Influence of the reaction medium on platinum surface formation

The effect of an organic reaction medium on the Pt-black surface formation has been studied. It has been shown that under the action of the reaction medium an irreversible change of the catalyst surface occurs.

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Kinetics of urethane formation from isophorone diisocyanate: The catalyst and solvent effects

The dependence of the kinetic parameters of urethane formation in the reaction between isophorone diisocyanate and alcohols of different structure (n-propanol, isopropanol, propargyl alcohol, 1,3-diazidopropan-2-ol, and phenol) in diluted solutions on the natures of solvent (toluene, carbon tetrachloride) and catalyst (dibutyltin dilaurate, diazobicyclooctane) was found using an original IR spectroscopic procedure. The ratio of the apparent rate constants for the reactions involving the aliphatic and cycloaliphatic NCO groups of isophorone diisocyanate was determined, and the efficiency of catalysis in these reactions was estimated. The reaction conditions under which the difference between the reactivities of isocyanate groups can reach 40 were determined.     

Influence of tin(II) chloride on the solvent extraction of platinum group metals with N,N-di-n-hexyl-N'-benzoylthiourea

Summary The solvent extraction of Rh(III) and Pt(IV) with N,N-di-n-hexyl-N'-benzoylthiourea (DHBT)/toluene is substantially accelerated in the presence of tin(II) chloride. Low concentrations of SnCl₂ from 0.02 to 0.03 mol/l and metal/ligand ratio of 1:4 (Pt) resp. 1:9 (Rh) lead to low residual metal concentrations below the detection limit of GFAAS. The extraction behaviour of Pt(II), Ru(III) and Ir(III) is not affected by the treatment with SnCl₂.     

Hydrosilylation of methylenecyclopropane and methylenecyclobutane on an immobilized platinum catalyst

The hydrosilylation of methylenecyclopropane on an immobilized platinum catalyst, in contrast to methylenecyclobutane, leads to the formation of linear 3-butenylsilane.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 958–960, April, 1991.     

Effect of solvent type on formation of PVC‐solvent complex

The effect of solvent type on the structural features of PVC gels prepared from the solutions of dibutyl phthalate(DBP) and butyl benzoate(BB) were investigated. The influence of solvent type, i.e., diester solvent or monoester solvent, on the mechanical responses was characterized by dynamic viscoelastic measurements as a function of temperature, aging time, and PVC concentration. In the case of PVC/DBP aged gels, the transition phenomenon appeared close to 50°C, whereas no such transitions were observed for PVC/BB aged gels. This transition is considered to be associated with polymer‐solvent complex which can be accounted for electrostatic interaction between C=O bond of solvent and H‐C‐Cl part of PVC. While BB solvent molecules can not form polymer‐solvent complex due to no capacity to act as a bridge between PVC molecular chains. FT‐IR spectroscopy and DSC investigation on either PVC/DBP gels or PVC/BB gels was reported and discussed together with the mechanical behavior. Electron micrographs of the dried gel prepared from the critical point drying technique reveal that the gel morphology consists of well developed three dimensional fibrous network structure independent of solvent type.     

Spontaneous formation of platinum particles on electrodeposited palladium

Pt particles have been spontaneously formed on the electrochemically deposited Pd layer on ITO substrate. SEM reveals that the Pt particles spontaneously formed on the Pd surface are uniformly distributed. The as-prepared material (denotes as Pt–Pd/ITO) as electrode shows a higher activity for ethanol oxidation than that of Pd/ITO. The mechanism is tentatively explained as that the H dehydrogenated from ethanol on Pt can efficiently spillover to the underneath Pd, resulting in an enhanced kinetics. The rapid removal of H on the Pt active sites accelerates the further adsorption of ethanol and dehydrogenation (oxidation). This work demonstrates a strategic method to spontaneous prepare small particles on the reductive species-containing substrates. The metal ion with a higher standard potential than that of hydrogen is theoretically possible to be spontaneously reduced to metal on hydrogenated Pd.     

Configurational is omerization of stereoisomeric 1,4-dimethoxycyclohexanes on a platinum catalyst

Russian Chemical Bulletin -     

Mesoporous platinum as sulfur-tolerant catalyst for PEMFC cathodes

The performance of proton exchange membrane fuel cell (PEMFC) is highly influenced by its operating conditions. One of the vital parameter is the purity of feed gases. The cathode of PEMFC is normally fed with air from the atmosphere containing certain impurities like CO₂, NO₂, and SO₂, which are the major contaminants for the electrocatalysts used in the fuel cell, causing both reversible and irreversible damages. The irreversible effect is caused due to adsorption of impurities like SO₂ on the conventionally used platinum catalyst supported on carbon (Pt/C). It has been observed that carbon facilitates the absorption of SO₂ on platinum. Hence, the present objective is to identify the catalysts containing no carbon and study their impurity tolerance in the fuel cell environment. In the present paper, we have attempted to synthesize unsupported mesoporous platinum by hard template method and studied its SO₂ impurity tolerance at the cathode side for oxygen reduction reaction (ORR) for PEMFC application. The mesoporous platinum showed for a higher tolerance towards SO₂ compared to its counterpart, viz., platinum black. Sulfur tolerance was evaluated by the sulfur coverage on the catalyst and its rate of recovery through electrochemical experiments. Mesoporous platinum has also exhibited a faster removal of adsorbed sulfur compared to the commercial microporous platinum black, revealing that the recovery is also fast comparatively for meso-structured platinum.

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Graphical abstract Morphology and cyclic voltammogram of mesoporous platinum (inset: sulfur coverage for mesoporous platinum and platinum black)

     

Influence of molar ratios of precursor,catalyst, solvent and water on monolithicity and physical properties of TMOS silica aerogels

In continuation to our earlier work on aerogels, the experimental results on the monolithicity and physical properties of silica aerogels as a function of the molar ratios of tetramethoxysilane (TMOS) precursor, catalyst (NH₄OH), methanol (MeOH) solvent and water, are reported. The molar ratios of NH₄OH/TMOS, MeOH/TMOS and H₂O/TMOS were varied from 7.1 × 10^–6 to 9.6 × 10^–1, 1 to 90 and 1 to 18 respectively. It has been found that larger molar ratios of NH₄OH/TMOS (10^–2), MeOH/TMOS (13 to 60) and H₂O/TMOS (>10) resulted in transparent but cracked aerogels, and very low molar ratios of these combinations gave monolithic but less transparent or opaque aerogels. The best quality silica aerogels, in terms of monolithicity, transparency and low density, have been obtained with TMOS:MeOH:H₂O:NH₄OH in the molar ratio of 1:12:4:3.7 × 10^–3 respectively. The aerogels have been characterized by density, optical transmission, surface area and porosity measurements. The results have been discussed by taking into account the hydrolysis and condensation reactions, and syneresis effects.     

Study on platinum(II) induced formation of dithiiranes

The reaction of a series of stable alpha-chlorinated oligosulfanes 2 and 3 with [Pt(eta(2)-C(2)H(4))(Ph(3)P)2] 1 have been investigated. Starting with the alpha-chlorodisulfanes 2 a,b, the platinum dichloride complex 5 and the side-on bonded thioketone platinum complexes 6 a,b were formed. Complex 1 was treated with corresponding trisulfanes 3 a,b to give 5, 6 a,b and the dithiolatocomplexes 7 a,b. We assume that the {Pt(0)(Ph(3)P)2}-complex fragment inserted along the S--S bond to form the unstable intermediate G, which decomposed to form the products described above. We could prove that the sterically crowded 1,2,4-trithiolane 8 was not involved in the reaction pathway by treatment of 1 with 8 under the same conditions; after 24 h, 8 was found to be unreacted. X-ray structure analyses were performed on complexes 6 a, 7 a and 7 b.     

Influence of solvent on propagation in cationic polymerization

The influence of solvent on cationic propagation steps-modelled by ethene homopolymerization—is investigated by means of theoretical models. In order to achieve cationation and the first three propagation steps, the influence of solvent was simulated by a continuum model after calculation for the gas phase at the MINDO/3 level. Characteristic changes, caused by the transition from the gas phase to solution (solvent: CH₂Cl₂), were obtained, e.g. an increase of activation barriers and specific alterations of heats of reaction depending on the cationic chain length. The shape of a special potential energy surface for the first propagation step is qualitatively affected by the solvent leading to a change of the character of the activated complex from educt-like (gas phase) to product-like (solution).     

Influence of solvent on carbene intersystem crossing rates

The influence of coordinating solvents on singlet-to-triplet carbene intersystem crossing (ISC) rates has been studied with diphenylcarbene (DPC) and para-biphenyltrifluoromethylcarbene (BpCCF 3) by using ultrafast time-resolved spectroscopy. DPC has a triplet ground state in all of the solvents considered, and the concentration of singlet carbene at equilibrium is too small to be measured. It is found that the lifetime of (1)DPC is extended in acetonitrile, benzene, tetrahydrofuran, dichloromethane, and halobenzene solvents relative to cyclohexane. The solvent effect does not well correlate with bulk measures of solvent polarity. The singlet-triplet energy separation of BpCCF 3 is close to zero. The data demonstrates that BpCCF 3 has a triplet ground state in benzene, fluorobenzene, and hexafluorobenzene. Halogenated solvents are found to dramatically retard the rate of ISC in (1)BpCCF 3. We postulate that the empty p orbital of a singlet carbene coordinates with a nonbonding pair of electrons of a halogen atom of the solvent to form a pseudoylide solvent complex, stabilize the singlet carbene, and decrease the singlet-triplet (S-T) energy gap. The "golden rule" of radiationless transitions posits that the smaller the energy gap between the two states, the faster their rate of interconversion. To explain the apparent violation of the golden rule of radiationless transitions for the carbene ISC processes monitored in this study, we propose that the significantly different specific solvation of the singlet and triplet carbenes imposes a Franck-Condon-like factor on the ISC process. Those solvents that most solvate the singlet carbene will also have the greatest structural difference between singlet carbene-solvent complex and their triplet spin isomer-solvent complex, the smallest S-T gap, and the slowest ISC rate. Alternatively, one can propose that a highly solvated singlet carbene must desolvate prior to ISC, and that this requirement decelerates the radiationless transition.     

Particle size effect on enantioselective hydrogenation of ethyl pyruvate over alumina-supported platinum catalyst

The effect of platinum particle size on the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate in the liquid phase was studied using a series of Pt/γ-Al₂O₃ catalysts modified with cinchonidine.The catalysts containing 5 wt.% platinum were prepared by the incipient wetness technique with H₂PtCl₆ as precursor.Reduction at different temperatures, redispersion in air and sintering were employed to change the platinum dispersion from 0.27 to 0.6, corresponding to a particle size range of 1.7–4.0 nm.Within this particle size range, activity (expressed as initial turnover frequency) and enantioselectivity were higher for the larger particles. However, an increase in selectivity was not observed when the platinum particle size was increased by sintering. This indicates that the method applied for particle size control has an influence on the performance of the catalysts.     

Effect of solvent quality on kinetics of tethered layer formation

We conducted a study of the effect of solvent quality on the kinetics of formation of a layer of polymer chains tethered to a solid substrate. In these experiments, tethering was accomplished by means of chemical bond formation between reactive sites on the surface and the end‐functional groups of the polymer chains in solution. All experimental variables were held constant except for the χ‐parameter between the polymer and solvent. Variation in the χ‐parameter was achieved by use of a series of nonpolar, organic solvents. The distinct three‐regime kinetics, typical of tethering reactions run in a good solvent and in the absence of segmental adsorption, was observed over the range of values for the χ‐parameter. As expected, an increase in the χ‐parameter (a decrease in solvent quality) did result in increased tethering density, but, contrary to expectation, no increase in tethering rate was observed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5530–5537, 2004     

Poisoning of a platinum catalyst with thiophene under reforming conditions

Russian Chemical Bulletin -