Kinetics of Fe(CN) :· /Fe(CN) :- redox reaction couple on oxidized
Ti, Ni and on Ti02 and NiO single crystals has been studied
by fast methods of polarization and current measurements. From
experimental values of the exchange-currents and transfer coefficients
a and ~. and knowing the reaction mechanism, the potential
distribution in the metal-oxide film - electrolyte system for Ti
has been evaluated. It has been shown, that the main difference
in properties of passive oxide films on Ni and Ti lies in the
character of potential distribution at the oxide-solution interface

0. G. Derjagina

A. Z. Fedotova

E. N. Paleolog

N. D. Tomashov

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CCA-678 CR 0 AT IC A CHEM I CA ACT A 44 (1972) 7 541.138 Confer1mce Paper Kinetics of Redox Reactions on Ti0'2 and NiO Single Crystals and Oxidized Metal Surfaces"' E. N. Paleolog, A. Z. Fedotova, 0. G. Derjagina, and N. D. Tomashov Institute of Physical Chemistry, Academy of Sciences of the USSR, Moscow, USSR Received November 2, 1971 Kinetics of Fe(CN) :· /Fe(CN) :- redox reaction couple on oxid-ized Ti, Ni and on Ti02 and NiO single crystals has been studied by fast methods of polarization and current measurements. From experimental values of the exchange-currents and transfer coef-ficients a and ~. and knowing the reaction mechanism, the potential distribution in the metal-oxide film - electrolyte system for Ti has been evaluated. It has been shown, that the main difference in properties of passive oxide films on Ni and Ti lies in the character of potential distribution at the oxide-solution interface. To investigate the nature of passive surface and to establish regularities of metal dissolution from the passive state it is of interest to study electro-chemical properties of a passive metal surface. There are a number of papers concerned the investigation of properties of passive layers on metals1- 1. Most authors proceed from the semiconductor nature of the oxide film, formed on the passive metal surface. The electrochemical behaV'ior of metals covered with a thin semiconductor film has theoretically been considered and the quantitative connection between the values of transfer coefficients of redox reactions and the potential distribution in this system has been established8. To characterize properties of the passive metal surface8, we have studied the kinetics of redox reactions on the oxidized metals and analysed funda-mental kinetic parameters. From these data, knowing the reaction mechanism, the po,tential distribution in the metal-oxide film- electrolyte system has been evaluated and conclusions have been drawn with regard to the properties of the oxide film. Ti and Ni passivated to different degrees, and compact single crystals of Ti0 2 and NiO, with a known concentration of free carriers, have been studied. TiO" (a rutile modification, n-type) with the concentration of free electrons 1018 cm-3 and NiO (p-type) with hole concentration 1020 cm-a have been used. 1 N Na"S04 with pH= 5.6 for Ti and pH= 10.6 for Ni was employed as a suipporti·ng solution. Under these conditions a stable passive state of these metals in a wide potential range was ensured and no chemical dissolution of oxide films took place. * Based on a lecture presented at the 22nd Meeting of the International Society of Electrochemistry, Dubrovnik, Yugoslavia, September 1971. 8 E. N. PALEOLOG ET AL. The ferri-ferrocyanide ionic couple with a one-electron transfer step was used as a model redox system. Measurements were carried out on rotating electrodes in argon atmosphere, at 25°. To maintain the oxidation state of electrode constant during measurements, fast methods of polarizatf m and current measurements were used. Q5 500 Fig. 1. The <p-i plot (60 v/min) for Ti oxidized at + 0.25 v (1, 1'); + 0.95 v (2, 2' ); Ti02 (3, 3') and pt (4, 4') in 1 N Na2SO, + 0.0025 N K aFe(CN)o + 0.15 N K,Fe(CN)o; 25' C. Fig. 1 shows nonsteady current'"'Potential curves characterizing the rate of redox reactions of a Fe(CN) :- /Fe(CN) !- couple on compact 'DiO" electrode and on Ti, .oxidized at a potential of + 0.25 v and a:t + 0.95 v* The cathodic reduction of Fe(CN)!- ions on Ti and Ti02 is shown to have a low overvoltage and the same Tafel slopes as on Pt electrode (b ..., 0.12 v) . Anodic oxidation of Fe(CN) !- ions on Ti and TiO,, is greatly inhibited and depends on the degree of electrode oxidation. Table I presents values of kinetic parameters of the redox reaction of Fe(CN) f /Fe(CN) :- ions for Ti, rutile and Pt. It follows, from the Table, that a passive titanium surface (as well as that of Ti02) is characterized by the low exchange current of the redox reaction (4-5 orders of magnitude less than on Pt) and by the low value of the apparent anodic transfer coefficient. Special experiments on the Ti02 electrode showed that mainly electrons from the conduction band participate in these redox reactions. In this case, the reaction rate of anodic oxidation of solution ions is determined by the potential drop value in the Helmholtz layer. Therefore, if the main part of the potential drop is localized not in the Helmholtz layer but * The potentials are referred to NHE. KINETICS OF REDOX REACTIONS g TABLE I Kinetic Parameters of the Fe(CN) :· + e ~ Fe(CN) :· Reaction for Ti and Ti02 Film forming Initial film Exchange Transfer current coefficients Electrode potential thickness (io) (volts) A A/cm' a. ~ Ti + 0.25 37 0.43 x 10·1 0.53 0.20 Ti + 0.95 55 0.10 x 10-7 0.53 0.08 Ti02 + 0.25 0.25 x 10·1 0.59 0.05 Pt 0.87 x 10-3 0.52 0.48 in the oxide, the anodic reaction rate should be very low and only slightly depending on the electrode potential. Measur ing the electrode differential capacity on the TiO" electrode in 1 N Na,S04 the linear dependence 1/C2 - cp has been obtained (Fig. 2). Hence, according to Shottky , the whole potential drop, under conditions· of c;nodic polarization, actually localizes in the space charge region of TiOc, which accounts for the great inhibition of anodic oxidation of Fe(CN) ~- ions on this electrode. 0 Fig. 2. The C-qi (1, 2) and 1/ C' - qi (3, 4) plots for Ti02 and NiO respectively in 1 N Na,so, 25' C. Assuming that the mechanism of these reactions on passive Ti is the same as that for rutile and using experimental values of exchange-current and transfer coefficients a and B, obtained for passive Ti, we made an attempt to evaluate, the portion of the ·potential drop in the passive fi lm and . in the 10 E . N . PALEOLOG ET AL. ion ic region of the double layer . We evaluated also the concentration of free electrons in the surface layer of the film-solution interface.* This evaluation has shown that at anodic polarization of Ti, passivated at potential 0.25 v, the potential dr op in oxid e film is equal to 600/o and increases when Ti was passivated at a more positive potential (0.95 v). The concentration of free electrons in oxide decreases at the contact with the electrolyte from 7 X 106 at the passivation potential of 0.25 v to 4 X 10" at potential 0.95 v. Similar measurements have been carried out on a Ni electrode, the compact oxide of which possesses hole conductivity. Fig. 3 shows cathodic and anodic reaction rates on the passive Ni surface and on the NiO single crystal in the same redox medium. The passive films on Ni were produced at the potentials + 0.30 v, + 0.50 v arnd + 0.70 volts. 0,5 a f,O I , s I 0 1 2 l , ,,,~, Fig. 3. The cp-i plot (4 v i min) for Ni oxidized at + 0.30 v (1); + 0.50 v (2, 2') ; + 0.70 v (3) ; NiO (4 , 4') and Pt (5, 5') in 1 N Na2SO, (pH ~ 10.6) + 0.005 N K 3Fe(CN)r. + 0.005 N K4Fe (CN),; 25° c. At close thicknesses of the passive film on Ti and Ni (of the order of 30-50 A) the behavior of Ni differs from that of Ti. The anodic oxidation reaction of Fe(CN)~- ions on passive Ni and on NiO proceeds with no inhibition. The cathodic reduction of Fe(CN) f ions at potentials up to + 0.35 v is . practi-cally not inhibited either. In the region of less positive potentials the inhibition of cathodic reaction, however, takes place and turns out to be stronger for the film produced at the potential + 0.30 v as well as for NiO electrode. In this case, the inhibition of the reaction manifests itself as the decrease of the »limiting« current on the cathodic i - cp curves. The Tafel slopes of anodic curves are near to those of Pt and are in-dependent of the potential of film formation. For cathodic curves the slopes change in the process of polarization, that is, they increase at a potential < + 0.35 v. Thus, the data obtained show that independently of the passivation degree, at potentials more positive than + 0.35 v the surface of passive Ni * In our calculation we 11.ave admitted that the potential in the film changes linearly. KINETICS OF REDOX REACTIONS 11 TABLE II s- ,_ Kinetic Parameters of Fe(CN) , + e ~ Fe(CN) , Reaction for Ni and NiO Film forming Initial film Exchange Transfer Electrode potential thickness, current coefficients (V) A (io), p A/cm2 a Ni + 0.30 41 0.4 x 10-3 0.26 Ni + 0.50 47 0.3 x 10-3 0.53-0.21 0.48 Ni + 0.70 0.5 x 10-3 0.58-0.22 NiO + 0.50 0.2 x 10-3 0.58-0.32 0.41 Pt 0.4 x 10-3 0.52 0.48 and NiO behaves itself as an effective inert metallic electrode with respect to retj.ox reactions in the system of Fe(CN) ~- /Fe(CN) :- ions. The absence -of inhibition of the anodic oxidation reaction on NiO and the value of anodic coefficient close to 0.5, independently of the reaction mechanism, indicate the localization of the potential in the Helmholtz layer under these conditions. Evidently, the potential redistribution and the increase of the part of potential drop in the oxide occur at potentials, less than + 0.35 v. In the case when the valent electrons participate in the cathodic reaction, the increase of the portion of the potential drop in the solid phase may be the cause of the a coefficient decrease. Our supposition is confirmed lby the data on the capacity measurement on the NiO electrode in 1 N Na2S04 (Fig. 2) . We have observed the linear dependence of 1/C2 - cp in the narrow region of potentials, namely from + 0.35 v to - 0.30 v, i. e., just where the inhibition of cathodic reduction of Fe(CN) f ions on the NiO electrode takes place. Comparison of the electrochemical behavi·or of the NiO electrode and the Ni passive surface allows to admit that for Ni in the potential range of + 0.35 to - 0.30 v the main potential drop is also localized rn an oxide film. This accounts for the peculiarity of electrochemical behavior of such electrodes. We have also studied the redox reaction 0£ oxygen reduction and OH-ions oxidation on passive Ni and NiO electrodes. The equilibrium potential of this reaction at pH = 10.6 is close to that of the Fe(CN) f /Fe(CN) :- couple. The overvoltage of the cathodic reaction on Pt is, however, higher and, there-fore cathodic reaction for oxygen proceeds in the potential range of 0 to - 0.30 v, i.e., in the range where the main potential drop is localized in the . oxide. As a result, on NiO and passive Ni the reaction of oxygen reduction is inhibited sharply up to potentials from - 0.50 to - 0.60 v. (Fig. 4). The kinetics of Ti and Ni anodic oxidation in 1 N Na2SO, at various potentials have been investigated together with the study of properties of oxide layers. The analysis of i-t curves (Fig. 5) shows that the current change with time is generally different for Ti and Ni. The curves for Ti plotted in double logarithmic coordinates give a straight line with slope (K) close to minus unity. The latter shows the fulfilment of the logarithmic law of Ti oxidation and the availability in the film of an electric field controlling its growth. 12 IJ1,J - 0, 5 0 0,5 f,0 \ ' f.5 ---E. N . PALEOLOG ET AL. ------2 _,... / / ,... 3 i ('"~/cm') Fig. 4. The cp-i plot for Ni oxidized at + 0.50 v (1) and the real curve of 02 reduction for Ni (2); NiO (3) Pt (4) in 1 N Na2S04 (pH = 10.6); atm 02 250 C. K,, ·O,H) •( 5 (tt,,:-0,98; K1 =-0.52} ·( ·( 0 . {Fig. 5. The lg i-lg t plot for Ni at + 0.30 v ; (1) in 1 N Na,so,; (2) + 0.50 v ; (3) + 0.70 v; (4) for Ti at + 0.25 v; (5) + 0.70 v (without stirring) . For Ni the fulfilment of the logarithmic oxidation law is observed during film formation at + 0.30 v, while the film growth at more positive potentials (0 .5 and 0.7 v) obeys the logarithmic law only in the initial period (during the first 5-7 minutes) and then it is replaced by the parabolic one. The change of the slope of lgi-lgt straight lines (K2 = - 0.5) corresponds to the latter. Diffusion becomes, in this case, the limiting stage of film growth. The absence KINETICS OF REDOX REACTIONS 13 of the effect of solution stirring (Fig. 5, curve 4) leads to the assumption that the diffusion of the particles controlling the growth of the oxide film takes place in the solid phase. The above mentioned differences in kinetics and the mechanism of the Ti and Ni oxidation are in accordance with the data on the estimation of potential distribution in these systems. Thus, the main difference in properties of a passive oxide film on Ni and Ti lies in the character of the potential distribution at the oxide-solution interface. It is the difference in the potential distribution at the interface that is responsible for the difference in the electrochemical behavior of pas-sive Ti and Ni. REFERENCES 1. I. F. Dew a 1 d, J. Phys. Chem. Solids 14 (1960) 155. 2. J. O' M. Bock r is, A. K. N. Reddy, and B. Rao, J. Electrochem. Soc. 113 (1966) 1133. 3. A. C. Macri des, J. Electrochem. Soc. 111 (1964) 392. 4. W. M. Nov a k o vs k y and W. I. 0 v ch are n k o, Zaschita Metal. 4 (1968) 656. 5. A. M. Suchotin and P. P. Stanishevsky, Tr. 3-go Mezhdunarodn. Kongressa po Korrosii Metal. I, p. 374. Moskwa (1968). 6. W. A. Lo v a ch o v, A. I. 0 ch e, and B. N. Ka b an o v, Elektrokhimiya 5 (1969) 958. 7. A. G. Akim o v and I. L. Rosen f e 1 d, Dokl. Akad. Nauk SSSR 193 (1970) 116. 8. A. M. Kuznetsov and R.R. Dogonadze, Izv. Akad. Nauk SSSR, Ser. khim. (1964) 1240. IZVOD Kinetika redoks reakcija na monokristalima TiOi i NiO i na oksidiranim povrsinama metala E. N. Paleolog, A. Z. Fedotova, O. G . Derjagina i N. D. Tomashov Mjereni su kineticki parametri za elektrokemijsku redoks reakciju Fe(CN) .. ' Fe(CN) :- na elektrodama od Ti, Ti02, Ni, NiO i Pt da bi se pronafao mehanizam prijenosa naboja. Ustanovljeno je da je za Ti02 (kao p-tip poluvodickog oksida) brzina reakcije oksidacije uvjetovana razlikom potencijala lokaliziranog u sloju pro-stornog naboja oksidne faze. Za NiO je utvrdeno da je najveci dio ukupnog pada potencijala lokaliziran u Helmholtzovom sloju, i da stoga ponasanje NiO slici onom inertnih metalnih elektroda, npr. platine. Pokazano je da se razlike u elektrokemij-skom ponasanju pasivnih oksidnih slojeva mogu uspjesno interpretirati razlikama u lokalizaciji potencijala u pojedinim dijelovima granicnih slojeva. INSTITUT ZA FIZIKALNU KEMI.JU AKADEMIJA NAUKA SSSR Primljeno 2. studenoga 1971. MOSKV A, SSSR 

Kinetics of Redox Reactions on Ti02 and NiO Single Crystals and Oxidized Metal Surfaces

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