Acceleration of fast redox reactions with upd-modified electrodes?
Numerous electrochemical reactions are influenced catalytically by underpotential deposition (upd) of foreign metals on the electrode. With upd in opposition to the bulk deposition only one until two monolayers of the metal were deposited. The upd-potential could be some 100 mV more anodically than the Nernst-potential. Experimental results are often restricted to qualitative observation of this effect.To seize quantitatively the modifications of electrode at a fast redoxreaction, the electrodes were examined after the upd-modification with the electrochemical impedance method.
upd is performed with cyclic voltammetry. The cathodic upd-peaks were integrated to determine the mass of the deposited metal. On platinum different masses of deposited metal could be found: Pb: 94 µC cm-2; Ni: 173 µC cm-2; Cu 270 µC cm-2.
The poorly defined peaks are difficult to integrate, under these conditions the otained charges have to ge treated carefully. Since no hydrogen-adsorption/desorption-peaks were found in the cyclic voltammetry, complete coverage of the platinum electrode could be suggested. From the recorded impedances, made after the upd, the charge tranfer resistance and the doublelayerconductivity could be determined. From the calculated charge tranfer resistance the exchange current density and by including the standard concentration c* with
j00 = j0 c*(caoxc1-ared)-1
the standard exchange current density could be calculated.
|RD/ Ohm cm2||4,57||7,35||5,72||3,18|
|j00/ A cm-2||0,56||0,35||0,45||0,81|
As can be seen in the table, upd has, with constant double layer capacity as a measure of the true surface area, a strong effect on the charge transfer resistance and therefore on the exchange current density. Thus a catalytic effect on the redox reaction can be quantified. It could be seen, that upd of lead results in an acceleration of the redox reaction by a factor of 1.4, on the other hand upd with copper or nickel leads to an inhibition of the redoxreaction with a factor of 0.6 (copper) and 0.8 (nickel).
It could be shown, that the upd has a catalytic effect on the fast redoxreaction Fe2+/3+. This influence could be quantified with the aid of electrochemical impedance measurements. upd with lead on platinum shows an acceleration of the redoxreaction with a factor of 1.4. With the upd from copper on platinum an inhibition of the redoxreaction with a factor of 0.6 and with nickel on platinum with a factor of 0.8 could be found.
In the future impedance measurements and SERS-examinations should be carried out with upd-modified goldelectrodes. For further examinations an IR-spectrometer is available. The examinations made so far should be repeated in the system Cr2+/3+.