2 3 4
(Poland) in the three-electrode cell with a Pt, Pd or Hg pool working elec-
trode, a Pt auxiliary electrode and the silver chloride reference electrode.
The electrode potential was scanned with the aid of an LP7 polarograph
(Czechoslovakia) and checked by a V7-16A digital voltmeter (USSR).
Current-voltage curves were recorded by a two-coordinate recorder NE-240
(Hungary).
Below the potentials are given relative to the normal hydrogen electrode.
Results and discussion
Analysis of the 'microelectrode' model of dihydrogen evolution on metals
The 'microelectrode' model in the form developed by McLendon can-
not be applied to the quantitative description of the catalytic dihydrogen
evolution for a broad class of metals and reductants, since it takes into
account only a single particular situation where the rate-determining step of
H2 evolution is the reaction of proton discharge on the metal, and the step
of A- oxidation is described by a reversible current-voltage curve for this
metal. Meanwhile, the mechanism of cathodic H2 evolution is known to
depend upon the nature of the metal, the rate-determining step being either
proton discharge (Volmer mechanism), or electrochemical desorption of
hydrogen atoms (Heyrovsky mechanism) or recombination of hydrogen
atoms on the metal suface (Tafel mechanism). In the two former cases, the
dependence of reaction rate on the electrode potential is the same at a con-
stant surface coverage with adsorbed hydrogen atoms. In all equations that
relate the rate of H: evolution to the potential of microelectrodes, we will
take into account only the direct reaction of proton reduction, thus neglect-
ing the back reaction of H2 oxidation. The validity of this supposition is
provided by the fact that in our experiments with introduction of a large
amount of the reductant the value of the overvoltage usually exceeds 50 mV,
and the concentration of H2 is initially negligibly low.
The rate of oxidation of the reductant A- is limited by the step of A-
discharge for most one-electron reductants [5]. Depending upon the ratio of
the mass-transfer coefficient mA- of the reductant to the rate constant kg-
of its discharge, a current-voltage curve for this reaction may be either
reversible (the situation mA-/kA- ~ l) or irreversible (the situation mA-/
kA->> 1). As a rule, kA- is high (>>10-4 cm s-1) and the current-voltage curve
is reversible; however some exceptions are known (kcr~/cr3 = 7 × 10-6 cm
s -~ [ 5 1 ) .
For example, the current-voltage curve of MV÷" oxidation registered
by us (Fig. 1) is reversible, and the haft-wave potential is close to the stan-
dard redox potential
0
( E M v + . / M v 2+ -- --0.44 V).
On the other hand, under the conditions of catalytic H2 evolution the
regularities of half-reactions (1) and (2) can differ somewhat from those
observed when the reactions are accomplished independently on bulk metal
electrodes. This difference may be due to the mutual effect of the haft-
Savinova
