Investigation of catalytic activity of ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> and ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles in the thermal decomposition of ammonium perchlorate

A new powder metallurgy technique was developed in order to increase the reinforcement proportion of aluminum with two different fractions of Al₂O₃. Aluminum powders were mixed with 20 % vol of alumina particles as primarily reinforcement, and additional alumina was produced in situ as a result of reaction between Al and additional 7.5 % vol of Fe₂O₃ powder. The three grades of powders were milled and hot-pressed into small preforms, and differential scanning analysis (DSC) was performed to determine the kinetics of microstructural transformations produced on heating. DSC curves were mathematically processed to separate the superposing effects of thermal reactions. Transformation points on resulting theoretical curves evidenced two distinct exothermal reaction peaks close to the melting point of aluminum that were correlated with formation of Fe–Al compounds and oxidation of aluminum. Microstructural investigations by means of SEM-EDX and XRD suggested that these exothermal reactions produced complete decomposition of iron (III) oxide and formation of Fe–Al compounds during sintering at 700 °C, and therefore, heating at higher temperatures would not be necessary. These results, along with calculation of activation energies, based on Kissinger’s method, could be used to optimize the fabrication of Al-Al₂O₃ composites by means of reactive sintering at moderate temperatures.     

MICRO-PHASE SAEPARATION IN EPOXY RESIN WATERBORNE PARTICLES DURING CURING PROCESS

Sub-micron sized phenolic epoxy resin waterborne particles were prepared by phase inversion emulsification. Micro-phase separation occurred during the curing process at high temperature. The as-prepared samples possessed one glass transition temperature （Tg） and two exothermal processes during DSC heating scannings. After being thermally treated above the exothermal peak temperature, they possessed two glass transition temperatures with the disappearance of exothermal peaks, whilst a core/shell structure was formed. This was likely related with the outward diffusion of reactive oligomers to the outer layer of particles.     

Role of thermal history on the thermal behavior of poly(<Emphasis Type="Italic">L</Emphasis>-lactic acid) studied by DSC and optical microscopy

Summary The thermal behavior of poly(L-lactic acid) (PLLA) was studied with differential scanning calorimetry (DSC) and polarized optical microscopy. For amorphous PLLA samples, double cold crystallisation peaks were observed in the DSC traces during heating process, being strongly dependent on heating rates. The observation was discussed based on the assumption that the quenched PLLA sample presented some remaining metastable or a precrystalline phase. A small exothermal peak was observed before the main melting peak at low heating rates. The probable reason was discussed through melt-recrystallisation mechanism. Influence of thermal history on the cold crystallisation and melting behavior was also performed on heating process for PLLA samples.     

Thermoanalytical Studies of Polyaniline ‘Emeraldine base’

The thermal behaviour of polyaniline-‘emeraldine base’ (PANI-EB) was studied using thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). We assume that during heating over 150°C three exothermal processes proceed - reorganization and crosslinking between PANI-EB chains followed by post-polymerization. The low temperature relaxation transition for PANI-EB was registered for the first time by DSC. We suppose that it might be due to the motion of polymer chains non-crosslinked during the first heating, chain fragments resulting from high-temperature decomposition over 300°C and chain ends of the already crosslinked polymer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure and catalytic properties of nanosized alumina supported platinum and palladium particles synthesized by reaction in microemulsion

Mixtures of nanosized platinum and palladium particles have been prepared by reduction of salt-containing microemulsion droplets using hydrazine as the reducing agent. To avoid possible negative effects of the presence of sulfur compounds during the preparation the microemulsion was made using the sulfur-free nonionic polyoxyethylene 4 lauryl ether surfactant. Transmission electron microscopy showed that the as-prepared mixtures contained crystalline platinum particles of fairly homogeneous size (20 to 40 nm) with adsorbed amorphous palladium particles 2 to 5 nm in size. Catalyst samples were prepared by depositing the nanoparticles on a gamma-Al(2)O(3) support followed by heating in air at 600 degrees C. Alloyed particles of platinum and palladium with sizes ranging from 5 to 80 nm were obtained during the heating. The majority of the particles had the fcc structure and their compositional range was dependent upon the Pt:Pd molar ratio of the microemulsion. A catalyst prepared from a microemulsion with a 20:80 Pt:Pd molar ratio showed the highest catalytic activity for CO oxidation, while pure platinum and palladium catalysts showed higher sulfur resistance. These results differ from the performance of conventional wet-impregnated catalysts, where a 50:50 Pt:Pd molar ratio resulted in the highest catalytic activity as well as the highest sulfur resistance.     

高压微波制备Pt／C催化剂的工艺研究

利用高压微波消解仪密闭带压、升温速率可控的特点,选择XC-72碳黑为载体,乙二醇为碳黑分散溶剂和还原剂,研究反应温度、升温速率、水含量、Pt含量及表面活性剂等工艺条件对Pt粒子尺寸及其分散度的影响．研究结果表明,快速升温到190℃、控制反应体系中水含量和Pt含量等工艺条件,可以得到Pt粒子平均粒径小于3．1nm的Pt／C催化剂,且Pt粒子分布均匀,无团聚．     

Exothermal Process in Miscible Polylactide/Poly(vinyl phenol) Blends: Mixing Enthalpy or Chemical Reaction?

Summary: In a previous paper (Macromolecules, 2005 , 38, 9221), the enthalpy of mixing in poly(DL ‐lactide) /poly(vinyl phenol) blends was directly measured by DSC. The first DSC scan for solution/precipitation blends showed phase separation, but miscibility was observed in the second DSC scan. Hence, miscibility was achieved after thermal treatment, an unusual behavior in polymer blend current research. However, the exothermal event observed during the first heating scan could also be the result of a chemical reaction. In this work, a new research study conducted to elucidate the nature of the exothermal heat observed in PDLA/PVPh during the first DSC heating scan. Since the single‐phase PDLLA/PVPh blend obtained after thermal treatment can be redissolved and reprecipitated, results obtained in the consecutive DSC scan prove that the process is completely reversible. Furthermore, GPC and ¹³C NMR results provide evidence that there is no change in the chemical structure of the studied polymers before and after the thermal treatment, which evidences the absence of transesterification reactions. Therefore, it can be concluded that the exothermal heat is the result of a mixing process, and miscibility is a consequence of specific interactions.

New structure expected for transesterfication reactions.     

Thin-film rotating disk electrode as a tool for comparing the activity of catalysts in the hydrogen oxidation reaction

A thin-film disperse rotating disk electrode is used to study the hydrogen oxidation reaction on platinum catalysts E-TEK with different purification degrees and on disperse palladium catalysts obtained from colloid solutions of organometallic complex precursors with subsequent thermal decomposition in an inert atmosphere or in hydrogen at diverse temperatures. Kinetic currents of the hydrogen oxidation reaction on these catalysts are determined at a potential of 0.025 V. The obtained values of currents may be utilized for performing a comparative estimation of the activity of various catalysts and the degree of their purification from the precursors or accidental impurities.     

Kinetics of exothermal decomposition of some ketone-2,4-dinitrophenylhydrazones

In this study, the thermal stability and exothermal decomposition of some ketone-2,4-dinitrophenylhydrazones have investigated using the DSC technique. The synthesized and purified crystalline solids are thermally stable and start to decompose after melting. Non-isothermal DSC curves, recorded at several heating rates, were used to evaluate the melting properties and the kinetics of thermal decomposition. The isoconversional and model-fitting methods were applied to determine the activation parameters from the common analysis of multiple curves measured at different heating rates. Based on the results of the model??free method, a kinetic model was derived, and the kinetic parameters were obtained by means of a multivariate nonlinear regression. The results are discussed in relation to the effect of the ketone structure.     

A new combined ETAAS method for the determination of platinum, palladium, and gold traces in natural samples

A new combined method is developed for determining trace platinum, palladium, and gold in natural materials of complex composition. The method involves sorption preconcentration with solid-phase extractants obtained by impregnating polymer supports (hypercrosslinked and highly crosslinked polystyrene resins) with an imidazolium ionic liquid (1-hexadecyl-3-methylimidazolium bromide), elution with acetone under normal conditions or with a solution of thiourea in 1 M HCl under microwave heating, and the ETAAS determination of analytes in the eluate. The efficiency of the method is confirmed by the analysis of various ores and rocks.     

Extraction chromatography of palladium and platinum complexes with nitroso-R-salt

The retention of palladium and platinum complexes with nitroso-R-salt on silica gel treated with Aliquat 336 has been investigated. The complexation of platinum with nitroso-R-salt (NRS) requires heating of H₂PtCl₆ with an excess of NRS at 100°. The affinity of the complexes for an Aliquat 336 stationary phase increases in the following order: PdCl₄²⁻ ˜ Pt-NRS < PtCl₆²⁻ Pd-NRS. The complexes of palladium and platinum can be separated by column chromatography on silica treated with Aliquat 336 and eluted with 0.25M perchloric acid (Pt) and 1M perchloric acid (Pd).     

Disperse Electrolytic Platinum and Palladium Deposits of Submicron Thickness on Polycrystalline Supports: An X-ray Diffractometry and Microscopy Study

The structure of model electrode materials broadly used in electrocatalysis (platinized platinum, palladized platinum, platinized gold) is studied by x-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. Disperse platinum and palladium coatings less than 1 m thick are obtained by potentiostatic or galvanostatic deposition from solutions of complex chloride salts. Lattice parameters of disperse metals are shown to be considerably smaller than those of bulky crystals. Some new tendencies of variations in structural parameters with the deposition potential are revealed. Special attention is paid to a reliable determination of the size of regions of coherent scattering (ROCS) and microdistortion; in connection with this, results of approximation of reflections by various functions and a harmonic analysis of reflections are compared. The latter are used for constructing ROCS distributions by size. Good agreement is found with the STM data for platinum deposits obtained earlier. Conversely, in the case of palladium deposits, ROCS are smaller than the particle size in an outer layer of the deposit by the STM data. This result is interpreted in terms of strong concrescence of palladium nanoparticles. To determine the true surface areas of deposits and estimate continuousness of covering the support by the deposit and regularities of platinum aging on gold in conditions of potential cycling, a voltammetry method is applied.     

The packing structure of side chains of liquid-crystalline side-chain polymers before and during heat treatment

Six varieties of liquid-crystalline side-chain polymers, poly (cholesteryl ω-(methacryloyloxy)alkanoates) (p-ChMO-n, n = 1,2,3,4,5,7, the carbon number of the alkyl chain), were studied by differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). On and after the first cooling run from the isotropic state, these polymethacrylates gave the same smectic phase. However, on the first heating run for virgin samples, unique phenomena were observed, similar to those frequently observed on the first heating run for thermotropic liquidcrystalline polymers. The thermal properties of the first heating run for these six polymers showed different tendencies between pChMO-n (n-1-3) and pChMO-n (4-7). Although the DSC curve of pChMO-n (n = 4, 5 and 7) exhibited the melting and clearing points on the first heating run, the DSC curve of pChMO-n (n = 1,2 and 3) exhibited an exothermal peak between the glass transition and the clearing point on the first heating run. This difference, probably due to the difference in the spacer length of these polymers, was investigated quantitatively by small-angle X-ray scattering by which the micro-change of the packing structure of the side chains of the polymers was traced.     

Propiconazole and Penconazole as Effective Extractants for Selective recovery and concentration of platinum(IV) and palladium(II) from hydrochloric acid solutions formed in leaching of spent aluminoplatinum and aluminopalladium catalysts

Selective recovery and concentration of platinum(IV) and palladium(II) from hydrochloric acid solutions of varied composition was studied using commercial reagents propiconazole and penconazole as extractants. The ranges of hydrochloric acid concentrations for effective extraction and highly selective separation of platinum metals from Al(III) and Ni(II) with propiconazole (toluene with 15 vol % n-decanol as deluent) and penconazole (chloroform) were determined. The conditions for 10-fold selective concentration of platinum metals with recovery of more than 99.9% of metal ions into the organic phase were found. The conditions for quantitative (>99%) stripping of platinum(IV) with a hydrochloric acid solution of thiourea and palladium(II) with ammonia solution were determined. The results obtained can be used for optimizing the modes of selective recovery of platinum(IV) and palladium(II) from hydrochloric acid solutions formed in leaching of alumina-supported platinum-rhenium, platinum-nickel, and palladium catalysts.     

Thermal behaviour of emulsions studied by differential scanning calorimetry

This article is a review about the ways in which solidification and the melting may occur within emulsions submitted to steady cooling and heating performed in a differential scanning calorimeter. Simple, multiple and mixed emulsions are considered. Due to nucleation phenomena creating supercooled and supersaturated liquids, the DSC curves obtained during cooling and heating are quite different. The influence of a solute in the disperse phase is described in detail. Some implications about the instabilities of emulsions due to mass transfer phenomena are described.     

Simultaneous Determination of Platinum and Palladium by Second Derivative Spectrophotometry Using 3-(2'-Thiazolylazo)-2, 6-Diaminopyridine as Chromophore Ligand

ABSTRACT

A second derivative spectrophotometric method has been developed for the determination of palladium and platinum in mixtures. The method is based on the formation of the platinum and palladium complexes with 3-(2-thiazolylazo)-2, 6-diaminopyridine, (2, 6-TADAP), in the presence of 1.7 M perchloric acid solution, upon heating at 90° C for 30 min and on the subsequent direct derivative spectrophotometric measurement. The zero-crossing approach and the graphic method were used for determination of platinum and palladium, respectively. Each analyte was determinated in the presence of one another in the ranges 8.9×10^-7 -3.1×10^-5 M for platinum and 4.6×10^-7 - 6.8×10^-5 M, for palladium. The detection limits achieved (3a) were found to be 2.7×10^-7 M of platinum and 1.4×10^-7 M of palladium. The relative standard deviations were in all instances less than 1.0%. In this work is included a study of effect of interferents and the application of the proposed method in synthetic mixtures.     

Synthesis of Mesoporous Platinum–Palladium Alloy Films by Electrochemical Plating in Aqueous Surfactant Solutions

Mesoporous platinum–palladium alloy films with different compositional ratios have been successfully synthesized by electrochemical plating in aqueous surfactant solutions. Scanning electron micrographs and transmission electron micrographs reveal that all of the platinum–palladium alloy films possess uniform mesopores with a narrow size distribution (around 7 nm). The alloy compositions in the pore walls can be controlled by changing the compositional ratios in the precursor solutions, as confirmed by inductively coupled plasma mass spectroscopy analysis and X‐ray photoelectron spectroscopy measurements. Due to large surface areas, the prepared mesoporous platinum–palladium films show distinctly enhanced electrocatalytic activity for methanol oxidation reactions, compared with the commercially available platinum black catalyst. Furthermore, compared with mesoporous platinum film, the alloying of platinum with palladium has a critical effect on the enhanced electrocatalytic activity. In particular, a mesoporous Pt₈₂–Pd₁₈ film exhibits highly enhanced electrocatalytic activity.     

Redispersion of Highly Disperse Powder of Titanium Dioxide in Aqueous Medium

Based on an analysis of interparticle interaction in highly disperse powders, a parameter is suggested allowing one to evaluate the tendency of dispersed particles to the formation of aggregates, the aggregation coefficient. It was shown that the particles of metal oxide powders with radius smaller than 10 m are present in their aggregated state. It was found that redispersion of highly disperse titanium dioxide powder in the aqueous medium is incomplete. The incomplete redispersion may be caused by the roughness of particles and/or the formation of the regions of entrapped air that are not destroyed by capillary pressure.     

Simultaneous Laser Thermal Lens Spectrometric Determination of Trace Platinum and Palladium in an Aquesous Solution

A selective and sensitive method for determination of platinum and palladium(Ⅱ）in an aqueous solution simultaneously by laser thermal lens spectrometry,based on the complex reaction of 2-(3,5-dichloropyridylazo)-5-dimethylaminoamiline(3,5-diCl-PADMA) with platinum and palladium,has been developed.It is shown that the palladium complex can be fromed at room temperature, while the platinum complex can be only formed after being heated in a boiling water bath.By using this difference of reaction temperature and the characteristic of the complexes mentioned above,the method for simultaneous determination of platinum and palladium was established in an aqueous solution without a pre-separation.The results show that the dynamic linear ranges of determination for platinum and palladium are 0.005-0.04μg/mL and 0.005-0.25μg/mL respectively,and that the detection limits are both 0.002/μg/mL.The method has been applied to the determination of platinum and palladium simultaneously in alloy and catalyst samples with satisfactory results.     

SERS at structured palladium and platinum surfaces

Palladium and platinum are important catalytic metals, and it would be highly advantageous to be able to use surface enhanced Raman spectroscopy (SERS) to study reactive species and intermediates on their surfaces. In this paper we describe the use of templated electrodeposition through colloidal templates to produce thin (<1 microm) films of palladium and platinum containing close packed hexagonal arrays of uniform sphere segment voids. We show that, even though these films are not rough, when the appropriate film thickness and sphere diameter are employed these surfaces give stable, reproducible surface enhancements for Raman scattering from molecules adsorbed at the metal surface. We report SERS spectra for benzenethiol adsorbed on the structured palladium and platinum surfaces of different thicknesses and void diameters and show that, for 633 nm radiation, enhancements of 1800 and 550 can be obtained for palladium and platinum, respectively.