Showing papers on "Platinum published in 1997"

Activation of c-h bonds by metal complexes

Abstract: ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Mesoporous Platinum Films from Lyotropic Liquid Crystalline Phases

Abstract: The lyotropic liquid crystalline phases of surfactants exhibit a rich polymorphism of structures that have long-range periodicities and whose characteristic repeat distances range from 2 to 15 nanometers. The electrochemical reduction of platinum salts confined to the aqueous environments of these phases leads to the deposition of platinum films that have a well-defined long-ranged porous nanostructure and high specific surface areas. These results suggest that the use of liquid crystalline plating solutions could be a versatile way to create mesoporous electrodes for batteries, fuel cells, electrochemical capacitors, and sensors.

Temperature-dependent oxygen electrochemistry on platinum low-index single crystal surfaces in acid solutions

Abstract: Using the rotating ring-disk technique (RRDPt(hkl)E), the oxygen-reduction reaction (ORR) was studied in sulfuric acid solution over the temperature range 298–333 K At the same temperature, the exc...

The influence of the preparation methods on the catalytic activity of platinum and gold supported on TiO2 for CO oxidation

Abstract: The influence of the preparation methods on the catalytic activity for CO oxidation was markedly large for Au-TiO2 and negligible for Pt-TiO2 catalysts. Platinum and gold were deposited on TiO2 by deposition-precipitation (DP), photodeposition (FD) and impregnation (IMP). The DP method gave the most active catalysts for both Pt and Au. Gold catalysts prepared by DP were active at temperatures below 273 K and showed a much greater activity than Pt catalysts.

Combustion of non-halogenated volatile organic compounds over group VIII metal catalysts

Abstract: Catalytic combustion of volatile organic compounds (VOCs), present in low concentrations (10–1000 ppm) in industrial effluent streams, is a promising air abatement technology. The oxidation of benzene, butanol and ethyl acetate over group VII metal catalysts supported on alumina carriers has been investigated. Pt, Pd and Co were found to be the most active among group VIII metals, while ethyl acetate was found to be the most-difficult-to-oxidize compound. Benzene and ethyl acetate oxidations over Pt/Al2O3 were found to be structure sensitive reactions with the turnover frequency (TOF) increasing with increasing mean metal particle size. The presence of chloride on the catalyst surface, originating from chloride-containing metal precursor compounds was found to exert an inhibiting effect on the activity of Pt. Apparent activation energies of the reactions over Pt and Pd catalysts were found to be in the 70–120 kJ/mol range while the reaction order with respect to the VOC was positive in all cases. During oxidation of benzene-butanol mixtures, benzene oxidation was completely suppressed as long as butanol was present in the reaction mixture.

Role of platinum deposits on titanium(IV) oxide particles : structural and kinetic analyses of photocatalytic reaction in aqueous alcohol and amino acid solutions

Abstract: Photocatalytic reaction at 298 K by platinum-loaded titanium(IV) oxide (TiO2−Pt) particles suspended in deaerated aqueous solutions of 2-propanol or (S)-lysine (Lys) was investigated. The TiO2 catalysts with various amounts of Pt loadings were prepared by impregnation from aqueous chloroplatinic acid solution onto a commercial TiO2 (Degussa P-25) followed by hydrogen reduction at 753 K. The physical properties of deposited Pt, e.g., particle size, surface area, and electronic state, were studied respectively by transmission electron microscopy, volumetric gas adsorption measurement, and X-ray photoelectron spectroscopy as well as infrared spectroscopy of adsorbed carbon monoxide. The increase in Pt amount mainly resulted in an increase of the number of Pt deposits, not of their size. The catalysts were suspended in the aqueous solutions and photoirradiated at a wavelength >300 nm under an argon (Ar) atmosphere. The overall rate of photocatalytic reactions for both 2-propanol and Lys, corresponding to the ...

Methanol Oxidation on Single‐Phase Pt‐Ru‐Os Ternary Alloys

Abstract: Methanol oxidation was studied on arc-melted Pt-Ru-Os alloys and on fuel cell catalysts prepared by the NaBH{sub 4} reduction of metal chloride salts. Both the arc-melted alloys and the high surface area catalysts have x-ray diffraction patterns indicative of single-phase face-centered cubic lattices. Hydrogen adsorption/desorption measurements on the polished alloy electrodes, in the presence of adsorbed CO (25 C), show that selected ternary alloys have significant hydrogen adsorption/desorption integrals at adsorption potentials where Pt:Ru (1:1) was fully blocked and higher integrals at all adsorption potentials studied up to 400 mV vs. the reference hydrogen electrode. In situ diffuse reflection Fourier transform infrared spectroscopy of the fuel cell anodes showed that the alloy catalysts had reduced CO coverage relative to Pt, with the ternary catalyst showing the least coverage. Steady-state voltammetry of the arc-melted alloys at 25 C confirmed that Pt-Ru-Os (65:25:10) is more active than Pt-Ru (1:1), particularly above 0.6 V. Pt-Ru-Os (65:25:10) methanol fuel cell performance curves were consistently superior to those of Pt-Ru (1:1) (e.g., typically at 90 C, 0.4 V; 340 mA/cm{sup 2} with Pt-Ru-Os vs. 260 mA/cm{sup 2} with Pt-Ru).

A thermal barrier coating for a superalloy article and a method of application thereof

Abstract: A multi-layer thermal barrier coating (42) for a superalloy article (40) comprises a platinum enriched superalloy layer (44), an MCrAlY bond coating (46) on the platinum enriched superalloy layer (44), a platinum enriched MCrAlY layer (48) on the MCrAlY bond coating (46), a platinum aluminide coating (50) on the platinum enriched MCrAlY layer (48), an oxide layer (54) on the platinum aluminide coating (50) and a ceramic thermal barrier coating (56) on the oxide layer (54). The platinum aluminide coating (50) and the platinum enriched MCrAlY layer (48) reduce movement of transition metals from the superalloy substrate (40) and the MCrAlY bond coating (46) to the oxide layer (54) so that the oxide layer is very pure alumina. The platinum enriched superalloy layer (44) reduces movement of aluminium from the MCrAlY bond coating (46) to the superalloy substrate (40) so that the oxide layer (54) remains pure alumina to improve long term adhesion of the ceramic thermal barrier coating (56), and additionally reduces movement of transition metals from the superalloy substrate (40) to the oxide layer (54).

Novel Multinuclear Catalysts for the Electroreduction of Dioxygen Directly to Water

Abstract: The electroreduction of O_2 to H_2O in aqueous acid at potentials close to the thermodynamically permitted value remains a daunting challenge for designers of superior fuel cells and batteries that utilize dioxygen as the reducible reactant. The four-electron reduction of O_2, which involves the rupture of the O-O bond and the formation of four O-H bonds, requires the use of catalysts to obtain useful rates at cathode potentials of interest in practical applications. The standard potential of the O_2/H_2O couple in solutions containing 1 M H^+ and saturated with O_2 at 1 atm is ca. 1.0 V (vs the saturated calomel electrode, SCE), but the highest cathode potentials achievable with currently available catalysts are closer to 0.55 V. (Molecules, functional groups, or metallic deposits that accelerate the rates of electrode reactions when they are confined to the surfaces of electrodes are often called electrocatalysts, a terminology that will be adopted in this Account.) Finely divided platinum supported on high area carbon is the electrocatalyst employed most frequently to achieve the electroreduction of O_2 to H_2O in presently available fuel cells. However, this type of electrocatalyst suffers from the disadvantages of high cost and gradual loss in catalytic activity as the surface area of the active platinum particles decreases because of sintering, dissolution, physical dislodgment, and/or adsorption of impurities. Searches for superior electrocatalysts for the reduction of O2 have often focused on cobalt porphyrins which are well-known to exhibit electrocatalytic activity toward the reduction of O_2, although H_2O_2 instead of H_2O is the usual product. However, it was discovered in recent years that a variety of molecular catalysts consisting of dimeric cofacial cobalt porphyrins adsorbed on the surface of graphite electrodes are able to catalyze the direct four-electron electroreduction of O_2 without passing through H_2O_2 as an intermediate. Both dimeric and monomeric iridium por phyrins have also been found to accomplish the electroreduction of O_2 to H_2O at unusually positive potentials. The mechanisms through which dimeric electrocatalysts are believed to operate involve the simultaneous interaction of both metal centers with the two oxygen atoms of the O_2 molecule as the O-O bond is severed. The ideas and strategies that underlay the development of these electrocatalysts have been described.

Evaluation of Platinum‐Based Catalysts for Methanol Electro‐oxidation in Phosphoric Acid Electrolyte

Abstract: Carbon-supported catalysts of Pt, Pt/Ru, Pt/Ru/W, and Pt/Ru/Pd were evaluated for the electro-oxidation of methanol in phosphoric acid at 180 C. These catalysts were characterized using cyclic voltammetry and x-ray diffraction. Addition of Ru to a 0.5 mg/cm{sup 2} Pt catalyst (1:1 atomic ratio) caused a large reduction in polarization. The open-circuit voltage was reduced by 100 mV and polarization at 400 mA/cm{sup 2} was reduced by 180 mV. A Pt/Ru (5:2) catalyst with the same Pt content lowered the open-circuit voltage 70 mV. Additions of W to form Pt/Ru/W (1:1:1, atomic ratio) and Pd to form Pt/Ru/Pd (2:2:1), all with the same platinum loading, gave the same performance as Pt/Ru (1:1) without the additions. All of the catalysts showed two Tafel slopes, 140 mV/dec at lower polarizations and 100 to 120 mV/dec at higher polarizations, indicating that the reaction mechanisms are the same for all of the catalysts. Methanol oxidation is greatly enhanced at 180 C in phosphoric acid compared to the lower operating temperatures of a perfluorosulfonic acid electrolyte. The exchange current density for methanol oxidation is higher than that for O{sub 2} reduction. Ru metal dissolves from catalysts at high potentials. Hydrogen oxidation in the presence ofmore » 1 mole percent carbon monoxide showed carbon monoxide tolerance in the order: Pt/Ru/Pd > Pt/Ru > Pt.« less

Hydrogen Oxidation at the Nickel and Platinum Electrodes on Yttria‐Tetragonal Zirconia Electrolyte

Abstract: H{sub 2} oxidation has been studied for Pt and Ni electrodes for different H{sub 2}/H{sub 2}O ratios at 1,000 C in solid oxide fuel cells using yttria-tetragonal zirconia electrolyte by galvanostatic current interruption and electrochemical impedance spectroscopy. The results clearly indicate that the mechanism and kinetics of the H{sub 2} oxidation reaction are strongly dependent on the catalytic activities of electrode materials, electronic conductivity of the electrolyte surface, and the water content in H{sub 2} gas. The effect of water vapor in H{sub 2} gas on the reaction kinetics is very much dependent on the electrode materials and is related to the partial pressure of oxygen. A reaction mechanism with two rate-limiting steps has been proposed and discussed.

Electrocatalytic oxidation of methanol on platinum nanoparticles electrodeposited onto porous carbon substrates

Abstract: To achieve methanol fuel cell electrodes allowing a high catalyst use, thin layers of various carbon powders and recast Nafion® were electrochemically plated with platinum. The resulting Pt deposits were characterized by hydrogen and carbon monoxide electrosorption, as well as by transmission electron microscopy. Methanol oxidation was then carried out using cyclic voltammetry. The influence of the amount of carbon surface oxides and the effect of Pt specific surface area on the Pt catalytic activity were thus investigated.

Ultrathin Films of Ruthenium on Low Index Platinum Single Crystal Surfaces: An Electrochemical Study

Abstract: We report on electrochemical properties of ultrathin films of ruthenium on platinum single crystal surfaces, Pt(100), Pt(111) and Pt(110), and demonstrate that such films can be obtained by spontaneous deposition. We also show that the spontaneously deposited ruthenium coverage is surface structure dependent. Using the spontaneous deposition process and a constant potential electrolysis, a variety of Pt/Ru surfaces up to ca. 0.4 monolayer of ruthenium were prepared. All such Ru films are stable in the electrode potential range that precedes platinum oxidation. A strong surface structure effect in the electrochemical properties of these thin films was found. On Pt(100)/Ru at a fixed Ru coverage, there is a transition from a reversible to irreversible surface redox behavior that is not observed on other platinum single crystal faces. In contrast to Pt(100)/Ru and Pt(110)/Ru, the individual voltammetric phases of the Pt(111)/Ru electrode are not resolved, and ruthenium surface oxides appear to be the most st...

Determination of vacancy concentrations in the bulk of silicon wafers by platinum diffusion experiments

Abstract: Diffusion of platinum at low temperatures is a convenient way to characterize vacancy profiles in silicon. This article summarizes the experiments performed to find a standard procedure, discusses the pitfalls and limitations, and shows the applicability of the method. The results of experiments with float-zone and Czochralski-grown samples in the temperature range from 680 to 842 °C were found to disagree with the predictions of models published in the literature. Therefore, parameters governing the diffusion of point defects and platinum in silicon were determined for this temperature range.

Complex pathways in dissociative adsorption of oxygen on platinum

Abstract: Gas adsorption on solid surfaces is the basis of heterogeneous catalysis. Gas–surface interactions may be complex and in many cases the fundamental mechanisms of the chemisorption process are hard to discern. The macroscopic kinetics of a heterogeneous catalytic reaction are usually modelled within the Langmuir model1, which assumes that free adsorption sites are occupied at random. The adsorption of oxygen on a platinum (111) surface has been studied extensively as a model system for surface chemical processes generally2,3,4,5,6,7,8,9,10,11,12,13,14,15, owing to its significance in platinum catalysed oxidation reactions such as that of CO and NO. Here we show that even for this well studied system the chemisorption process may be much more complicated than the Langmuir model implies. Our observations with the scanning tunnelling microscope show that the dissociation probability for an oxygen molecule becomes affected by chemisorbed species in the vicinity that have dissociated already. This introduces a dynamic heterogeneity in the adsorption mechanism which leads to kinetically limited ordering of the adsorbate. This effect is likely to be quite general and to affect the bulk kinetics of catalytic reactions conducted at the high temperatures and pressures of most industrial heterogeneous catalysis.

The chemistry of aqua ions : synthesis, structure, and reactivity : a tour through the periodic table of the elements

Abstract: Partial table of contents: Main Group Elements: 1,2,13,14,15,16,17 and 18. Group 4 Elements: Titanium, Zirconium and Hafnium. Group 6 Elements: Chromium, Molybdenum and Tungsten. Group 8 Elements: Iron, Ruthenium and Osmium. Group 10 Elements: Nickel, Palladium and Platinum. Group 11 Elements: Copper, Silver and Gold. Appendices.

Photooxidation of Platinum(II) Diimine Dithiolates

Abstract: The violet color of Pt(bpy)(bdt) (bpy = 2,2‘-bipyridine; bdt = 1,2-benzenedithiolate) is due to a Pt/S → diimine charge-transfer transition; the emission originates from the corresponding triplet state (τ = 460 ns). Photochemical oxidation of Pt(bpy)(bdt) occurs in the presence of oxygen in N,N-dimethylformamide, acetonitrile, or dimethyl sulfoxide solution; the reaction has been investigated by 1H NMR and UV−visible absorption spectroscopy. Singlet oxygen produced by energy transfer from the excited complex is implicated as the active oxygen species, in sequential formation of sulfinate, Pt(bpy)(bdtO2), and disulfinate, Pt(bpy)(bdtO4), products. Both products have been characterized by X-ray crystallography. The rate of photooxygenation is strongly dependent on water concentration, and transient absorption spectra are consistent with the formation of at least one intermediate. As a whole, our data suggest that the photooxidation chemistry of platinum(II) diimine dithiolates is similar to that of organic ...

Mixed manganese oxide/platinum catalysts for total oxidation of model gas from wood boilers

Abstract: Mixed manganese oxide/platinum catalysts in the form of monoliths have been prepared by the deposition-precipitation method. The influence of the platinum content and the calcination temperature has been investigated by activity measurements for total oxidation of methane, naphthalene and carbon monoxide in the presence of steam and carbon dioxide, and by the temperature programmed reduction (TPR) technique. Manganese oxide behavior is influenced by the presence of even very small amounts of platinum especially when the catalyst is calcined at a higher temperature due to favorable synergetic effects.

Assessment of Dowex 1-X8-based anion-exchange procedures for the separation and determination of ruthenium, rhodium, palladium, iridium, platinum and gold in geological samples by inductively coupled plasma mass spectrometry

Abstract: Synthetic multielement solutions of the platinum group metals (PGE: Ru; Rh; Pd; Ir; Pt) and gold, with analysis by ICP-AES and ICP-MS, have been used to study the behaviour of the precious metals on Dowex 1-X8 resin. Simple solutions of precious-metal chlorocomplexes showed near-complete adsorption (>99%) of most elements, and only minor breakthrough of Ru and Ru (≈5%). Solutions pre-treated with acid mixtures typically used to decompose geological samples, demonstrated that perchloric acid adversely affects the adsorption of the PGEs on the resin. Solutions treated with HF–HNO 3 –HCl maintained good retention of Ir, Pt, Au (>99%), Pd (>94%) and Ru (>90%), but displayed significant loss (up to 40%) of Rh. A two-step procedure was necessary to elute the precious metals from the resin: 0.3 mol l - 1 thiourea prepared in 0.1 mol - 1 HCl removed Ru, Pd, Pt, Au, and some Rh: 12 mol l - 1 HCl eluted remaining Rh and all Ir. Recoveries ranged from 50 to 100%. At low levels, the determination of PGE and Au in the thiourea fraction by ICP-MS was compromised by high levels of total dissolved solids (TDS), which necessitated dilution of the eluate prior to analysis. The TDS was reduced by decomposing thiourea with HNO 3 and removing SO 4 2 - by precipitation of BaSO 4 , but this led to lower and more erratic results, and increased contamination. An assessment of the optimised procedure employing geological reference materials PTM-1, PTC-1 and SARM7, indicated that acceptable results should be attainable for ICP-MS determination of most elements in geological samples containing high concentrations (>1 µg g - 1 ) of the PGE, for which decomposition of thiourea is unneccessary. The addition of a decomposition step led to low recovery of all elements except Ir, which was present entirely in the HCl eluate. The method is viable for the determination of Ir in a range of geological materials, but modifications will be required if it is to be extended to the other precious metals.