Lj.M. Vračar

Adsorption and corrosion inhibitive properties of some organic molecules on iron electrode in sulfuric acid

Adsorption and corrosion inhibitive properties of three different organic molecules: 2-naphthalenesulfonic acid, 2,7-naphthalenedisulfonic acid and 2-naphthol-3,6-disulfonic acid are investigated on Armco-iron electrode cathodically polarized, in 0.5 mol dm−3 H2SO4 solution. The examinations show that the three organic molecules behave as a cathodic corrosion inhibitors. The inhibitive efficiency, changes with the number of functional groups substituted on benzene ring and increases with concentration. The experimentally obtained adsorption isoterms follow the Frumkin equation [θ/n(1−θ)n]exp(−2aθ)=Bc and the best fit is obtained for n=2 (for 2-NSA) and n=3 (for 2,7-NDSA and 2N-3,6-DSA). The calculated values for the parameter a are 2.1–2.9 and Gibbs energies are 17–21 kJ mol−1. The adsorption rate constants are very small, proving that iron surface permanently changes due to the corrosion processes.

Different views regarding the kinetics and mechanisms of oxygen reduction at Pt and Pd electrodes

Abstract Two diametrically different pH dependences have been reported for oxygen reduction at prereduced Pt electrodes. In the current density (cd) region with the Tafel slope of −2.3 RT/F , the pH dependence expressed as d E /dpH is, according to one group of workers, −90 mV, ie −3 × 2.3 RT /(2 F ) and − 30 mV, ie −2.3 RT /(2 F ) in acid and alkaline solutions, respectively. The same dependences, according to another group of workers are −60 mV, ie −2.3 RT/F in both acid and alkaline solutions. In the cd region with the Tafel slope of −2 × 2.3 RT/F , d E /dpH according to the first group of workers is − 120 mV and 0 mV in acid and alkaline solutions, respectively. However, according to the second group, this dependence is −60 mV in both acid and alkaline solutions. The reasons for the variation in pH dependences is discussed. A previous analysis for the pH dependence of − 2.3 RT/F at all pHs in both cd regions is in error. An account of the pH dependences of −3 × 2.3 RT /(2 F ) and −2.3 RT /(2 F ) in terms of Temkin conditions of adsorption is satisfactory and allows for the change of Tafel slopes and pH dependences with increasing cd.

Apparent enthalpies of activation of electrodic oxygen reduction at platinum in different current density regions—I. Acid solution

Oxygen reducton (OR) at Pt electrodes in aqueous HClO4 solution (pH = 1.9) is examined at temperature from 5 to 45°C. At all temperatures, two distinct Elogi regions are observed. At low current densities (cd), the Tafel slopes are close to −2.3 RT/F, while at high cds they are close to −2 × 2.3 RT/F. Though the apparent enthalpy of activation at the reversible potential in the low cd region (=60 kJ mole−1) is higher than the apparent enthalpy of activation in the high cd region (=43 kJ mol−1), it is shown that the actual enthalpies of activation, ie the enthalpies at †φ = 0, are the same in both cd regions. The rate determining step of the reaction is, therefore, the same in both cd regions and the observed changes in the Tafel slopes arise from different conditions of adsorption of reaction intermediates in these regions, as previously suggested.

Hydride formation at Ni-containing glassy-metal electrodes during the H2 evolution reaction in alkaline solutions

A group of Ni-containing glassy metal alloys that are of interest as electrocatalysts for cathodic hydrogen production in water electrolysis have been examined with regard to the role of hydride formation and the participation of sorbed H during cathodic H2 evolution in alkaline solutions. Hydride formation is important as it may determine, in part, the electrocatalytic properties of the alloys at their surfaces, for cathodic H2 evolution. Characteristic time-dependences of currents observed near, but negative to the H2 reversible potential in response to a stepwise change of potential, together with potential-relaxation measurements on open-circuit following current interruption, lead to the conclusion that a three-dimensional hydride is formed in the near-surface region of the metals during the H2 evolution process. Complementary cyclic-voltammetry experiments show that hydrogen deposition-absorption, or H oxidation processes, can be distinguished from possible spontaneous oxidation of the glassy metals, over somewhat overlapping potential regions.

Influence of chloride ion adsorption on hydrogen evolution reaction on iron

Abstract In order to investigate the effect of chloride ions on the hydrogen evolution reaction (HER) on iron, electrochemical polarization measurements have been carried out in aqueous sulphuric acid with the addition of different concentrations of NaCl. Data for the surface coverage have been calculated from the observed shift of the cathodic polarization curve. The Temkin isotherm has been found to fit the experimental results. The adsorption kinetic analysis shows that adsorption is quite slow, fitting a linear relationship between surface coverage and logarithm of time. Simultaneous a.c. impedance measurements in the HER potential region do not show any sign of pseudocapacitance, indicating that chloride ions adsorb without partial charge transfer.

The kinetics of inhibitor adsorption on iron

Abstract The kinetics of adsorption of Cl − and Br − ions and 2-naphthol-3,6-disulphonic acid (NDSA) on Armco iron electrodes cathodically polarized in 0.5 M H 2 SO 4 was carefully analyzed. Contrary to the initial expectation that the adsorption of these species should be faster on such cathodically protected surfaces as compared to the rates of adsorption during anodic polarization when the metal surface changes permanently, the obtained adsorption rate constants were about 10 −2 mol −1 dm 3 s −1 both for halogen ions and NDSA with a small dependence on electrode potential for halogen ions. The kinetics were analyzed by using a computerized curve fitting procedure based on a Frumkin-type isotherm for the quasi-equilibrium state and Roginskii—Zeldovich-type equations for the adsorption and desorption processes. A computer analysis of the experimental θ vs. time data provided good estimations of k a and a , while the estimation of k d was less reliable, depending on the precision of the experimental data. For halide ion adsorption it was found that n = 1 and a n = 3 and a > 0 (attractive forces). The slow adsorption rates during cathodic polarizations are explained by the “anomalous” dissolution of iron during cathodic polarization by a mechanism similar to that for the anodic polarizations.

Invariance with pH of enthalpies of activation for O2 reduction at Pt electrodes in acid solutions

The apparent enthalpies of activation, ΔH*a, for O2 reduction at Pt electrodes are determined in acid solutions of different pH. In the high current density region, characterized by a Tafel slope close to − 120 mV, ΔH*a,h is lower than ΔH*a,l in the low current density region characterized by a Tafel slope close to − 60 mV. Although ΔH*a,l > ΔH*a,h it is concluded that at every pH the enthalpies of activation at the zero Galvani potential difference are the same in both current density regions. Therefore, irrespective of the different Tafel slopes, the same mechanism is operative in both regions. In all solutions, ΔH*a for either current density region are independent of pH when they are determined at constant potentials vs rhe. This invariance with pH is unexpected in view of the suggested variance of the Galvani potential difference with pH. Alternative causes for the invariance are discussed.

Anomalous temperature dependence of the hydrogen evolution reaction on iron

Abstract Cathodic polarizations for the hydrogen evolution reaction on Armco iron in perchloric acid measured in the temperature range 278 −348 K and pH range 0.5–2.0 follow the Tafel relationship but with anomalous temperature dependence. Instead of varying widely with temperature, the measured values of the slope were between 115 and 140 mV dec −1 , i.e., ±10%. This is ascribed to an anomalous change of the apparent transfer coefficient α c with temperature. Various possible reasons for this behaviour, such as adsorption and absorption of hydrogen, mechanism change and impurity effects, are ruled out on the grounds of experimental findings, leading to changes in the symmetry factor, β, as the probable reason. Following the Conway approach, numerical values of the enthalpic and entropic part of the symmetry factor are evaluated.

Underpotential deposition of hydrogen on MoPt4 intermetallic phase in acid solution: temperature dependence

Abstract Potentiodynamic studies of the underpotential deposition of hydrogen (Hupd) on MoPt4 electrode in 0.5 mol dm −3 HClO4 aqueous solution in the range of temperature from 275 to 312 K are made and thermodynamic state functions for the hydrogen adsorption are determined. Theoretical treatment of the experimental results is derived from Hupd electrochemical adsorption isotherm. It is determined that Δ G ads θ (H upd ) , as a function of temperature, varies with the surface coverage from ∼−20 kJ mol −1 (θ=0) . The increase of Δ G ads θ (H upd ) with the surface coverage indicates the repulsive interactions between Hupd adatoms. From the temperature dependence of the Gibbs energy of adsorption, the enthalpy and entropy of adsorption are calculated. The values of these functions are determined to be Δ H ads (θ=0) θ =4.5 kJ mol −1 and Δ S ads (θ=0) θ =82 J mol −1 K −1 . The value of ΔHadsθ allows determinations of the bond energy between electrode surface and Hupd, which is found to be E M–H =213 kJ mol −1 for θ=0. The lateral repulsion interactions are the reasons why M–Hupd bond energy decreases significantly with the increase of coverage so the saturation coverage is much less than 1 in the UPD potential region.

Comparative Potentiodynamic Study of Nickel and Hydrogen Underpotential Deposition at Polycrystalline Platinum Electrode in Weak Acid and Neutral Solutions

Underpotential deposition of nickel and hydrogen on polycrystalline platinum in weak acid and neutral solutions (3.5 ≤ pH ≤ 7.0), with and without Ni2+-ions, has been examined using cyclic voltammetric technique in the range of temperature from 274 to 313 K. The nickel and hydrogen ad-atom surface coverages were calculated from the voltammetric adsorption and desorption charges. The Temkin isotherm was fitted for both underpotential depositions, and thermodynamic adsorption parameters were calculated for both atoms. The value of the bonding energy between hydrogen and surface in the presence of underpotentially deposited nickel was calculated and found to amount to 287 kJ mol-1, which is 40 kJ mol-1 stronger bond than the same of pure platinum. The presence of nickel on the surface facilitates interfacial transfer of hydrogen into the metal bulk and consequent hydride formation significant in design of hydrogen based fuel cells.