Showing papers on "Platinum published in 1982"

Kinetic Oscillations in the Platinum-Catalyzed Oxidation of Co

Abstract: Oscillations of the steady-state rate of catalytic CO oxidation on clean Pt surfaces were observed under low-pressure conditions and are related to the adsorption properties of the reactants. Evidence is presented for a model in which the reversible (5\ifmmode\times\else\texttimes\fi{}20)\ensuremath{\rightleftarrows}(1\ifmmode\times\else\texttimes\fi{}1) phase transition of the Pt(100) surface (which is associated with a marked variation of the oxygen sticking coefficient) is responsible for the oscillations.

The chemistry of heteroarylphosphorus compounds. Part 15. Phosphorus-31 nuclear magnetic resonance studies of the donor properties of heteroarylphosphines towards selenium and platinum(II)

Abstract: The donor properties of a series of heteroarylphosphines (bearing 2- and 3-furyl, 2- and 3-thienyl, and 1-methylpyrrol-2-yl groups directly bound to phosphorus) towards selenium and platinum(II) acceptors have been investigated by 31P n.m.r. studies of the one-bond coupling constants 1J(77Se–31P) and 1J(195Pt–31P). It is shown that the respective coupling constants increase as the heteroaryl groups become more electron withdrawing, indicating an increased s character for the phosphorus lone pair. The implications of this for the relative donor properties of the heteroarylphosphines and PPh3 are considered.

Relationship between the area of L2,3 x‐ray absorption edge resonances and the d orbital occupancy in compounds of platinum and iridium

Abstract: The relationship between the intensity of the L 2 and L 3 x‐ray absorption threshold resonances and d orbital occupancy has been investigated for a series of compounds of platinum and iridium and the pure metals, using Xα‐SW molecular orbital calculations to obtain the unoccupied d orbital states. The change in d orbital occupancy caused by formation of the core hole was found to be fairly constant in different compounds and the pure metal. The difference between the d orbital occupancy in platinum metal and the d orbital occupancy in a given platinum compound agrees well with the calculated effective charge on the platinum atom in the compound. The areas of the threshold resonance lines were obtained by a deconvolution of the absorption edge into a Lorentzian component and an underlying ’’step’’ representing the onset of absorption to continuum states. For a series of platinum compounds, a linear relationship was obtained between the unoccupied d orbital states calculated in the core hole potential, and the sum of the areas of the L 2 and L 3 threshold resonance lines (corrected for the different degeneracies of the initial state). However, this relationship could not be applied to IrO2 because of the absence of transitions to empty d states at the top of the valence band.

Photosensitised dissociation of water using dispersed suspensions of n-type semiconductors

Abstract: n-Type semiconductor powders (TiO2 and SrTiO3) were used to sensitise the photochemical cleavage of water using u.v. light (λ < 400 nm) under conditions of room temperature and pressure. Although several methods of platinising these powders were used, one method in particular (involving precipitation of a platinum sol, by addition of an inert electrolyte, in the presence of the semiconductor powder) was found to produce an efficient photocatalyst for water reduction. Many different photocatalysts were tested for water reduction activity, using EDTA as an electron donor, and for water oxidation activity, using Fe3+ as an electron acceptor. In the absence of EDTA and Fe3+, u.v. irradiation of these photocatalysts liberated H2 but O2 evolution was not observed. Reasons for these observations are discussed.

Dehydrogenation processes on nickel and platinum surfaces. Conversion of cyclohexane, cyclohexene, and cyclohexadiene to benzene

Abstract: Described is the surface coordination chemistry of cyclohexane, 1,3- and 1,4-cyclohexadiene, and cyclohexene on the low Miller index planes and a stepped surface of nickel and on the platinum (111) and stepped 6(111)X(111) surfaces as established by thermal desorption spectroscopy, chemical displacement reactions, and isotopic labeling studies. Cyclohexane did not react with Ni(111) at 0 to 70/sup 0/C, Ni(110) at 20-90/sup 0/C, Ni(100) at 25-200/sup 0/C, and Ni(9(111)X(111)) at 20-70/sup 0/C. A similar behavior was observed for Pt(111) at -35 to +135/sup 0/C, although a small degree of dehydrogenation was evident on this surface. Definite evidence for cyclohexane conversion to benzene was obtained for the stepped platinum surface; the reactivity of this stepped surface toward cyclohexane was substantially higher than that of the platinum (111) plane. Cyclohexene and 1,3- and 1,4-cyclohexadiene were converted, at least partially, to benzene on all of the nickel and platinum surfaces. Hydrogen-deuterium (C-H) exchange during the conversion of a mixture of chemisorbed C/sub 6/H/sub 10/ and C/sub 6/D/sub 10/ and of a mixture of chemisorbed cyclochexadiene and deuterium atoms to benzene was evident only for the platinum surfaces. Carbon significantly altered the cyclochexene chemistry on Ni(110);Ni(110)-C was far more effective than the clean surface formore » dehydrogenation of cyclohexene to benzene. A substantial carbon effect of the chemistry of the other surfaces was not evident.« less

Surface structure and temperature dependence of light-alkane skeletal rearrangement reactions catalyzed over platinum single-crystal surfaces

Abstract: The structure sensitivity of isobutane, n-butane, and neopentane hydrogenolysis and isomerization catalyzed over a series of flat, stepped, and kinked platinum single-crystal surfaces was investigated near atmospheric pressure and at 540 to 640/sup 0/K. The atomic structure and surface composition of the active catalyst were determined before and after reaction studies by using low-energy electron diffraction and Auger electron spectroscopy. Catalytic activities for butane isomerization and consecutive rearrangement reactions were maximized on platinum surfaces with high concentrations of (100) microfacets. Maximum rates for the competing hydrogenolysis reactions were obtained on platinum surfaces that contain high concentrations of steps and kinks. Hydrogenolysis product distributions varied markedly with terrace structure. The symmetries of d orbitals that emerge from the surfaces with different atomic structure have been used to rationalize the structure sensitivity of light-alkane isomerization.

Catalytic partial oxidation process

Abstract: A method and apparatus for carrying out partial oxidation of hydrocarbon feeds to produce hydrogen rich gases suitable for steam reforming, utilizes a monolithic platinum and palladium containing catalyst. The process comprises introducing an oxygen containing oxidant gas (e.g., air), steam and a hydrocarbon feed into a catalytic partial oxidation apparatus comprising an adiabatic reaction vessel. The oxidation catalyst preferably comprises platinum, palladium and optionally rhodium distended upon a stabilized alumina washcoat. At least one half by weight of the hydrocarbon feed is catalytically oxidized in the monolith at a high throughput rate to produce an effluent suitable for further processing, such as steam reforming.

Electrochemical Studies of Coal Slurry Oxidation Mechanisms

Abstract: Oxidation of a coal slurry solution at platinum electrodes in strong acidic media has been studied in detail. The results indicate that the iron ions leached out from coal are responsible for major anodic currents. The Fe(II) is electrolytically oxidized to Fe(III) at the anode, which oxidizes coal to various products including CO/sub 2/. The rate constants for the catalytic reaction are determined to be 3.0*10/sup -5/ and 1.1*10/sup -4/ sec/sup -1/ for Fe(III) and Ce(IV), respectively. The activation energy for the processes are 13.3 and 7.0 kcal/mol for Fe(III) and Ce(IV). Current efficiencies for CO/sub 2/ production are approximately 15-30%. 6 refs.

Platinum metal surface chemistry of benzene and toluene

Abstract: The coordination chemistry of benzene and toluene on Pt(111) and Pt(6(111)X(111)) has been defined by thermal desorption spectrometry, isotopic labeling studies, and chemical displacement reactions. Benzene chemisorption was largely molecular (nondissociative) on Pt(111) but less so on the stepped surface. At temperatures above 100/sup 0/C, reversible benzene desorption and benzene decomposition were competing reactions. More than one differentiable benzene chemisorption state was present on both surfaces. Exchange experiments established that the rate of surface migration of chemisorbed benzene between states on these surfaces was very low. One sharp distinction in the benzene chemistry of the two platinum surfaces was that reversible C-H bond breaking occurred on the stepped surface but not on Pt(111). Whereas toluene chemisorption on nickel surfaces is fully irreversible, toluene chemisorbed on Pt(111) was partially desorbed as the toluene molecule at 70-110/sup 0/C. Studies with C/sub 6/H/sub 5/CD/sub 3/ and C/sub 6/D/sub 5/CH/sub 3/ suggested that the faster low-temperature C-H bond breaking process is centered on the methyl-group C-H bonds. Chemisorption of mesitylene and m-xylene of Pt(111) was partially reversible.