J. Lipkowski

Investigations of SO42− adsorption at the Au(111) electrode by chronocoulometry and radiochemistry

Abstract Chronocoulometry was employed to measure charge density at the Au(111) electrode surface. Thermodynamic analysis of the charge density data for ionic adsorption from a solution with an excess of a supporting electrolyte is described and used to study SO 4 2− adsorption at the Au(111) electrode from a 0.1 M HClO 4 solution. The Gibbs excesses determined from the charge density data are compared with the Gibbs excesses determined from radiochemical measurements using 35 S-labelled sulphate solutions. Very good agreement between the Gibbs excesses determined by the two techniques is observed. Essin-Markov coefficients for SO 4 2− adsorption from two series of solutions were determined: (i) at constant pH and variable K 2 SO 4 concentration; (ii) at constant K 2 SO 4 concentration and variable pH. The Essin-Markov coefficients were used to identify the nature of the adsorbed species (HSO 4 − or SO 4 2− ). Our results indicate that SO 4 2− ion is the adsorbed species even if HSO 4 − predominates in the bulk of the solution. Chronocoulometry and radiochemistry were used to study SO 4 2− adsorption at the Au(111) electrode. Very good agreement between the Gibbs excesses determined from chronocoulometric and radiochemical measurements was observed. This agreement is an important verification of the reliability of these two in-situ techniques for measuring surface coverages. Although the two techniques give the same physical quantity, which is the Gibbs excess of adsorbed ions, each of them probes different properties of the interface and therefore has different limitations. The quantity measured by chronocoulometry is the electrode charge density. The surface concentration is calculated from σ M employing one integration step and one differentiation step and assuming the validity of the electrocapillary equation. Therefore the chronocoulometric technique can be applied only within the double-layer region of the gold electrode. However, since both the Gibbs excess and the charge density are measured simultaneously, this technique provides not only the surface coverages but also an additional electrical variable needed for the interpretation of the coverage data. In contrast, radiochemistry provides information about the Gibbs excess from direct counts of the radioactivity when the electrode with adsorbed radiolabelled material is pressed against the scintillator window. Therefore this technique can be used to study adsorption on both reactive and non-reactive materials. However, the technique is restricted by the efficiency in squeezing out the solution containing radioactive species from the thin layer between the electrode surface and the scintillator window. When the count rates from solution trapped in that gap are high with respect to surface counts, the adsorption measurements are difficult and ultimately become impossible to conduct. In fact, the method cannot be employed for adsorption measurements when the bulk concentration of labelled solute is higher than 10 −2 M [1]. In addition, the radiochemical technique provides information about the Gibbs excess only.

Adsorption of pyridine at the Au( 110)-solution interface

Chronocoulometry has been used in order to characterize quantitatively the energetics of pyridine adsorption onto a gold (100) single crystal electrode surface. Over the potential region investigated (−0.8 to+0.6 V), three orientations of the pyridine molecules on the gold surface have been observed. The pyridine orientation is influenced strongly by the electrode potential. At a positively charged surface, the pyridine assumes a vertical orientation with the nitrogen atom facing the gold surface. A limiting surface concentration corresponding to 6 × 10−10 mol cm−2 was determined for the vertical orientation. At a negatively charged surface and at low surface concentrations (Γ Γ > 1 × 10−10 mol cm−2) and for potentials close to zero charge, a third orientation, presumably intermediate between the flat and the vertical orientations, was observed. Reorientation from the intermediate to the vertical orientation takes the form of a phase transition. The potential of the phase transition coincides approximately with the potential of zero charge. The Gibbs energies of adsorption, electrosorption valencies, potentials and charges of the maximum adsorption were determined for the flat and the vertical orientation of pyridine. Adsorption of pyridine on the Au (100) surface is compared with the adsorption at polycrystalline Au and mercury electrodes.

Adsorption of pyridine at the polycrystalline gold—solution interface

Abstract Chronocoulometry has been employed in order to investigate quantitatively the adsorption of pyridine from aqueous solution onto a polycrystalline gold electrode. It was found that at a negatively charged surface, pyridine adsorbs in the flat orientation, reaching a limiting surface concentration of 3 × 10 −10 mol cm −2 . Under the influence of the electric field pyridine reorients so that on the positively-charged surface the pyridine molecules assume the vertical orientation, presumably with the nitrogen facing the gold surface. A limiting surface concentration of 7 × 10 −10 mol cm −2 is reached in the vertical orientation. The standard Gibbs energy of adsorption was found to reach fairly large values (−38 kJ mol −1 ). The electrosorption valency was also determined and found to be −0.6 indicating that a partial charge transfer is involved in the pyridine adsorption.

The influence of surface crystallography on the rate of hydrogen evolution at Pt electrodes

Abstract The kinetics of the hydrogen evolution reaction from HClO 4 solutions on the (100), (111) and (511) single crystal planes of platinum and on the polycrystalline electrode have been investigated. Cyclic voltammetry has been used to study hydrogen adsorption. The adsorption isotherms have been determined and the standard Gibbs energies of adsorption Δ G A ° have been calculated. The values of Δ G A ° display dependence on the crystallographic orientation which spans over 16 kJ mol −1 . The potential step technique has been used to measure the rate of hydrogen evolution. The Tafel plots have been constructed and the exchange currents of the hydrogen reaction have been calculated. It has been found that, in contrast to the strong variation of Δ G A °, the rate of the hydrogen evolution reaction is not sensitive to the surface crystallography.

Determination of the acid dissociation constant for bisulfate adsorbed at the Pt(111) electrode by subtractively normalized interfacial Fourier transform infrared spectroscopy

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was applied to determine the acid dissociation constant for bisulfate adsorbed at the Pt(111) electrode surface. The experiments were carried out in NaF+HF buffer solutions with pH varying between 0.4 and 6.1. The SNIFTIR spectra were used to identify adsorbed bisulfate and sulfate ions and to monitor changes in the concentration of the adsorbed species with pH. The pKa for the adsorbed bisulfate was determined from the changes of the intensity of IR bands assigned to sulfate and bisulfate with pH. The values of pKa depend on the electrode potential and their values range between 3.3 and 4.7. They are significantly higher than the value of pKa in the bulk of the solution. The magnitude of pKa indicates that bisulfate is the predominantly adsorbed species at pH 4.7.

Comments on the thermodynamics of solid electrodes

The thermodynamics of solid electrodes are discussed in light of the recent measurements of surface stress. Interfacial tension and surface stress are not even approximately equal, and they generally exhibit a different dependence on the electrode potential. The variation of the interfacial strain with potential is small so that the Lippmann equation for a solid is practically the same as for a liquid electrode. Changes in the interfacial tension can be obtained by integrating the charge density over the potential.

ADSORPTION OF PYRIDINE ON THE (210) FACE OF SILVER

Abstract The adsorption of pyridine at an Ag (110) single crystal electrode has been studied. In the range of potentials explored, adsorption, maximum coverage of the surface and the beginning of desorption are observed. At potentials close to the potential of zero charge the pyridine molecules assume a tilted orientation with the nitrogen atom facing the metal. The adsorption isotherms have been determined for the first time for a silver face.

AN EXAMINATION OF THE RELATIONSHIP BETWEEN SURFACE ENHANCED RAMAN SCATTERING (SERS) INTENSITIES AND SURFACE CONCENTRATION FOR PYRIDINE ADSORBED AT THE POLYCRYSTALLINE GOLD/AQUEOUS SOLUTION INTERFACE

Surface enhanced Raman scattering (SERS) data are presented for pyridine adsorbed onto both smooth and roughened polycrystalline gold electrode surfaces. It was found that, for smooth surfaces, time independent (stable to within 10% of the initial value) intensities of the totally symmetric ring breathing mode of pyridine (1010 cm−1 band) could be measured by applying slow oxidation-reduction cycles between each intensity measurement. The potential and concentration dependence of the intensity of the 1010 cm−1 band was therefore determined. Independently, the isotherms for pyridine adsorption at smooth polycrystalline gold surfaces were obtained using chronocoulometry. The SERS data are compared to the surface concentration data. The SERS intensities are directly proportional to surface concentration at low coverages and deviate, becoming inverse, above half coverage concentrations. Lastly, the effect of surface roughness on the potential and concentration dependence of pyridine SERS was studied. No correlation between the SERS data from rough electrode surfaces and adsorption isotherms determined at a smooth surface was found.

Specific adsorption at Au(111) electrodes studied by second harmonic generation

Abstract It will be shown that the main source of second harmonic generation (SHG) (including isotropic and anisotropic contributions) arises from an intermediate layer covering the same region where electrochemical processes take place. Therefore, SHG exhibits an enormous potential for surface studies and is very useful as an in situ spectroscopic technique in electrochemistry. For instance, the formation and lifting of surface reconstruction can be monitored in situ under the influence of both potential and adsorption of ions or small organic molecules. For the Au(111) surface, the Au(111) - (√3 × 23) reconstruction leads to a symmetry change of the type C 3v → C s . At the Au(100) electrode, reconstruction gives rise to a significant contribution of a three-fold symmetry to the observed anisotropy pattern and a complex potential dependence of the anisotropy is observed. In addition, adsorption and potential controlled changes of the symmetry pattern of the SHG intensity can be separated from the effect of (re)charging the electrode. The variation in the static, interfacial electric field alters the surface electron distribution and, hence, their net polarizability in the z direction. This effect can be used to study—during the course of increasing coverage—possible phase transitions within the adsorbate layer.

The driving force for (p×√3)↔(1×1) phase transition of Au(111) in the presence of organic adsorption: a combined chronocoulometric and surface X-ray scattering study

Abstract In situ X-ray scattering and chronocoulometric experiments were performed to assess the influence of adsorption of pyridine, 2,2′-bipyridine and uracil on the driving force for the (p×√3)→(1×1) transition of the Au(111) surface. We have shown that the overall driving force is a combination of the driving force due to charge and the driving force due to the adsorbate. We have estimated the magnitude of the two driving forces and have given the upper and lower limits to this estimate. Our results show that the two driving forces are of comparable magnitude and that the interpretation of the surface reconstruction phenomena given in terms of either purely charge or a purely adsorbate effect is an oversimplification.