Transport and kinetics at microheterogeneous electrodes: Part V. The methylviologen/platinum system

Results are reported for the reaction of methylviologen radical cation, MV⁺ with platinum colloidal particles, studied by stopped flow spectrophotometry. The rate of the reaction depends on the gaseous pretreatment of the particles. For particles reduced by hydrogen, the kinetics are usually first order with respect to MV⁺. The reaction is also first order in the concentration of platinum, and is inhibited in a first order manner by the product MV²⁺. This inhibition suggests that MV²⁺ is adsorbed on the particle surfaces, and this has been confirmed by ac, ring—disc electrode studies on macroscopic platinum electrodes. At high concentrations of MV⁺ some deviation from first order kinetics is observed. These results are all explained by a kinetic model in which either the desorption of MV²⁺ or the adsorption of MV⁺ is the rate limiting process. The rate of consumption of MV⁺ on an oxidised surface is an order of magnitude faster than that on the reduced surface. Ring—disc studies show that this is because the MV⁺ is not producing H₂ but is reducing the surface oxide. The results are shown to fit a simple model which takes into account this titration of the oxide layer. The model also explains why the rate on partially oxidised surfaces will appear to have an order greater than one in [Pt].     

Synthesis and properties of a platinum catalyst supported on plasma chemical silicon carbide

A CO oxidation catalyst based on β–SiC and Pt nanoparticles has been synthesized and studied. The average size of Pt clusters on the surface of the plasma-chemical silicon carbide nanoparticles is close to 4 nm. It has been found that the rate of the CO oxidation reaction at low concentrations (100 mg/m³) in air at room temperature over the catalyst based on platinum and silicon carbide nanoparticles is 60–90 times that over a platinum black-based catalyst with a specific surface area of 30 m²/g. The Pt/SiC catalyst containing 12 wt % Pt has been found to provide the maximum CO oxidation rate.     

Substoichiometric isotope dilution analysis of copper traces in zinc by adsorptive sampling

The spontaneous deposition of Cu²⁺ from aqueous solutions on a hydrogen-saturated platinum surface as a possibility for the substoichiometric sampling of copper is shown. A sensitive isotope dilution method has been developed for the determination of traces of copper in high purity zinc making use of the spontaneous deposition of microquantities of Cu²⁺ from aqueous solutions of zinc salts on a cathodically polarized platinum surface. If the same platinum surface is used in each run, the saturation values of the deposited copper are practically independent of the concentrations and contents of copper and zinc in the solution. The copper traces were labelled with the product of the⁶⁴Zn(n,p)⁶⁴Cu nuclear reaction. A copper content of 0.18±0.03 ppm in zinc was determined by this method.     

Electrochemical behavior of ellipticine derivatives: Part I. Oxidation of 9-hydroxy-ellipticine

The electrochemical one-electron oxidation of 9-hydroxy-ellipticinium cations at a platinum electrode has been examined with particular attention to the thermodynamic redox potential and to the dimerization rate of the radical species produced. Both the reactant and the product of the electrochemical reaction are strongly adsorbed at the solution/electrode interface. The initial step of the anodic process is a reversible electron transfer accompanied by a fast deprotonation; E^0′=900?53 pH mV vs. NHE. The resulting neutral radical dimerizes, the rate constant of the surface dimerization being ca. 2.5×10⁹ mol^?1 cm² s^?1.     

Isotope exchange between NO and H2O on a platinum-containing catalyst based on fiberglass

The dynamics of ¹⁸O isotope exchange between NO or H₂O and a catalyst and the dynamics of ¹⁸O label transfer from NO to H₂O have been studied under conditions of sorption-desorption equilibrium. The occurrence of a reaction of oxygen exchange between NO and water sorbed in the bulk of the catalyst was detected. This reaction occurs at platinum sites with the participation of acid sites of the glass matrix. The rate constants of the reaction of NO with platinum sites and the diffusion coefficients of water in the bulk of the glass matrix are evaluated.     

Zum nukleophilen Abbau von Tris(pentasulfido)platinat(IV), [Pt(S5)3]2−, und Bis(pentasulfido)platinat(II),[Pt(S5)2]2−

On the Nucleophilic Degradation of Tris(pentasulfido)platinum(IV), [Pt(S₅)₃]^2?, and Bis(pentasulfido)platinum(II), [Pt(S₅)₂]^2? The behaviour of [Pt(S₅)₃]^2?, ( I ), towards sulfite, arsenite, sulfide, hydroxide, and triphenylphosphine has been studied qualitatively and quantitatively. With stoichiometric amounts of nucleophile one ring is degraded; the reaction product [Pt(S₅)₂]^2?, ( II ), can be isolated. With excess of nucleophile all sulfur atoms are taken off from the platinum; with triphenylphosphine, however, (PPh₃)₂PtS₄, ( III ), is formed. A mechanistic interpretation of the course of the reaction is given and supported by kinetic studies.     

Rapid Extraction Spectrophotometric Determination of Platinum Using Anisaldehyde-4-Phenyl-3-Thiosemicarbazone as a Chromogenic Reagent

Abstract

The synthesis, spectral characteristics and analytical applications of anisaldehyde-4-phenyl-3-thiosemicarbazone (APT) are described. A simple, rapid, selective and sensitive spectrophotometric method for the determination of platinum was developed based on the colour reaction between platinum (IV) and anisaldehyde-4-phenyl-3-thio-semicarbazone (APT) in the pH range 1.7 - 3.0. The yellow coloured species has an absorption maximum at 360 nm. The complexation is complete within 1 min. A five-fold excess of the reagent is required for complete complex formation. Beer's law is obeyed over the concentration range 0.1 -20 ppm of Pt(IV). The mtilar absorptivity and Sandell's sensitivity are 1.58 × 10⁴ l. mol^?1 cm^?1 and 0.0123 ug of Pt(IV) cm^?2, respectively. The effects of pH, time concentration of reagent, order of addition of reagents and the interference from various ions were investigated. The method has been employed for the determination of platinum in synthetic mixtures whose composition correspond to some alloys.     

Formation of Ethers from Alcohols with Chloride Complexes of Platinum(II) as Catalysts

A new catalytic reaction of the etherification of alcohols in the system ROH-PtCl ₄ ²⁻ has been observed. At 70 °C in the presence of catalytic amounts of chloride complexes of platinum(II) methanol gave dimethyl ether. Methyl tert-butyl ether and di-tert-butyl ether were formed analogously from a mixture of methanol and tert-butanol. In the reaction with ethanol the products were diethyl ether and a π-ethylene complex of platinum(II). It is suggested that the step-wise mechanism includes the oxidative addition of the alcohol with the intermediate formation of an alkyl complex of platinum(IV), the decomposition of which by reductive elimination under the influence of a second molecule of alcohol or an alkoxide anion gives an ether and regenerates the catalyst, a chloride complex of platinum(II).__________Translated from Teoreticheskaya i Eksperimental’naya Khimiya, Vol. 41, No. 3, pp. 190–193, May–June, 2005.     

Spectrophotometric studies on the platinum(IV)-prochlorperazine bismethanesulfonate complex

We have studied the formation of a platinum complex and developed a simple, rapid and sensitive spectrophotometric method for the determination of platinum in solution. The method is based on the complexation reaction of the chromogen, prochlorperazine bismethane-sulfonate (PCPMS), with platinum(IV) in phosphoric acid medium which forms a reddish brown 1 1 complex with an absorption maximum around 528 nm. The reaction is fast in the presence of copper(II) and goes to completion within 1 min. Beer's law is obeyed over the concentration range 0.3–7.2 g/ml of platinum(IV) with an optimal range of 1.2–6.6 g/ml. The molar absorptivity is 2.65 × 10⁴1 mol^–1 cm^–1 and the Sandell's sensitivity is 7.8 ng cm^–2. The stability constant, logK, of the complex is 4.96±0.1 at 25 ° C. The effects of time, temperature, concentrations of acids, PCPMS and copper(II), and the interference by various ions are investigated. The method has been successfully applied to the determination of platinum content in alloys and minerals.     

CO oxidation over the Pt-Rh system. 3. Reaction on a heterophase surface

The reaction of the CO oxidation over a Pt-Rh heterophase surface, which represents 10-20 nm platinum particles located on a rhodium film, which in turn is deposited on an inert support, has been investigated at low pressures (P < 2'10^-5 mbar). The results are compared with the data for the clean surfaces of Pt and Rh. In the high-temperature range, the rate of CO₂ formation on the heterophase surface is found to be higher than the sum of the rates on individual metals corrected by the surface area of the different metals. The nature of the synergistic effect in the CO oxidation is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Oxidative cleavage of α, β-unsaturated compounds by pentachlorohydroxoplatinate(IV) in alkaline medium

The kinetics of oxidation of α,β-unsaturated compounds by platinum(IV) in the presence of alkali {[OH⁻]= (1–9) × 10⁻³ mol dm⁻³} have been investigated over the 303–318 K temperature range . The rate of the reaction is dependent on the first power of the concentrations of substrates, oxidant, and alkali. The rate constant increases with an increase in ionic strength and also with increasing dielectric constant of the medium. The oxidation rates follow the order: –CN > –CONH₂ > –COO⁻. The values of the third order rate constant (k₃) for the oxidation of acrylonitrile, acrylamide and acrylate are 1.24, 0.826 and 0.628 mol⁻² dm⁶ s⁻¹ respectively, at 303 K. The oxidations of the substrates by PtCl₅(OH)²⁻ take place by an inner-sphere mechanism. Platinum(IV) is reduced to platinum(II) by the substrates in a one-step two-electron transfer process to give reaction products. The major reaction product, HCHO, is identified from the reaction mixture using i.r. spectrometry, n.m.r. and C, H, N analysis. A tentative reaction mechanism, leading to the formation of products, has been suggested. The activation parameters of the reaction have been evaluated.     

Cathodic reduction of carbon disulfide in aprotic medium

The cathodic reduction of CS₂ at platinum electrodes has been studied in aprotic media (DMF and MeCN). It has been established that in an irreversible one-electron reduction CS and CS₃^2? are formed. Evidence for the former has been obtained via the formation of the RhCl(CS)(PPh₃)₂ complex and for the latter via u.v. and i.r. spectra. Furthermore evidence has been gained for a slow chemical reaction, following the charge transfer step, between CS and CS₃^2? to form a cyclic dianion.     

195Pt-Kernresonanz in PtII-und PtIV-Verbindungen

The ¹⁹⁵Pt-nmr spectra of several Pt^IV- and Pt^II-complexes have been measured in aqueous solutions. It has been shown that this method has great potentialities for the evaluation of the bonding character in platinum complexes. The ¹⁹⁵Pt-resonance spectra of solutions containing mixed chloro-bromo-complexes of Pt^IV have been used for the estimation of relative stabilities of these complexes. It is pointed out that this method is generally applicable to determine equilibrium constants of platinum compounds.     

Pt掺杂对Mo-Ru-Se簇合物电催化氧还原性能的影响

直接甲醇燃料电池(DMFC)是理想的移动电源,但因金属Pt阴极催化剂的选择性较差,甲醇在阴极产生“混合电位”,导致电池效率降低。抗甲醇氧电还原催化剂可降低“混合电位”,是解决该问题的有效的方法。     

Extraction-spectrophotometric determination of palladium with triphenylphosphine (TPP)

Palladium is extracted with triphenylphosphine (TPP) solution in benzene from hydrochloric acid medium as the PdCl₂ · 2TPP complex showing maximum absorption at 346 nm and a molar absorptivity of ? = 2.26 × 10⁴.The conditions of palladium extraction have been examined and the composition of the extracted species has been found to be PdCl₂ · 2TPP.A 20-fold excess of other platinum and transition metals has no effect on the palladium extraction. Palladium can be determined at platinum concentrations up to 5 mg/ml provided that the result is corrected for the blank. The elaborated method has been applied to the analysis of platinum samples containing not less than 10^?3% of palladium.     

Deuterium nuclear magnetic resonance spectroscopy. II—distribution of deuterium in some labelled nitrogen heterocyclic compounds

Pyridine, methylpyridines, quinoline and isoquinoline have been labelled with deuterium using pre-reduced platinum dioxide (PtO₂·2H₂O) and heavy water. Their ²H chemical shifts from monodeuteriated TMS have been assigned. The extent of the labelling has been determined directly by ²H NMR spectroscopy.     

Kinetics of processes occurring at a H<Subscript>3</Subscript>PW<Subscript>12</Subscript>O<Subscript>40</Subscript>/Pt,H<Subscript>2</Subscript> interface depending on the platinum content on the electrode

Kinetics of processes occurring at H⁺/solid electrolyte/Pt, H₂ three-phase interface are studied subject to the platinum content on the electrode. The study was performed with model electrochemical cells PbO₂/H₃PW₁₂O₄₀/Pt with different platinum content at the working electrode that consisted of platinum deposited onto the E-Tek LT1200-N carbon-nanotubes paper. On the basis of the obtained results, the occurring processes were practically fully separated. It is shown by the analyzing of relaxation curves that there exist at least two processes in the system: the faster one corresponds to the hydrogen reaction; the slower, to the oxygen one. The rates of both processes depend on the platinum content at the working electrode; they have an extreme at the platinum concentration of 0.5 mg/cm². Impedance data allowed revealing the processes’ limiting stages. The experimental data allowed suggesting that at low platinum content the relaxation time is determined by the electrochemical reaction rate; at higher content, by gas diffusion through the platinum dense layer.     

Investigations into the catalytic activity of rhodium(III) in red-ox reactions by capillary zone electrophoresis

Speciation of rhodium(III) in different acidic media has been studied by capillary zone electrophoresis (CZE). Depending on the nature of the acid, rhodium was shown to occur in the form of positive, neutral and/or negatively charged complexes. The relationship between the distribution of rhodium forms and its catalytic action on the oxidation of N-methyldiphenylamine-4-sulfonic acid by periodate ions has been investigated. It was found that only positively charged complexes of rhodium, such as those dominating in perchloric acid solutions, catalyzed a given reaction to form a colored oxidation product. The rate of the catalyzed reaction was optimized with respect to the pH, reagent and oxidant concentration levels, ionic strength, concentration of the catalyst, as well as the presence of interfering ions. The developed kinetic spectrophotometric method features rather high sensitivity (limit of determination 10 μg l⁻¹) and tolerance for most platinum metals and was applied to a complex industrial sample of a platinum concentrate.     

Synthesis and properties of ms- and β-substituted Pt(II) and Pt(IV) tetraphenylporphyrinates

The interaction of 5,10,15,20-tetraphenylporphyrin, 5,10,15,20-tetra-(4-chlorophenyl)porphyrin, 2-bromo-5,10,15,20-tetraphenylporphyrin, and 2,3,12,13-tetrabromo-5,10,15,20-tetraphenylporphyrin with platinum(II) chloride in boiling phenol has been studied. The corresponding platinum(II) porphyrinates have been synthesized; their subsequent treatment with bromine in chloroform resulted in platinum(IV) porphyrinates. The Pt(II) and Pt(IV)(Br)₂ porphyrinates have been identified by elemental analysis, electron absorption, IR, and ¹H NMR spectroscopy.     

Perfluorinated membranes as catalyst supports

The perfluorinated polymer Nafion and porous PTFE/Nafion composite membranes have been employed as supports for nickel complexes or for platinum and palladium metal particles. The resultant materials have been employed as catalysts in various olefin conversion processes. Supported platinum and palladium metal systems were evaluated as catalysts for the hydrogenation of cyclohexene. Rates of reaction are better than those of commercially available catalysts; turnover numbers in excess of 6000 have been obtained with no poisoning apparent. Catalysts may be regenerated many times. The reduction rate approaches a limit at high pressures of hydrogen and has an activation energy of 13 kJ mol^?1 in neat cyclohexene. Nafion was employed as a strong acid cocatalyst to activate and then support a nickel complex catalyst. The resultant catalyst was active for double-bond-shift isomerization.