Structural Peculiarities of Anomalous Electrolytic Palladium Deposits Prepared under Hydride Formation Conditions

The peculiarities of bulk and surface structures of electrolytic palladium deposits formed under hydride formation conditions are studied by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy techniques. For anomalous deposits with high hydrogen capacities, the evolution of the XRD behavior is traced as a function of thermal treatment. The material is shown to comprise two cubic phases with different lattice parameters.     

Stacking faults and mechanisms strain-induced transformations of hcp metals (Ti,Mg) during mechanical activation in liquid hydrocarbons

The evolution of the structure and substructure of metals Ti and Mg with hexagonal close-packed (hcp) lattice is studied during their mechanical activation in a planetary ball mill in liquid hydrocarbons (toluene, n-heptane) and with additions of carbon materials (graphite, fullerite, nanotubes) by X-ray diffraction, scanning electron microscopy, and chemical analysis. The temperature behavior and hydrogen-accumulating properties of mechanocomposites are studied. During mechanical activation of Ti and Mg, liquid hydrocarbons decay, metastable nanocrystalline titanium carbohydride Ti(C,H) _x and magnesium hydride β-MgH₂ are formed, respectively. The Ti(C,H) _x and MgH₂ formation mechanisms during mechanical activation are deformation ones and are associated with stacking faults accumulation, and the formation of face-centered cubic (fcc) packing of atoms. Metastable Ti(C,H)x decays at a temperature of 550°C, the partial reverse transformation fcc → hcp occurs. The crystalline defect accumulation (nanograin boundaries, stacking faults), hydrocarbon destruction, and mechanocomposite formation leads to the enhancement of subsequent magnesium hydrogenation in the Sieverts reactor.

     

Corrosion–Electrochemical Behavior of Chromium Deposits Obtained from Sulfuric Acid Solutions Containing Oxalates

The corrosion–electrochemical behavior of chromium electrodeposits, which are formed in sulfuric acid solutions of Cr(III) containing oxalates, is studied by taking steady-state polarization measurements in a 0.5 M H₂SO₄ solution. No region of the metal's active dissolution is observed for such coatings, and the open-circuit potential is localized in the passivity region, i.e. it is substantially displaced in the positive direction as compared with that for metallurgic chromium or the coatings deposited from standard chromium-plating electrolytes. According to XPS data, the surface layer of the passive metal a few nanometers thick includes oxide compounds of chromium and also carbides formed during the coating electrodeposition. Specific features of the corrosion–electrochemical behavior of the deposits are attributed to the presence of carbide compounds of chromium in them, with the compounds playing the role of a cathodic alloying agent.     

Modeling the Relaxation of Surface Layers on Renewable Electrodes Made of Alloys of the Solid Solution Type

A model is suggested for describing the kinetics of a diffusion exit of atoms of the surface-active component of binary alloys of the solid solution type into a surface layer. An impedance method is used to study the dependences of the differential capacitance of the electrical double layer on the potential on renewable-by-mechanical-cut electrodes made of alloys of the system Ag–Sn (1 and 3 at. % Sn) in solutions of NaF. It is established that with increasing duration of contact of a renewed surface of electrodes with a solution of an electrolyte there takes place a very rapid process of predominant exit of tin atoms into a surface layer. It is shown that the observed temporal effects are quite satisfactorily described by the model suggested. Additionally, direct data about the accumulation of tin atoms in a surface layer of alloys studied are obtained by an XPS method.     

Oxidation of Methanol and Other Low-Molecular-Weight Alcohols on the RuNi Catalysts in an Alkaline Environment

The results of investigations of the RuNi catalysts, which are designed for the electrooxidation of methanol and other low-molecular-weight alcohols in alkaline solutions, are presented. It is shown that the maximum catalytic activity in this reaction is exhibited by a catalyst, which was synthesized thermochemically on acetylene black AD100 containing 15 wt % RuNi at a 68 : 32 atomic ratio (in at. %) between the metals. The structure of the synthesized catalysts is studied by the methods of x-ray photoelectron spectroscopy and x-ray diffraction analysis (XRDA). The area of the metal surface is determined on the basis of the magnitude of the adsorption of CO from the voltammetric curves. An analysis of the data obtained in this work leads to the conclusion that ruthenium in the composition of the catalyst exists in metallic and partially oxidized states and nickel exists in the form of a nonstoichiometric oxide. In addition it is found that the insertion of nickel into the system leads, as follows from the XRDA data, to the dispersion of ruthenium and, as follows from the voltammetric curves, to a decrease in the specific surface area accessible to the adsorption of CO. This is probably connected with the decoration and blockade of a portion of the ruthenium surface by some nickel oxides. Data on the influence, which is exerted by the concentration of methanol, alkali, and temperature on the electrocatalytic activity of the AD100 + 15 wt % RuNi catalyst at a 68 : 32 atomic ratio (in at. %) between the components, are presented. Rates of the oxidation of methanol, ethanol, n-butanol, and ethylene glycol in identical conditions on the catalyst AD100 + 15 wt % RuNi (68 : 32 at. %) between the metals are compared with one another. The oxidation currents, which are observed at a potential of 0.3 V, are equal to 5.48, 2.67, 0.48, and 0.47 A per gram of the catalyst for ethanol, ethylene glycol, methanol, and n-butanol, respectively.__________Translated from Elektrokhimiya, Vol. 41, No. 7, 2005, pp. 829–839.Original Russian Text Copyright © 2005 by Tarasevich, Karichev, Bogdanovskaya, Kapustin, Lubnin, Osina.     

Salient features of the electrical double layer structure on mechanically renewed gold-silver electrodes in aqueous solutions of a surface-inactive electrolyte

The behavior of electrodes, which are made of binary Au-Ag alloys (Ag content 1–15 at %) and renewed by mechanical cutting in aqueous solutions of sodium fluoride, is studied with the aid of cyclic voltammetry and impedance methods. It is established that, in the region of potentials corresponding to ideal polarizability, the differential capacitance of the electrical double layer rapidly changes with time elapsed after the renewal of the surface of the electrodes. The change in the capacitance is brought about by the exit of silver atoms into a surface layer. The implication is that silver is the surface-active component in these alloys. The rate of the surface segregation of silver atoms depends on the electrode potential. The segregation rate substantially increases upon going into the region that corresponds to positive charges of the silver electrode surface and to the beginning of adsorption of atomic oxygen on the electrode. Based on phenomenological models, a method for processing capacitance curves is realized, which links experimentally observed time effects to variations that occur in the surface composition, and assumptions concerning the mechanism of relaxation processes that are responsible for the observed time effects are put forth. Explicit data on the effect, which is exerted by mechanical renewal on the composition of the surface layer of Au-Ag alloys at different distances from the interface with a vacuum, are obtained with the aid of an x-ray photoelectron spectroscopy method. It is established that the surface layer (～0.5 nm) is enriched by silver atoms as compared with the alloy’s bulk.     

The Microstructure of Poly(Isobutylene-co-p-Methylstyrene) by NMR Spectroscopy

Abstract

The microstructure of isobutylene-para-methylstyrene (IB-pMeSt) copolymers was studied by NMR spectroscopy. ¹H- and ¹³C-NMR spectra were used to obtain overall copolymer compositions. ¹³C-NMR signals were assigned in terms of triad monomer sequences, and triad distributions were obtained over a wide copolymer composition range. According to statistical tests, the IB-pMeSt copolymerization cannot be described by zero- (Bernoullian) or first-order Markov models because reactivity ratios r _IB and r _pMeSt were found to change with the monomer feed composition. Additional insight into the microstructure of IB-pMeSt copolymers was gained by calculating sequence numbers, run numbers, and sequence lengths from triad distributions. Further, the Kelen-Tüdös plot showed a distinct curvature indicating that the Kelen-Tüdös method, applied over the entire monomer feed composition range, cannot give meaningful reactivity ratios for this monomer pair. Evidently the simple two-parameter Mayo-Lewis model is inadequate to describe the IB-pMeSt copolymerization system.     

Identification of Initiator Fragments in Polyisobutylene by Nmr Spectroscopy

Abstract

Deuterated polyisobutylenes carrying protonated initiator fragments were prepared by the living polymerization technique employing perdeuterated isobutylene [CD₂=C(CD₃)₂] and select protonated initiators (see Scheme 1). The polymers were analyzed by ¹H- and ¹³C-NMR spectroscopy, and the resonances due to the protic initiator fragments were unequivocally assigned. The assignments of ¹³C-NMR signals were affected by the distortionless enhancement by polarization transfer mode of spectra accumulation.     

Composition of Chemisorption Layers Forming on a Chromium Electrode in Sulfuric Acid Solutions of Formaldehyde and Formic,Oxalic, and Glyoxylic Acids

Surface layers that form on a chromium electrode after its contact with sulfuric acid solutions of formaldehyde, formic, oxalic, and glyoxylic acids at an open-circuit potential and in conditions of cathodic polarization at a potential of ?1.05 ± 0.10 V (NHE) are examined by an x-ray photoelectron spectroscopy method. The structure of the Cls line, which characterizes the binding energy of electrons of that level with the nucleus in products of interaction of said organic compounds with metal, is analyzed in detail. It is established that chemisorption products include substances with reduced oxygen-containing functional groups and hydrocarbon fragments. The degree of carbonation of chemisorption products, whose appearance is probably connected with electrocatalytic reduction and processes of polymerization of organic substances, is given an estimate.     

Living carbocationic polymerization of isobutyl vinyl ether and the synthesis of poly[isobutylene-b-(isobutyl vinyl ether)]

The MeCH(O-i-Bu)Cl/TiCl₄/MeCONMe₂ initiating system was found to induce the rapid living carbocationic polymerization (LC^⊕Pzn) of isobutyl vinyl ether (IBuVE) at ?100°C. Degradation by dealcoholation which usually accompanies the polymerization of alkyl vinyl ethers by strong Lewis acids is “frozen out” at this low temperature and poly(isobutyl vinyl ether)s (PIBuVEs) with theoretical molecular weights up to ca. 40,000 g/mol (calculated from the initiator/monomer input) and narrow molecular weight distributions (M?_w/M?_n ≤ 1.2) are readily obtained. According to ¹³C-NMR spectroscopy, PIBuVEs prepared by living polymerization at ?100°C are not stereoregular. The MeCH(O-i-Bu)Cl/TiCl₄ combination induces the rapid LC^⊕Pzn of IBuVE even in the absence of N,N-dimethylacetamide (DMA). The addition of the common ion salt, n-Bu₄NCl to the latter system retards the polymerization and meaningful kinetic information can be obtained. The kinetic findings have been explained in terms of TiCl₄. IBuVE and TiCl₄ · IBuVE and TiCl₄ · PIBuVE complexes. The HCl (formal initiator)/TiCl₄/DMA combination is the first initiating system that can be regarded to induce the LC^⊕Pzn of both isobutylene (IB) and IBuVE. Polyisobutylene (PIB)–PIBuVE diblocks were prepared by sequential monomer addition in “one pot” by the 2-chloro-2,4,4-trimethylpentane (TMP-Cl)/TiCl₄/DMA initiating system. Crossover efficiencies are, however, below 35% because the PIB^⊕ + IBuVE → PIB-b-PIBuVE^⊕ crossover is slow. © 1993 John Wiley & Sons, Inc.