Adsorption of 1-pentanol on bismuth single-crystal plane electrodes

Cyclic voltammetry, impedance and chronocoulometry have been employed for the quantitative study of 1-pentanol (n-PenOH) adsorption at the bismuth single-crystal plane | aqueous Na₂SO₄ solution interface. The adsorption isotherms, Gibbs energies of adsorption ΔG _A ^∘, the limiting surface excess Γ_max and other adsorption parameters, dependent on the crystallographic structure of the electrodes, have been determined. The adsorption of n-PenOH on Bi single-crystal planes is mainly physical and is limited by the rate of diffusion of organic molecules to the electrode surface. Comparison of the adsorption data for n-PenOH with 1-propanol (n-PrOH), 1-butanol (n-BuOH), cyclohexanol (CH) and 1-hexanol (n-HexOH) shows that the adsorption characteristics depend on the structure of the hydrocarbon group. The adsorption activity of adsorbates at the bismuth | solution interface increases in the sequence n-PrOH < n-BuOH < CH ≤ n-PenOH < n-HA as the adsorption activity at the air | solution interface increases. For all the compounds studied, the adsorption activity increases in the sequence of planes (111)<(001)<(011ˉ). Received: 1 July 1998 / Accepted: 2 October 1998     

Adsorption Kinetics of Normal-Heptanol on the Bismuth Single Crystal Planes

Cyclic voltammetry and impedance methods are employed for a quantitative study of normal-heptanol (n-HepOH) adsorption kinetics at the bismuth single-crystal plane/aqueous Na₂SO₄ solution interface. The results of nonlinear regression analysis show that the Frumkin–Melik-Gaikazyan (FMG) or Frumkin–Melik-Gaikazyan–Randles (FMGR) equivalent circuits can be used for the simulation of experimental impedance data. The dependences of adsorption capacitance (caused by the potential dependence of surface coverage), Warburg diffusion impedance, and adsorption resistance on the electrode potential and organic-compound concentration are established. Analysis of impedance data demonstrates that the adsorption of n-HepOH is mainly limited by the rate of diffusion of organic compound to the electrode surface. Small deviations toward mixed adsorption kinetics are established at very high frequencies. In the region of maximum adsorption in more concentrated n-HepOH solutions, the slow reorganization or two-dimensional association of adsorbed molecules is possible. However, the very low adsorption or partial charge transfer resistance values indicate that then-HepOH adsorption at Bi planes is a practically reversible process and thus there is no noticeable partial charge transfer between adsorbed n-HepOH molecules and Bi surface atoms.     

Adsorption of alkanes on a bismuth electrode from an alcoholic medium

The adsorption of higher alkanes (from C₁₀H₂₂ to C₂₁H₄₄) on a polarized bismuth electrode was studied from methanolic, ethanolic and n-butanolic solutions of LiClO₄ by differential capacity measurements. Alkane molecules, which do not contain either polar groups or double bonds, are compounds of low chemical activity. The adsorption of this group of compounds is purely physical. Nevertheless, a considerable difference in their adsorption activity on the various areas of the electrode surface caused by the two types of bond (metallic and covalent) between the atoms in the bismuth crystal lattice was discovered. Such a large difference indicates the different wettabilities of the separate single-crystal faces by alkanes. A weak dependence of the interaction parameter in the Frumkin isotherm on the length of the alkane molecule as well as on the length of the hydrocarbon chain of the solvent molecule was established. Various adsorption effects at high bulk concentrations of adsorbate (desolvation of the cations of the supporting electrolyte, thickening of the adsorption layer, maximum in the isotherm) were analysed.     

Estimation of the specific adsorption of I−, Br−, and Cl− ions in systems (Tl-Ga)/[N-methylformamide + 0.1m M KA + 0.1(1 − m) M KClO4] (KA is KCl, KBr, and KI) based on the analysis of two-dimensional pressure curves obtained by integration of differential capacitance

By means of an ac bridge, the differential capacitance vs. potential curves are measured in systems (Tl-Ga)/[N-MF + 0.1m M KI + 0.1(1 ? m) M KClO₄], (Tl-Ga)/[N-MF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (Tl-Ga)/[N-MF + 0.1m M KCl + 0.1(1 ? m) M KClO₄] for the following fractions m of the surfaceactive anion: 0, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. Based on the analysis of curves of two-dimensional pressure found by integrating the differential capacitance, it is shown that the data on the specific adsorption of anions I^?, Br^?, and Cl^? in the mentioned systems can be quantitatively described by the Frumkin isotherm. The main adsorption parameters of I^?, Br^?, and Cl^? anions at the (Tl-Ga)/N-MF interface are determined. It is found that on the (Tl-Ga)/N-MF interface, the same as on the (In-Ga)/N-MF interface, the adsorption energy of ions increases in the sequence Cl^? < Br^? < I^?, in contrast to the Ga/N-MF interface, where the energy increases in the reverse sequence: I^? ≈ Br^? < I^?. For all halide ions (Hal^?), the adsorption energy and the energy of metal-Halinteraction increase in the sequence (Tl-Ga) < (In-Ga) < Ga.     

Determination of porphyrin dimer in a mixed monolayer of porphyrin/phospholipid transferred on ITO electrodes

The Langmuir–Blodgett method has been used to transfer mixed monolayers of a porphyrin (TMPyP) and a phospholipid (DMPA) from the air|water interface onto optically transparent indium–tin oxide (ITO) electrodes. The surface concentration of porphyrin, Γ, transferred on the ITO surface, has been obtained by integration of the reduction current from the cyclic voltammograms. The experimental Γ values ranged from Γ_m=5.19×10⁻¹¹ mol cm⁻², and corresponding to a compact monolayer of porphyrin monomers in a plane orientation with respect to the surface, to Γ_d=8.65×10⁻¹¹ mol cm⁻², and equivalent to the total amount of the porphyrin molecules at the air|water interface under a compression of 35 mN m⁻¹. Prior to the electrochemical experiments, the transmission spectrum was recorded. The surface concentration obtained of the porphyrin is not directly proportional to the transmission of the film, ΔT. This phenomenon is assigned to the dimer formation and, depending on the surface activity of the ITO electrodes, toward porphyrin adsorption. The dimer structure of TMPyP on an intact ITO electrode is altered with respect to that found at the air|water interface. A simple model has been developed to evaluate the contribution of monomer and dimer phases of the porphyrin in the mixed monolayer. Furthermore, spectroscopic measurements with linearly polarized light under oblique incidence have been performed in order to infer the plane orientation of the TMPyP molecules with respect to the ITO surfaces.     

A preliminary investigation of ionic adsorption at the mercury—hexamethyl phosphoric triamide interface

A reference electrode is described that is reversible and reproducible in hexamethyl phosphoric triamide; it was used to study the adsorption of 1 : 1 electrolytes at the mercury—solution interface. The differential capacity curves show features similar to those found for water and the degree of specific adsorption of the anions follows the same sequence: PF^?₆ < BF^?₄, ClO^?₄ < Br^?. There appears to be no specific adsorption of the alkali metal cations at negative charge values whereas the tetraalkyl ammonium ions are strongly adsorbed.     

Dynamic surface tension and dilational viscoelasticity of adsorption layers of alkylated chitosans and surfactant–chitosan complexes

The kinetics of the adsorption at the air-water interface and the processes of the structure formation inside the adsorption layers of hydrophobically modified systems [alkylated chitosans and sodium dodecyl sulfate (SDS)–chitosan (Ch) complexes] have been studied by the tensiometric method based on the axisymmetric rising-bubble-shape analysis as a function of the bulk concentration of polymers and the ageing time of their adsorption layers. The kinetics of the adsorption of chitosan, alkylated chitosans (ChC₃, ChC₈, and ChC₁₂), and surfactant–polyelectrolyte (PE) complexes formed by the chitosan and the polysoaps with oppositely charged anionic surfactant SDS is characterized by an induction time (the so-called lag time), τ_lag, corresponding to the diffusion stage of the formation of adsorption layers. During this time, the decrease in the surface tension (or the increase in the surface pressure π) does not exceed several millinewtons per meter that corresponds to the “gaseous” state of adsorption layers. The postlag stage of the formation of the adsorption layer is characterized by the remarkable rate of increase in the surface pressure π that corresponds to the conformational rearrangement of PEs inside the adsorption layer by increasing the number of hydrophobic groups (adsorbing centres) in contact with the non-polar phase at the interface. It has been found that during the lag time, the adsorption of alkylated chitosans (cationic polysoaps) increases with increasing alkyl chain length, whereas during the postlag time, the adsorption of the ChC₃ is maximal with regard to other polysoaps. It has been confirmed that at equal content of alkyl groups in the system, the surface activity of the SDS–Ch complexes is much higher with regard to that of the polysoaps. The viscoelasticity of adsorption layers of individual PEs and their complexes continuously increases with the ageing time, giving evidence for the interaction between the polymers inside the adsorption layers. It has been found that the rate of increase in the dilational storage module E′ of the adsorption layers of SDS–Ch complexes is much higher than for the polysoaps that correlates with the higher surface activity of the former with regard to the latter. For the mentioned systems, the module E′ is much higher than the loss module E″ that confirms the solid-like properties of their adsorption layers. On the other hand, the adsorption layers of the chitosan are liquid-like, while E′<<E′′.     

Adsorption of Cl−, Br−, and I− ions from 0.1 M solutions in methanol on Ga, (In-Ga), and (Tl-Ga) electrodes

Adsorption of Cl^?, Br^?, and I^? (Hal^?) ions from 0.1 M solutions in methanol (MeOH) is studied on the liquid renewable Ga, (In-Ga), and (Tl-Ga) electrodes by the methods of differential capacitance and jet electrode. It is shown that the adsorption parameters and the series of surface activity of halide ions in MeOH essentially depend on the metal nature. On the (In-Ga) and (Tl-Ga) electrodes, as well as on the Hg electrode, the surface activity of halide ions increases in the series: Cl^? < Br^? < I^?; on the Ga electrode, it varies in another series: Br^? < Cl^? < I^?. The data for the Ga/MeOH interface support the result, which was first obtained on the Ga/N-methyl formamide (N-MF) interface, that the effect of inversion of surface activity series can be observed not only in the aprotic solvents, but also in the protic solvents. The data, which were obtained in MeOH, are compared with the corresponding data, which were obtained in N-MF, dimethyl formamide (DMF), acetonitrile (AN), and water. For Ga, (In-Ga), and (Tl-Ga) electrodes, the adsorption of Hal^? varies in the series: H₂O < MeOH ≈ N-MF < DMF < AN. The data obtained in MeOH indicate that the energy of metal-Hal^? interaction (ΔG _M-Hal) increases in the series (Tl-Ga) < (In-Ga) < Ga as the electronic work function increases. This is in agreement with the data, which were obtained in other solvents, and is the evidence for the donor-acceptor nature of metal-Hal^? interaction, where the Hal^? ions are the donors of electron pair with respect to the metal.     

Estimating of surface activity of Cl− and Br− ions in the (In-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4] and (In-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] systems by different methods

Differential capacitance curves in the (In-Ga)/[N-methylformamide + mc KCl + (1 ? m)c KClO₄] and (In-Ga)/[N-methylformamide + mc KBr + (1 ? m)c KClO₄] systems are measured using an ac bridge for the following molar portions m of the surface-active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The Cl^? and Br^? anions specific adsorption in the systems can be described quantitatively by the Frumkin isotherm. The principal parameters of Cl^? and Br^? anions adsorption at the (In-Ga)/N-methylformamide interface are determined by different methods. Unlike Ga/N-methylformamide interface, where the adsorption energy increased in the sequence I^? ≈ Br^? < Cl^?, at the (In-Ga)/N-methylformamide interface it increased in the reverse sequence: Cl^? < Br^? < I^?. The adsorption parameters at the charge density q = 0, obtained by three different methods, are close to each other. However, the parameters α₁ and α₂, which characterize the charge effect on the adsorption energy, when determined by the analyzing of dependences of adsorption potential drop E _ads on ln(mc), differ from those determined by two other methods. The error may be caused by the assuming that the adsorption potential drop is proportional to the coverage of dense layer with the specifically adsorbed ions.     

Study of adsorption of tetraethylammonium ions on Bi single crystal planes from solutions in ethanol

The adsorption of tetraethylammonium (TEA⁺) ions on the (001) and planes of the bismuth single crystal from solutions in ethanol has been investigated by impedance measurement method. The experimental data were obtained in 0.02 M mixed-electrolyte solutions and the calculations performed with electrode potential as the independent electrical variable. The Gibbs energy of adsorption of TEA⁺ ions has been calculated using the simple virial adsorption isotherm, and it was found that the adsorption of TEA⁺ cations is weaker than the adsorption of halide anions. The electrosorption valency evaluated has a nearly constant value in the potential region studied. It was concluded that the formed effective surface dipole is significantly screened by the solvent molecules and the metal electron gas. The analysis of the impedance spectra was performed by fitting the experimental data to the various equivalent circuits. It was found that the behaviour of TEA⁺ ions at Bi(hkl)∣ethanol interface can be described with the equivalent circuit, corresponding to the classical Frumkin–Melik-Gaikazyan model. The results obtained indicate that only weak interaction between TEA⁺ ions and bismuth surface takes place, and there is no remarkable partial charge transfer from the adsorbed ions to the Bi surface atoms.     

Effect of Bismuth Additives on the Properties of Vanadium–Phosphorus Oxide Catalyst in the Partial Oxidation of n-Pentane

The selective oxidation of n-pentane on vanadium–phosphorus oxide (VPO) catalysts with bismuth additives (Bi/V = 0–0.30) is studied. The catalysts are characterized by XRD, XPS, and specific surface area measurements using nitrogen adsorption. Their acidic properties are studied (using ammonia TPD and the 2-methyl-3-butyn-2-ol reaction). It was found that the introduction of bismuth insignificantly affects the specific surface area but increases the surface concentration of phosphorus and changes the acidic properties of the catalysts. The specific catalytic activity of samples in n-pentane oxidation correlates with the effective charge of surface oxygen (E _b of O1s electrons). The selectivity to citraconic anhydride increases with an increase in the general surface acidity. The selectivity to maleic anhydride increases with an increase in the Brønsted acidity of the surface. The selectivity to phthalic anhydride increases with an increase in the Lewis acidity. The pathways of product formation in the partial oxidation of n-pentane are proposed.     

Synergetic effect in PdAu/CeO<Subscript>2</Subscript> catalysts for the low-temperature oxidation of CO

Gold-palladium catalysts supported on cerium oxide were synthesized with the double complex salts. X-ray photoelectron spectroscopy (XPS) and other physicochemical methods (TEM, TPR) were used to demonstrate that synthesis of highly active palladium catalysts requires the oxidative treatment stimulating the formation of a catalytically active surface solid solution Pd _x Ce_1?x O₂, which is responsible for the lowtemperature activity (LTA) in the reaction CO + O₂. In the case of gold catalysts, active sites for the lowtemperature oxidation of CO are represented by gold nanoparticles and its cationic interface species. Simultaneous deposition of two metals increases the catalyst LTA due to interaction of both gold and palladium with the support surface to form a Pd_1?x CexO₂ solid solution and cationic interface species of palladium and gold on the boundary of Pd-Au alloy particles anchored on the solid solution surface.     

The surface adsorption and micelle formation of the mixed aqueous solutions of fluorocarbon and hydrocarbon surfactants: II. Sodium perfluorooctanoate-sodium decylsulfate system

The adsorption and micellar behavior of the mixed solutions of sodium perfluorooctanoate (7CFNa) and sodium decylsulfate (C₁₀SNa) have been studied at constant ion strength of 0.1m. The adsorption was calculated from the surface (and interfacial) tension-concentration curves by applying Gibbs equation. It was found that the cmc's of 7CFNa and C₁₀SNa are of nearly the same value (1.66 × 10⁻²m and 1.45 × 10⁻²m, respectively), but γ_cmc of 7CFNa solution is ∼23 mNm⁻¹, which is much lower than that of C₁₀SNa solution. This implies that 7CFNa would have a much higher surface activity than C₁₀SNa and be adsorbed preferentially; for instance, 7CFNa has a surface mole fraction of about 0.8 in the saturated adsorption layer of the 1:1 mixed solution. The mole fractions of 7CFNa at the surface are always greater than those in the bulk solutions. The adsorption at the n- heptane-aqueous solution interface is quite different from that at the air-solution surface. Here C₁₀SNa is preferentially adsorbed owing to “Mutual phobicity” between the HC-chain of n-heptane and the FC-chain of 7CFNa at the interface. All the cmc's obtained from the γ-log m relations of 7CFNa (or C₁₀SNa) in the mixed solutions have nearly the same value and the cmc-x curves show a positive deviation from the ideal case. This further indicates that in the mixed solutions of fluorocarbon and hydrocarbon surfactants no completely miscible micelle but essentially the individual micelle of each surfactant exist due to the “Mutual phobicity” between FC- and HC-chain in the micellization process.     

An oxygen deficient fluorite with a tunnel structure: Bi8La10O27

A new lanthanum bismuth oxide, Bi₈La₁₀O₂₇, has been synthesized. It crystallizes in the Immm space group with the following parameters: a = 12.079 (2) Å, b = 16.348 (4) Å, c = 4.0988 (5) Å. Its structure was determined from powder X-ray and neutron diffraction data. It can be described as an oxygen deficient fluorite superstructure (a ≈ 3a_F/√2, b ≈ 3a_F, c ≈ a_F/√2) in which bismuth and lanthanum, as well as oxygen vacancies, are ordered. The structure consists of fully occupied (110) or lanthanum planes (La) which alternate with mixed planes and fully occupied oxygen planes (A) which alternate with two sorts of oxygen deficient (110) or planes (B and C) according to the sequence . The anionic distribution determines tunnels where the bismuth ions are located, forming diamond-shaped based tunnels. The coordination of bismuth and lanthanum is discussed. The high thermal factor of some oxygen atoms suggests that this oxide exhibits ionic conductivity.     

Synthesis of halide anion‐doped bismuth terephthalate hybrids for organic pollutant removal

Halide anion‐doped bismuth terephthalate hybrids were synthesized using a facile solvothermal method. Four series of hybrids doped with halide anions X^? (F^?, Cl^?, Br^? and I^?) were produced by varying the molar ratios (n) of X^? relative to Bi(NO₃)₃ (n = 0.25, 0.5, 0.75 and 1) in dimethylformamide solution. The results indicated that 0.25 equiv. of different halide anion‐doped bismuth terephthalate hybrids, especially BiBDC‐Cl(0.25) and BiBDC‐Br(0.25), exhibited excellent photocatalytic activity under visible light and UV light irradiation. They also exhibited excellent adsorption performance for Rhodamine B which could be attributed to high surface areas and negative charge on the surface of the catalysts. Moreover, the degradation of Rhodamine B under visible light irradiation is a photosensitization process and ^?O₂^? is the most important active species. The halide anion‐doped bismuth terephthalate hybrids are promising photocatalysts for removal of organic pollutants. Copyright © 2016 John Wiley & Sons, Ltd.     

Influence of nanoporous carbon electrode thickness on the electrochemical characteristics of a nanoporous carbon|tetraethylammonium tetrafluoroborate in acetonitrile solution interface

Electrochemical characteristics for the nanoporous carbon|Et₄NBF₄+acetonitrile interface have been studied by cyclic voltammetry and impedance spectroscopy methods. The influence of the electrolyte concentration and thickness of the nanoporous electrode material on the shape of the cyclic voltammetry and impedance curves has been established and the reasons for these phenomena are discussed. A value of zero charge potential, depending slightly on the structure and concentration of the electrolyte, the region of ideal polarizability and other characteristics have been established. The nanoporous nature of the carbon electrodes introduces a distribution of resistive and capacitive elements, giving rise to complicated electrochemical behaviour. Analysis of the complex plane plots shows that the nanoporous carbon|Et₄NBF₄+acetonitrile electrolyte interface can be simulated by an equivalent circuit, in which two parallel conduction paths in the solid and liquid phases are interconnected by the double-layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte concentration and electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. The value of the solid state phase resistance established is independent of the thickness of the electrode material.     

The role of anions in oxygen reduction in neutral and basic media on gold single-crystal electrodes

Oxygen reduction on well-defined single-crystal electrodes in different basic and neutral media has been studied using the impinging jet system. The results obtained with this system in 0.1 M NaOH are comparable to those reported in the literature for rotating disk electrodes, indicating that the impinging jet system behaves as a system in which the thickness of the diffusion layer is stationary. The activity of the Au(100) electrode is considerably higher than the rest of the basal planes in all media and yields water when E>–0.2 V and hydrogen peroxide for E<–0.2 V on the SHE scale. For Au(111) and Au(110) the activity of the electrode is much smaller and the final product is always hydrogen peroxide. The transition between both mechanisms for the Au(100) is, essentially, independent of the solution pH. In acid media the final product is always hydrogen peroxide for all the electrodes studied. The differences between the activities of the electrodes have been explained in the light of the different adsorption properties of the surfaces in relation to HO₂ ^–. In the case of the Au(100) electrode, the existence of a negative charge density on the metal inhibits further reduction of HO₂ ^–.Dedicated to Prof. Dr. Wolf Vielstich on the occasion of his 80th birthday in recognition of his numerous contributions to interfacial electrochemistry     

Adsorption of ions in the diffuse part of an electrical double layer at ionic surfactant solution-air interfaces in the presence of background electrolytes

The equations are derived for the calculation of adsorption values Γ _± ^d of coions and counterions in the diffuse part of an electrical double layer characterized by Ψ_d potential in the presence of a background electrolyte. The case of arbitrary |Ψ_d| values is considered. Based on the known experimental data, the contributions of adsorption values Γ _± ^d to the surface excesses of ions, as determined by the Gibbs method for the solution-air interfaces, are quantitatively estimated. It is shown that the adsorption of counterions in the diffuse part of the electrical double layer is significantly lower than that in its dense part; however, the orders of these values are comparable. At potentials |Ψ_d| > 25 mV, surface-active ions are mainly located near the interface, and their adsorption values Γ _? ^d cannot noticeably affect the calculated surface excesses.     

Anion-induced adsorption of bismuth(III) on mercury from chloride medium

The adsorption of bismuth(III) on a mercury electrode in chloride perchloric acid medium has been measured by double-step chronocoulemetry. The amount of adsorption of Bi(III) increases with the concentration of chloride, a maximum value being reached around c_C1 = 80 mM, to diminish at greater concentrations of the halide. The adsorption of Bi(III) also shows maximum values when the electrode charge varies. The results allow the existence of several coordinated Bi-Cl^? species to be postulated, which would be responsible for the adsorption process and the subsequent reduction of Bi(III).