Specific Adsorption Of Anions Research Articles

Effect of specific adsorption of anions on the kinetics of the current relaxation after pulsed anodic activation of dissolution of hydrophilic metals

Based on the model of spatial separation of the dissolution and passivation of hydrophilic metals, the effect of strong specific adsorption of anions on the shape of curves for the relaxation of anodic current of metals in acid solutions after switching an anodic activation pulse off is considered by using iron as an example.

Capacitance of the Double Layer at Polycrystalline Au-Ag Electrodes: Influence of Preparation Technique and Specific Anion Adsorption

We present electrochemical impedance spectra made on gold alloy containing 30% silver electrodes of various roughnesses in aqueous perchlorate acid solution as supporting electrolyte in the absence and in the presence of mM of specifically adsorbed halide ions X (X = Br - , Cl - , I - ), at potentials where the dominant electrode process is the adsorption of the above anions. Efforts were mainly concentrated on the importance of surface preparation technique of the electrode and its influence on impedance spectra. Atomic scale inhomogeneities are introduced by mechanical treatment and can be decreased by annealing. Due to the annealing the double layer behaves as (almost) an ideal capacitance in the absence of specific adsorption though the surface roughness remains the same. A study of the related impedance behaviour in the presence of adsorbates even at very low concentrations (10 -4 M), revealed capacitance dispersion increasing with the extent of specific anion adsorption at the gold/silver surface.

Adsorption phenomena on polycrystalline gold electrode modified by Zn adatoms

In this paper, experimental results obtained by the in-situ radiotracer and voltammetric studies of the competitive adsorption of the HSO4-/SO42- (labeled with 35S) and Cl- (labeled with 36Cl) anions on bare Aupoly substrate and Aupoly surfaces modified by the Zn adatoms in the lower pH region (pH≤4.5) are presented and discussed. In addition, some data on the formation of Zn adlayers (labeled with 65Zn) are reported to demonstrate the complex features of the underpotential deposition (UPD) phenomenon. It has been revealed that (1) the relative adsorption strength of Cl- ions on bare Aupoly is higher than that of H2PO4-/PO43- and HSO4-/SO42- ions in the entire pH region studied; (2) anomalous tendency of the specific adsorption of anions on polycrystalline gold modified by Zn adatoms occurs at pH 4.5 as follows: PO43->SO42->Cl->>ClO4- and (3) the UPD of Zn2+ ions on polycrystalline gold is measurable even from solutions containing 5 . 10-8M Zn2+ over the entire potential range where the hydrogen evolution takes place.It is, however, plausible to assume that the coverage of the gold surface by Zn adatoms measured even at solution concentrations of c≥5 . 10-4M does not exceed one monolayer.

Thermodynamic Analysis of the Temperature Dependence of OH Adsorption on Pt(111) and Pt(100) Electrodes in Acidic Media in the Absence of Specific Anion Adsorption

The effect of temperature on the voltammetric OH adsorption on Pt(111) and Pt(100) electrodes in perchloric acid media has been studied. From a thermodynamic analysis based on a generalized adsorption isotherm, DeltaG degrees , DeltaH degrees , and DeltaS degrees values for the adsorption of OH have been determined. On Pt(111), the adsorption enthalpy ranges between -265 and -235 kJ mol(-1), becoming less exothermic as the OH coverage increases. These values are in reasonable agreement with experimental data and calculated values for the same reaction in gas phase. The adsorption entropy for OH adsorption on Pt(111) ranges from -200 J mol(-1) K(-1) (low coverage) to -110 J mol(-1) K(-1) (high coverage). On the other hand, the enthalpy and entropy of hydroxyl adsorption on Pt(100) are less sensitive to coverage variations, with values ca. DeltaH degrees = -280 kJ mol(-1) and DeltaS degrees = -180 J mol(-1) K(-1). The different dependence of DeltaS degrees with coverage on both electrode surfaces stresses the important effect of the substrate symmetry on the mobility of adsorbed OH species within the water network directly attached to the metal surface.

Specific adsorption of bromide and iodide anions from nonaqueous solutions on controlled-surface polycrystalline silver electrodes

Abstract A systematic investigation was performed on the specific adsorption of bromide and iodide anions on controlled-surface polycrystalline silver electrodes, from three organic solvents (acetonitrile, propylene carbonate, and dimethylformamide), combining differential capacity and impedance experiments with a recently proposed “indirect” voltammetric method based on the monitoring of the negative shift of the reduction peak potential of a “probe” organic halide molecule induced by progressive additions of halide anions, resulting in increasing adsorption competition. Bromide and iodide ions are specifically adsorbed onto polycrystalline silver electrodes in the three organic solvents studied. The adsorption process, which is slow and it is characterised by a partial charge transfer and by a slow diffusion step, is modulated by the different coordination abilities of the solvents for the species involved in the adsorption. The three experimental methods provide consistent results. In particular, the strength of halide adsorption increases in the halide sequence Br−

Disposal of domestic sludge and sludge ash on volcanic soils

Column leaching experiments were conducted to test the ability of Chilean volcanic soils in retaining the mineral constituents and metals in sewage sludge and sludge ash that were incorporated into the soils. Small or negligible amounts of the total content of Pb, Fe, Cr, Mn, Cd, and Zn (0 to <2%), and more significant amounts of mineral constituents such as Na (7–9%), Ca (7–13%), PO 4 (4–10%), and SO 4 (39–46%) in the sludge and sludge ash were readily soluble. When they were incorporated on the surface layer of the soils and leached with 12 pore volumes of water over a 3 month period of time, less than 0.1% of the total amount of heavy metals and PO 4 in the sludge and sludge ash were collected in the drainage water. Cation exchange selectivity, specific anion adsorption and solubility are the processes that cause the reduction of leaching. The volcanic soils were capable of retaining the mineral constituents, P, and metals in applied sewage sludge and sludge ash and gradually released them as nutrients for plant growth.

Effects of phthalic and salicylic acids on Cu(II) adsorption by variable charge soils

In the present study, the effect of two substituted benzoic acids on Cu(II) adsorption onto two variable charge soils was investigated, with the emphasis on the adsorption and desorption equilibrium of Cu(II). Results showed that the presence of organic acids induced an increase in Cu(II) adsorption onto the two soils. The extent of the effect was related to the initial concentrations of Cu(II) and organic acid, the system pH, and the nature of the soils. The effect of organic acids was greater for Oxisol than for Ultisol. Phthalic acid affected Cu(II) adsorption to a greater extent than salicylic acid did. The effect of organic acids varied with pH. The adsorption of Cu(II) induced by organic acids increased with increasing pH and reached a maximum value at approximately pH 4.5, and then decreased. It can be assumed that the main reason for the enhanced adsorption of Cu(II) is an increase in the negative surface charge caused by the specific adsorption of organic anions on soils because the desorption of Cu(II) adsorbed in organic acid systems was greater than that for the control. The desorption of Cu(II) absorbed in both control and organic acid systems also increased with increasing pH; it reached a maximum value at pH ∼5.25 for control and salicylic acid systems and at pH ∼5.1 for a phthalic acid system, then decreased. This interesting phenomenon was caused by the characteristics of the surface charge of variable charge soils.

The solvent effect in the electrocatalytic reduction of organic bromides on silver

Abstract The electrochemical reduction of two model organic bromides, acetobromoglucose and benzyl bromide, on glassy carbon (assumed as non-catalytic reference material) and silver (the best electrocatalytic material for organic halide reduction) is investigated in six popular organic solvents of widely different properties (acetonitrile, dimethylformamide, dimethylsulfoxide, propylene carbonate, acetone and methanol). Analysis of the solvent effect on the electrocatalysis at Ag required the use of an appropriate reference system for a reliable intersolvental comparison of the electrode potentials. The redox behaviour of two ferrocene derivatives to be used as internal reference systems was therefore investigated in the selected solvents. Also the role of specific bromide anion adsorption on the silver surface was taken into consideration in the assessment of solvent effects on the electrocatalytic process. In all solvents tested, Ag exhibits a remarkable electrocatalytic effect for the reduction of both compounds. In particular, its catalytic activity is outstanding in the protic solvent methanol. For both compounds the electrocatalytic activities linearly increase with decreasing primary medium effect on the bromide anion ( Δ G Br - , W → S ∘ ) , i.e. with increasing solvent ability in bromide anion coordination.

Effect of Low Molecular Weight Organic Anions on the Adsorption of NO3 − by Variable Charge Soils

Low molecular weight organic acids exist widely in soils and have been implicated in many soil processes. The results in the present paper showed that the presence of organic anions led to a decrease in the adsorption of NO3 −. The effect of citrate was much larger than that of oxalate and malate. Among different soils, the effect for Hyper-Rhodic Ferralsol and Rhodic Ferralsol was larger than that for Haplic Acrisol, which was related to the content of iron oxides in these soils. The effect of organic anions decreased with the increases in pH value and the amount of organic anions added. The organic anions depressed the adsorption of NO3 − through two mechanisms, the competition of the organic anions for adsorption sites with NO3 − and the change of soil surface charge caused by the specific adsorption of organic anions.

Formation of cellular films consisting of wurtzite-type zinc oxide nanosheets by mediation of phosphate anions

We successfully prepared a wide variety of wurtzite-type zinc oxide films exhibiting columnar, cellular and densely packed morphologies in a simple aqueous solution system containing phosphate anions. As the phosphate concentration increased, the shape of crystalline units composing the films varied from hexagonal needles into seaweed-like sheets. A novel type of open cellular structures was obtained by assembly of nanoscale zinc oxide sheets covered with phosphate. Specific adsorption of phosphate anions on (001) of the wurtzite-type crystal flattened the crystal grains, and then induced the structural evolution into a cellular form. A blue shift of the absorption edge suggested that the quantum size effect occurred in the nanoscale platy crystals composing the cellular films.

CO oxidation on stepped Rh[ n(1 1 1) × (1 1 1)] single crystal electrodes: Anion effects on CO surface mobility

The influence of anion adsorption on the CO electrooxidation reaction on stepped Rh[ n(1 1 1) × (1 1 1)] electrodes was investigated by comparing voltammetric and chronoamperometric data obtained in 0.5 M HClO 4 with previously published results obtained in 0.5 M H 2SO 4. Compared to sulfuric acid media, complete stripping of the CO adlayer requires fewer cycles in perchloric acid and the resulting stripping voltammetry peaks are shifted to considerably lower potentials, attesting to the reduced influence of more weakly adsorbed anions. The absence of a shoulder prior to the main oxidation peak and the higher symmetry of the main peaks implies that the electrooxidation reaction in perchloric acid is not only faster than in sulfuric acid but probably also not diffusion limited, which suggests the mean field approximation as the best mathematical model for the reaction kinetics rather than nucleation and growth. The chronoamperometric transients recorded at various potentials show only a single oxidation peak with a slight tailing at longer times. Only the Rh(1 1 1) transients display tailing, which is Cottrellian in nature. The surface diffusion coefficient of CO deduced from the Cottrell plots is more than four orders of magnitude larger in perchloric than in sulfuric acid (HClO 4: 1 × 10 −12 < D < 2 × 10 −11 cm 2 s −1 vs. H 2SO 4: 1 × 10 −16 < D < 8 × 10 −16 cm 2 s −1), which also suggests a mean field model for HClO 4 rather than nucleation and growth. Apparently, specific anion adsorption on rhodium surfaces not only affects the rate, but also the dynamics of the CO electrooxidation reaction. Thus, by varying the adsorption strength of the anion, we could, in principle, influence the diffusion rate of adsorbates on the surface and, therefore, the reaction dynamics and the overall reaction rate.

In situ scanning tunnelling microscopic study of the initial stages of growth and of the structure of the passive film on Ni(111) in 1 mM NaOH(aq)

In situ electrochemical scanning tunnelling microscopic (STM) measurements of the growth mechanism of the passive film on Ni(111) single-crystal surfaces in an alkaline 1 mM NaOH aqueous solution are reported. Stepping the potential from −1,050 mV/SHE, where the terrace topography of the bare Ni(111) surface is observed, to −800 mV generates a continuous step flow resulting from a slow dissolution of the surface localized at step edges of the surface. Islands are observed on the terraces that possibly correspond to the specific adsorption of hydroxide anions, without formation of an ordered structure. The nucleation of the passivating oxide is preferentially located at the step edges at this potential. The ongoing dissolution at the nearby step edges not yet passivated leads to the formation of isolated 2D islands of the passivating oxide. At −780 mV, the formation of extended islands constituted of 2D nanocrystals (~2 nm) is observed on the substrate terraces and blocks the dissolution at the step edges. The nanocrystals have an hexagonal lattice assigned to the formation of a monolayer of Ni(OH)2(0001) in strained epitaxy on Ni(111). At E≥−630 mV, the formation of a facetted topography is observed as a result of the 3D growth of the passive film in tilted epitaxy on the substrate. Its structure is in excellent agreement with that expected for an unstrained 3D β-Ni(OH)2(0001) layer. This 3D layer of β-Ni(OH)2 is not reduced by a cathodic sweep of the potential down to −1,500 mV whereas the 2D islands are reduced under similar conditions. Ageing of the passive film shifts cathodically the potential of non-reversibility.

Determination of O and OH Adsorption Sites and Coverage in Situ on Pt Electrodes from Pt L23 X-ray Absorption Spectroscopy

The adsorption of atomic oxygen and hydroxide on a platinum electrode in 0.1 M HClO4 or H2SO4 electrolytes was studied in situ with Pt L(2,3) X-ray absorption spectroscopy (EXAFS and XANES). The Pt L3 edge absorption data, mu, were collected at room temperature in transmission mode on beamline X-11A at the National Synchrotron Light Source using a custom built cell. The Pt electrode was formed of highly dispersed 1.5-3 nm particles supported on carbon. A novel difference procedure (delta mu = mu(O[H]/Pt) - mu(Pt)) utilizing the L3 XANES spectra at different applied voltages was used to isolate the effects of O[H] (O or OH) adsorption in the XANES spectra. The Deltamu results are compared with results obtained from real-space full-multiple scattering calculations utilizing the FEFF8 code on model clusters. The experimental results, when compared with theoretical calculations, allow the adsorption site to be identified. At low coverages OH adsorbs primarily in 1-fold coordinated atop sites. As the coverage increases, O binds in the bridge/fcc sites, and at still higher coverages above 1.05 V RHE, O adsorbs into a higher coordinated n-fold or subsurface site, which is thought to be the result of Pt-O site exchange and oxide formation. These results are similar to those found in the gas phase. Direct specific adsorption of bisulfate anions in H2SO4 is spectroscopically seen in both the EXAFS and XANES data and is seen to impede oxygen adsorption consistent with previous reports.

Effect of cadmium and lead adatoms on the reduction kinetics of peroxodisulfate anions at platinized platinum in acid solutions

The reduction of peroxodisulfate anions on a rotating Pt/Pt disk electrode in solutions of sulfuric and perchloric acids with or without cadmium and lead salts is studied. At E r > 0.2V (RHE) the reduction rate in HClO4 exceeds that in H2SO4, but at E r < 0.2V, the process in HClO4 is strongly inhibited upon approaching E r = 0. Lead adatoms catalyze the process, while cadmium adatoms inhibit it in H2SO4 and accelerate it at E r < 0.2V in HClO4. The results are interpreted by taking account of the specific adsorption of sulfuric-acid anions and their co-adsorption with cadmium cations, as well as the adsorption of peroxodisulfate anions on the Pt surface modified by lead and cadmium ions in perchlorate solutions.

Effect of low-molecular-weight organic anions on exchangeable aluminum capacity of variable charge soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes, such as nutrient availability, translocation of metals, fate of heavy metals, and mineral weathering. In this paper, the effect of the LMW organic anions on the exchangeable aluminum of two variable-charge soils was examined. The results showed that the organic anions induced an increase or a decrease in the exchangeable Al, and the extent and direction of the effect depended on the nature of organic anions, surface chemical properties of soils, and pH. For example, at pH 4.5, the quantity of exchangeable Al of Oxisol in the control system was 2.65 mmol kg −1, whereas the values in the citrate, oxalate, malonate, malate, tartarate, salicylate, and lactate systems increased by 3.25, 1.93, 1.95, 1.82, 1.28, 0.88, and 0.45 times, respectively. In contrast, the quantity of the exchangeable Al of Ultisol at pH 4.5 in the oxalate and the citrate systems decreased by 8.8 and 19.6%, respectively. The increase in the exchangeable Al was caused mainly by the increase in negative surface charge of the soils due to the specific adsorption of organic anions. The ability of organic anions at low concentrations to increase exchangeable Al for Oxisol followed the order citrate > oxalate and malonate > malate > tartarate > salicylate > maleate > lactate. This order is consistent with that of the effect of the adsorption of anions on the increase in the negative surface charge and/or the decrease in the positive surface charge of the soil. On the other hand, the organic anions could depress the exchangeable Al through the formation of soluble Al–organic anion complexes under certain conditions. The anions with small stability constants of Al–organic anion complexes, such as lactate, caused an increase in exchangeable Al with the change in surface charge of the soils, while those with large stability constants, such as citrate and oxalate, caused an increase in the exchangeable Al at low concentration and a decrease at high concentration.

The stability and electro-oxidation of carbon monoxide on model electrocatalysts: Pt(1 1 1)–Sn(2 × 2) and Pt(1 1 1)–Sn(√3 × √3)R30°

The surface redox behaviour, the stability, and the electro-oxidation of CO on model platinum–tin surface alloy catalysts has been studied by ex situ electrochemical measurements. The Pt(1 1 1)–Sn(2 × 2) and Pt(1 1 1)–Sn(√3 × √3)R30° surfaces have been prepared and characterised in UHV by vapour deposition of Sn on Pt(1 1 1) and the surfaces subsequently transferred for electrochemical investigation. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V RHE/0.15 V RHE in H 2SO 4 electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V RHE, and an oxidised state at potentials above 0.28 RHE. Specific adsorption of sulphate anions is not associated with the surface process since there is no evidence from photoemission of sulphate adsorption, and the same surface couple is observed in HClO 4 electrolyte. A second surface redox couple, associated with further oxidation of the Sn sites in the alloy surfaces, is observed at 0.8 V RHE/0.58 V RHE. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 ± 0.05 ML and 0.2 ± 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites at the lower potential of 0.28 V RHE. This strong promotional effect is unambiguously associated with the bi-functional mechanism. The Sn induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V RHE. The instability of the alloy surface at higher potentials is associated with the second redox couple. The redox behaviour at higher potentials is modified by the presence of CO in stripping experiments, with couples now observed at 0.78 V RHE/0.56 V RHE, and 0.9 V RHE/0.70 V RHE. This behaviour may be associated with enhanced oxidation through the exclusion of sulphate anions at the surface by CO.

Atomic structure and tip-induced reconstruction of bromide covered Cu(1 1 0) electrodes

The behavior of specifically adsorbed bromide on Cu(1 1 0) in 10 mM HBr has been studied using cyclic voltammetry in combination with high resolution in situ electrochemical scanning tunneling microscopy (EC-STM). At cathodic potentials near the onset of the hydrogen evolution reaction the structure of the bare copper surface was observed by EC-STM. A variation of the electrode potential in positive direction causes a specific anion adsorption leading to the formation of a highly ordered superstructure with a quasi-hexagonal symmetry. The two-dimensional lattice of this adlayer can be described by a c(3 × 2) unit cell. The bromide anions are arranged parallel to the close packed copper rows and are located in three different types of adsorption sites. These inequivalently bound bromide species are imaged with different brightness in the STM-pictures. As a result the bromide adlayer exhibits a long-ranged wavy superstructure superimposed on the atomic corrugation. Tip induced copper corrosion is used to obtain highly ordered nanostructures due to a locally confined electrochemical annealing process proceeding along the [0 0 1]-direction of the substrate. This annealing process results in the formation of Cu(1 0 0)-facets.

Kinetics of dissociative adsorption of ethylene glycol on Pt(1 0 0) electrode surface in sulfuric acid solutions

The dissociative adsorption of ethylene glycol (EG) on Pt(1 0 0) electrode surface cooled in air after flame annealing was investigated by using programmed potential step technique and in situ FTIR spectroscopy. The stable adsorbates derived from EG dissociative adsorption on Pt(1 0 0) were determined by in situ FTIR spectroscopy as linear- and bridge-bonded CO. The quantitative results demonstrated that the average rate of dissociative adsorption of EG on Pt(1 0 0) surface varies with electrode potential, yielding a volcano-type distribution with a maximum value located near 0.10 V versus SCE. From the variation of the quantity of CO adsorbates generated in EG dissociative adsorption with the adsorption time t ad, the initial rate ( ν i) of this surface reaction was evaluated quantitatively. The maximum value of ν i has been determined to be 2.64 × 10 −11 mol cm −2 s −1 in a solution containing 2 × 10 −3 mol L −1 EG. The influence of the surface structure of Pt(1 0 0) electrode obtained by different pretreatment as well as of the specific adsorption of (bi)sulfate anions on the kinetics of EG dissociative adsorption has been also investigated and discussed. In comparison with a Pt(1 0 0) surface cooled in air atmosphere after flame treatment, the Pt(1 0 0) surface cooled in an Ar–H 2 stream or subjected to a treatment of fast potential cycling decreased significantly the initial rate ν i of EG dissociative adsorption. Similar effect was also observed for the specific adsorption of (bi)sulfate anions. However, the maximum attainable coverage ( θ DA S ) of adsorbates derived from EG dissociative adsorption is not affected either by the surface structure of Pt(1 0 0) or by (bi)sulfate anions adsorption.

Anion specific adsorption on Fe 2O 3 and AlOOH nanoparticles in aqueous solutions: comparison with hematite and γ-Al 2O 3

The specific adsorption of radiolabeled sulfate and phosphate ions from perchlorate supporting electrolyte onto nano-AlOOH and nano-Fe 2O 3 powder has been investigated. The pH dependence of the adsorption of anions onto nanopowders was compared with that of the same ions onto γ-Al 2O 3 and hematite. It was demonstrated that the character of the pH dependence of the adsorption is very similar in the comparable cases. It was found, however, that in contrast to the behavior of γ-Al 2O 3, nano-AlOOH dissolves at a significant rate at low pH values (pH<2). Thus the study of the pH dependence of the anion adsorption encounters difficulties at these pH values. Disregarding this fact, it can be concluded that no special effects can be observed in the anion adsorption onto the nano-oxides studied.

The Oxygen Reduction Reaction on Thin Palladium Films Supported on a Pt(111) Electrode

The oxygen reduction reaction (ORR) in acid and alkaline solutions is investigated on thin Pd films (0 < θPd < 1.5 ML) supported on a Pt(111) electrode. In 0.1 M KOH, the general form of the current vs ϑPd relationship for the ORR has a volcano shape, with the maximum catalytic activity exhibited by a surface modified with 1 ML of Pd. However, if the amount of Pd deposited exceeds 1 ML, the reaction is inhibited and 1.5 ML of Pd are less active than unmodified Pt(111). In 0.1 M HClO4, none of the Pd films are more active than Pt(111). In solution containing Cl- (ca. 106−10-5 M), the inhibition on the Pt(111)−Pd electrode is much more severe than on bare Pt(111) due to stronger anion adsorption on Pd. We present an interpretation of the kinetics of the ORR on Pt modified by thin Pd films in alkaline and acid solution based on the electronic properties of the Pd films and how the electronic properties effect specific anion adsorption, the formation of OHad and the adsorption of the reaction intermediates.