Zum anodischen und kathodischen Verhalten des Eisens in neutralen und alkalischen Lösungen

The anodic and cathodic behaviour of iron in neutral and alkaline solutions To investigate the behaviour of iron in the p_H range from 7 to 14 in the presence of chloride ions, the authors make use of the electrochemical affinity/overpotential diagram. At p_H =7, the activation of the iron by the destruction of the FeO primary layer is possible. With increasing alkalinity, the passive zone is reduced by the direct effect of Cl^? ions, by the oxidation of FeCl₂, and by the formation of the iron complexes FeO₄^2?. The decomposition of Fe(OH)₂ by Cl^? ions is thermodynamically possible up to p_H = 10.5 only. At p_H > 10.5, there occurs the equilibrium Fe(OH)₂ = HFeO₂^? + H⁺.     

para-Sexiphenylene as a model compound of poly(para-phenylene) during the electrochemical intercalation of lithium and sodium ions in ethylene carbonate-based electrolyte

The electrochemical intercalation of alkaline ions (Na⁺, Li⁺) into para-sexiphenylene (PSP) has been carried out in ethylene carbonate (EC) using MClO₄ as the alkaline salt (M-Na or Li). Cyclic voltammetry and galvanostatic experiments reveal different behaviour for lithium and sodium. Intercalation of Na⁺ into PSP occurs with formation of successive stages of Na_0.33(C₆H₄) and Na_0.5(C₆H₄). After the first cycle, where the irreversible side reactions (such as the decomposition of the electrolyte, the reduction of the binder, etc.) occurred giving important capacity loss, the system exhibits a good reversibility. On the other hand, intercalation of Li⁺ into PSP is partially reversible whatever the cycle. A reversible capacity of 0.2 is reported in this case. The galvanostatic curves do not show any well-defined plateaux as in the case of the intercalation of Na⁺.     

Synthesis,structure and luminescent properties of two-dimensional lanthanum(III) porous coordination polymer based on pyridine-2,6-dicarboxylic acid

The novel porous coordination polymer {[La₄(PDA)₁₀(H₂O)₈]·2H₂O}_n with two-dimensional framework has been prepared by hydrothermal synthesis of pyridine-2,6-dicarboxylic acid (H₂PDA) and lanthanum nitrate under mild conditions and characterized by elemental analyses, IR spectroscopy, thermal analyses and single crystal X-ray diffraction. The complex crystallizes in triclinic system, space group P-1. La(III) ions in the coordination polymer are nine-coordinated and have N₁O₈ or N₂O₇ polyhedral units connected to the PDA^2? anions forming tricapped trigonal prism geometries. The H₂PDA ligands coordinate to the central La(III) ions in bridging tetradentate and pentadentate modes. The thermal decomposition of the complex has been predicted with the help of thermal analyses (TG and DTG). The complex exhibits luminescent property in the near-UV at room temperature in the solution of DMSO due to the π^* → n or π^* → π transitions of PDA^2? anions.     

Individual contributions of overpotentials at iron mixed electrodes

The Nagel formulation of overpotential for a single electrode from an E_H (pH, log a_i) equilibrium diagram has been developed in the case of iron as a mixed electrode. A detailed discussion is presented concerning the possibilities of determining the kinetics of corrosion processes in terms of irreversible thermodynamics. The distribution of overpotential for Fe + Fe²⁺; 2H⁺ + 2e^? = H₂ and its division into activation, reaction, concentration and diffusion factors is represented graphically as a function of pH at constant a_Fe²⁺ (= 10^?5M).     

Deterioration and film forming abilities of the slurries used for preparing La0.8Sr0.2MnO3 films through composite sol-gel method

The slurries, used for preparing La_1−xSr_xMnO₃ (LSM) films through composite sol-gel method with citric acid, were made up of LSM particles and precursor sol with different pH values. The reasons and key factors for the deterioration of the acidic slurry were analyzed by XRD, SEM and TG/DSC. Furthermore, the LSM films were fabricated with different slurries for studying their film forming abilities. It has been found that the deterioration of the acidic slurry is determined by the abundant H⁺ ions, generated from ionization of the citric acid. Discoloration of the alkaline sol and slurry is considered as a result of complicated complexes of Mn²⁺ ions, which have a good effect on the film forming ability of the alkaline slurry, together with the complexes of La³⁺ and Sr²⁺ ions. The alkaline slurry has better long-term stability than the acidic slurry, and the films made from it are more homogeneous, thicker and have lower sheet resistance.     

Effect of sulphate ion on the electrochemical polymerization of pyrrole and N-methylpyrrole

Pyrrole and N-methylpyrrole were electrochemically polymerized in an identical manner in aqueous electrolytes containing (Et₄N)BF₄, LiClO₄ or Na₂SO₄ to give films of the corresponding p-doped polymers. The polymer obtained from the aqueous Na₂SO₄ electrolyte, [(poly N-methylpyrrole)^+y (SO₄)_y/2^2-]_x, differed from the other polymers in that it exhibited a very low conductivity (≈ 10^?7 S/cm), contained a high concentration of > C=O groups and had an EPR linewidth, ΔH_pp, more than an order of magnitude greater than that found in the corresponding polymers obtained from the (Et₄N)BF₄ and LiClO₄ electrolytes. A sample of [(polyN-methypyrrole)^+y(BF₄)_y^?]_x electrochemically synthesized in CH₃CN and subsequently converted electrochemically to the corresponding sulphate in the Na₂SO₄ aqueous electrolyte exhibited normal behaviour. A mechanism is proposed for the introduction of > C=O groups into the poly(N-methylpyrrole) polymer during its electrochemical polymerization in the Na₂SO₄ aqueous electrolyte.     

Stabilization of supported platinum nanoparticles on γ-alumina catalysts by addition of tungsten

The thermal stabilization of Al₂O₃ using W⁶⁺ ions has been found useful to the synthesis of Pt/Al₂O₃ catalysts. The simultaneous and sequential methods were used to study the effect of W⁶⁺ upon Pt/γ-Al₂O₃ reducibility, Pt dispersion, and benzene hydrogenation. The W/Pt atomic ratios were from 0.49 to 12.4. In the first method we found that the W⁶⁺ ions delayed reduction of a fraction of Pt⁴⁺ atoms beyond 773 K. At the same time, W⁶⁺inhibited sintering of the metallic crystallites once they were formed on the surface. For the sequential sample with a W/Pt atomic ratio of 3.28 W⁶⁺ did not inhibit the H₂ reduction of Pt oxides even below of 773 K, the Pt oxides were reduced completely. After reduction at 1073 K, sequential samples impregnating Pt on WO_x–γ-Al₂O₃ were more active and stable during benzene hydrogenation. TOF of the reaction did not change when the W/Pt atomic ratio, preparation technique and reduction temperature changed and its value was of 1.1 s⁻¹. W⁶⁺ ions promoted high thermal stability of Pt crystallites when sequential catalysts were reduced at 1073 K and decreased their Lewis acidity_.     

Effects of Na+ doping on crystalline structure and electrochemical performances of LiNi0.5Mn1.5O4 cathode material

Pristine LiNi_0.5Mn_1.5O₄ and Na-doped Li_0.95Na_0.05Ni_0.5Mn_1.5O₄ cathode materials were synthesized by a simple solid-state method. The effects of Na⁺ doping on the crystalline structure and electrochemical performance of LiNi_0.5Mn_1.5O₄ cathode material were systematically investigated. The samples were characterized by XRD, SEM, FT-IR, CV, EIS and galvanostatic charge/discharge tests. It is found that both pristine and Na-doped samples exhibit secondary agglomerates composed of well-defined octahedral primary particle, but Na⁺ doping decreases the primary particle size to certain extent. Na⁺ doping can effectively inhibit the formation of Li_xNi_1–xO impurity phase, enhance the Ni/Mn disordering degree, decrease the charge-transfer resistance and accelerate the lithium ion diffusion, which are conductive to the rate capability. However, the doped Na⁺ ions tend to occupy 8a Li sites, which forces equal amounts of Li⁺ ions to occupy 16d octahedral sites, making the spinel framework less stable, therefore the cycling stability is not improved obviously after Na⁺ doping.     

Syntheses and characterization of new low-band gap polymers containing 4H-cyclopenta[def]phenanthrene unit and 4,7-di(thien-2-yl)-2H-benzimidazole-2-spirocyclohexane for photovoltaic device

Two new low-band gap polymers, poly[(2,6-(4,4-bis(2′-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane))] (PCPP-DTCHBI) and poly[(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane))] (PBEHPCPP-DTCHBI), were synthesized and characterized for the photovoltaics. These polymers showed typical characteristics of low-band gap polymers through the internal charge transfer (ICT) between 4H-cyclopenta[def]phenanthrene as the electron-rich unit and di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane as the electron-deficient unit. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are ?5.52 and ?3.82 eV for PCPP-DTCHBI, and ?5.36 and ?3.76 eV for PBEHPCPP-DTCHBI, respectively. Optical band gaps of PCPP-DTCHBI and PBEHPCPP-DTCHBI are 1.70 and 1.60 eV, respectively. As compared to the case of poly[(2,6-(4,4-bis(2′-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2,1,3-benzothiadiazole))] (PCPP-DTBT), PCPP-DTCHBI shows deeper HOMO energy levels by 0.12 eV and lower band gap by 0.3 eV. The FET mobilities of PCPP-DTCHBI and PBEHPCPP-DTCHBI are 1.19 × 10^?4 and 5.11 × 10^?5 cm²/V s, respectively, and the power conversion efficiencies of the solar cell devices of the PCPP-DTCHBI and PBEHPCPP-DTCHBI blended with [6,6]phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) are 1.01% and 0.53%, respectively. The newly designed DTCHBI unit can be used as the electron-deficient moiety inducing efficient ICT for low band gap generation while keeping deep HOMO energy level of the polymer.     

Electrocatalytic efficiency of polyaniline by cyclic voltammetry and electrochemical impedance spectroscopy studies

Electrochemical redox reactions of ferrous/ferric (Fe²⁺/Fe³⁺) and hydroquinone/quinone (H₂Q/Q) were studied on Pt and polyaniline (PANI)-deposited Pt electrodes in 0.5 M H₂SO₄-supporting electrolyte by cyclic voltammetry and ac impedance spectroscopy. A comparison of the experimental data obtained with the Pt and PANI/Pt electrodes suggested that the reactions were catalyzed by the PANI. Based on a relative increase in peak currents of cyclic voltammograms, catalytic efficiency (γ_cv) of the PANI was defined. There was an increase in γ_cv with an increase of scan rate and a decrease of concentration of Fe²⁺/Fe³⁺ or H₂Q. The complex plane impedance spectrum of the electrode consisted of a semicircle in high frequency range and a linear spike in low frequency range. The exchange current density (i₀) calculated using the semicircle part of the impedance showed Butler–Volmer kinetics with respect to concentration dependence. From a relative increase of i₀ on the PANI/Pt electrode, catalytic efficiency (γ_eis) was evaluated.     

Humidity sensitive properties of substituted polyacetylenes

Thin film resistive humidity sensors have been prepared based on a series of homogeneously doped soluble substituted polyacetylenes synthesized using palladium acetylide complex catalyst. The humidity sensitive properties of the polymers (poly(propiolic acid) (PPA); poly(propiolic acid-co-ethynylbenzene) (PA-co-EB); poly(propiolic acid-co-p-diethynylbenzene) (PA-co-DEB); poly(propiolic acid-co-propargyl alcohol) (PA-co-OHP)) doped with FeCl₃, H₂SO₄ and HClO₄ have been investigated and compared. The chemical structures of polymers and the doping agents have great influence on the sensing properties of the sensors. A sensor based on PA-co-OHP doped with HClO₄ shows the best response. Its logarithm of impedance varies linearly with relative humidity (RH) for four orders of magnitude (10⁷–10³ Ω) over a wide range of 30–95%RH, and the response time is <6 and <15 s for absorption and desorption, respectively. Furthermore, the effect of temperature on the sensing behaviour of the polymers has also been described.     

Chemical and physical sputtering of aluminium and gold samples using Ar–H2 DC-glow discharges

We present a series of sputtering experiments on aluminium samples performed with an Ar–H₂ DC-glow discharge at varying Ar–H₂ gas-composition, driven at a discharge voltage of –300 V and a pressure of 0.2 mbar, in conjunction with measurements of the corresponding ion-energy distributions of the ions bombarding the discharge cathode (Ar⁺, Ar²⁺, ArH⁺, H₂⁺ and H₃⁺). Similar measurements on gold samples, which have been published, have shown that the Au-sputtering efficiency of an Ar–H₂ glow discharge as a function of gas-composition could be adequately described by the corresponding change in the measured ion-energy distributions, under the assumption of a purely physical sputtering process. The experiments presented here show that this is not the case for aluminium (effectively Al₂O₃). In this case, a measured optimal gas-composition of 80% H₂ was found for Al-sputtering, while the energy-distributions suggest an optimum at 20% (as for gold). This clearly suggests that hydrogen-enhanced chemical sputtering is taking place.     

Studies of photoexcitations in conducting polymers mixed with C60

We have studied photoexcitations in mixed compounds of C₆₀ with derivatives of poly (p-phenylenevinylene) and poly(phenyleneacetylene) using a variety of CW and picosecond (ps) transient spectroscopies. The CW techniques used include photoinduced absorption (PA), photoluminescence (PL) and optically detected PA (ADMR), whereas the ps transient techniques used were time-resolved PA, PL and stimulated emission (SE). Compared with the pristine polymers, all mixed C₆₀ compounds have shown enhancement of PA bands associated with charged photoexcitations. These PA bands have shown a single spin-half ADMR resonance, identified as due to spins on the polymer chains; no spin-half ADMR signal from photogenerated C₆₀⁻ has been observed. In the ps time domain we have observed a pronounced reduction of the transient PL and SE in the mixed compounds. Due to the similarity found between the ps transient and CW PA bands, we have identified the ps photoexcitations in the mixed compounds as polarons (P⁺−P⁻ ) on the polymer chains, which are indirectly photogenerated as a result of exciton dissociation in C₆₀⁻ related defect centers. The ultrafast charge transfer, claimed to exist between the polymer chains and C₆₀ molecules, has not been observed in the ps time domain.     

Effect of ascorbic acid on hydrogen peroxide decomposition into an environmentally friendly mixture for pickling of 316L stainless steel

One of main disadvantages of using environmentally friendly chemical mixtures for pickling, is stabilization of hydrogen peroxide (H₂O₂), due to its decomposition is affected by the presence of metal ions. In previous studies, p-toluenesulfonic acid (C₇H₁₀O₄S) was used as a hydrogen peroxide stabilizer; however, benzene rings in its structure have a certain level of toxicity. In this work, ascorbic acid (C₆H₈O₆) was tested as a stabilizer agent for H₂O₂ in a pickling mixture composed by sulfuric acid (H₂SO₄) and hydrofluoric acid (HF)-H₂O₂, this mixture was tested on 316L stainless steel (SS). Decomposition of H₂O₂ in pickling solution was evaluated for different ferric ions concentration between 0 to 40 g/L and at different temperatures from 25° to 60°C. Pickling rates at 25°C for 316L SS were 26873 mg/dm² and 27799 mg/dm² using ascorbic acid and p-toluenesulfonic acid, respectively. Ascorbic acid has a positive influence on stabilization processes of H₂O₂.     

Corrosion of a cast iron boiler in a model central heating system

The rate of corrosion of a grey, cast iron boiler in a model central heating system was measured by chromatographic analysis of evolved gases (H₂ + N₂). The rate of H₂ evolution rose to a maximum and then declined parabolically as a protective Fe₃O₄ scale formed. Comparison with published results on corrosion of steel at ～ 300°C suggests that the rate-controlling process is the diffusion of FeOH⁺ ions through pores in the scale. A reaction mechanism is proposed which is similar to the established mechanism for conversion of Fe to Fe₃O₄ in aerated waters except that H⁺ ion reduction is the cathodic process. The relevance of these findings to the corrosion of central heating systems in service is discussed.     

Determination of acid-base properties of hcl acid activated palygorskite by potentiometric titration

The surface acidity of raw and acid activated palygorskite clay was studied by acid-base potentiometric titration. The Gran plot method was applied for the hydroxide titration, and the total surface sites (Hs) and the average number of protons reacted per surface site (Z) of the palygorskite samples at a given ionic strength were calculated. Acid treatment increases the clay acidity and modifies its surface charge. The point of zero charge value, which was determined by the common crossing point of Z vs. the pH curves performed at different ionic strengths, decreased from 8.8 to 3.5 with the acid treatment period. For illustrating the acidic characteristics of the treated and untreated palygorskite surface, three surface protonation models were tested: (a) the one site one pK_a model, ≡SOH?≡SO^?+H⁺; (b) the two sites two pK_as model,≡S_IOH?≡S_IO^?+H⁺ and S_IIOH?≡S_IIO^?+ H⁺; and (c) the one site two pK_a model,≡SOH?≡SO^? + H⁺ and SOH + H⁺?≡SOH ₂ ⁺ . The three surface protonation models sufficiently describe the surface properties and their evolution with the acid treatment.     

AC and DC study of the temperature effect on mild steel corrosion in acid media in the presence of benzimidazole derivatives

The effect of benzimidazole derivatives on mild steel corrosion in 1 M HCl at five different temperatures has been studied. Impedance, polarization resistance, polarization curves measurement and gravimetric methods have been used. The inhibiting efficiency and the apparent activation energy (E_a) have been calculated in the presence and in the absence of the organic inhibitors.The comparative investigations carried out in 1 M H₂SO₄ and 1 M H₂SO₄+1×10⁻⁴ M KI verify the assumption that the halogen ions facilitate the inhibitors adsorption. The electrostatic absorption of the compound cation form seems more probable. The behaviour of 2-mercaptobenzimidazole (2-SH-BI) contrasts that of the rest of the substances investigated. Chemisorptive adsorption is more probable in this case. The lower value of activation energy E_a in the presence of 1-benzyl-benzimidazole when compared to that in 1 M HCl is explained with a partial compensation effect.     

Investigation of the Interaction Between Cu2-x S +So Coatings and Pd(II) Ions by Cyclic Voltammetry and X-ray Photoelectron Spectroscopy

The interaction between Pd²⁺ ions and Cu_2-xS coating formed by three cycles and containing ~30 at.% of elementary S has been investigated by the methods of cyclic voltammetry and photoelectron spectroscopy (one cycle of coating formation includes treatment of the surface with Cu(I)+Cu(II) ammoniate solution, hydrolysis of the adsorbed copper compounds and sulphidation of copper oxygen compounds in Na₂S_n solution). After exposure of such a coating to Pd²⁺ ions (1.7 mM PdCl_2’ pH-2), an exchange as well as a redox interaction between the coating components and Pd²⁺ ions has been shown to occur. Due to this the amount of copper in the coating decreases from 2 to 4 times and that of sulphur from 1.5 to 5 times. The coating modified in such a way has been found to contain up to 75 at.% of palladium, ~90% of it being in a metallic state.

It has been determined that at the beginning S^o is bound into a soluble compound:

2Pd²⁺ + S^o + 3H₂O → 2Pd^o + H₂SO₃ + 4H⁺.

The Cu₂S present in the coating is considered to interact with Pd²⁺, with the formation of Pd⁰ and CuPdS_2’, while CuS reacts most likely according to the reaction:

CuS + 3Pd²⁺ + 3H₂O → 3Pd^o; + H₂SO₃ + Cu²⁺ + 4H⁺.

The Cu_2-xS +S^o coating formed on a dielectric and modified with Pd²⁺, contrary to the initial Cu_2-xS +S^o coating, can be plated with copper from any electrolyte for copper deposition.     

Effect of Cathodic Polarization on Hydrogen Diffusion across a Steel Membrane from HCl Ethylene Glycol Solutions

Hydrogen diffusion across a steel (C3) membrane from 0.01–1.0 M HCl in ethylene glycol containing water (0.1 and 5 wt %) was studied. The effect of the solvate form of the discharging proton (C₂H₄(OH)₂H⁺ and H₃O⁺), the concentration of H⁺ _solv, and the cathodic polarization on the process was revealed.     

Conducting polymers synthesized by dopant-induced polymerization of insulating charge-transfer crystals

Conductive polymer complexes are formed by a one-step process resulting in successive doping and dopant-induced solid-state polymerization of insulating molecular complexes involving the donor biphenyl or terphenyl and acceptors such as TCNQ (7,7,8,8-tetracyano-p-quinodimethane) and DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone). The conductor resulting from the gas-phase doping of the terphenyl/TCNQ complex with AsF₅ contains low molecular weight poly(p-phenylene) chains complexed with AsF₆^? ions. Results are consistent with the presence of neutral TCNQ molecules as ‘molecular spacers’ within the conducting phase. The solid-state reaction and resulting conducting polymer are examined using spectroscopic and diffraction techniques, as well as anisotropic conductivity measurements.