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Electroreduction of cobalt-amino peroxo complexes. Part 2.—The reduction of oxygen in the Co(II)-ethylenediamine and Co(II)-triethylenetetramine systems

 

作者: Armand Bettelheim,  

 

期刊: Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases  (RSC Available online 1977)
卷期: Volume 73, issue 1  

页码: 150-156

 

ISSN:0300-9599

 

年代: 1977

 

DOI:10.1039/F19777300150

 

出版商: RSC

 

数据来源: RSC

 

摘要:

Electroreduction of Cobalt-Amino Peroxo ComplexesPart 2.-The Reduction of Oxygen in the Co(II)-Ethylenediamine andCo(I1)-Triethylenetetramine SystemsBY ARMAND BETTELHEIM," M. FARAGGI, 1. HODARA AND J. MANASSEN tAtomic Energy Commission, Nuclear Research Centre-Negev, P.O.B. 9001,Beer-Sheva, IsraelReceived 19th March, 1976The cathodic reduction and the optical properties of the peroxo-complexes [LCO-O~--COL]+~with L = en (ethylenediamine) and L = trien (triethylenetetramine) have been studied. It wasfound that four electrons are involved in the reduction of the peroxo-complexes. The diffusioncoefficients of the above complexes were calculated to be 7.0 x cm2 s-l respectively.The redox potentials are -0.57 V and -0.50 V (us. s.c.e.) respectively. These complexes areunsuitable as catalysts for the cathodic reduction of oxygen because they have lower redox potentialsthan that of oxygen.A different behaviour was found in the cobalt-ethylenediamine system whenthe ratio [en] : [Co(n)] = 1. The redox potential of the complex formed in this case is - 0.17 Vand its optical spectrum showed two absorption bands (Amax at 350 and 310nm.) Therefore, itis suggested that a CO(III) en complex is formed, one which was found to be a suitable homogeneouscatalyst for the cathodic reduction of oxygen.and 8.8 xPeroxo complexes of Co(n)-ethylenediamine (en) and Co(I1)-triethylenetetramine(trien) have been prepared. The interaction between oxygen and CO(II) complexesof en and trien were studied by spectroscopic, potentiometric and kinetic r n e t h ~ d s .~ ~ ~The spectrum of the complex [(en)2Co-0,-Co(en)2]+4 is characterized by two ab-sorption bands : Amax = 355nm and A,,, = 270nm, both with an approximate extinctioncoefficient of 5 x lo3 dm3 mol-1 ~ m - l . ~ Those of the complex [trien Co-0,-Cotrien]+4 are : A,,, = 360 nm and A, = 220 nm with extinction coefficients of 6.4 x lo3and 1.5 x lo5 dm3 mol-1 cm-l respectively.6 From oxygen absorption and cryoscopymeasurements, it was concluded that the peroxo complexes are bin~clear.~~Kinetic studies indicate that the reaction of the CO(II) complexes and oxygen is firstorder with respect to oxygen.4- 9 9 lo Magnetic susceptibility measurements indicatethat the peroxo complexes are diamagnetic1 Miller and Wilkins assumed p hydroxobridged intermediates in the formation of the oxygen complex of Co(~r)-trien.~Also Michailidis and Martin have indicated that a p-hydroxo bridge may be presentin the oxygen complex of bis ethylenediamine CO(II).Stability constants of theoxygenated and oxygen-free complexes were found 2* using potentiometrictechniques.The increasing reactivity of molecular oxygen upon coordination (activation ofthe oxygen molecule) was suggested. According to Valentine,l three explanationsare possible :1. Coordinated oxygen being diamagnetic, reactions with diamagnetic substratesto form diamagnetic substrates are not hindered by the requirement for spinconservation.t present address : The Weizmann Institute, Rehovoth, Israel.15A .BETTELHEIM, M. FARAGGI, I . HODARA A N D J . MANASSEN 1512. The metal may hold the oxygen molecule and the substrate in a cis position,lowering the activation energy for the oxidation of the substrate.3. Coordinated O2 is, in most cases, partially reduced; increasing the electrondensity of the oxygen may activate it.In this paper, the Co(II)-en and Co(u)-trien systems were studied by electro-chemical methods. In view of the suggested activation mechanism, these systemsmight be used as possible catalysts for the cathodic reduction of oxygen in aqueoussolutions.EXPERIMENTALWater was triple distilled. Merck pro analysis CoCl2, Baker " analysed " trien andFluka " puriss " en were used without further purification. All other materials were ofanalytical grade.Matheson " extra dry " oxygen and " high purity " nitrogen were usedfor the saturation of the solutions.The working electrode for the coulometric measurements was a mercury pool ( A =20 cm2) or a platinum gauze ( A = 30 cm2). The electrochemical apparatus has beendescribed previously. Spectroscopic measurements were performed using a Cary 17spectropho tometer.The peroxo complexes were prepared by mixing 75 cm3 of water containing appropriateamounts of electrolyte with en or trien. These solutions were brought to pH 5 under oxygenbubbling. Co@) ions were added and the pH 5 adjusted by dropwise addition of 1 mol dm-3NaOH. After pH stabilization, the solutions were diluted to 100 cm3 in volumetric flasks.All potentials were measured against the s.c.e.as reference at 25 +O.l"C.RESULTSTHE CO(II)-EN SYSTEMPOLAROGRAPHYPolarography of the peroxo complex [(en>,C~-O~-Co~(en),]+~ in alkalinesolutions and in excess en shows a single polarographic wave with a half wavepotential of -0.57 V [fig. l(B)]. This is to be compared with the two polaro-graphic waves of oxygen saturated alkaline solutions not containing Co(n) ionIV against s.c.e.FIG. 1.-Polarograms of: (A) an oxygen saturated solution at pH 9 containing 1 mol dm-3 NaCI.(B) mol dm-3 peroxo complex [(en)zCo-02-Co(en)2]+4 and 1 mol dm-3 NaCl at pH 9.Polarogram B was recorded after removal of the free dissolved oxygenI52 ELECTROREDUCTION OF 0, I N C O ( I I ) + N H ~[fig. I(A)]. These waves are attributed to the reduction of O2 to hydrogen peroxideand to water.14 The single wave of the cathodic reduction of the peroxo complexis explained as a redaction involving the transfer of four electrons.This was shownby a coulometric collecting charge at a controlled potential of -0.9 V with a mercurypool as the working electrode. The charge collected for 25 pmol of the peroxocomplex was 12.7k0.2 C as compared with the theoretical value of 3.1 C per electron.Other pzroxo complexes of Co(11) behaving similarly have been investigated bySprieck.lcquation The diffusion polarographic current as given by the iniproved IlkovicThis equation differs from the usual Ilkovic equation by the term 1 +39 D3 f* nr*,which represents the influence of the electrode curvature. In this equation, id is thediffusion current (in PA); n, the number of electrons; f , the drop time (in seconds);D, the diffusion coefficient (in cm2 s-I); 712, the weight of the mercury flowing perunit time (in mg s-l) and C, the buIk concentration (in mol ~ m - ~ ) .For the peroxocomplex [(en)2Co-02 - C~(en),]+~, the value of the diffusion coefficient (D),calculated from the above equation, is 7.0 x cm2 s-I.ROTATING DISC ELECTRODE ( R . D.E.) EXPERIMENTSThe polarographic study shows that the peroxo complex in solutions where[en]/[Co(xr)] > 2, is reduced at a more negative potential than that of oxygen.Changes in the redox potential of the peroxo complex as function of the en concentra-tion at a constant value of CO(II) ion concentration were recorded by r.d.e.voltammo-grams.The redox potential of the yeroxo complex of CO(II) (which is partially at theCO(III) state) is expected t o shift towards that of oxygen, when the [en] : [Co(11)]ratio is decreased.17~ l8 Thus, Rock l7 stated that the redox potential of the @o(II~)complex is related to the thermodynamic stabilization of the CO(III) ions. Henneyet d . l S found thzt the potential shifts towards more negative values when the[en] : [CO(III)] ratio is increased.Fig. 2 shows the r.d.e. voltammograms obtained for [en]/[Co(~r)] = 2 (curve A)and for [en]/[Co(11)1 = 1 (curve B). Curve A shows the single wave of the pzroxoV against s.c.e.FIG. 2.-Pt -r.d.c. voltaminograim ( w = 340 r.p.m.) of : (A) lop3 tnol d n r 3 peroxo complex[(en)2Co-f2,-Co(en),]-4 and 1 rnoI d ~ r a - ~ NaCI at pH 9. (B) 2 x 1W3 mol dm-3 Co(III) en com-plex and 1 12201 dm-3 NaCl at pH 9.Voltaiixnograms were recorded after removal of the freedissolved oxygenA . BETTELHEIM, M. FARAGGI, 1. HODARA AND J . MANASSEN 153complex. In agreement with our spectroscopic and electrochemical results, the firstwave in curve B (curve B,) is attributed to the mononuclear salt of Co(rr1)-en. Thesecond wave (wave B,) is suggested to be due to H202 reduction, hydrogen peroxidebeing a product of the reaction of Co(rI)-en with oxygen (wave B2 appears at the samepotential as that of H20J :The half wave potentials of the various species are given in table 1.2Co(11)-en+0, +2H,O 3 2Co(r11)-en+H,O,+20H-. (1)TABLE 1 .-EXPERIMENTAL DATA OF VARIOUS CQ(1II) SPECIES AND OXYGENsystemEL no. of D X 106 experimenta 1/V againzt s.c.e.electrons ,kmz s- method[(NH~)~CO-O~-CO(NH~)~]+~ - 0.58 4 8.6 r.d.e.4 7.0 d.m.e.[trien Co-02--Co trien]+' - 0.50 4 8.8 d.m.e.Co(nI)eii -0.17 1 2.0 r.d.e.0 2 - 0.38 4 26 l4 r.d.e.[Cen)2Co-02 - C ~ ( e n ) ~ ] + ~ - 0.57In a separate experiment, a nitrogen-saturated solution of the Co(r1)-en complex(cquimolar concentrations) was oxidized electrolytically in the absence of oxygen :The oxidation was carried out at a controlled potential of +0.9 V on a Pt gauze asthe working electrode.Co(II)-en + Co(rrr)-en + e (2)SPECTRQSCOPIC MEASUREMENTSThe spectrum of the peroxo complex (en)2Co-02-Co(en)z is shown in fig. 3(curve A). It is similar to that given in the literat~re.~ When the [en]/[Co(rr)]ratio is reduced to 1 : 1, the spectrum obtained is shown in fig.3, curve B. It ischaracterized by two absorption bands, one at Amax = 350 nm and the other at Amax =310 nm. The spectrum obtained by electrolytic oxidation of a nitrogen saturatedsolution containing equimolar concentrations of Co(11) ions and en, as describedabove, has similar absorption bands (Amax at 350 and 310nm, fig. 3 curve C).Spectroscopic titratioii at II, = 355 nm of Co(11) ion oxygen-saturated solutions as afunction of en concentration shows a two step curve. The first occurs at a ratio of[en] : [Co(rr)] = 1 and the second at a ratio of 2. Thus it is suggested that twodifferent species are formed in the two [en] : [CO(II)] ratios.When the ratio is 2,the peroxo complex is formed. When it is 1 , the Co(rI1)-ethylenediamine complexis generated. Similar results have been recently reported by Bijl and De V r i e ~ . ~Moreover, acidifying the peroxo complex (pH = 2) liberates bound oxygen ashydrogen peroxide or free oxygen.6 Acidification causes the disappearance of theabsorption band at il = 350 nm. This is not the case when the ratio equals unity,no changes are observed upon acidifying to pH 2. This strengthens the abovesuggestion.THE CO(II)-TRIEN SYSTEMThe peroxo complex with trien as ligand ([trien Co-0,-Co t r i e ~ ~ ] + ~ ) shows thesame characteristics as found in the polarography of the peroxo complex with en asthe ligand. One polarographic wave is formed (E3 = -OSOV, table 1) and fourelectrons are involved in the complex reduction, as confirmed by constant potentialcouloinetry at -0.9 V.The reduction potential was found to be independent o154 ELECTROREDUCTION OF 0 2 I N CO(II)+NH,the ligand-to-Co(n) ratio. The limiting current of the polarographic wave for solu-tions containing CO(II) ions increased linearly with trien concentration. A constantcurrent value was obtained in solutions where [trien] 2 [CO(II)]. This is similar toresults obtained by spectroscopic methods in a similar system.lgThe 1 : 1 trien/Co(rr> +02 complex is polarographically equivalent to the 2 : 1en/Co(u)+O, complex. This similarity can be explained by the fact that at leastthree nitrogen atoms are needed for the formation of a stable cobalt peroxo complex.'In the present study, the ligands can supply four nitrogen atoms, by a single trien ord01I250 300I 1350 4A /nm0FIG.3.-U.V. spectra ( I = 1 cm) of (A) 2 xand 1 mol dm-3 NaCl at pH 9, (3) 4 xpH 9, (C) a nitrogen saturated solution containingmol dm-3 peroxo complex [(en)zCo--02-Co(en)]~+4mol dm-3 Co(1II) en complex and 1 mol dm-3 NaCl atmol dmd3 CoflII) en obtained by constantpotential electrolysis at +0.9 V (against s.c.e.) at a Pt-gauze (A = 30 cmz).by two en molecules. These results suggest that only liganded CO(II) ions react withoxygen to form the peroxo complex. Decreasing the ligand to CO(II) ratio causes adecrease of peroxo complex concentration; no other species are formed when[trien] : [CO(II)] < 1.Using the improved Ilkovic equation (I), the calculated valueof the diffusion coefficient of the peroxo complex of CO(II) ion with trien as ligand is8.8 x cm2 s-l.HOMOGENEOUS CATALYSIS OF OXYGEN CATHODIC REDUCTIONThe peroxo complexes of CO(II) with ammonia,l ethylenediamine and triethylene-tetramine are reduced at a more negative potential than that of oxygen (table 1).This makes them unsuitable as possible catalysts for oxygen reduction. However,the complex Co(m)-en formed in oxygen-saturated solutions containing equimolarconcentrations of Co(n) ions and en is reduced at a more positive potential than thatof oxygen (table 1). Moreover, in these solutions ([en] : [CO(II)] = 1), coulometricexperiments show that it is possible to keep a constant value of cathodic current at apotential where free dissolved oxygen is insignificantly reduced at the electrode(fig.4). At a potential of -0.2 V in solutions at pH = 9, the cathodic current iA . BETTELHEIM, M. FARAGGI, I . HODARA AND J. MANASSEN 155stable, even when the number of Coulombs recorded is 10 times the charge requiredto reduce all CO(III) species present in solution (10 C).The steady state current was plotted as a function of the potential. Fig. 5,curve A, shows the results obtained in oxygen saturated solutions at pH 9. Curve Bin fig. 5 represents the potentiostatic current against potential curve for solutionscontaining equimolar concentrations of Co(11) ions and en. At a current density of0.05 mA c r r 2 an overvoltage decrease of 200 mV is observed.Thus, this Co(II)-ensystem is a suitable homogeneous catalyst for the cathodic reduction of oxygen.0 ,. f6 34 52 70FIG. 4.-Coulometric plots of-0.2 V (against s.c.e.) and pHflhsolutions (50 cm3) continuously saturated with oxygen (bubbling) at9 containing : (A) 1 mol dm-3 NaCl ; (B) 2 x mol dm-3 Co(m) encomplex and 1 rnol dm-3 NaCl.r IV against s.c.e.FIG. 5.-Current density against potential curves with a Pt-gauze as the working electrode ( A =30 cm2) of solutions at pH 9 continuously saturated with oxygen (bubbling) and containing :(A) 1 mol dnr3 NaCl ; (B) 2 x rnol dm-3 Co(m) en complexland Ibniol dm-3 NaCl.DISCUSSIONThe diffusion coefficients, the number of electrons involved in the reduction andthe reduction potentials of the peroxo complexes [(NJ33),Co-0,-Co(NH3)5]+4, a[(en),Co--O,-C~(en),]+~ and [trien Co-02 -Co trier^]+^ as well as the complexCo(m)-en are summarized in table 1 and are compared to that of oxygen.A mechanism commonly mentioned in the literature is called " oxygen activa-tion ' ' .1 2 5 20* 21 This term means that oxygen bound in the complex is partiallyreduced. A partial electron transfer from the metal ion to the n* orbitals of theoxygen causes a negative polarization of the oxygen molecule.21 However, th156 ELECTROREDUCTION OF 0 2 I N CO(II)+NH~peroxo complexes we investigated are reduced at more negative potentials than thatof free oxygen, so that they cannot be used as homogeneous catalysts for oxygenreduction.The only catalytic effect in these cobalt complexes was found in the cobalt-ethylenediamine system when the ratio [en] : [CO(II)] = 1.In this catalytic cycle,the reducing agent (the electron from the electrode) reacts with the oxidized state,giving the reduced state. The reduced state of the catalyst then reacts with oxygenreducing it to hydrogen peroxide [reaction (l)] and regenerating the oxidized state ofthe catalyst to react with the primary reducing agent once again. Coulometricexperiments show that the steady state current obtained in oxygen-saturated solutionsis obtained after collecting a certain amount of charge. In the experimental set-updescribed in this study, this charge (which may depend on the cell geometry) wasabout 20 C .It is kiiown that the first step of oxygen reduction to produce the HOzradical :02+H++e 3 HQi (3)is unfavourable from thermodynamic considerations (E" = - 0.32 V).22 However,the two-equivalent reduction of oxygen to produce H202 is more favourable(E" = +0.68 V) :It is suggested that the coulometric results indicate that a constant current value isobserved after a steady state concentration of species is obtained. This species isreduced with two electrons :Q2+2H++2e -+ H202. (4)2Co(11r)en+2e -+= 2Co(11)en ( 5 )-0;- J. + 0 2L- [en CO(III)-O~---CO(III)~~]+~The species [en Co(rn)-0$--Co(rrr) en]+" seems to be an unstable intermediate incontrast to the well known stable peroxo coniplex [(en)2Co(~r~)-Qq--Co(~~~)(en)2]+4.A. G.Sykes and J. A. Weil in Inorganic Reaction Mechanisms, Progress in Inorganic C/iemistry,ed. J. 0. Edwards (Interscience, N.Y., 1970), vol. 13, p. 1 .R. Nakon and A. E. Martell, J. Inorg. Nuclear Cheni., 1972, 34, 1365.R. Nakon and A. E. Martell, J. Amer. Chem. SOC., 1972, 94, 3026.F. Miller and R. G. Wilkins, J. Amer. Chenz. SOC., 1970, 92, 2687.P. Bijl and G. De Vries, J.C.S. Dalton, 1972, 303.S. Fallab, Chimia, 1970, 24, 76.R. S. Nakon, Ph. D. Thesis (Texas A and M University, 1971).S. G. Abrahamson, Ph.D. Thesis (University of Idaho, 1964).J. Simplicio and R. G. Wilkins, J. Amer. Chem. SOC., 1967, 89, 6092.lo F. Miller, J. Simplicio and R. G. Wilkins, J. Amer. Chem. Soc., 1969, 91, 1962.l1 M. S. Michailidis and R. B. Martin, J. Amer. Clzem. Suc., 1969, 91,4683.l2 J. S. Valentine, Chem. Rev., 1973, 73, 235.l3 A. Bettelheim, M. Faraggi, I. Kodara and J. Manassen, J.C.S. Faraduy I , 1977,73,143.l4 J. M. Kolthoff and J. J. Lingane, Polarography (Interscience, N.Y., 1952), vol. I, P. 105.l5 T. Sprieck, Ph.D. Thesis (University of Nebraska-Lincoln, 1972).l6 Ref. (14), p. 44.l7 P. A. Rock, Inorg. Chem., 1968,7, 837.l 9 A. Bondoli and V. Carunchio, J. Iizorg. Nuclear Chem., 1972, 34, 3491.2o G. Henrici Olive and S. Oliv6, Angew. Chem. Internat. Edn, 1974, 13, 29.22 P. Georgz in Oxidases and Reluted Redox Systems, ed. T. E. King (John Wiley, N.Y., 19651,R. C. Henney, H. F. Holtzclaw Jr. and R. C. Larson, J. Electroanalyt. Clzem., 1967, 14, 435.H. Alt, H. Binder and G. Sandstede, J. Catalysis, 1973, 28, 8.vol. 1, p. 3.(PAPER 6/530

 

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