on TiO/sub 2/ particles by measuring the hydrogen isotope effect of the gases produced by the photodecomposition of water using a series of metal-loaded TiO/sub 2/ suspensions (separation factor analysis). The mechanism of the hydrogen evolution process on metal-loaded TiO/sub 2/ particles is discussed in relation to the hydrogen electrode reaction of the same metals. By means of separation factor analysis the hydrogen evolution process on Pt/TiO/sub 2/, Pd/TiO/sub 2/, and Rh/TiO/sub 2/ is assigned to a slow recombination mechanism. 27 references, 2 figures, 2 tables.

Catalyzed diesel exhaust particulate filter

Abstract: A self-cleaning diesel exhaust particulate filter is disclosed wherein burn-off of collected particulate matter is accomplished at certain exhaust gas temperatures, the filter being provided with a catalyst mixture consisting essentially of a platinum group metal and an alkaline earth metal oxide. The platinum group metal may be one or both of platinum and rhodium and the alkaline earth metal oxide may be magnesium oxide. The presence of the catalyst mixture serves to lower the temperature at which ignition of the collected particulates is initiated.

Activity of platinum(II) intercalating agents against murine leukemia L1210.

Abstract: Four series of intercalating, square-planar Pt(II) complexes derived from the ligands 2,2'-bipyridine, 2,2':6',2"-terpyridine, 1,10-phenanthroline, and 3,4,7,8-tetramethyl-1,10-phenanthroline were synthesized and aspects of their activity against murine leukemia L1210 cells investigated. The 2,2':6',2"-terpyridine-thiolato complexes are growth inhibitory in culture, with IC50 values in the range 6-32 microM, and cause cell lysis at high concentrations. Of the remaining three series, the 2,2'-bipyridine complexes are the least potent in their effects. There is a general enhancement in activity on moving from the 1,10-phenanthroline complexes to the 3,4,7,8-tetramethyl-1,10-phenanthroline analogues. Flow cytometric analysis on representative complexes shows that they are not cell cycle specific. Alkaline elution experiments indicate no damage to DNA of cells exposed to (thiophenolato)(2,2':6',2"-terpyridine)platinum(II) chloride monohydrate and (ethylenediamine)(1,10-phenanthroline)platinum(II) dichloride dihydrate although (ethylenediamine)(3,4,7,8-tetramethyl-1,10-phenanthroline)platinum(II) dichloride dihydrate causes both single-strand breaks and DNA cross-links. Compounds 2a, 5a, and 6a showed no antitumor activity against L1210 in mice.

Triangular platinum and nickel clusters: the "tinker-toy" construction of chains with high nuclearity

Abstract: The triangular metal carbonyls formed from the Ni group transition metals are remarkable in their propensity to form high nuclearity clusters just by associating into chains. Such chains are typified by the compounds [Pt3(w2-CO),(CO),]?( n = 2, 3 , 4, 5 , 6, 10). However, there is a characteristic structural difference between the “tinker-toy” construction of stacks using Ni rather than Pt; the Ni carbonyl chains have a screw axis along the main axis, whereas the Pt chains have a simple threefold axis. Reasons for stacking and the requirements of the 2charge for all chain lengths as well as the structural differences between Ni and Pt chains are discussed in terms of the calculated electronic structure of the dimer. As well as a necessary electron count for oligomerization, there exist monomers which have specific valence electron counts. For 42 valence electrons and acceptor ligands as bridges (e.g., Pt,(CNR),) an equilateral triangle of metals is expected. When an extra two electrons are added, calculations show that distortion to an isosceles triangle or to a larger equilateral triangle is favored. Differences in electron count of various monomers are related to the types of bridging and terminal ligands present. For instance for bridging phosphido, PR2, trimers the 44-electron species should be the stable one. The reasons for further deformation in such 44-electron trimers are addressed. In addition to oligomers, intercalation complexes are also known. For Ni there is the series [NiI2(CO),,H,,]“ where n = 2,3,4 and for Pt there are the compounds [Pt3(fi2-CNR)3(CNR)3]2Hg where R = 2,6-dimethylphenyl and [Pt(p2-CO)3(PPh-i-Pr2)3]2Hg2. The reasons for the stability of these compounds are discussed in terms of the electronic structure of the Pt dimers at large monomer-monomer separation. On the basis of the known monomers, intercalates, and oligomers the existence of long-chain polymers is postulated. For [Pt3(C0)6] the calculated bond structure indicates that oxidation or reduction of the neutral chain should result in stabilization. In many ways it is similar to the tetracyanoplatinate chains discussed previously in the literature. Partially occupied bands may result in Peierls distortions leading to structures similar to those observed for the pentamer and presumably the decamer. The band structure of ‘[Pt,(CO),Hg], shows a valence band with much greater dispersion than the other bands. Partial occupation of this band (by replacing Hg with Cu, Ag, or Au) will most likely result in a pairing distortion to give a semiconductor. The attention paid to polyhedral, multinuclear metal clusters derives both from the aesthetically pleasing act of creating polyhedral clusters with increasingly higher nuclearity and also from the desire to use these clusters as models of crystalline metal surfaces. Many reactions catalyzed on metallic surfaces are also seen to occur on smaller metal complexes.’ The smallest multinuclear metal cluster which can be considred as a model (albeit a highly simplistic one) of a metal surface is a triangular, trinuclear cluster. This unit forms the basis of many polyhedral clusters, indeed three out of the five platonic solids can be built up from this simple unit, viz., tetrahedron, octahedron, and icosahedron. Triangular units can be recognized in polyhedra having 4-12 vertices, and for each of these molecular examples are known.2 Once the polyhedra (closo structures) have been enumerated, more open frameworks having the same number of vertices can be found (nido, arachno, and hypho structures). A beautiful example of the opening of a closed structure is given by Lewis, Johnson, and co-workers in their discussion of the raft-like os6(co)17[P( O M e ) 3 1 ~ ~ There are still further ways to construct high nuclearity clusters from the basic “tinker-toy”, triangular building block. If the building block is flat, and the M3(k2-L’)3(L)3 unit 1 is particularly suited for this, then it can be simply stacked in one dimension. Two layers if eclipsed lead to a trigonal prism and if staggered to an octahedron of metals. The extension to oligomers and to a one-dimensional polymer is obvious. And not only on paper. These are real systems. We have discussed elsewhere4 the beautiful CSIRO Division of Applied Organic Chemistry. * Cornell University. t Osaka University. $The Institute of Physical and Chemical Research. 0002-7863 /85 /1507-5968$01.50/0 L