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Electrocatalysis of oxygen reduction on organic metallic complexes
BioeCectrocltettristv?l
am2 Bioeneugetics
Electrocatalysis Complexes
of Oxygen
by JI. R. TAR~SEVICH, Institute
of Electrochemistry
Manuscript
received
March
4,
1Q--2g
Reduction
I<. A.
on Organic Metallic
RIDIYSCHGIS_X
of the Academy 16th
(1977)
and S. I. AZ~DROUSEVA
of Sciences
of the USSR,
3foscow
1976
The influence of the nature of the central ion and ligand on the mechanism and kinetics of osygen electroreduction at metal (Fe, Co, Nn) phthalocyanines and tetraphenJ-lporphins in the pH range of o,z-x_~,a The oxygen reduction reaction to hydrogen peroside in was studied. acid and neutral solutions proceeds at a faster rate in the presence of organic compleses of metals than in the case of carbon black. In addition, in alkaline solutions a direct reaction occurs in the case of MnPhc, &lfiPhci, CoPhci, CoTMPP, involving the breaking of the O-O bond. The kinetic parameters d U/d log I, d U/d pH of the osygen reduction reaction to hydrogen peroside are the same for both the organic comples studied and carbon black. This points to a similar mecharkm for the slow stage: the addition of the first electron to an adsorbed oxygen molecule.
Introduction The eleetrocatalysis on organic semiconductors is a new trend in electrochemical research,lv” and is of special importance in the investigation of the relationship between the chemical structure of a substance and its catalytic acti\-ity_ Electrocatalysis on organo-metallic complexes is of particular interest since they can sert-e as models for the most perfect catalysts-enzymes. Natural compounds of a number of porphins are of great importance in biochemical processes_ Molecular compleses of metals with porphin groups are active centers of protein molecules - enzymes and electron carriers in respiratory chains. The possibilities for varying the chemical structure and electrophysical properties of organic semiconductors are unlimited. Therefore, these substances hold considerable scientific and practical promise. This paper presents the results of systematic studies on the mechanism and kinetics of o_sygen3 reactions on organometallic complexes, which are of importance for practical use.
Osvgen
Reduction
on Organic
Metatlic
Compleses
19
Experimental The following organic compleses were used in the esperiments : I. monomeric phthalocyanines of bivalent Fe, Co, Mn and phthalocyanine without metal, prepared from phthalonitrile and salts of the respective metals ;j
2. polymeric phthalocyanines of Fe, Co, Mn. These compounds were synthesized by sintering, pyromellitic acid dianhyclride phthalic anhydride, urea, ammonium molybdate and salts of the respective metals; 3_ cobalt tetraphenyland tetra(p-methoxyphenyl)-porphins. These compounds were obtained by the condensation of pyrrole with benzaldehyde and anisaldehyde, respectively, using cobalt acetate as complesing agent .I In our investigations we used mainly the disk-ring electrode methThe esperimental set-up was descrrbed in Ref. 3, 4 and 6, and od.” the electronic equipment and esperimental technique in Ref. 7. The theory of parallel-consecutive osygen and hydrogen peroxide reactions and the limits of its applicability are given in Ref. S and g_ The application of this theory to the calculation of the constants of partial osygen and hydrogen peroxide reactions was described in Ref. 3, 4. 6 and IO. We shall consider here only some specific features of the technique used, associated with the investigation of organometallic complexes. The disk was made of pyrographite, to which a thin layer of active mass was applied, which contained carbon black, 20 *!Jometallophthalocyauine and Co-tetraCp-methoxyphenyl)-porphin, (or 5 */* Co-tetraphenylporphin IO */*teflon. Metallophthalocyanines were deposited respectively) and on carbon black from a solution in concentrated H&SO,. while tetraphenylporphin compleses came from a solution in dimethylformamide. After the misture was applied to the electrode, it was sintered in air at 330 *C. Special esperiments 11 showed that at the current densities esceeding one half of the limiting value of the diffusion current, the electrochemical reduction reaction of oxygen was displaced to the external surface. Under these conditions, the disk surface can be considered as *being At smaller current densities, the porous nature of the equally accessible. In calculating the kinetic active layer should be taken into account. it is very difficult to introduce a correction for the electroparameters, chemical activity of the carrier (carbon black). Therefore, the constants of individual reactions were calculated by means of the theory of parallelconsecutive reactions (from the data obtained by the disk-ring method) only in the cases where in the presence of an organic catalyst the polarizatran curve was shifted by not less than 50-60 mV more positively in comparison with that for pure carbon black. The esperiments were carried out in the pH range of '0,3-L&2 The solutions were prepared from using phosphate buffer solutions. distilled and recrystallized reagents and purified by cathodic polarization
20
Tarasevich,
Eadiyschkina
and
Androuseva
in hydrogen atmosphere. The potentials were referred to the hydrogen electrode in the same solution (UJ or to the standard hydrogen electrode ( U~I,) .
Me&a&m
of oxygen and hydrogen peroxide reactions
The main object of this part of the investigation \;as to elucidate the influence of the central ion and of the &and nature on the mechanism The mechanism, or path, of the oxygen reduction on of the process. organometallic comple.ues was analyzed by means of the scheme : I_
K3I
‘Z/7
ILO-+
O,-+
H,O
-
/
-k,
H-0 0,
kl This scheme was discussed more than once in our works.3~4~6~8-11 The investigations carried out so far with a wide variety of electrode The ratio k,/k2 in this-scheme materials confirm its general character_ characterizes the ratio of the reactions proceeding directly to water and via an intermediate formation of hydrogen pero_tide. zIe_ with and without a break of the O-O bond. The subsequent transformation of hydrogen pero,side is characterized by the constants k3 (cathodic reduction) and These constants were calculated from k, (catalytic decomposition). results obtained with the disk-ring electrode method. Fig. I shows the polarization curves of osygen reduction on the monomeric phthalocyanines of Fe, Co, Mn and without metal, as well
Fig.
I.
Polarization curves of oxygen reduction on monomeric phthalocyanines of iron (3). cobalt (3). manganese (_I) and without metal (I), and the corresponding dependences of the limiting currents of hydrogen peroxide oxidation (~'-4') on the ring in 0.1 N KOH. w = g6o r.p.m. Dashed line - carbon black.
O_xygen
Reduction
on Organic
Metallic
Cotiplextis
-2’I
-..
as the corresponding dependences of the limiting currefits-of hy&o&n peroxide oxidation on the ring from the disk potential.in. 0.1 N Q?H_ In the case of FePhc, just as in Ref. IZ, at potenti~sIlrlore_cathodic-than; : 0,7 V, the value of the current on the disk is close. to that_of,the:-lim~tiiig current fdr a four-electron reaction. -In this case; small amounts :of ’ H,O, are recorded on the ring electrode only at Ur < 0,4 V. Iri the presence of CoPhc and MnPhc, the currents on the disk increase!s@nificantly, as compared to carbon black, while the cuqents_ on the’.eng ~. decrease by about twice as much. On the curve of H,O&ogda@n, the maximum shifts in the a.nOdic direction by N 300 mV. :.:: . The dependence of the constants of the reduction reactions-of 0, to H,O (R,) and H,O,(R,) on the disk potential is $hqwn.-oti J?ig.’ i. We can observe that the ;ntrqluction of monoqleric CqPhc;Mtq &rboii -_ black increases the. constant -k, twice_ as much, as complied to pure tirbon : black. Moreover, in the c&se- df _Mn soPhc, a parallel reaction arises whithont2 forming hydrogen peroxide.
_ Fig. 2. Dependence of the constants k, (@) and Rs (0). on the disk potential for oxygen reduction on.. monomeric phthaiocyanines of cobalt (2). manganese (3,3’), without metal (I) and carbon black (4) in 0.1 N KOH.
0.8
0.7
0.6
OS
_.
. I .
Fig. 3 and 4 show similar dependences for polymeric _metallo; By comparing Fig. I with Fig. 3 it appears- that the phthalocyanines. introdudtion of &In and Co into the polymeric chain increases considerably the currents on the disk and decreases the currents on the *g. me polymerization of CoPhc leads to a change in the mechanismi i_%:-tb-$reaction of 0, reduction directly to water takes place--~ Thus,-deperiding on the central ion nature the phthalocyanines investigated form-s a&tie solution the series: monomers: Fe >Co >Mn >H,; polymers: Fe >Co >Mn. It is known.13 that the length of the O-O bond is very-sensitive to a change in the ligands bound with the metal. - Ins this connedtidn, it was of interest to study the 0, ionization.reaction ori organic complexes in which the same central atom (Co)- is bound with difEei@;lig&ti&. The polarization curves of 0, reduction on CoTPP, CoTMPPand (CoPhc)f in alkaline solution are shown in Fig. 5: This figure also shdws Jthen.&%
Tarasevich,
22
Radi!-schkina
and
_Androuseva
0.25 t
_/----,. --0.0
.
2”
1
‘\
3’
U,(V) I 1.0 Fig.
3.
Polarization curves of osygen reduction on polymeric phthaloq-anines of iron (2). cobalt (3), manganese (I) and corresponding dependences of the limiting currents of hydrogen pero_xide osidation on the ring in 0.1 X KOH. (I’-39 w = g6o r.p.w. Dashed line - carbon black.
Fig.
4.
Dependence of the constants R, (e) and k, (0) on the disk potential for osygen reduction on polymeric phthalocyanines of cobalt (I) and manganese (z.z’) in 0.1 _L- KOH.
0
I
0.8
0.7
I
0.6
UJ
VI 1
0.5
pendence on these compounds of the limiting currents of H,O, osidation to the total process on the ring. -1s can be seen in Fig. 6. the contribution of the direct reaction to water (k,) is 50 y/o for (CoPhc)i and 30 ",/o for CoTJIPP. On CoTPP, just as on monomeric CoPhc, the reaction occurs In alkaline solution the actkit!only via intermediate fomration of H,O,. forms the series : (CoPhc)i >CoTMPP >CoPhc >CoTPP. The polarization curves of 0, reduction on organic Co compleses in acid solution and the dependences on the disk potential of the currents studied the electroon the ring are given in Fig. 7_ For all the compleses reduction reaction of molecular osj-gen undergoes activation, as com-
Osygen
Reduction
on
Organic
Metallic
Complexes
23
Polarization curves of oxygen reduction on polymeric Co-phthalocyanine (I). Co tetraphenylporphin (2) and Co tetra (p-metho_-phenyl)-porphin (3) and corresponding dependences of the limiting currents of hydrogen peroxide oxidation on the ring in 0.1 N KOH. ( “-3’) -ZI= 960 r_p_~n_
50
Fig.
6.
Dependence of the constants k, (g) and R, (0) on the disk potential for osygen reduction on polvmeric Co-phthalocyanine (I). Co-tetraphenylporphin (2) and Co-tetra(P-methosyphenyl)porphin (3.3’) in 0.1 N KOH.
o.
U,(V)
O”?
0.8
0.7
0.6
0.5
pared to pure carbon black. The calculations show that in all cases the reaction in acid solution occurs vin intermediate hydrogen peroside formation. The value of the constant k, decreases in the series : CoTUPP> We see that in acid solution (CoPhc)i is less active COTPP >(CoPhc)i. than tetraphenylporphin compleses.
24
Tarasevich,
Radiyschkina
Fig.
and
Xndrouseva
7_
Polarization curves of osl;gen reduction on polymeric Co-phthalocpanlne (I), Co-tctraphenylporphin (2). Co-tetra(+methox>-phenyl) dependences porphin (3). and corresponding of the limiting currents of hydrogen peroside osidation (“-3’) on the ring in 0.1 N H,SO,. Dashed fine - carbon black. -a = 960 r.p.w.
Kinetics of oxygen electroreduction The elucidation of the slow step in the series of the partial reactions of the global process of reduction of the molecular osygen necessitates These studies were carried out for a kinetic study in a wide pH range. As it follows from. the data preH,Phc, CoPhc, (CoPhc)i and COTPP. in the presence of these compounds sented in the preceding section, [escept for (CoPhc)i in alkaline medium] the reduction of osygen occurs This in two steps vin intermediate formation of hydrogen peroxide. paper is concerned with the kinetics of the first step : reduction of 0, The kinetics of the second step, including the chemical and to H,O,. electrochemical hydrogen peroside reactions, for some phthalocyanines depenwere analyzed in Ref. 14. Fig. S and g and Table I give typical 0.6 U”*W
\
COTPP CoPhCi
\ Fig. 5. The log (I /Ilk - I) cs. o‘,, - dependences for osygen on organic compleses for pH = 0.3 ; 7,6 ; I+Z.
reduction
Oxygen
Reduction
on Organic
Metallic
Complexes
Fig. g. Dependence of the half-wave potential on pH for osygen reduction on carbon black (0). monomeric (A) and polymeric (X) Cophthalocyanine, phthalocyanine without metal (A) and Co-tctnphenylporphin (0).
Table I. Kinetic parameters peroxide on carbon materials Electrode
i
Pyrographite
i I
Carbon
black
H,Phc
.
of the reduction reaction of oxygen to hydrogen and in the presence of organic compleses. dU/d
PH 2-g
pH,
mV 130-160
0
9-14
60-30
2-10 IO-14
60-30
30-40 go-120
0
2-10
40-50 100-120
0
30-40
10-I-f
40-50
I
I
CoPhc
Z-10
0
30-40
IO-I-$
1
1
(CoPhc)i
/
CoTPP
dences
of
the
2-10
IO-15
IO-I-#
40-50 II0
O-Q 4-10 IO-14
kinetic
pararneters
O-10 30
dU/d
I'O-If0 50
I
IOO--120 IOO-120
40-60
and dU/d pH for the cataconditions. Over the whole of organic compleses, the rate
log I
lysts studied under different experimental pH range from 0,~ to 14 ,z in the presence
I ! / 1 i I
Tarase\-ich,
16
Radiyschkina
and
Xndrouseva
of osygen reduction to hydrogen peroside is higher than for carbon to neglect Therefore, to the first approsimation. it is possible black. the cqntribution of the carrier current to the total current_ Consider the kinetical dependences for alkaline (pH > IO), neutral (4 < pH < IO) and acid (pH < 4) solutions separately_ In alkaline solutions dU,/d pH and (dU/d pH)~=~,,m are 60-30 mV. The latter The polarization slope dU/d log 1 is value is obsen-ed at pH > 12. - 40-60 mV in the current range where dU/d pH was above the mentioned \-alue. In acid and neutral solutions dU/d pH E O-IO mV and dU’/d log I lies between 90 and 150 mV. There are two esceptions to these data. For (CoPhc)i at low current densities dU/d pH is - 40-50 mV In this case the points with I = IOWA along the whole pH range I-I+ lie on the sections with a slope smaller than go-150 mV. For CoTPP in acid solutions dU/d pH = 100-110 mV. Thus. escept for CoTPP in acid
medium
(CoPhc)i at low current
and
of the reduction reaction of 0, to those for carbon black.
Discussion
densities,
to H,O,
the kinetic
on different
parameters
catalysts
are close
of results
method The in\-estigations by means of the disk-ring electrode showed which reactions in the comples process of reduction of the molecular osygen are catalyzed by organometallic compleses and how the change of the chemical structure of the catalyst affects their rates ratio. Data are listed in Table 2. In acid solution in the presence of organometallic compleses, the rate of the reaction leading to H,O, increases. In alkaline solutions in the case of MnPhc, (AlnPhc)i, (CoPhc)i and CoTMPP a direct reaction to water with the break of the O-O bond occurs in addition. The dependence of the activity of phthdlocyanines on the central ion nature has been repeatedly discussed in the literature1-a*15 and can be qualitatively interpreted in terms of the theory of the molecular orbitals. The adsorption of an oxygen molecule on phthalocyanines is accom-
Constants
partial
reactions compleses
H,Phc
iCarbon ; black PI
.I R,x
IO3 cm/s!
-
! CoPhc
,
’ (CoPhc)i’
,
&\[nPhc I>In Phci\ I I I
-
-
55
-
I 12
CoTJIPPi
_-’
/ j
CoTPP
30
i
32
.
55
15
IS
10
1
I4
'
Osygen
Reduction
on Organic
Metallic
Compleses
27
panied by a partial electron transfer from the central ion of metal to antibonding Tc-orbitals of oxygen and by a weakening of the O-O bond in the molecule_ This accelerates the cathodic reduction of 0,. The increase in the number of collective x-electrons (polymerization of phthalocyanines or introduction of methosy-groups into tetraphenylporphin) results in a greater electron density of the central ion. This facilitates the electron transfer to osygen or to hydrogen peroside molecules and accelerates their reduction. On the other hand, an increase in the degree of electron localization on an adsorbed 0, molecule leads to the weakening of the O-O bond up to its breaking point : the reaction will then occur directly to water. This treatment of the problem is undoubtedly only a qualitative one and does not take into account a number of esperimental facts. The explanation of these facts necessitates more accurate theoretical and experimental studies of the electrophysical properties of organometallic compleses. M7e shall discuss now the kinetics of the reduction of 0, to H,O, on different organic compleses. The coincidence of the kinetic parameters of the 0, reduction on organic compleses and on carbon black. obser\red in most cases, testifies to a similarity in the nature of the slow step. Therefore, we shall first consider the processes occurring on carbon black. We studied earlier quantitatively the 0, reduction on p,yrographite in We showed that the cathodic reaction of the the pH range of I-I+~~ osygen reduction is controled by the slowness of the step of the first electron transfer to an adsorbed oxygen molecule (and in the case of the reverse reaction of anodic hydrogen peroside oxidation - from the HO,--ion) : 0,
+
%lb,,k MO.,.
JIHO, HO,-
M -+ NO,,&. + e+ + +
+
H,O
MO,f
e- s H,O
(slow step)
MHO.,
+
OH-
ATHO,f
H,O,
+
OH-
The other reaction steps leadin, m to the formation of H,Oz (in acid) or HO,(in alkaline solution) occur rapidly. In acid and neutral solutions the reaction is irreversible and (dU/d pH)rZconst = 0, dU/d log I = 120 mV at o! = 0.5. When the pH rises, the rate of the reverse reaction increases This and the system O,/H,?,(HO,-) approaches the reversible state: leads to a potential shift toward the negative direction and a decrease Table I shows clearly that in the value of dU/d log I to 30 mV. there is practically complete agreement between the data on pyrographite The only esception is the higher value of and those on carbon black. XI/a log I in the region of the cut-reuts where the O,/HO,system This is apparently due to the porous nature approaches equilibrium. of the active layer of carbon black and to the change of the H,O, conIt would be quite reasonable to assume that on centration across it.
Radiyschkina and
25
For organic catalysts the reduction of 0, follows the same mechanism. (CoPhc)i (dU/dpH)r= lo~ = 40-50 rnk at pH from 2 to 14. Apparently, owing to a large anodic osidation rate of H,O, on (CoPhc)irq, the equilibrium of the O,--H,O, system is maintained at low currents throughout the whole pH range. +neThus, on organic cobalt compleses the reaction 0, w H,O, occurs much faster than on a carbon carrier and without any change in the nature of the slow step- The reason for this is a faster electron transfer to an osygen molecule of the adsorption site.
due to an increase in the electron
density
The 0, reduction on CoTPP in acid solution does not proceed according to the above scheme (Table I). The kinetics observed in this case can be explained on the basis of the foliowing mechanism supposedly responsible for the slow step : 310,
+
Ht
+
e-
-+
MHO,
At a = 0.5 au/a log Z = 120 mV and au/a pH = 120 mV. According to the data reported in the literature ,17a18 tetraphenylporphin complexes can undergo protonation both in the molecule center and on benzene rings, whereas in the case of phthalocyanines the same may happen only on It is possible that this-difference in the the periphery of the molecule. chemical properties of the compleses mentioned above accounts for the change in the reduction mechanism in acid solutions on cobalt tetraphenylporphin and tetra(p-methosyphenyl)porphin. In further studies on the electrocatalysis on organic complexes it will be necessary to take into account the properties of the actual system If the thickness of conductor(carrier)-semiconductor film (catalysts)_ the semiconductor film is not greater than the screening depth, then the Such injection of current carriers into the semiconductor may occur. phenomena were obsen-ed in the heterogeneous reactions of decomposition of hydrogen peroside and of the osidation of cumenelg on metal powders wrth a film of metal phthalocyanine. At present organometallic complexes are not only of an academic, but also of a certain practical interest. Some methodGo of activation of carbon oxygen electrodes were developed on the basis of these compounds, thus ensuring a stable operation during several thousands of hours. However, the discussion of the practical application of organic compleses is beyond the scope of the present work.
References H. JAHNKE and 31. SCH~XBORS, Comptes Rendars, Tvoisidnze Jouvde Interwationde d’Etzrde des Piles ci Combustible. Presses Academiques EuropCennes. Bruselles (1969) p_ 60 n_ BI. SAVY, P. XXDRO and C_ BERSXRD. Electrochim. Acta 19, 403 (1974) 1
Oxygen 3
4
5 6
7
8 9
10
11
12
13 14 15 16
17
18 19
40
Reduction
on Organic
K.ARADIYSCHKINA,M.R.TARASEVICH
Metallic
Complexes
and S-1. ANDROUSEVA, -
:
29.
Elekfrokkimiya
11, 1079 (1975) !%I. ASDROUSEVA. M.R.TARAsE~IcH~~~ K._~.RADIYSCHICI~INA. Elekfrokkimiya _. __ in press _ A.i\F. FRUMKIX and L.N. XEKRASOV, Dokl. Akad. Xuuk USSR 126,..115 (1959) M.~.TARASEVICH,K.A.RADIYSCHKISA, V.Y~.FILIXOVS~Y~~~R.H. I+RSHTEI~~.,Elektvokhimiya 6, 1522 (1g7oj R.H. BURSHTEXX,V.S. VILINSKAYA,L.L. KxoTs,V.V. KYSHNEV, B-1. LEXZ-_ E;ER and 35-R. TARASEVLCH, Elektrokhimiya 8, 1rS3 (1972) V.S. BA~OTSI;Y, M.R. TXRASE~XH and V.Yu. VILIXO~SECY, Ekktrokhimiya 5.121s (1969) VS. BAGOTSICY, b1.R. TARASEV~CH and V.Yu. VILINOVSKY. EZektrbkhimiya 8, s* M.R.
(197”) TARASEVICH,
V.A.
BOGDANOVSIWYA,
Bioelectrockem.
(1975) R.H. BURSHTELN, M.R. TARASE~ICH, A.M. KHIJTORNOY, F.Z. SABIROV, 1-I. ASTAKHOV and G.G. TEPLITSKAYA. 10% km) A. KOZAWA, V.E.
ZLLIONIS and
R. J_ BRODD,
Bioenerg.
2,
69
V.S..VILINSKAYX, EZektrokkimiycc.fI,
J. EZectrochettz. Sot.
117,
(1970) -4-V. CAVIZKY and V.I. XELIYBIN, Usp; KAitn. 44, 214 (1975) G.I. ZAKBARKLS and M.R_ TARASEVICH, Elektrokhimiya 11, rorg (1975) H. ALT. H. BIXDER and G. SXSDSTEDE, J. CataZ. 28, 8 (1973) MR. TXRASEVICH, F.Z. SABIROV and R.H. B~RSHTEIX, Etektrokhimiya
1470
7.
404 (r9ir) J.E. FALK in Porpkyrins and MetaZZofiovphirins. Elsevier Publishing Comp., New York (1964) IA. AKIMOV and G.A. KORSYNOVSW, Opt. Spectvos. (USSR) 8, 427 ‘(1960) SD. LEVIXA, T-1. AXDRIAXOVA, MN. SAKHAROV, O.A. GOLO~INA, K-P. LoB_~XOV_~ and Z.X. ROTESBERG, Zh. Fiz-. Khim. 40, rzzg (1966) K._4. RADIYSCHKINA, R.H.BuRsH~EIN,B.D.BEREz~N,~CI.R.TARASEVICH~~~ SD. LEVIXA, EZektrokhimiya 9, qro (1973)
am2 Bioeneugetics
Electrocatalysis Complexes
of Oxygen
by JI. R. TAR~SEVICH, Institute
of Electrochemistry
Manuscript
received
March
4,
1Q--2g
Reduction
I<. A.
on Organic Metallic
RIDIYSCHGIS_X
of the Academy 16th
(1977)
and S. I. AZ~DROUSEVA
of Sciences
of the USSR,
3foscow
1976
The influence of the nature of the central ion and ligand on the mechanism and kinetics of osygen electroreduction at metal (Fe, Co, Nn) phthalocyanines and tetraphenJ-lporphins in the pH range of o,z-x_~,a The oxygen reduction reaction to hydrogen peroside in was studied. acid and neutral solutions proceeds at a faster rate in the presence of organic compleses of metals than in the case of carbon black. In addition, in alkaline solutions a direct reaction occurs in the case of MnPhc, &lfiPhci, CoPhci, CoTMPP, involving the breaking of the O-O bond. The kinetic parameters d U/d log I, d U/d pH of the osygen reduction reaction to hydrogen peroside are the same for both the organic comples studied and carbon black. This points to a similar mecharkm for the slow stage: the addition of the first electron to an adsorbed oxygen molecule.
Introduction The eleetrocatalysis on organic semiconductors is a new trend in electrochemical research,lv” and is of special importance in the investigation of the relationship between the chemical structure of a substance and its catalytic acti\-ity_ Electrocatalysis on organo-metallic complexes is of particular interest since they can sert-e as models for the most perfect catalysts-enzymes. Natural compounds of a number of porphins are of great importance in biochemical processes_ Molecular compleses of metals with porphin groups are active centers of protein molecules - enzymes and electron carriers in respiratory chains. The possibilities for varying the chemical structure and electrophysical properties of organic semiconductors are unlimited. Therefore, these substances hold considerable scientific and practical promise. This paper presents the results of systematic studies on the mechanism and kinetics of o_sygen3 reactions on organometallic complexes, which are of importance for practical use.
Osvgen
Reduction
on Organic
Metatlic
Compleses
19
Experimental The following organic compleses were used in the esperiments : I. monomeric phthalocyanines of bivalent Fe, Co, Mn and phthalocyanine without metal, prepared from phthalonitrile and salts of the respective metals ;j
2. polymeric phthalocyanines of Fe, Co, Mn. These compounds were synthesized by sintering, pyromellitic acid dianhyclride phthalic anhydride, urea, ammonium molybdate and salts of the respective metals; 3_ cobalt tetraphenyland tetra(p-methoxyphenyl)-porphins. These compounds were obtained by the condensation of pyrrole with benzaldehyde and anisaldehyde, respectively, using cobalt acetate as complesing agent .I In our investigations we used mainly the disk-ring electrode methThe esperimental set-up was descrrbed in Ref. 3, 4 and 6, and od.” the electronic equipment and esperimental technique in Ref. 7. The theory of parallel-consecutive osygen and hydrogen peroxide reactions and the limits of its applicability are given in Ref. S and g_ The application of this theory to the calculation of the constants of partial osygen and hydrogen peroxide reactions was described in Ref. 3, 4. 6 and IO. We shall consider here only some specific features of the technique used, associated with the investigation of organometallic complexes. The disk was made of pyrographite, to which a thin layer of active mass was applied, which contained carbon black, 20 *!Jometallophthalocyauine and Co-tetraCp-methoxyphenyl)-porphin, (or 5 */* Co-tetraphenylporphin IO */*teflon. Metallophthalocyanines were deposited respectively) and on carbon black from a solution in concentrated H&SO,. while tetraphenylporphin compleses came from a solution in dimethylformamide. After the misture was applied to the electrode, it was sintered in air at 330 *C. Special esperiments 11 showed that at the current densities esceeding one half of the limiting value of the diffusion current, the electrochemical reduction reaction of oxygen was displaced to the external surface. Under these conditions, the disk surface can be considered as *being At smaller current densities, the porous nature of the equally accessible. In calculating the kinetic active layer should be taken into account. it is very difficult to introduce a correction for the electroparameters, chemical activity of the carrier (carbon black). Therefore, the constants of individual reactions were calculated by means of the theory of parallelconsecutive reactions (from the data obtained by the disk-ring method) only in the cases where in the presence of an organic catalyst the polarizatran curve was shifted by not less than 50-60 mV more positively in comparison with that for pure carbon black. The esperiments were carried out in the pH range of '0,3-L&2 The solutions were prepared from using phosphate buffer solutions. distilled and recrystallized reagents and purified by cathodic polarization
20
Tarasevich,
Eadiyschkina
and
Androuseva
in hydrogen atmosphere. The potentials were referred to the hydrogen electrode in the same solution (UJ or to the standard hydrogen electrode ( U~I,) .
Me&a&m
of oxygen and hydrogen peroxide reactions
The main object of this part of the investigation \;as to elucidate the influence of the central ion and of the &and nature on the mechanism The mechanism, or path, of the oxygen reduction on of the process. organometallic comple.ues was analyzed by means of the scheme : I_
K3I
‘Z/7
ILO-+
O,-+
H,O
-
/
-k,
H-0 0,
kl This scheme was discussed more than once in our works.3~4~6~8-11 The investigations carried out so far with a wide variety of electrode The ratio k,/k2 in this-scheme materials confirm its general character_ characterizes the ratio of the reactions proceeding directly to water and via an intermediate formation of hydrogen pero_tide. zIe_ with and without a break of the O-O bond. The subsequent transformation of hydrogen pero,side is characterized by the constants k3 (cathodic reduction) and These constants were calculated from k, (catalytic decomposition). results obtained with the disk-ring electrode method. Fig. I shows the polarization curves of osygen reduction on the monomeric phthalocyanines of Fe, Co, Mn and without metal, as well
Fig.
I.
Polarization curves of oxygen reduction on monomeric phthalocyanines of iron (3). cobalt (3). manganese (_I) and without metal (I), and the corresponding dependences of the limiting currents of hydrogen peroxide oxidation (~'-4') on the ring in 0.1 N KOH. w = g6o r.p.m. Dashed line - carbon black.
O_xygen
Reduction
on Organic
Metallic
Cotiplextis
-2’I
-..
as the corresponding dependences of the limiting currefits-of hy&o&n peroxide oxidation on the ring from the disk potential.in. 0.1 N Q?H_ In the case of FePhc, just as in Ref. IZ, at potenti~sIlrlore_cathodic-than; : 0,7 V, the value of the current on the disk is close. to that_of,the:-lim~tiiig current fdr a four-electron reaction. -In this case; small amounts :of ’ H,O, are recorded on the ring electrode only at Ur < 0,4 V. Iri the presence of CoPhc and MnPhc, the currents on the disk increase!s@nificantly, as compared to carbon black, while the cuqents_ on the’.eng ~. decrease by about twice as much. On the curve of H,O&ogda@n, the maximum shifts in the a.nOdic direction by N 300 mV. :.:: . The dependence of the constants of the reduction reactions-of 0, to H,O (R,) and H,O,(R,) on the disk potential is $hqwn.-oti J?ig.’ i. We can observe that the ;ntrqluction of monoqleric CqPhc;Mtq &rboii -_ black increases the. constant -k, twice_ as much, as complied to pure tirbon : black. Moreover, in the c&se- df _Mn soPhc, a parallel reaction arises whithont2 forming hydrogen peroxide.
_ Fig. 2. Dependence of the constants k, (@) and Rs (0). on the disk potential for oxygen reduction on.. monomeric phthaiocyanines of cobalt (2). manganese (3,3’), without metal (I) and carbon black (4) in 0.1 N KOH.
0.8
0.7
0.6
OS
_.
. I .
Fig. 3 and 4 show similar dependences for polymeric _metallo; By comparing Fig. I with Fig. 3 it appears- that the phthalocyanines. introdudtion of &In and Co into the polymeric chain increases considerably the currents on the disk and decreases the currents on the *g. me polymerization of CoPhc leads to a change in the mechanismi i_%:-tb-$reaction of 0, reduction directly to water takes place--~ Thus,-deperiding on the central ion nature the phthalocyanines investigated form-s a&tie solution the series: monomers: Fe >Co >Mn >H,; polymers: Fe >Co >Mn. It is known.13 that the length of the O-O bond is very-sensitive to a change in the ligands bound with the metal. - Ins this connedtidn, it was of interest to study the 0, ionization.reaction ori organic complexes in which the same central atom (Co)- is bound with difEei@;lig&ti&. The polarization curves of 0, reduction on CoTPP, CoTMPPand (CoPhc)f in alkaline solution are shown in Fig. 5: This figure also shdws Jthen.&%
Tarasevich,
22
Radi!-schkina
and
_Androuseva
0.25 t
_/----,. --0.0
.
2”
1
‘\
3’
U,(V) I 1.0 Fig.
3.
Polarization curves of osygen reduction on polymeric phthaloq-anines of iron (2). cobalt (3), manganese (I) and corresponding dependences of the limiting currents of hydrogen pero_xide osidation on the ring in 0.1 X KOH. (I’-39 w = g6o r.p.w. Dashed line - carbon black.
Fig.
4.
Dependence of the constants R, (e) and k, (0) on the disk potential for osygen reduction on polymeric phthalocyanines of cobalt (I) and manganese (z.z’) in 0.1 _L- KOH.
0
I
0.8
0.7
I
0.6
UJ
VI 1
0.5
pendence on these compounds of the limiting currents of H,O, osidation to the total process on the ring. -1s can be seen in Fig. 6. the contribution of the direct reaction to water (k,) is 50 y/o for (CoPhc)i and 30 ",/o for CoTJIPP. On CoTPP, just as on monomeric CoPhc, the reaction occurs In alkaline solution the actkit!only via intermediate fomration of H,O,. forms the series : (CoPhc)i >CoTMPP >CoPhc >CoTPP. The polarization curves of 0, reduction on organic Co compleses in acid solution and the dependences on the disk potential of the currents studied the electroon the ring are given in Fig. 7_ For all the compleses reduction reaction of molecular osj-gen undergoes activation, as com-
Osygen
Reduction
on
Organic
Metallic
Complexes
23
Polarization curves of oxygen reduction on polymeric Co-phthalocyanine (I). Co tetraphenylporphin (2) and Co tetra (p-metho_-phenyl)-porphin (3) and corresponding dependences of the limiting currents of hydrogen peroxide oxidation on the ring in 0.1 N KOH. ( “-3’) -ZI= 960 r_p_~n_
50
Fig.
6.
Dependence of the constants k, (g) and R, (0) on the disk potential for osygen reduction on polvmeric Co-phthalocyanine (I). Co-tetraphenylporphin (2) and Co-tetra(P-methosyphenyl)porphin (3.3’) in 0.1 N KOH.
o.
U,(V)
O”?
0.8
0.7
0.6
0.5
pared to pure carbon black. The calculations show that in all cases the reaction in acid solution occurs vin intermediate hydrogen peroside formation. The value of the constant k, decreases in the series : CoTUPP> We see that in acid solution (CoPhc)i is less active COTPP >(CoPhc)i. than tetraphenylporphin compleses.
24
Tarasevich,
Radiyschkina
Fig.
and
Xndrouseva
7_
Polarization curves of osl;gen reduction on polymeric Co-phthalocpanlne (I), Co-tctraphenylporphin (2). Co-tetra(+methox>-phenyl) dependences porphin (3). and corresponding of the limiting currents of hydrogen peroside osidation (“-3’) on the ring in 0.1 N H,SO,. Dashed fine - carbon black. -a = 960 r.p.w.
Kinetics of oxygen electroreduction The elucidation of the slow step in the series of the partial reactions of the global process of reduction of the molecular osygen necessitates These studies were carried out for a kinetic study in a wide pH range. As it follows from. the data preH,Phc, CoPhc, (CoPhc)i and COTPP. in the presence of these compounds sented in the preceding section, [escept for (CoPhc)i in alkaline medium] the reduction of osygen occurs This in two steps vin intermediate formation of hydrogen peroxide. paper is concerned with the kinetics of the first step : reduction of 0, The kinetics of the second step, including the chemical and to H,O,. electrochemical hydrogen peroside reactions, for some phthalocyanines depenwere analyzed in Ref. 14. Fig. S and g and Table I give typical 0.6 U”*W
\
COTPP CoPhCi
\ Fig. 5. The log (I /Ilk - I) cs. o‘,, - dependences for osygen on organic compleses for pH = 0.3 ; 7,6 ; I+Z.
reduction
Oxygen
Reduction
on Organic
Metallic
Complexes
Fig. g. Dependence of the half-wave potential on pH for osygen reduction on carbon black (0). monomeric (A) and polymeric (X) Cophthalocyanine, phthalocyanine without metal (A) and Co-tctnphenylporphin (0).
Table I. Kinetic parameters peroxide on carbon materials Electrode
i
Pyrographite
i I
Carbon
black
H,Phc
.
of the reduction reaction of oxygen to hydrogen and in the presence of organic compleses. dU/d
PH 2-g
pH,
mV 130-160
0
9-14
60-30
2-10 IO-14
60-30
30-40 go-120
0
2-10
40-50 100-120
0
30-40
10-I-f
40-50
I
I
CoPhc
Z-10
0
30-40
IO-I-$
1
1
(CoPhc)i
/
CoTPP
dences
of
the
2-10
IO-15
IO-I-#
40-50 II0
O-Q 4-10 IO-14
kinetic
pararneters
O-10 30
dU/d
I'O-If0 50
I
IOO--120 IOO-120
40-60
and dU/d pH for the cataconditions. Over the whole of organic compleses, the rate
log I
lysts studied under different experimental pH range from 0,~ to 14 ,z in the presence
I ! / 1 i I
Tarase\-ich,
16
Radiyschkina
and
Xndrouseva
of osygen reduction to hydrogen peroside is higher than for carbon to neglect Therefore, to the first approsimation. it is possible black. the cqntribution of the carrier current to the total current_ Consider the kinetical dependences for alkaline (pH > IO), neutral (4 < pH < IO) and acid (pH < 4) solutions separately_ In alkaline solutions dU,/d pH and (dU/d pH)~=~,,m are 60-30 mV. The latter The polarization slope dU/d log 1 is value is obsen-ed at pH > 12. - 40-60 mV in the current range where dU/d pH was above the mentioned \-alue. In acid and neutral solutions dU/d pH E O-IO mV and dU’/d log I lies between 90 and 150 mV. There are two esceptions to these data. For (CoPhc)i at low current densities dU/d pH is - 40-50 mV In this case the points with I = IOWA along the whole pH range I-I+ lie on the sections with a slope smaller than go-150 mV. For CoTPP in acid solutions dU/d pH = 100-110 mV. Thus. escept for CoTPP in acid
medium
(CoPhc)i at low current
and
of the reduction reaction of 0, to those for carbon black.
Discussion
densities,
to H,O,
the kinetic
on different
parameters
catalysts
are close
of results
method The in\-estigations by means of the disk-ring electrode showed which reactions in the comples process of reduction of the molecular osygen are catalyzed by organometallic compleses and how the change of the chemical structure of the catalyst affects their rates ratio. Data are listed in Table 2. In acid solution in the presence of organometallic compleses, the rate of the reaction leading to H,O, increases. In alkaline solutions in the case of MnPhc, (AlnPhc)i, (CoPhc)i and CoTMPP a direct reaction to water with the break of the O-O bond occurs in addition. The dependence of the activity of phthdlocyanines on the central ion nature has been repeatedly discussed in the literature1-a*15 and can be qualitatively interpreted in terms of the theory of the molecular orbitals. The adsorption of an oxygen molecule on phthalocyanines is accom-
Constants
partial
reactions compleses
H,Phc
iCarbon ; black PI
.I R,x
IO3 cm/s!
-
! CoPhc
,
’ (CoPhc)i’
,
&\[nPhc I>In Phci\ I I I
-
-
55
-
I 12
CoTJIPPi
_-’
/ j
CoTPP
30
i
32
.
55
15
IS
10
1
I4
'
Osygen
Reduction
on Organic
Metallic
Compleses
27
panied by a partial electron transfer from the central ion of metal to antibonding Tc-orbitals of oxygen and by a weakening of the O-O bond in the molecule_ This accelerates the cathodic reduction of 0,. The increase in the number of collective x-electrons (polymerization of phthalocyanines or introduction of methosy-groups into tetraphenylporphin) results in a greater electron density of the central ion. This facilitates the electron transfer to osygen or to hydrogen peroside molecules and accelerates their reduction. On the other hand, an increase in the degree of electron localization on an adsorbed 0, molecule leads to the weakening of the O-O bond up to its breaking point : the reaction will then occur directly to water. This treatment of the problem is undoubtedly only a qualitative one and does not take into account a number of esperimental facts. The explanation of these facts necessitates more accurate theoretical and experimental studies of the electrophysical properties of organometallic compleses. M7e shall discuss now the kinetics of the reduction of 0, to H,O, on different organic compleses. The coincidence of the kinetic parameters of the 0, reduction on organic compleses and on carbon black. obser\red in most cases, testifies to a similarity in the nature of the slow step. Therefore, we shall first consider the processes occurring on carbon black. We studied earlier quantitatively the 0, reduction on p,yrographite in We showed that the cathodic reaction of the the pH range of I-I+~~ osygen reduction is controled by the slowness of the step of the first electron transfer to an adsorbed oxygen molecule (and in the case of the reverse reaction of anodic hydrogen peroside oxidation - from the HO,--ion) : 0,
+
%lb,,k MO.,.
JIHO, HO,-
M -+ NO,,&. + e+ + +
+
H,O
MO,f
e- s H,O
(slow step)
MHO.,
+
OH-
ATHO,f
H,O,
+
OH-
The other reaction steps leadin, m to the formation of H,Oz (in acid) or HO,(in alkaline solution) occur rapidly. In acid and neutral solutions the reaction is irreversible and (dU/d pH)rZconst = 0, dU/d log I = 120 mV at o! = 0.5. When the pH rises, the rate of the reverse reaction increases This and the system O,/H,?,(HO,-) approaches the reversible state: leads to a potential shift toward the negative direction and a decrease Table I shows clearly that in the value of dU/d log I to 30 mV. there is practically complete agreement between the data on pyrographite The only esception is the higher value of and those on carbon black. XI/a log I in the region of the cut-reuts where the O,/HO,system This is apparently due to the porous nature approaches equilibrium. of the active layer of carbon black and to the change of the H,O, conIt would be quite reasonable to assume that on centration across it.
Radiyschkina and
25
For organic catalysts the reduction of 0, follows the same mechanism. (CoPhc)i (dU/dpH)r= lo~ = 40-50 rnk at pH from 2 to 14. Apparently, owing to a large anodic osidation rate of H,O, on (CoPhc)irq, the equilibrium of the O,--H,O, system is maintained at low currents throughout the whole pH range. +neThus, on organic cobalt compleses the reaction 0, w H,O, occurs much faster than on a carbon carrier and without any change in the nature of the slow step- The reason for this is a faster electron transfer to an osygen molecule of the adsorption site.
due to an increase in the electron
density
The 0, reduction on CoTPP in acid solution does not proceed according to the above scheme (Table I). The kinetics observed in this case can be explained on the basis of the foliowing mechanism supposedly responsible for the slow step : 310,
+
Ht
+
e-
-+
MHO,
At a = 0.5 au/a log Z = 120 mV and au/a pH = 120 mV. According to the data reported in the literature ,17a18 tetraphenylporphin complexes can undergo protonation both in the molecule center and on benzene rings, whereas in the case of phthalocyanines the same may happen only on It is possible that this-difference in the the periphery of the molecule. chemical properties of the compleses mentioned above accounts for the change in the reduction mechanism in acid solutions on cobalt tetraphenylporphin and tetra(p-methosyphenyl)porphin. In further studies on the electrocatalysis on organic complexes it will be necessary to take into account the properties of the actual system If the thickness of conductor(carrier)-semiconductor film (catalysts)_ the semiconductor film is not greater than the screening depth, then the Such injection of current carriers into the semiconductor may occur. phenomena were obsen-ed in the heterogeneous reactions of decomposition of hydrogen peroside and of the osidation of cumenelg on metal powders wrth a film of metal phthalocyanine. At present organometallic complexes are not only of an academic, but also of a certain practical interest. Some methodGo of activation of carbon oxygen electrodes were developed on the basis of these compounds, thus ensuring a stable operation during several thousands of hours. However, the discussion of the practical application of organic compleses is beyond the scope of the present work.
References H. JAHNKE and 31. SCH~XBORS, Comptes Rendars, Tvoisidnze Jouvde Interwationde d’Etzrde des Piles ci Combustible. Presses Academiques EuropCennes. Bruselles (1969) p_ 60 n_ BI. SAVY, P. XXDRO and C_ BERSXRD. Electrochim. Acta 19, 403 (1974) 1
Oxygen 3
4
5 6
7
8 9
10
11
12
13 14 15 16
17
18 19
40
Reduction
on Organic
K.ARADIYSCHKINA,M.R.TARASEVICH
Metallic
Complexes
and S-1. ANDROUSEVA, -
:
29.
Elekfrokkimiya
11, 1079 (1975) !%I. ASDROUSEVA. M.R.TARAsE~IcH~~~ K._~.RADIYSCHICI~INA. Elekfrokkimiya _. __ in press _ A.i\F. FRUMKIX and L.N. XEKRASOV, Dokl. Akad. Xuuk USSR 126,..115 (1959) M.~.TARASEVICH,K.A.RADIYSCHKISA, V.Y~.FILIXOVS~Y~~~R.H. I+RSHTEI~~.,Elektvokhimiya 6, 1522 (1g7oj R.H. BURSHTEXX,V.S. VILINSKAYA,L.L. KxoTs,V.V. KYSHNEV, B-1. LEXZ-_ E;ER and 35-R. TARASEVLCH, Elektrokhimiya 8, 1rS3 (1972) V.S. BA~OTSI;Y, M.R. TXRASE~XH and V.Yu. VILIXO~SECY, Ekktrokhimiya 5.121s (1969) VS. BAGOTSICY, b1.R. TARASEV~CH and V.Yu. VILINOVSKY. EZektrbkhimiya 8, s* M.R.
(197”) TARASEVICH,
V.A.
BOGDANOVSIWYA,
Bioelectrockem.
(1975) R.H. BURSHTELN, M.R. TARASE~ICH, A.M. KHIJTORNOY, F.Z. SABIROV, 1-I. ASTAKHOV and G.G. TEPLITSKAYA. 10% km) A. KOZAWA, V.E.
ZLLIONIS and
R. J_ BRODD,
Bioenerg.
2,
69
V.S..VILINSKAYX, EZektrokkimiycc.fI,
J. EZectrochettz. Sot.
117,
(1970) -4-V. CAVIZKY and V.I. XELIYBIN, Usp; KAitn. 44, 214 (1975) G.I. ZAKBARKLS and M.R_ TARASEVICH, Elektrokhimiya 11, rorg (1975) H. ALT. H. BIXDER and G. SXSDSTEDE, J. CataZ. 28, 8 (1973) MR. TXRASEVICH, F.Z. SABIROV and R.H. B~RSHTEIX, Etektrokhimiya
1470
7.
404 (r9ir) J.E. FALK in Porpkyrins and MetaZZofiovphirins. Elsevier Publishing Comp., New York (1964) IA. AKIMOV and G.A. KORSYNOVSW, Opt. Spectvos. (USSR) 8, 427 ‘(1960) SD. LEVIXA, T-1. AXDRIAXOVA, MN. SAKHAROV, O.A. GOLO~INA, K-P. LoB_~XOV_~ and Z.X. ROTESBERG, Zh. Fiz-. Khim. 40, rzzg (1966) K._4. RADIYSCHKINA, R.H.BuRsH~EIN,B.D.BEREz~N,~CI.R.TARASEVICH~~~ SD. LEVIXA, EZektrokhimiya 9, qro (1973)