Methodological Aspects of the Partial Charge Transfer in the Adsorption of Anions. Part II

The partial charge transfer coefficient and the electrosorption valence of anions on metals, as well as the role of electron tunneling in the formation of contact electric resistance (CER) signal are discussed. The CER value is shown to depend on the potential (E), the surface coverage with adsorbate particles, and their apparent charge. Bell-shaped CER–E curves of copper, silver, and gold in solutions containing halide ions are obtained. The maxima of these curves (E _max) correspond to the beginning of a substantial charge transfer and depend on the metal, anion, and its concentration. At E < E _max, halides adsorb without a noticeable charge transfer; while in a range of E _max + 0.1 V, the transfer is almost complete. On a chosen metal, E _max increases in a series of anions: I^– < Br^– Cl^–. For a chosen anion (e.g., I^–), it increases in a series of metals: Cu Ag Au. A correlation between the charge transfer of the adsorbed anions and the adsorption of the solvent, as well as the surface reconstruction of monocrystal electrodes, is discussed. The results are compared to the literature data.     

CO<Subscript>2</Subscript> electrochemical reduction via adsorbed halide anions

The electrochemical reduction of CO₂ was studied utilizing halide ions as electrolytes, specifically, aqueous solutions of KCl, KBr, KI. Electrochemical experiments were carried out in a laboratory-made, divided H-type cell. The working electrode was a copper mesh, while the counter and reference electrodes were a Pt wire and an Ag/AgCl electrode, respectively. The results of our work suggest a reaction mechanism for the electrochemical reduction of CO₂ where the presence of Cu-X as the catalytic layer facilitates the electron transfer from the electrode to CO₂. Electron-transfer to CO₂ may occur via the X⁻ _ad(Br⁻, Cl⁻, I⁻)-C bond, which is formed by the electron flow from the specifically adsorbed halide anion to the vacant orbital of CO₂. The stronger the adsorption of the halide anion to the electrode, the more strongly CO₂ is restrained, resulting in higher CO₂ reduction current. Furthermore, it is suggested that specifically adsorbed halide anions could suppress the adsorption of protons; leading to a higher hydrogen overvoltage. These effects may synergistically mitigate the over potential necessary for CO₂ reduction, and thus increase the rate of electrochemical CO₂ reduction.     

Dependence of the surface charge and the fluorine adsorption by γ-aluminum oxide on the solution temperature

The dependence of the surface charge of the γ-aluminum oxide and the fluorine adsorption on the solution temperature (20, 30, and 40°C), the pH (3.5–10), and the equilibrium concentration of fluorine in the solution (from 1.0 × 10⁻³ to 1.5 × 10⁻¹ M/l) is studied by the method of potentiometric titration and adsorption variations with the view to elucidate the nature of the processes that take place upon the removal of fluorine with the use of ECDM sludge of an aluminum alloy that was calcined at a temperature of 800 °C. The adsorption isotherms were processed using the Freundlich, Langmuir, and BET equations. The best coincidence with the experimental data is obtained with the use of the Langmuir equation. It is shown that both the solution temperature increase and the amount of fluorine adsorbed by the sample surface shift the pH_PZC to a more acid range. The fluoride adsorption occurs due to the exchange of the OH⁻-groups of the hydrated oxide surface for fluorine ions due to the interaction of the charged AlOH₂⁺ centers of the surface with F⁻ ions and due to the formation of hydrogen bonds of F⁻ and uncharged AlOH centers.     

Effect of solution temperature and ph on adsorption and density of surface charge at aluminum oxyhydroxide /NaF aqueous solution interface

The effect of temperature, pH, and adsorbed fluorine on the density of a surface charge at the aluminum oxyhydroxide /NaF aqueous solution interface is investigated using the method of potentiometric titration. The equilibrium of fluorine ion adsorption in analyzed. Products of electrochemical dimensional machining (ECDM) of an AMG-6 aluminum alloy thermally treated at 200°C are used as an adsorbent. The experiments on the adsorption have been carried out from an acetated buffer with the initial fluorine solution concentrations from 1.0 × 10⁻³ to 1.5 × 10⁻¹ M/l. The adsorption isotherms were plotted for solution temperatures of 20, 30, and 40°C. Adsorption is shown to obey the Langmuir adsorption model. It is demonstrated that, at all solution temperatures, the pH is 5 for the maximal fluorine adsorption on aluminum oxyhydroxide. It has been revealed that the surface charge at the aluminum oxyhydroxide /NaF aqueous solution interface depends on the solution temperature; i.e., the pH of a point of a zero charge point (pH_PZC) of the aluminum oxyhydroxide surface decreases with an increase in temperature. As is the case with temperature, adsorbed fluorine shifts pH_PZC toward smaller values, which is typical for specific adsorption. The temperature dependence of pH_PZC has made it possible to determine the standard mole enthalpy and entropy of surface-charge formation. The interaction of fluorine with aluminum oxyhydroxide is shown to be the result of the chemical affinity of F with surface centers of AlOH₂ ⁺ and AlOH and electrostatic attraction.     

Studying the Contact Electric Resistance of Platinum in Aqueous Solutions

The contact electric resistance (CER) R of platinum is studied depending on the potential E in the pH range from 0 to 14. In all the solutions examined, the R–E curves are bell-shaped with the maximum around 0.4 to 0.5 V (EHE). This shape is relatively insensitive to the concentration and nature of the electrolyte and changes only upon adding strongly adsorbing anions to the solution or upon replacing water as a solvent with dimethyl sulfoxide. In alkaline solutions, the CER of platinum increases with an increase in the concentration of hydroxide ions, which is explained by the sensitivity of the CER to the adsorption of hydroxide ions on platinum. Their adsorption is shown to begin in the hydrogen range of potential. A sharp decrease in the CER of platinum in acidic, neutral, and alkaline solutions is observed at E 0.4 V (EHE) and accounted for by the beginning of the noticeable charge transfer from the adsorbed hydroxide ions to the metal. The isotherms of the adsorption of hydroxide ions on platinum in alkaline solutions at E 0.4 V (EHE) are considered.     

On the Coadsorption of Mephenaminate and Phenylundecanoate Anions at Passive Iron

Adsorption of sodium mephenaminate (SMEP), sodium phenylundecanoate (SPU), and their mixtures by oxidized iron from a borate buffer (pH 7.4) at the constant potential E = 0.2 V was studied by ellipsometry. Like the adsorption of the previously studied SMEP, that of SPU obeys the Frumkin equation with the higher Gibbs energy of adsorption (− ΔG _A ⁰ = 33.1 kJ/mol). The Frumkin equation fails for the adsorption of an equimolar SPU + SMEP mixture (inhibitor IFKhAN-31), which is described by the Bockris equation. The calculations showed that −Δ G _A ⁰ = 41.7 ± 0.2 kJ/mol and the number of water molecules displaced by an adsorbed inhibitor species from the metal-solution interface is n = 4. It was found that SMEP anions preadsorbed at oxidized iron do not affect the shape of the SPU adsorption isotherm, yet altering its constants. In the case of preadsorbed SPU, the SMEP adsorption isotherm changes its shape and cannot be described by any known adsorption equation. In addition, SMEP begins to adsorb at the electrode in lower concentrations; i.e., PU anions stimulate the adsorption of SMEP. __________ Translated from Zashchita Metallov, Vol. 41, No. 6, 2005, pp. 573–578. Original Russian Text Copyright ? 2005 by Kuznetsov, Andreeva.     

Kinetics of nitrogen absorption in molten aisi 316 stainless steel during immersion nitrogen blowing

Nitrogen absorption in molten metal for stainless steel AISI316 has been investigated by immersion nitrogen blowing through an immersed alumina nozzle with an internal diameter of 3 mm. Based on these experimental data, some kinetic parameters of nitrogen absorption, such as reaction order, rate constant and apparent activation energy of nitrogen absorption reaction, have been obtained. Effect of stirring by immersion nitrogen blowing through an immersed alumina nozzle on nitrogen absorption reaction has been observed. Results show the following: (1) Nitrogen absorption reaction is the −1.5th order reaction. The rate constant k _N is of the order of 10⁻⁵ wt%^2.5·min⁻¹. Nitrogen absorption reaction for AISI 316 has negative apparent activation energy of −92.40 kJ·mol⁻¹. This indicates that the nitrogen absorption reaction has a complex and multistep reaction mechanism. (2) The rate of nitrogen absorption reaction in molten stainless steel is mixture control by the adsorption of monatomic nitrogen on the surface of molten stainless steel and mass transfer in molten metal. (3) A rate equation of nitrogen absorption reaction has been derived based on a mixed control mechanism by both the -1st order nitrogen absorption reaction and mass transfer in molten metal.     

Corrosion behavior of mild steel in the presence of scale inhibitor in sulfuric acid solution

Surfactants such as non-ionic polyethylene glycol tert-octylphenyl ether (Triton X-114) have been studied as efficient corrosion inhibitors in acid medium. In this study inhibition performance of Triton X-114 has been evaluated as corrosion inhibitor for mild steel in 0.5 mol l⁻¹ H₂SO₄. The electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and linear polarization (LPR) techniques has been applied to determine the electrochemical behaviour of Triton X-114. The maximum efficiency of Triton X-114 was found as 96% at the concentration 5.0 × 10⁻⁵ mol l⁻¹. The adsorption isotherm of inhibitor on the mild steel surface was found to be in a good agreement with the Langmuir and the standard free energy value (ΔG _ads^°) was calculated as −50.1 kJ mol⁻¹, which shows that adsorption of Triton X-114 on the mild steel surface improves the inhibition characteristics in 0.5 M H₂SO₄.     

Kinetics of hydrogen evolution from acidic solutions on pressed micro graphite electrodes modified with carbon nanotubes. II. Impedance studies

The kinetics of hydrogen evolution on pressed micro graphite electrodes modified with multi-walled carbon nanotubes (MWCNTs) is studied in chloride environments with a constant ionic strength equal to unity with the use of impedance spectroscopy. The impedance of electrodes containing from 0.1 to 8 wt % MWCNTs in solutions of a general composition x M HCl + (1 − x) M NaCl at a currentless potential and at E = −0.6 V (N.H.S.) is interpreted as impedance of porous electrodes. The data obtained indicate the absence of diffusion limitations of the processes that proceed at a currentless potential and of the hydrogen evolution reaction. When the content of MWCNTs in a micro graphite electrode increases to 8 wt % at E = −0.6 V (N.H.S.), the increase in the contribution of the double layer capacitance and the adsorption capacitance to the Faraday impedance is the most pronounced, which means that MWCNTs promote a specific interaction of the electrolyte ions and atomic hydrogen with the surface of the cathodically polarized electrode.     

Methodological Aspects of the Partial Charge Transfer in the Adsorption of Anions. Part I

Appealing to the modern discussions, some problems of applying the Lippman and Gibbs equations to the adsorption on solid electrodes are considered. The possibility of using nonthermodynamic approaches for solving certain thermodynamic problems (particularly, the role of elastic and plastic deformations of the electrode surfaces in the adsorption processes, the orientation of surface bonds during the adsorption of anions, and others) is discussed. The coefficient of partial charge transfer and the electrosorption valence of anions on metals, as well as the role of electron tunneling in the formation of a contact electric resistance (CER) signal are considered. The relation between the charge transfer during the adsorption of an anion and the adsorption of the solvent and the surface restructuring of monocrystal electrodes is analyzed. The results are compared to the available literature data.     

Electrochemical synthesis of polypyrrole on aluminium in different anions and corrosion protection of aluminium

In this study, polypyrrole was deposited on the aluminium in different anions (CO₃²⁻, NO₂⁻, CrO₄²⁻, DBS⁻). The contribution of anions to formation of polypyrrole film was investigated by using cyclic voltammetry (CV) technique. The effect of polypyrrole film on the corrosion of aluminium was searched in 0.1 M HCI solution by using potentiostatic method. For this purpose, polarization curves were obtained, corrosion current density (icorr), corrosion potential (Ecorr), polarization resistance (Rp) were determined from the polarization curves. Moreover, the percent efficiency of coating was calculated. The complexes of pyrrole pentamers and different anions (CO₃²⁻, NO₂⁻, CrO₄²⁻, DBS⁻) were studied using ab initio quantum chemical at the Hartree-Fock (HF) levels with STO-3G, 3-21G, 6-31G(d,p) [13] basis sets and HOMO-LUMO energy gap is calculated by B3LYP method with 3-21G* and 6-31G(d,p) basis sets. The polypyrrole film obtained in CrO₄²⁻ anion is determined to be the most effective in prevention to pitting corrosion of aluminium as experimental and theoretical.     

Physicochemical study of the nanodiamond surface

Suspensions of five types of ultrafine diamond of a detonation synthesis in distilled water and 0.9 mol/l NaCl solution were found to have pHs in the range of 3–6, which results from the intrinsic acidity of their surfaces. The possibility of quantitatively determining the protogenic groups of a surface is found using the pH-potentiometric method. The data on the kinetics of the varying pH of suspensions in water and 0.9 mol/l NaCl solution and the data on the curves of the alkali titration of the suspensions of nanodiamond in the indicated systems make it possible to infer the presence of one or two types of acid groups on the surface of a diamond. The constants of dissociation (pK ₁) of these groups on the assumption of their single-base nature are estimated. From 1 × 10⁻³−5 × 10⁻² mol/l chloride solutions of H[AuCl₄] and RhCl₃ on the surface of diamond, gold(III) and rhodium(III) are found to be adsorbed; the adsorption of methylene blue from its 2.5 × 10⁻⁴−5 × 10⁻⁴ mol/l solutions reaches more than 90% (upon adsorption from 5 ml of the initial solution by a 0.05-g nanodiamond).     

On the mechanism of electron-stimulated formation of a nanosize C-Sn coating on a tin surface

The effect of electrons with an energy E _p = 1400, 1600, and 1800 eV on the condition of an Sn (99.999 at %) surface under a residual pressure P = 10⁻⁶ Pa obtained using oilless pumps have been studied by the Auger electron spectroscopy. It has been demonstrated that the electron irradiation of tin stimulates processes of adsorption of carbon-containing particles from the residual gas, which dissociate prior to the precipitation onto the surface. The adsorption layer of about 2 nm thick consists mainly of elementary carbon, through which oxygen atoms diffuse and form an oxide layer at the C-Sn interface during the final stage of the formation of the adsorption layer upon the bombardment by electrons with an energy E = 1800 eV.     

Inhibition efficiency of benzidine for mild steel in acidic media

In this study, the inhibition effect of different concentrations of benzidine possessing amine groups in its structure on the corrosion behavior of mild steel (MS) in 1.0 M HCl solution at 293 K temperature was practiced in both short and long immersion times by measuring electrochemical impedance spectroscopy (EIS), hydrogen evolution (V _H2 − t) and change of open circuit potential (E _ocp − t). For short-term tests, potentiodynamic polarization and linear polarization resistance (R _lp ) were also studied. Polarization data indicate that this compound act as mixed-type inhibitor for mild steel in 1.0 M HCl. With regard to the results cited, it was identified that the increase in the inhibitor efficiencies with concentration was emanated from the adsorption of benzidine molecules on the electrode surface. Pertinent to this adsorption, it followed Langmuir isotherm. Adsorption equilibrium constant and adsorption free energy were determined. The effect of temperature on the corrosion behaviour in the absence of any inhibitor and in the presence of 75 mM benzidine was studied in the range of 293–323 K. Activation energies (E _a ^*) were calculated from the obtained corrosion rates at different temperatures. Furthermore, in order to identify the adsorption mechanism of the inhibitor on the metal surface, the zero-charge potential (PZC) was determined by EIS measurement technique. Surface morphology of mild steel electrodes were emerged by means of the scanning electron microscope.     

The effect of sulfate and nitrate ions on the passivation and activation of silver in alkaline solutions

Based on the concept of a competitive adsorption of hydroxide ions with SO₄²⁻ and NO₃⁻ activating anions, some peculiarities of the anodic behavior of silver in alkaline solutions containing sulfates or nitrates are explained. At the ascending branches of the anodic peaks, a co-adsorption of OH⁻ and SO₄²⁻ or NO₃⁻ results in the formation of mixed adsorption complexes, which are soluble better than hydroxide ones. An increase in the part of the soluble oxidation products of silver is observed if a rotating disc electrode with a ring is used. Passivation of silver is explained by a change in the electronic structure of the adsorption complexes when certain potential values are reached, while a local activation, by the destruction of these complexes at the passive state potentials between the anodic peaks. At the depassivation, pH value of the solution in pits decreases, which results in the formation of Ag₂SO₄ or AgNO₃ salts. The presence of the salts in a deposit on the electrode is confirmed by the appearance of a C ₃ cathodic peak. Original Russian Text ? N.N. Lesnykh, N.M. Tutukina, I.K. Marshakov, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 472–477.     

Initiation and repassivation of crevice corrosion of type 444 stainless steel in chloride solution

Effects of chloride ion concentrations, solution temperature, and crevice-forming materials on the crevice corrosion of type 444 stainless steel were investigated using a potentiostatic method. Critical crevice potential (E_crev) and repassivation potential (E_r) of the creviced alloy decreased with an increase in chloride concentration [Cl⁻], satisfying the logarithmic relationship between E and [Cl⁻]. In addition, E_crev and E_r of the alloy with silicone crevice former were measured to be higher than those of the alloy with an EPDM (Ethylene Propylene Diene Monomer) crevice former, suggesting that silicone was more effective in preventing water from penetrating crevices between a stainless steel sheet and the crevice former. In electrochemical current transient measurements with an applied potential, the intensity of current transients corresponding to the initiation of metastable pits increased abruptly near the E_r of the alloy, indicating that the stability of crevice corrosion is associated with the initiation of metastable pits.     

Thermodynamic calculation of the critical potentials of the selective dissolution of Ag-Au and Cu-Au alloys

A thermodynamic approach to estimating the critical potential of the selective alloy dissolution E _c at a vacancy concentration N _v(s) in the superficial layer is considered. The N _v(s) dependence on the potential and the concentration of gold N _Au in the alloy is calculated. The E _c potential corresponds to the critical concentration of vacancies N _v(s) ≈ 10⁻², and the E _c value itself is determined chiefly by the zero-point potential of the alloy, which is a pronounced function of the surface enrichment in gold. The calculated E _c-N _Au functions for Ag-Au and Cu-Au alloys satisfactorily coincide with the experimental dependences. Original Russian Text ? A.E. Kutyrev, Yu.Ya. Andreev, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 2, pp. 152–159.     

Kinetics of forward extraction of Ti（IV） from H_2SO_4 medium by P_（507） in kerosene using the single drop technique

The kinetics of forward extraction of Ti(IV) from H2SO4 medium by P507 in kerosene has been investigated using the single drop technique.In the low concentration region of Ti(IV),the rate of forward extraction at 298 K can be represented by F(kmol·m-2·s-1)=10-5.07 [TiO 2 + ][H+]-1 [NaHA 2 ](o)·Analysis of the rate expression reveals that the rate determining step is(TiO)(i)2+ +(HA 2)(i)-[TiO(HA2)](i)+.The values of Ea,H±,S±,and G±298 are calculated to be 22 kJ·mol-1,25 kJ·mol-1,-218 J·mol-1·K-1,and 25 kJ·mol-1,respectively.The experimental negative S± values indicate that the reaction step occurs via SN2 mechanism.     

Anodic dissolution of copper in perchlorate solutions of different ionic strength

By means of the potentiodynamic method, the anodic behavior of copper is studied in perchlorate solutions in a wide range of pH and ClO ₄ ⁻ ion concentrations. The activating action of ClO ₄ ⁻ ions in the anodic process is associated with their weak ability to form hydrates, which facilitates their incorporation into the electrical layer and displacement of OH⁻ ions from the surface complexes. The schemes of mechanisms proposed for the copper anodic dissolution imply the possibility of the formation of intermediate adsorption complexes with participation of ClO ₄ ⁻ ions. By the technique of nonlinear regression analysis, the constants in the kinetic equations are calculated for the proposed schemes.