Temperature influence on hydrogen sorption in palladium limited-volume electrodes (Pd-LVE)

The effect of temperature on hydrogen and deuterium electrosorption into a palladium LVE (limited-volume electrode) has been investigated. A decrease in hydrogen capacity (H/Pd ratio) with increasing temperature has been observed. Temperature strongly influences the plots of measured H(D)/Pd values vs. potential scan rate. In addition, hydrogen absorption was found to be dependent on the composition of the surrounding electrolyte solution. These results have confirmed the hypothesis that two different mechanisms of hydrogen desorption from the palladium electrode take place, namely electrochemical oxidation and non-electrochemical recombination. Further, the ratio between the rate constants for these two processes has been found to change with temperature. Electronic Publication     

Study of the mutual influence of hydrogen and carbon monoxide in their joint oxidation on palladium

The joint oxidation of hydrogen and carbon monoxide in excess oxygen on palladium was investigated in a reactor for separate calorimetry and a gradientless microreactor at atmospheric pressure. An initiating influence of hydrogen m the oxidation of carbon monoxide, as well as inhibitory effect of carbon monoxide on the heterogeneous-homogeneous oxidation of hydrogen, was detected. The conditions of transition of the reactions into the volume according to a heterogeneous-homogeneous mechanism were determined. It is suggested that the joint oxidation of CO and H₂ occurs with the participation of the same active intermediate products formed from hydrogen and oxygen.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 4, pp. 504–508, July–August, 1985.     

A quartz crystal microbalance study on a metallic nickel electrode

Electrochemical processes taking place on a Ni electrode have been investigated with the electrochemical quartz crystal microbalance. At potentials negative of ca. –500 mV vs. SCE, a closed frequency loop is observed without irreversible changes in the mass of the electrode. The phase transition - -Ni(OH)₂, taking place at potentials positive to –500 mV vs. SCE, is accompanied by an irreversible increase in the mass of the electrode. When Ni(OH)₂ is further oxidized, the frequency increase is followed by a decrease, indicating the transport of various species in both directions, i.e. from and into the electrode. During the Ni(OH)₂ oxidation reaction the transport of species responsible for the mass increase is slower than the charge transfer process.Contribution to the 3rd Baltic Conference on Electrochemistry, GDASK-SOBIESZEWO, 23–26 April 2003. Dedicated to the memory of Harry B. Mark, Jr. (February 28, 1934–March 3rd, 2003)     

The investigation of redox properties of palladium-zeolite catalysts

Palladium catalysts based on A, X, Y, chabazite, erionite, mordenite, TsVM, and TsVK zeolites are studied by the methods of temperature-programmed reduction and hydrogen desorption. Palladium is shown to be located in zeolite pores in all catalysts except Pd/mordenite for which 24 % of the palladium is located on the external surface of the zeolite. The dispersity of the palladium on the zeolites is determined by the amount of H₂ evolved in the decomposition of the -phase of PdH and by the value of the O/Pd ratio obtained by palladium oxidation in the 100 to 500 °C temperature range. According to the O/Pd ratio, the catalysts are divided into two groups: samples based on X, Y, chabazite, erionite, and mordenite (O/Pd=0.4 to 0.7), and catalysts containing A, TsVM, and TsVK zeolites (O/Pd=0.2 to 0.3). The catalysts of the first group are weakly active in the oxidative acetoxylation of propylene, while all of the catalysts of the second group except Pd/NaA are very active. The conclusion is drawn that the oxidation of palladium is not the limiting stage in the oxidative acetoxylation of propylene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 642–648, April, 1995.     

States of Adsorbed Hydrogen and Their Effect on the Reaction of CO Oxidation on Pd and Ta

Various states of hydrogen are identified on the foil and film surfaces of palladium and tantalum by photoelectric, conductivity, and thermal desorption methods. They are formed in the course of ₂ diffusion through a membrane and in the course of adsorption from the gas phase. The effect of an ethylene pyrolysis product, pyrocarbon, on the activity in CO oxidation on the palladium surface with and without _ads is determined. The presence of hydrogen is found to weaken the effect of pyrocarbon. A study of hydrogen adsorption on the tantalum foil showed that hydrogen adsorption drastically declines in the presence of chemisorbed CO, but the H–Ta binding strength doubles. The fact that the sorption ability of tantalum is completely restored upon CO adsorption and partially restored upon ₂ chemisorption is achieved by thermochemical treatment in hydrogen.     

Chemisorption and oxidation of carbon monoxide on palladium alloys

Kinetic and adsorption data (programmed thermal desorption in situ) for the oxidation of carbon monoxide on palladium alloys with silver and gold have been discussed. It has been shown that the local reaction plays a determining role and that active cluster sites are best for catalysis which are monoatomic with respect to palladium, weakly binding the chemisorbed CO molecule. The role of silver is to activate the second component: oxygen. The cluster model makes it possible to predict the optimum composition of Pd-Ag catalysts for the oxidation of CO and to explain the dependence of Pd-Au activity on the composition.Kiev University. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 5, pg. 574–578 September–October, 1991. Original article submitted July 12, 1991.     

Microstructure and corrosion resistance of electrodeposited zinc alloy coatings

A heat treatment effect on the microstructure and corrosion properties of electrodeposited Zn, Zn-Co, Zn-Fe and Zn-Ni alloy coatings was studied. Surface morphology examinations were carried with AFM, while XRD was used to determine metal lattice parameters, texture and phase composition. Low-temperature annealing (at 225 °C) caused the formation of intermetallic Fe/Zn compounds, a transformation of amorphous oxide inclusions to the crystalline form and a decrease in the Zn lattice parameter for Zn-Co and Zn-Fe alloys. The mentioned structural modifications were not accompanied, however, by corrosion behavior changes of these coatings. On the Zn-Ni alloy, the annealing caused a significant reduction in the diffraction peak width and simultaneous considerable augmentation of the corrosion current. This effect is related to the formation of a less disordered lattice for this alloy.Contribution to the 3rd Baltic Conference on Electrochemistry, GDASK-SOBIESZEWO, 23–26 APRIL 2003.Dedicated to the memory of Harry B. Mark, Jr. (February 28, 1934–March 3rd, 2003)     

Cyclic voltammetric behavior of Pd–Pt–Rh ternary alloys

The electrochemical behavior of Pd–Pt–Rh alloys has been investigated using cyclic voltammetry (CV). The alloys were prepared by electrochemical codeposition as limited volume electrodes (less than 1 m in thickness). The morphology of the alloy surface and bulk compositions were examined by the SEM/EDAX method. Surface oxides generation (oxygen adsorption) and oxides reduction (oxygen desorption) currents together with hydrogen adsorption and hydrogen absorption signals can be distinguished on CV curves. During potential cycling through the full hydrogen–oxygen potential range Rh and Pd are preferentially dissolved, which is reflected in a dramatic transformation in the voltammogram shape. The composition changes involve not only the surface but also some atomic layers beneath the surface.     

Synthesis of hydroxybiphenyls

Cyclohexylphenols were converted into hydroxybiphenyls via dehydration over large-scale palladium catalysts. The process involves slow reaction of the substrate with active centers of the catalyst with subsequent fast product formation and hydrogen desorption.Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 9, 2004, pp. 1337–1339.Original Russian Text Copyright © 2004 by Kuznetsova, Postnova, S. Koshel, Lebedeva, Yunkova, G. Koshel.     

Electrooxidation of phenol at exfoliated graphite electrode in alkaline solution

The present paper reports on exfoliated graphite (EG) used for the cyclic electrochemical process of phenol oxidation in alkaline solution. It is shown that the electrochemical activity of anode-produced EG decreases considerably in the second cycle due to the deposition of an oligomer film, composed of the products of phenol oxidation, on the EG surface. Thermal treatment of the inactive graphite anode in air at 500 °C provided a regenerated material of activity three times higher for the first cycle and 2.6 times higher for three cycles as compared to the original anode. The reason for such a behavior is assigned to a carbon film formed on the EG surface during the carbonization/oxidation processes involving the products of phenol oxidation. Comparative studies showed that electroactivity of the original EG can also be enhanced if before the process of phenol oxidation the original EG is activated by heat treatment. Unfortunately, the electrochemical activity of the product of such a treatment is higher only for the first cycle of phenol oxidation and drops dramatically in the following cycles.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdask-Sobieszewo, 23–26 April 2003. Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

Optical and Chromaticity Characteristics of Cobalt and Palladium 4-(2-Pyridylazo)resorcinates

Optical and chromaticity characteristics (diffuse reflectances, chromaticity coordinates, lightness, color saturation, yellowness, color tint, and whiteness) of cobalt and palladium complexes of 4-(2-pyridylazo)resorcinol were determined at pH 7.2–7.9 and in 1 M H₂SO₄. The conditions of the separate determination of cobalt and palladium in the presence of each other by changing the order of the addition of reagent and using bifunctional chromaticity measurements and two-wavelength spectrophotometry are specified. The advantages of the chromaticity measurements are demonstrated. The proposed procedures were used for the analysis of model mixtures, binary cobalt–palladium alloys, and cobalt and palladium catalysts.     

Electrochemical deposition: the concept of residence times in structure development

The concept of the residence time of an atom in a kink position was introduced to predict the structure of electrochemically deposited surface coatings. The residence time is obtained from the reciprocal value of the rate constant for separation from the kink position. Residence times and deposition rates determine the density of kink positions [ksp] on the growing metal film. For equilibrium conditions (Nernst potential), the density of kink positions [ksp]₀ depends on the exchange current density i₀ and on the residence time by the equation:

Vapor-phase hydrogenation of acetone on applied metals of the platinum group

Ruthenium, rhodium, iridium, and platinum, applied on -Al₂O₃, show a high catalytic activity in the vapor-phase hydrogenation of acetone to isopropanol at 337–393K; under these conditions palladium is inactive. A reaction mechanism has been derived from kinetic measurements, adsorption and thermal desorption data, and IR spectra includes as a limiting stage the reaction of acetone adsorbed on the metal with atomic or molecular hydrogen. The kinetic and thermodynamic characteristics of the reaction have been established. The order of the specific catalytic activity of the metals has been determined, the mechanism of adsorption of alcohols on transition metals has been discussed, and the inhibition of the reaction by isopropanol adsorbed on the Ru catalyst has been interpreted.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 698–706, November–December, 1986.     

Formation of palladium hydride complexes upon the reduction of Pd(II) by hydrogen

Conclusions The reduction of palladium(II) acetate by hydrogen in the presence of 1,10-phenanthroline or 2,2-dipyridyl gives palladium hydride clusters containing a massive metal-like nucleus with interstitial hydrogen atoms. The ligands are found to be peripheral palladium atoms, while the acetate groups are found in the external sphere.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2381–2384, October, 1985.The authors express their gratitude to S. G. Ellert for measuring the magnetic susceptibility and to A. L. Chuvilin for the electron microscopic data.     

Reversible sorption of hydrogen by colloidal palladium in aqueous solutions

Colloidal palladium was prepared by -irradiation or by H₂ reduction of Pd(NH₃)₄l₂ in aqueous solutions containing sodium polyacrylate as a stabilizer. The sots contain spherical particles 2–4 nm in diameter. Their optical spectra contain a band at 230 nm ( = 6.1 · 10³ L mol^–1 cm^–1) smoothly descending toward the visible range. The sots reversibly absorb chemically from 0.35 to 0.15 g-mol of H₂ per g-atom of Pd. The spectra of ultrafine metal particles saturated with molecular hydrogen exhibit an absorption band at 265 nm ( = 4.5 · 10³ L mol^–1 cm^–1). One-electron reduction of methylviologen by hydrogen is catalyzed by the colloidal palladium prepared.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 313–316, February, 1996.     

Reactivity of methyl derivatives of nitrogenous heterocycles in vapor-phase catalytic oxidation

A study has been made of the reactivity of methylpyridines, methylpyrazines, and methylquinolines in oxidation in the vapor phase in the presence of -VO (PO₃)₂. Relationships have been found between the overall reaction rates of heterocyclic compounds and the charge on the ring nitrogen, and between the partial oxidation rate and the charge on the ring carbon atom adjacent to the methyl group. The partial oxidation rate of methylpyridines is given to a first approximation by the Hammet-type expression lnWa = –3.5 + 4.6 , with a correlation coefficient of 0.93.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 75–79, January, 1986.     

Indirect complexometric determination of palladium(II) using sodium nitrite as a selective masking reagent

A selective complexometric method is described for the determination of palladium, sodium nitrite being used as masking reagent. Palladium(II) in a given sample solution is initially cornplexed with an excess of EDTA and the surplus EDTA is titrated with zinc sulfate solution at pH 4.5–5.5 (acetic acid-sodium acetate buffer), using xylenol orange as indicator. An excess of sodium nitrite is then added, the mixture is shaken well and the EDTA released from the Pd-EDTA complex is titrated with a standard zinc sulfate solution. Results are obtained for 2.5–27.5 mg of Pd with relative errors 0.5% and standard deviations 0.05 mg. The interferences of various ions are studied. The method is applied for the determination of palladium(II) in alloys and complexes.     

PEDOT films: multifunctional membranes for electrochemical ion sensing

Properties of electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) films were studied from the point of view of direct use as ion-sensing membranes in potentiometric or amperometric sensors. Stable and reproducible potentiometric characteristics were obtained for PEDOT doped by poly(4-styrenesulfonate) ions, PEDOT(PSS) (cationic characteristics), and PEDOT doped by hexacyanoferrate(II) anions, PEDOT(HCF) (anionic characteristics). As shown by voltammetric and EDAX results, the anion exchange properties of the latter polymer result from gradual replacement of HCF ions by Cl^– anions from solution. The zero-current potentiometric detection limit of PEDOT(PSS), equal to 3×10^–6 M, can be shifted to 7×10^–7 M by polarization using a cathodic current density of 3×10^–7 A cm^–2. PEDOT films doped by Cl^– or PSS^– ions can be used as membranes for sensing anions or cations, respectively, under pulse amperometric conditions, within the range from 10^–4 to 1 M, comparable with that accessible by zero-current potentiometry. Dissolved oxygen (redox interferent of low charge transfer rate) exerts a minor influence on the slope of the potentiometric and amperometric characteristics of PEDOT films. Although the presence of redox reactants characterized by a high rate of charge transfer [Fe(CN)₆ ^3–/4–] results in the disappearance of the potential dependence on KCl concentration, this disadvantageous effect is much less significant under pulse amperometric conditions.Contribution to the 3rd Baltic Conference on Electrochemistry, GDASK-SOBIESZEWO, 23–26 April 2003. Dedicated to the memory of Harry B. Mark, Jr. (February 28, 1934–March 3rd, 2003)     

Effect of the chemical nature of the support on the properties of platinum catalysts for the combined oxidation of CO and H2

The steady-state rate of hydrogen oxidation catalyzed by platinum on an inert support (corundum) is greater than the rate of oxidation of carbon monoxide, while the rate of hydrogen oxidation on a platinum-vanadium catalyst is less than the rate of oxidation of carbon monoxide. The conditions for the complete selective oxidation of the reaction mixture components were determined. Hydrogen is oxidized on the Pt/corundum cataljst at 273K in a non-steady-state mode, while CO is oxidized on the same catalyst at 273 K and on Pt/BaSO₄-V₂O₅ at 383–393K in a steady-state mode.Deceased.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 31, No. 2, pp. 81–85, March–April, 1995.