-Oi (WM'WE)
Fig. 1
The typical response to laser beam pulses of p-alanine system contai ning O.Oi M Cu(II), 0.1 M 1igand and 0.3 M K„SO.
at pH = 7
The response of electrode potential exactly reiterates the shape of laser signal,the intensity of which is modula
ted up to the frequency of 3 kHz (higher frequencies have not been investigated).
In contrast to the p-alanine solutions the positive photopotentials and negative photocurrents prevail at low overvoltages in the case of glycine and «-alanine systems.
The photoeffects are weaker, which is in accordance with the evaluation of thermodynamic probability of CUjO formation.
The different photoelectrochemical properties observed in the systems under investigation can be related to the deviations from stoichiometry of the surface oxide layers.
Both the degree and the sign of these deviations depend on the ligand nature as well as on the electrode cathodic polarization value.
References
1. A.A. Survila. Electrode processes in systems of labile complexes of metals. Vilnius: Mokslas, 1989.
2. A.A. Survila, V.A. Uksiene, Elektrokhimiya,
25(1989)952.
3. V.A. Uksiene, V.D. Reipa, A.A. Survila. Investigations in the metal deposition field. Vilnius, 1989. P. 33.
195 25*
HYDROGEN EVOLUTION REACTION ON THE METALS OP IRON GROUP J.Tamm, L.Teimm, P.Vares, J.Arold
Tartu University, Tartu
The hydrogen evolution reaction on the metals of iron group has been studied in many works, but it is very diffi cult to generalize the experimental results obtained. The пгшЬег of factors'which influence the kinetics of the catho-dic hydrogen evolution on iron, cobalt and nickel is much larger them for mercury type metals, however this fact is often not been taken into account. A systematic study of the effect of the chemical nature and structure of the metal, the surface state of the electrode,the electrolyte composi
tion and temperature has been carried out in the work to be reported. The electrodes were prepared from different kinds of metals: poly- and single crystal iron and nickel, elec-trochemically deposited cobalt and nickel. The sTorface of the electrodes has been polished mechanically,chemically or electrochemically.
It has been established that the rate of the hydrogen evolution reaction depends not only on the nature and struc
ture of the metal, but also on the method of preparing the electrode's surface and on the polarization treatment.In the case of nickel, the most essential factor is the pretreat-ment of the electrode: the rate of the hydrogen evolution on mechanically polished surface exceeds that on chemically polished surface 50-100 times /1/. The rates on iron elec
trodes with differently prepared siirfaces differ ' only 5 times, but the purity of the metal has a much stronger in
fluence /2/. If the purity of iron or nickel is high enough, the overvoltage depends only slightly on the structure and piupity of the metal.
According to the great number of experimental data nickel is considered to have highest catalytic activity among the metals of iron group. However,the results of our measu
rements on chemically polished electrodes show that the ex
change current densities increase in the sequence Ni Co Pe.
These results can be explained if to assume that hydrogen absorbs into the metal during the cathodic polarization.Hyd
rogen absorption should decrease the metal-hydrogen bond energy and increase the hydrogen overvoltage. Some experi
mental data support this supposition.lt has been establish
ed /1/ that the special surface state with low hydrogen ovei*-voltage can be realized on polycrystalline nickel electrodes, annealed in hydrogen. After the polarization with high cat
hodic cxirrent density this surface state disappeared. A re
markable rise of the hydrogen overvoltage in time after switching on the cathodic polarization is probably also con
nected with the absorption of hydrogen /3/»
It has been established that for all the electrodes in vestigated, the dependence of the hydrogen overvoltage on the pH of the solution is less than that proposed by the theory of slow discharge of protons. Obviously the nature of the metal-solution interface in the case of the metals of iron group is mainly determined by the metal-water bond ra
ther than by the electrostatic interactions between ions and the metal surface.
Essential differences in the regularities of the hyd
rogen evolution on the metals of iron group become also evi
dent in the different influence of the temperature on the hydrogen overvoltage. According to the theory of the elemen
tary act of electrochemical reactions the overvoltage might have an effect mainly on the value of apparent heat of ac
tivation. The most vinusual results were obtained on mecha
nically polished nickel: the overvoltage has a major effect on the value of the preexponential factor and'influences ve
ry slightly the apparent heat of activation. The results on iron electrodes were as proposed by the theory - overvolt
age influences mainly the apparent heat of activation.
References
1. L.Tamm, J.Tamm, V.Past, Trans. Teu'tu State Univ., 332 (1974) 3.
2. J.Tamm, P.Vares, Elektrokhimiya, 23 (1987) 1269.
3. J.Arold, J.Tamm, Elektrokhimiya, 25 (1989) 418.
197
ОХУ(ЖМ REDUCTION ON GOLD
К. Tammeveski,Т. Tenno Tartu University,Tartu
At present time in the case of amperometric oxygen sen
sors , cathodes are mainly made of noble metals,of platinum and gold preferably. In the present work the electroreduction of oxygen on gold-covered glassy carbon electrodes has been examined,using the rotating disc electrode technique. A thin layer of gold was applied by means of vacuum evaporation. The measurements were carried out in KCW and KCl solutions. All potentials are given versus saturated calomel electrode CSCE3.The solutions were saturated by air oxygen. To obtain reproducible results the working electrode was cycled several times between potentials +0. 1 and -1.2 V.
Voltammetry curves for oxygen reduction are shown in the Figure. They are characterized by two waves in O. 1 M and l.O M solutions of KOH. The second wave is not well formed. In the potential range from -O. 4 to -0.8 V the experimental values of current are higher than required for 2 e process of oxygen reduction.Additional current was considered due to partial reduction of HO^ in the region of the first wave •'1,2/. The results obtained by Adžic et ai.on gold single crystal electrodes testify to the essential dependence of oxygen and hydrogen peroxide reduction process on the surface structure/3-5/. The second wave in cur г ent-potential' Ci,ED curve,which begins at potential -O. 8 V is characterized by a gradual rise of the current. At the potential of -1.2 V current gains the value,close to the diffusion limited 4e process.
I n the sol uti ons of O. 1 M and l.O M KCl the same measurements were carried out. In the i,E curve only one wave can be observed. There is a large region of potentials from -O. 6 V to -1.2 V,where oxygen reduction current is dif
fusion 1 imi ted. Tilis is evident from the plot of i * vs. w Cw-rotation rate3 .where linear dependence is gained,the
Approximation of which will pass through the zero.
-W EMSCE)
Fig. Voltammetry curves for oxygen reduction on gold covered glassy carbon electrodes in air saturated solutions of: 1-0.1 M KCl; 2- 1.0 M KCl; 3-0.1 M KOH; 4- 1.0 M KOT.
Sweep rate lO mV-s rotation rate 18SO rpm.'
Ref erences
1. R. W. Zurilla, R. K..Sen,E. Yeager , J. Electrochem. Soc. ,125 C1978D 1103.
2. M. R. Tarasevich, K. A. Radyushlcina, V. Yu. Filinovslcii ,R. Kh. Bwrs-tein, Elektrokhi mi ya, R Г1970!) 1522.
3. K. R. Adžic , N. M. Markovi с , V. B. VeSovic , J. Electroanal. Chem. ,165 C1984:> ICS.
•4. N. M. Markovic.R. R. Adžic, V. В. Vežovic, J. Electroanal. Chem. ,165 C1984D 121.
5. N. A. Anastasi jevic,S. Strbac,R. R. Adžic, J. Electroanal. Chem. , 240 С1988Э 239.
199