Influence of inhibitor structure and metal/solution interface on the corrosion resistance of the protected metal

Seven systems of more than 60 compounds with possible inhibiting properties are investigated. Several methods are used: electrochemical, gravimetric, XPS, and SEM analyses. The inhibition efficiency Z is related to the chemical structure of inhibitors (sequences of compounds with regard to Z are found), their electronic structure, the surface area of the inhibiting molecule, and the structure and composition of the metal/solution interface (impedance, adsorption equilibrium parameters, etc.). The most efficient of the investigated inhibitors have Z = 94–99%. Conclusions are drawn which allow identification of compounds with prognosticated inhibiting action. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1352–1364. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Study on the preparation and structure of positive sol composed of mixed metal hydroxide

Using precipitation method mixed metal hydroxide (MMH) positive sol was prepared. The preparation process and the properties of the sol were studied with powder XRD, TEM, particle size distribution determination system and microelectrophoresis instrument.The preparation of MMH was made as follows: Diluted ammonia water was added to the mixed solution of aluminum and magnesium chlorides which was prepared in the molar ratio of 1 1 or 1 2, or 1 3; then the pH values of the suspension at different amounts of ammonia water were measured. After that, the precipitate was aged for 5 h in the mother solution at room temperature, and washed after filtering. Finally, the filter cake was peptized at constant temperature of 333 K.The results showed that 1) preparation reaction was completed in three steps, 2) pH value was decisive factor, and 3) both the contents of magnesium and the -potential of MMH sol particles increased with pH values and finally remained constant. The mechanism of the reaction was that magnesium ions intercalated Al(OH)₃ crystal lattice, forming mixed metal hydroxide. The results also showed that positively charged MMH colloidal particle belonged to hexagonal system and three-layer superposition structure.     

Weak alkali and alkaline earth metal complexes of low molecular weight ligands in aqueous solution

This work is aimed at reviewing the chemical literature dealing with thermodynamic aspects of the weak complex formation (species with log K values less than about 3) between alkali and alkaline earth metal ions with low molecular weight inorganic and organic ligands in aqueous solution. The following ligands (up to hexavalent anions) were examined in detail: (i) hydroxide, chloride, sulfate, carbonate and phosphate as inorganic, and (ii) carboxylates, amines, amino acids, complexones and nucleotides as organic ligands. The paper also identifies the main reasons responsible for the dispersion of the stability data on ion pairs in the literature. When possible, the trend of stability for the different metal ions interacting with the same ligand will be considered to find predictive interaction relationships. Since the stability of weak alkali and alkaline earth metal complexes are mainly due to electrostatic interaction, simple empirical relationships were obtained between log K and the charge of the anionic ligand. The interest for alkali and alkaline earth cations rises since they are used in study of basic science as components of the supporting electrolyte and are widely diffused in natural fluids. Some examples of application of this science were presented too, to show the role of weak complex formation in the modelling process of natural systems.     

Phosphomolybdic acid supported on silica gel and promoted with alkali metal ions as catalysts for the esterification of acetic acid by ethanol

Different ratios of phosphomolybdic acid PMA supported on silica gel (1–30 wt%) and promoted with alkali metal hydroxide have been prepared by an impregnation method and calcinated at 350 °C for 4 h. The catalysts were characterized by thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction, FT-IR spectroscopy and N₂ adsorption measurements. The surface acidity and basicity of the catalysts were determined by adsorption of pyridine and the dehydration–dehydrogenation of 2-propanol. The gas-phase esterification of acetic acid by ethanol was carried out in a conventional flow bed reactor. The results clearly revealed that among the PMA loading, the use of 10 wt% catalyst showed maximum yield of ethyl acetate. This catalyst also improved on addition of Na or K-hydroxide. These results were correlated with the structure and the acid–base properties of the prepared catalysts.     

Enhancement of chemoselectivity in epoxidation reactions over TS-1 catalysts by alkali and alkaline metal ions

Alkali metal ions, when present during the synthesis of TS-1, lead to inactive oxidation catalysts. However, when added, in small amounts, to the reaction medium during the epoxidation of allyl alcohol or allyl chloride by H₂O₂ over TS-1, they increase the selectivity for the epoxide. To probe this phenomenon in detail, the influence of pH and alkali and alkaline earth ions on the structure and catalytic activities of the oxo-Ti species generated in H₂O₂. TS-1 and TiMCM-41 systems have been investigated using EPR and diffuse reflectance UV-Vis spectroscopies. In acidic and neutral pH, two types of superoxo-Ti species (Ti(O₂√⁻)), A and B, are observed over TS-1. In the presence of alkali and alkaline metal ions or at high pH, Ti³⁺ ions and a new type of Ti(O₂√⁻) species, A′ are observed. Only the B-type species, however, is observed on TiMCM-41. The A-type are more reactive than the B-type. Epoxide selectivities approaching 100% can be achieved by a proper control of the reaction medium.     

State of the art in the electrochemical characterization of the surface structure of shape-controlled Pt,Au, and Pd nanoparticles

The application of shape-controlled metal nanoparticles in electrocatalysis has improved significantly the activity, selectivity, and even the stability of many relevant electrocatalytic reactions. It is well accepted that, by controlling the shape of the nanoparticles, it is possible to provide nanoparticles with a preferential surface structure. However, to fully understand the capabilities of these nanomaterials, it is extremely relevant to correlate shape, surface structure, and electrocatalytic reactivity. Particularly, establishing the correlations between surface structure and reactivity is the key point to be studied and understood. Consequently, having tools to characterize the surface structure of these nanoparticles is of critical importance. In this short review, we discuss about the progress in the in situ characterization of the surface structure of shaped Pt, Au, and Pd nanoparticles by electrochemical probes. The results here included clearly demonstrate the potentialities of the electrochemical tools to gain detailed information of the surface structure of these shaped nanomaterials.     

The role of the solvation shell decomposition of alkali metal ions in their selective complexation by resorcinarene and its cavitand

2-Methylresorcinarene and its methylene-bridged cavitand derivative as host compounds were investigated in selective complexation of alkali metal ions as guests in methanol media by photoluminescence measurements. These host molecules possess either flexible (2-methylresorcinarene) or rigid (cavitand) molecular skeleton. The Benesi–Hildebrand method and the van't Hoff theory have been applied to determine the stability constants and the thermodynamic parameters, respectively. Considerable interactions between 2-methylresorcinarene and Li⁺ or Na⁺ ions have been observed while the rigid cavitand derivative can interact only with K⁺ or Cs⁺ ions. Neither the complexes of 2-methylresorcinarene with K⁺ or Cs⁺ nor those of the cavitand derivative with Li⁺ or Na⁺ ions are stable at room temperature in methanol media. Quantum-chemical investigations justified that only solvated Li⁺ and Na⁺ ions can form stable complexes with 2-methylresorcinarene while unsolvated K⁺ and Cs⁺ ions form stable complexes with the methylene-bridged cavitand. These results highlight that the stability of the guest solvation shell and its size could play a key role in the selectivity behaviour of host molecules.     

Influence of 5-chloro and 5-methyl benzotriazole on the corrosion of copper in acid solution: an experimental and a theoretical approach

The inhibition of copper corrosion in aerated 0.1 mol l⁻¹ hydrochloric acid solutions was studied using electrochemical polarization in the presence of different concentrations of benzotriazole and its two derivatives, 5-chloro and 5-methyl benzotriazole. The inhibition efficiencies obtained from cathodic Tafel plots increased markedly with increase in the additive concentration. Benzotriazole and 5-methyl-benzotriazole were found to be cathodic type corrosion inhibitors for concentrations higher than 10⁻⁴ mol l⁻¹ . However, the 5-chloro-benzotriazole was found to be a mixed inhibitor for concentrations up to 10⁻³ mol l⁻¹, above this concentration the inhibitor behaves as an anodic type inhibitor. The inhibitors are physisorbed on the copper surface following a Langmuir’s isotherm. The inhibition efficiencies depended on the inhibitor concentration and follows the order 5-chloro-benzotriazole > 5-methyl-benzotriazole > 1-H-benzotriazole. From the theoretical calculations, the change in the inhibition mechanism observed for 5-chloro-benzotriazole at concentrations higher than 10⁻³ mol l⁻¹ is associated with the electronic acceptor characteristic of chloro, which increases the benzotriazole acidity allowing the formation of CuBTA.     

Dynamics of the filling of a porous cathode by the deposited metal: Modeling the process and analyzing the case of high cathode conductance and low solution depletion degree

A dynamic model for a porous electrode is designed on the basis of a one-dimensional representation of the electrode in the form of parallel filaments. The method takes into account the alterations in the local values of the filament diameter (and, correspondingly, in the effective conductances of phases), porosity, the velocity of a linear flow, and the mass transfer coefficient for the deposited metal ions, which occur in the course of the metal electrodeposition. For the simplest version of dynamics, at a high initial conductance of the electrode and a small solution depletion degree, the method predicts the following specific features: (i) the development of the working surface area and an increase in the current efficiency for the metal associated with it, (ii) a decrease in the metal penetration depth into the electrode with time and the metal localization near the most loaded end, and (iii) an irregular change in the current efficiency and concentration of the metal at the exit out of the electrode.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 333–342.Original Russian Text Copyright © 2005 by Maslii, Poddubnyi, Medvedev.     

Role of the surface heterogeneity in adsorption of hydrogen ions on metal oxides: theory and simulations

In this article we study the effect of energetic heterogeneity of a crystalline surface on the adsorption of hydrogen ions (protons) from the liquid phase. In particular, we examine the influence of the shape of the adsorption energy distribution on the equilibrium isotherms of hydrogen ions. To that purpose, a few popular distribution functions, including rectangular, exponential, and asymmetric Gaussian are considered. Additionally, multimodal distribution functions, which may correspond to the adsorption on different crystal planes of the oxide, are also used. Lateral interactions between adsorbed charges are modeled using the potential function proposed by Borkovec et al., which accounts also for polarization of the liquid medium. The results presented here are obtained using both Monte Carlo (MC) simulations and theoretical calculations involving Mean Field Approximation (MFA). They indicate that increased energetic heterogeneity of the adsorbing surface may, in general, considerably change the behavior of the adsorption isotherms, regardless of the assumed distribution function. It is also shown that the predictions of the proposed theory are consistent with the data obtained from the MC simulations.     

Visible spectrophotometric determination of metal ions: the influence of structure on molar absorptivity value of copper(II) complexes in aqueous solution

In this study, the structure-absorptivity relationship problem is dealt with, on the basis of a considerable mass of data (117 complexes) concerning equatorial complexes of copper(II) in aqueous solution. Some spectrophotometric measurements have been carried out and as well as some molar absorptivity calculations on copper(II)-ammonia (ε₆₄₀=77 M⁻¹ cm⁻¹ for the Cu(NH₃)₄²⁺ species) and copper(II)-biuret (ε₅₀₅=46 M⁻¹ cm⁻¹ for the Cu(C₂H₃N₃O₂)₂²⁻ species) systems have been made, in order to complete the quantitative information necessary for the discussion. It was found that the qualitative relationship between ε_max and λ_max, for the complexes in question is traceable back to a hyperbolic type of path; the dispersion of the data in the cartesian plane (ε_max, λ_max) is analyzed and discussed. It has been found that the dissociated peptide nitrogen is, amongst those under study (namely: amino nitrogen; peptide nitrogen; imidazole nitrogen; carboxylate oxygen; alcoholate oxygen; hydroxide oxygen; water oxygen), that which corresponds to the major increase of ε_max for copper(II). On the contrary, the one which corresponds to the minor increase is the imidazole donor (that is pyridine nitrogen of imidazole residue), which manifests the ability to lower the ε_max values of the equatorial complexes of copper(II).Moreover, contrary to what has been previously demonstrated for λ_max, for ε_max it does not appear to be correct to schematize the ligand with its donor groups in order to assess spectrum-structure correlations. Consequently, this study contains a type of methodological proposal, even beyond the metal ion specifically in question, for the analyses of the selection criteria of the molecular characteristics that a ligand must possess in order to form a co-ordination compound of analytical interest with a determined metal ion.     

Hydrolytic condensation of trialkoxysilanes in the presence of alkali metal and copper(ii) ions. Influence of the reaction conditions on the structure of Cu/M organosiloxanes

The formation of polyhedral copper/sodium(potassium) organosiloxanes was examined as a result of hydrolytic condensation of organotrialkoxysilanes in the presence of copper(ii) and sodium or potassium ions. High selectivity of the synthesis of copper/sodium(potassium) organosiloxanes having desired structures can be achieved by choosing the reaction conditions.     

Dynamics of variations in the metal deposit distribution in porous flow-through electrodes: Effect of solution flow velocity and direction

Dynamics of variations in the metal deposit distribution in a porous flow-through electrode (PFE) and a number of integral indicators of the process are studied as a function of the velocity of solution flow and the direction of its supply with the aid of a dynamic model for PFE. It is established that qualitative character of variations in the above parameters with time depends on which of two factors (distribution of polarization or concentration of metal ions) predominantly defines the metal electrodeposition process inside PFE. It is shown that employing reverse of solution flow through PFE with the aim of increasing the weight of metal deposited in it and the uniformness of its distribution gives positive effect only in the case where the metal distribution for opposite supply directions is different.     

Comments on the paper of A.T. D'Agostino and P.N. Ross: “on the importance of surface structure in electrochemistry: the well-defined Au (111) single crystal electrode surface”

The electroreduction of dimethylpyrrolidinium (DMP⁺) in diglyme on mercury cathodes was investigated. It was found to proceed via a fast one-electron transfer resulting in an insoluble “amalgam”. The DMP⁺ amalgam was reactive and transferred electrons to suitable substrate with regeneration of DMP⁺. Thus DMP⁺ could be used as a catalyst for electroreduction and this possibility was studied. It was found to be a suitable catalyst for the reduction of several difficult-to-reduce substrates (E_1/2 < ?2.8 V vs. SCE). Among others it effected the reduction of fluorobenzene, an aliphatic chloride, and simple conjugated dienes. A possible mechanism and the utility of the reaction are discussed.     

Effect of rate and direction of solution flow on metal deposition in porous electrodes: The final weight and distribution of the deposit

Using an earlier-developed dynamic model for a porous flow-through electrode (PFE) with a high initial conductivity, the effect of the solution’s flow rate (0.05–10 cm/s) and direction on the final metal weight m _f and uniformity of the metal distribution in the porous matrix is studied. It is found that m _f increases with increasing flow rate. However, the dependence is nonmonotonic: it peaks at intermediate flow rates. The peak is most pronounced in the case of rear supply. At high and very low flow rates, m _f is independent of the flow direction. In the first case, the metal distribution profiles almost coincide, while in the second case they are mirror-opposite. The deposit weight correlates well with the index of uniformity of its distribution: all other factors being equal, the more uniform the deposit distribution in PFE, the larger the m _f. These effects are explained by taking into account the joint effect of profiles of cathodic polarization and concentration of metal ions in PFE.     

Tantalum oxide effect on the surface structure and morphology of the IrO<Subscript>2</Subscript> and IrO<Subscript>2</Subscript> + RuO<Subscript>2</Subscript> + TiO<Subscript>2</Subscript> coatings and on the corrosion and electrochemical properties of anodes prepared from these

Alterations in the phase composition, porosity, and surface morphology of coatings are examined following the insertion of a quantity of Ta₂O₅ into active coatings prepared from IrO₂ or IrO₂ + RuO₂ + TiO₂ (OIRTA). It is shown that even an insignificant concentration of Ta₂O₅ in a coating renders it substantially amorphous and leads to the appearance of a large number of wide protracted cracks in the coating. The latter extends the surface of anodes and boosts their apparent catalytic activity in the chlorine evolution reaction. In addition, this accelerates the diffusion of chloride ions toward the front surface of anodes, which noticeably reduces the overvoltage of the chlorine evolution reaction when manufacturing sodium chlorate. The coatings’ amorphization and the development of their surface substantially reduce the corrosion resistance of these anodes as compared with OIRTA.     

Chelate and intramolecular ion-pair formation: A guiding concept for structure/selectivity relationships in the liquid-liquid extraction of alkali and alkaline earth metal ions by anionic crown ether derivatives

Crown ether dyes with pendent anionic side-arms were synthesized for extractionspectrophotometry of alkali and alkaline earth metal ions. Dramatic changes in metal selectivity were obtained simply by changing the nature of the anionic side-arm on the same crown ether skeleton. A structure/metal selectivity relationship is discussed in detail in terms of “chelate” and “intramolecular ion-pair” formation. Small metal cations (high charge density) are preferred in the extraction by a crown ether reagent with a charge-localized anionic side-arm through the formation of a “chelate”. Large metal cations (low charge density) are preferred in the extraction by reagents with a charge-delocalized anionic side-arm through the formation of an “intramolecular ion-pair”. Steric restrictions imposed by the side-arm on the metal ion approaching the crown ether are also important factor in controlling the selectivity of these reagents.     

Chelation of vanadium(IV) by a natural and edible biopolymer poly(gamma-glutamic acid) in aqueous solution: structure and binding constant of complex

The naturally occurring edible biopolymer poly(gamma-glutamic acid) (gamma-PGA) is shown to be an efficient chelating agent of vanadium(IV). The structure of poly(gamma-glutamic acid)oxovanadium(IV) (VO-gamma-PGA) complex in solution has been analyzed by electron spin resonance and UV-visible absorption spectra. The equatorial coordination sphere of vanadium(IV) is proposed to be [2 x carboxylate (2O)-VO-(OH2)2]. The binding isotherm is determined for suspensions of gamma-PGA in vanadium(IV) oxide sulfate (VS) solutions of different concentrations, and the data have been adjusted to fit the modified Langmuir equation. The maximum amount of vanadium bound per gram of gamma-PGA is estimated to be 141 mmol . g(-1) with a binding constant of 22 L . g(-1) at pH 3.     

Formation of metal complex ions from amino acid in the presence of Li+, Na+ and K+ by electrospray ionization: metal replacement of hydrogen in the ligands

Alkali metal cations easily form complexes with proteins in biological systems; understanding amino acid clusters with these cations can provide useful insight into their behaviors at the molecular level including diagnosis and therapy of related diseases. For the purpose of characterization of basic interaction between amino acids and alkali metal, each of the 20 naturally occurring amino acids were ionized in the presence of lithium, sodium and potassium cations by electrospray ionization, and the resulting product ions were analyzed. We focus our attention on the gas phase alkali metal ion-proton exchanged complexes in current study, specifically complexes with serine, threonine, asparagine and glutamine, which share characteristic pattern unlike other amino acids. All amino acids generated [M + H](+) and [M + Na](+) ions, where M stands for the neutral amino acid. Serine, threonine, asparagine and glutamine generated cluster ions of [nM - nH + (n + 1)Na](+) and [nM - (n - 1)H + (n - 1)Na + K](+) , where n = 1-7. While the (M - H + Li) and (M - H + K) species were not observed, the neutral (M - H + Na) species formed by proton-sodium cation exchange had a highly stable cyclic structure with ketone and amine ligand sites, suggesting that (M - H + Na) serves as a building block in cluster ion formation. Cluster ion intensity distributions of [nM - nH + (n + 1)Na](+) and [nM - (n - 1)H + (n - 1)Na + K](+) showed a magic number at n = 3 and 4, respectively. Extensive B3LYP-DFT quantum mechanical calculations were carried out to elucidate the geometry and energy of the cluster ions, and they provided a reasonable explanation for the stability and structure of the cluster ions.     

Changes in the surface structure of Pd/Ta2O5 by oxygen and CO studied using X‐ray Photoelectron Spectroscopy (XPS)

Behaviors of Pd structures with different thicknesses supported by Ta₂O₅/Ta in the reaction with oxygen and CO were studied by XPS and SEM. For the samples with a Pd thickness of 3 nm, a new low‐binding‐energy component appeared in the Pd 3d level upon O₂ exposure at ～200 °C and was reduced in intensity after a subsequent CO exposure at 150 and 200 °C. The change in the Ta 4f state could also be found upon oxygen and CO exposure, indicating that both Pd and the Ta‐oxide substrate participate in the chemical reactions. For the sample with a higher Pd thickness, a positive shift in the Pd 3d level due to the oxidation of Pd was observed after exposure to O₂ at a higher temperature (280 °C). A subsequent CO exposure at ～150 °C could not reduce Pd‐oxide layers, as confirmed by the unchanged Pd 3d spectra after CO treatment, i.e. Pd‐oxide was not reactive for CO oxidation. Copyright © 2010 John Wiley & Sons, Ltd.