In this thesis the influence of the metal phase upon the
electrical double layer and electrode kinetics is investigated. This subject is somewhat
underexposed in electrochemistry, in contrast with for instance phenomena connected with
varying the solution phase that have been studied more extensively. The majority of the
studies that deal with the influence of the electrode material have been performed at
solid metal electrodes. From an experimental point of view this is strange, because
results with solid electrodes are less accurate and reproducible than those obtained at
liquid electrodes, e.g. the dropping mercury electrode (DME). The results of the former
highly depend upon the method of pretreatment for the electrode material, while at liquid
electrodes these problems are avoided. Therefore we have selected liquid amalgams as model
electrodes. Solute metals have been selected as for their high solubility, viz. indium,
thallium and (to a much lesser extent) zinc.
In Chapter 2 the physics of liquid indium and thallium amalgams and their components are
discussed. Here, the attention is focussed upon those physical properties expected to be
the most relevant for the study of electrode kinetics (Chapters 5 and 6) and double layer
phenomena (Chapters 3 and 4), viz. the electronic density of states and the electronic
work function.
In Chapter 3 the adsorption of indium and thallium at the amalgam/aqueous solution
interface is studied using impedance measurements and the dropping amalgam micro electrode
method. The relative and absolute surface excesses of the amalgams have been determined as
a function of composition at different potentials and surface charge densities. The
minimum thickness of the non-homogeneous interfacial layer, i.e. the layer of special
interest in the study of heterogeneous kinetics, is estimated to be about 6 monolayers.
In Chapter 4 the relationship between the potential of zero charge, Epzc, of
indium and thallium amalgams and the electronic work function W is studied. For the
amalgams the relation Epzc = W - constant is found to be valid over a wide
concentration range. The potentials at constant negative charge density also depend upon
the work function, a dependency that is interpreted by taking into account desorption of
the solute metal (i.e. indium or thallium) from the metal/solution interface and the mean
orientation of the dipoles of the water molecules in the compact double layer. Also the
concept of "electrochemical work function" is discussed. They have been
determined for indium and thallium amalgams. For indium they agree surprisingly well with
literature values of the work function from photo-emission experiments and contact
potential measurements. For thallium amalgams, unfortunately, no such determinations exist
in the region xTl = 0.1 - 0.4. The W-values have been used in the following
chapters dealing with the influence of the electrode material to investigate correlations
with kinetic parameters.
In Chapter 5 the Zn2+ reduction from aqueous 1M NaClO4 solution is
studied at liquid indium and thallium amalgams of varied compositions. We attempted to
correlate kinetic parameters, obtained with impedance voltammetry, with physical
properties of the metal phase and with the results described in Chapters 3 and 4. The Zn2+
reduction is known to proceed via two consecutive one-electron transfers. For the first
electron transfer step, over a wide potential range, a remarkable correlation with the
surface charge density was found. Even more remarkable is the linear relation with the
electric field strength in the compact double layer. Deviations from this linearity,
occurring at mercury and diluted amalgams, could be ascribed to non-free-electron like
behaviour of the metal phase. We think it is possible to understand this behaviour within
the framework of the theory of Levich and Dogonadze.
Chapter 6 deals with the Yb3+ reduction from aqueous 1M NaClO4
solution at liquid indium, thallium and zinc amalgams. This reaction proceeds via a
one-electron transfer and is, unlike the Zn2+ reduction, uncomplicated by
amalgamation. Also for this reaction the correlation between the rate constant and the
electric field strength in the compact double layer is observed and deviations from
linearity are similar to that of the Zn2+ reduction for indium and thallium
amalgams.
With these two examples we believe for the first time to have found solid proof that the
density of states and the electric field strength in the compact double layer co-determine
the rate of electron transfer at a metal electrode.
A detailed double-layer analysis of liquid indium and
thallium amalgams in contact with 1 M NaClO4 is performed, using impedance
measurements to obtain the differential double-layer capacitance, and the dropping mercury
micro electrode (DMµE) method to obtain directly the potentials of zero charge and the
charge density vs potential curves.
These data are combined with the early interfacial tension values published by Frumkin and
coworkers, to obtain the relative and the absolute surface excesses of the amalgam
constituents, both at constant potential and at constant charge density. The minimum
thickness of the non-homogeneous interphasial layer is estimated to be 1.8 nm
(approximately 6 monolayers) in the case of indium, and 1.65 nm (approximately 5.5
monolayers) in the case of thallium. The negative adsorption of thallium and indium is
tentatively related to the respective electronegativities of mercury, thallium and indium.
The relationship between the experimental zero charge
potential, Epzc, of indium and thallium amalgams and their electronic work
function, W, is studied and discussed. For indium amalgams, the linear relationship, Epzc
= W - const., is found to be obeyed in the surprisingly wide composition range of 0.02
It appears that these amalgams constitute a most useful continuous series of metal phases
to study the metal dependency of interfacial properties.
The kinetic parameters of both electron transfer steps in the Zn2+ reduction are obtained at dropping indium and thallium amalgam electrodes, as a function of their composition. Empirically it is attempted to correlate the observed dependences with several physical properties of the metal phase. The best correlation found was with the electrical field strength in the inner layer in the amalgam concentration range where the metal phase behaves according to the Sommerfeld free electron model. At lower concentrations the rate constant is decreased due to the low electronic density of states. The factor with which the rate is decreased for pure mercury agrees very well with the Mott factor for mercury.
The reduction reaction of Yb(III) to Yb(II) is studied at dropping indium, thallium and zinc amalgam electrodes in aqueous 1M NaClO4 solutions by the impedance method. This reaction proceeds according to a single-step mechanism: the electron- transfer is rate determining at all potentials. The rate constant at mercury is influenced by the addition of solute metal. A remarkable correlation between the rate constant and electric field strength at the metal surface has been observed. Departure from this correlation was observed for the amalgams with a solute metal content up to about 0.2. This coincides with the deviation from free-electron like behaviour for these amalgams. The behaviour of the Yb(III) reduction at liquid amalgams resembles that of the Zn(II) reduction we discussed in our previous publication about kinetics at amalgam electrodes.
© 1995, 1996 L. Koene, Utrecht, The Netherlands
© 1995, 1996 L. Koene / M. Sluyters-Rehbach and J. H. Sluyters, Utrecht, The Netherlands
(for the joint publications)