L. M. Doubova

Crystal-face specificity of electrical double-layer parameters at metal/solution interfaces

The structure of the boundary region between a metal and a solution (the so-called electrical double layer) depends on the chemical nature of the metal and of the solvent, the type and amount of solute, and the structure of the metal surface. This article is focussed on the effect of varying the crystallographic orientation of the metal surface of an electrode on ‘classical’ double-layer parameters such as (i) potential of zero charge, (ii) its temperature coefficient, (iii) the interfacial permittivity (capacitance), (iv) and interfacial intermolecular interactions (adsorption). Similarities and differences between UHV experiments and related electrochemical situations are first scrutinized. The components of the electrode potential are analysed and their relevance to gas-phase experiments is discussed. Discrepancies between UHV and electrochemical data related to the same situation are explained in terms of temperature of experiment, lack of potential control and deviation of the structure of the surface solvent layer from bulk properties. A few experimental problems related to the preparation of single crystals, their use in an electrochemical cell, and the instrumental technique are then described, pointing to some ambiguous aspects. An interfacial parameter, ΔX, measuring the extent of the modifications occurring in the surface potential at the surface regions of the two phases as they are brought into contact, is derived from potential of zero charge–work function plots. For metal electrodes in aqueous solutions, ΔX is proportional to the metal–water interaction strength, although it is not only related to the surface water dipole orientation. ΔX is shown to correlate quite well with other double-layer parameters for different metals. The crystal-face specificity of the double-layer parameters is also correlated to ΔX, which thus turns out to be a unifying parameter for predictive analysis. The crystal-face specificity of capacitance and adsorption is discussed on the basis of several examples.

Surface screening effects by specifically adsorbed halide anions in the electrocatalytic reduction of a model organic halide at mono- and polycrystalline silver in acetonitrile

Abstract The silver surface screening effects by specifically adsorbed halide anions in the electrocatalytic reduction of a model organic bromide (acetobromoglucose) have been studied by cyclic voltammetry on controlled mono- and polycrystalline silver surfaces in acetonitrile+0.1 M tetraethylammonium perchlorate medium as a function of the concentration c X of added TEAX (X=Cl, Br, or I, TEA, tetraethylammonium). The reduction peak potentials, E p , are regularly shifted in the negative direction with increasing c X , typically tending to an asymptotic value for c X ≈0.1 M. Several literature models describing adsorption/desorption equilibria have been applied to justify the above experimental E p versus c X trends (being logarithmic in the iodide cases) for the three halides and the four silver surfaces tested.

Electrodeposited Polycrystalline Silver Electrodes: Surface Control for Electrocatalysis Studies

An advantageous procedure is developed, allowing preparation of electrodeposited silver electrodes of highly controlled polycrystalline surface (characterized by both electrochemical techniques and SEM), to be employed as cathodes for interphase and electrocatalysis studies. Such electrodes, tested in halide adsorption experiments in parallel with the more demanding single-crystal and polycrystalline silver rod ones, acting as a reference, perform competitively in terms of both reproducibility and stability. The same experiments allow further evaluation of the surface roughness factors of the electrodeposited silver electrodes, based on (i) the Parsons–Zobel criterion and (ii) the comparison of their capacitance minima with those of the (110) single-crystal ones, both approaches resulting in very good agreement with the standard UPD and capacitive methods.

The minimum value of the differential capacity of gallium at negative charges

Abstract The minimum values of the differential capacity of Ga at negative charges has been found to be systematically higher than that of Hg by about 20%. These results, in close agreement with recent chronocoulometric experiments, cause the relation between the structure of the interface at Hg and that at Ga to be looked at in a different light.

Adsorption of n-pentanol from KPF6 aqueous solution on the (100) and (110) faces of Ag single crystal electrodes

Abstract The potential dependence of the adsorption of n -pentanol (NP) on the (100) and (110) faces of Ag single crystal electrodes from aqueous KPF 6 solutions has been studied at 10 mV s −1 potential scan rate by measuring the impedance both at constant frequency ( f ) and by sweeping f from 11 kHz to 0.1 Hz. The adsorption of NP has been found to be strongly dependent on crystal orientation. The results have been compared with those obtained on Ag(111) with the same kind of single crystal preparation, as well as with Ag(100) electrolytically grown in a Teflon capillary [A. Popov, O. Velev, T. Vitanov, J. Electroanal. Chem. 256 (1988) 405].

Acetonitrile as Adsorbate or Solvent to Probe the Crystal Face Specificity of Metal–Water Interaction at Silver Electrode/Solution Interfaces

The crystal face specificity of metal–water interaction at Ag electrode/solution interfaces is investigated by using acetonitrile (ACN) as a probe molecule of the water interfacial structure or as a solvent in which water is a solute. Capacitance and voltammetric curves suggest that ACN is weakly adsorbed from aqueous solution on Ag in the order (111) > (100) > (110). Apparent inconsistencies of adsorption parameters are explained by the occurrence of two ACN adsorption modes: (i) directly on the metal surface and (ii) on the water layer adsorbed on the metal surface. Ag surface oxidation in ACN in the presence of variable amounts of water suggests that water has an inhibiting effect on Ag oxidation, the diminution of the water content in ACN leading to free anodic dissolution of the metal surface.

Crystal Face Specificity of Incipient Oxidation of Ag Single Crystal Electrodes in Acidic Aqueous Solution

The onset of anodic oxidation of Ag single crystal face electrodes in acidic solution is investigated by means of capacitance and voltammetric curves. The potential of incipient oxidation is found to depend only slightly on the atomic density of the surface. On the other hand, a pre-monolayer oxidation peak is identified only for the (110) face in HClO4 while in H2SO4 it is suppressed. Results are interpreted in terms of anodic oxidation vs. anodic dissolution interference and anion adsorption vs. water oxidation competition. The behavior of the (110) face is explained on the basis of a model for water adsorption proposed for UHV experimental data. The higher reactivity of the (110) face toward water molecules supports the “hydrophilicity” scale of Ag crystal faces based on ionic and non-ionic adsorption data.

Nitrite oxidation on RuO2 electrodes

An electrochemical kinetic investigation of nitrite oxidation to nitrate on RuO2 is discussed. The process is studied by cyclic voltammetry, steady-state measurements and potential step measurements. The overall oxidation reaction is a two-electron process where the first step involves a reversible charge transfer: NO2− ⇔ NO2 + e− The one-electron oxidation of nitrite yields adsorbed NO2 which is further oxidized to adsorbed (NO2)+ and subsequently desorbed via a chemical reaction. In the general case, fit of experimental data is obtained with adsorption described by a Temkin isotherm unless the electrode is pre-treated at a cathodic potential where the (NO2)ads is removed. This treatment lowers the degree of coverage by intermediates but not the nature of the slow step.