DOI
Electro-oxidation of Cyclohexanol on a Copper Electrode Modified by Copper-dimethylglyoxime Complex Formed by Electrochemical Synthesis
Mohammad. HasanzadehJ*，* Nasrin.ShadjouJ*Lotfali. saghatforoush,tBalal. Khalilzadeh,§ and Isa. Kazeman*
‘Department of Chemistry,FacultyofScience,PayameNoor University (PNU), P. O. Box 58168-45164,Khoy, Iran
*E-mail: mhmmd_hasanzadeh@yahoo.com
* Departmentof Chemistry,Facultyof Science, K. N. Toosi UniversityTechnology, Tehran, Iran
§ Departmentof Chemistry,Faculty of Science, ArakUniversity, Arak, Iran Received August 15, 2009, AcceptedOctober9, 2009
Copper-dimethylglyoxime complex (CuDMG) modified Copper electrode (Cu/CuDMG) showed a catalytic activity towards cyclohexanol oxidation in NaOH solution. The modified electrode prepared by the dimethylglyoxime anodic deposition on Cu electrode in the solution contained 0.20 M NHqCl + NH4OH (pH 9.50) and 1 x 10-4 M dimethylglyoxime. The modified electrode conditioned by potential recycling in a potential range of -900 〜900 mV vs. Ag/AgCl by cyclic voltammetry in alkaline medium (1 M NaOH). The results show that the CuDMG film on the electrode behaves as an efficient catalyst for the electro-oxidation of cyclohexanol in alkaline medium via Cu (III) species formed on the electrode.
KeyWords: Cyclohexanol, Electrocatalysis, Copperdimethylglyoxime, Alkalinemedia
Introduction
A detailed knowledge of the electrochemical behavior of organic compounds bears primaryimportancefordevelopment of appropriatesensors and foradvanced electrochemicalsyn- thesis.1-3 Numerousstudies deal withelectrocatalytical oxida tion of organic substance shouldhavethe followingproperties (i) Theenergyof adsorptionofsubstrate molecules and/orin termediate species formed onthe catalystsurface should be sufficientlyhighto constantly decrease the activation energy of the dehydrogenation step, constantly butit should be suffi cientlylowforintermediate and final productstobedesorbed easily,and(ii)theeneigyrealized byformation ofwatermole cular as oneofthe reaction products shouldbe sufficientto compensate for theenergyneeded energy for thedesorption of dehydrogenated organic intermediatesfromthemetal surface.
Copperelectrode is the mostactive and the mostoften used for electrocatalysts.4-12
Theliterature on electrochemical oxidation ofalcoholscan roughly be grouped into threesets:fuel cells, hydrogenousand homogenous catalysis.13-16The inorganic community's investi gationof the electrooxidation of alcoholsprimarilyconsisted of homogenoussystems. The alcoholsubstratesaretypically either cyclicalcohol such asthe reaction stops after 2e- oxida
tion to the ketone orprimary alcohols. Thus competitionbe tween 2e- oxidation to thealdehyde and 4e- oxidation to car
boxylicacid becomes possible.Alcohols canalso undergo 6e-
oxidation toCO2； rarely appear in the homogeneous studies.15-16 Adipic acid as an oxidation product of cyclohexanol,cyclo- hexen orcyclohexanon, has been widely used not only as an importantintermediate in manufacturing of nylon, but also a plasticizerand a food additive. Since cyclohexanol is animpor tant material used in producing adipicacid, it worths tostudythe reaction mechanism of electro-oxidation ofcyclohexanol.17-19
Recently, we conducted the electrochemical oxidationof cyclohexanol on a copper electrode.20Inthe present work,the copper electrode is electrochemically modified with dime thylglyoxime and theefficiency of theelectrode,towardsthe electrocatalytic oxidation of cyclohexanol in alkaline medium, is investigated.
Experimental Details
Allused chemicalswere analytical grade fromMerck(Darm stadt, Germany) and were used without furtherpurification.
All solutions werepreparedwithdoubly distilled water. Electro chemical measurements carried outin a conventionalthree- electrodecell poweredby an electrochemical system comprising an AUTOLAB system withPGSTAT12 boards (EcoChemie, Utrecht,The Netherlands).The system was run ona PC using GPES 4.9 software. A saturated Ag/AgCl(fromMetrohm)and platinumwire were reference and counter electrodes, respe
ctively.Allpotentialsweremeasured with respect totheAg/
AgCl and positioned, bymeans ofa Luggin capillary,as close tothe working electrodeaspossible. Cylindrical copper bars withthepurity of 99.9%werefittedintoTeflonexposing cir
cular areas having 4mmdiameters(surface area is 0.126cm2) to preparestationaryandworking electrodes, respectively. The copper surfaces polished with sand paper and 0.05 卩m alumina to a mirrorfinish and were subsequentlyrinsed with distilled water. Films of DMG were formedonthe Cu electrode with surface area 0.126cm2bythemethod previouslyreportedby Bobrowski.21 The modified electrodes were prepared by cyclic the potential of the workingelectrode in the range of一 900 to 900 mV Ag/AgClat a scan rate of 100mV/s for70 cycles.
The surface concentration ofDMG film which were con
trollingby thenumber of cycles appliedin thedeposition pro cess is electrochemically evaluated in 0.1 MNaOH solution.
Results and Discussion
Fig. 1 (image a), showsthe structure of the copper surface immediatelyafter polishing with sand paperand 0.05gm alu mina. As seen in thisimagethe surfaceisalmostsmooth and uniform.Theimage b in Fig.1 shows thestructures of theCu/
CuDMG electrode after formation of film on Cu electrode.In these conditions some cracks appear onthefilm. Therefore the obtainedfilmshows irregular shapesand is notuniform.
Onthewhole, Cu surfacehasan approximately smooth mor
phology,while relatively no uniform and irregular shapesof DMG film with haveanaverage size of 10 gm are formed on Cu surface.
The electrocatalytic activity ofCu/CuDMGelectrode for cyclohexanol oxidationwasevaluatedbycyclic voltammetry and compared with copper electrode. Fig. 2A showsthe CVs of electro-oxidationof cyclohexanolonCu and Cu/CuDMG electrodes in thepresence and absenceof cyclohexanol in 0.1 M NaOH solution. At Cu/CuDMG electrode, oxidation ofcyclo
hexanolresulted in atypicalelectrocatalytic response.In the presence of thecyclohexanol,itwasobserved thattheanodic currentincreased drastically. Inthepresence ofcyclohexanol
(a)
Figure 1. SEM images of Cu electrode surface immediately after polishing (a), SEM images of Cu/CuDMG electrode after formation of film on Cu electrode.
그 200
-—
—100- 0 -100
-200
-300
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
E/V
B
<
600
500
400
300
200
100
0-
-100
0 0.2 0.4 0.6 0.8 1
E/V
Figure 2. A: Cyclic voltammograms (CVs) of Cu electrode (b) and Cu/CuDMG electrode (c) in the absence (a) and the presence (b and c) of 0.2 M cyclohexanol in 0.1 M NaOH solution at a potential sweep rate of 10 mV/s. B: Cyclic voltammograms of Cu/CuDMG electrode in 0.1 M NaOH solution in the presence of different concentrations of cyclohexanol in Cu/CuDMG electrode. From inner to the outer: 0.03 (a) 0.08 (b), 0.1 (c), 0.15 (d), 0.2 (e), and 0.25 M (f), respectively. The potential sweep rate is 10 mV/s. C: Dependency of peak current on the concentration of cyclohexanol in the range of 0.01 - 0.1 M.
the ratio ipa/ipc was 74.16. In absence of it the ratio decreased to29.01.Theresult indicates that cyclohexanol is oxidizedby active Cu (III) moiety through a cyclic mediation redox process.20 The Cu2+/Cu3+ redox coupleactsas a catalyst for the oxidationof cyclohexanol inbasic solutions. It can be clearlyseen inFig. 2A which wasrecorded in0.1 M NaOH/
0.2 M cyclohexanol onCu electrode(curveb)and Cu/CuDMG electrode (curve c), respectively. Furthermore, the currentof
0.15 0.35 0.55 0.75 0.95
E / V
<
A
0 1 9
7
5 3
2
2
2 45
c
D3 0
(m V 5 ov s( 。
"0刀 >
트( 幺드 등 흐 느 -
u 은
=6
3 0 0
0
0
0
0
0 0
0
0
0
0
0 7
6
5
4
3
2
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Figure 3. A: Cyclic voltammograms of the Cu/CuDMG electrode in 0.1 M NaOH solution in the presence of cyclohexanol in various potential sweep rates of 2, 5, 10, 20, 30, 40, 50, 75, 100, 150, 200, 250 and 300 mV/s. B: Dependence of the peak potential on Ln v for the oxidation of cyclohexanol at Cu/CuDMG electrode obtained from the data of panel (B). C: Dependence of anodic peak current during the forward sweep on the square roots of potential sweep rate. D: Current function vs. v 0.5 for 0.1 M NaOH solutions in the presence of 0.1 M cyclohexanol.
the Cu/CuDMG electrode is muchhigher than Cu electrode.
For example, at thepotential0.7V the Cu/CuDMG electrode presents acurrent of about 500 卩A, nearly fivetimes higher thanthat of theCuelectrode(~100 卩A). Onthe basisof the observationwe presume that theCu/CuDMG electrode shows significantly greater electrocatalytic activity towards cyclo
hexanol oxidation than Cu electrode.
Fig.2B showsthe effectof cyclohexanol concentration on the anodic peak current at Cu/CuDMG electrode in0.1 M NaOH.It is clearlyobservedthatasthe cyclohexanol concen
trationincreases, the peak heightincreases linearly with cyclo hexanolconcentrationupto0.25M.It can be assumedthatthe increase is due to the presence of a diffusioncontrolled process that appears to play an importantroleatlow cyclohexanol con
centrations. Whilethecyclohexanol concentrationexceeds it limits,therate of the wholeoxidationprocess whichseems to belimitedbythat ofthe catalytic process in origin andits rate depends on the reaction between cyclohexanol and CuDMG species,which is present in the film.
In thehigh range of concentrations thisdependency is signi
fying the dominance of thediffusion controlledprocesses(lin ear). Thisdependencyobserved in therange of0.03 -0.25 M.
Fig. 2B showsthat upon increasingcyclohexanol concen
trationits irreversibleoxidation develops in the regionof the electrochemical formation of CuIII. Thus, it is likely thatthe electro-generated CuIIIspecies is theactivemoiety which effi ciently speeds up the oxidation ofcyclohexanol. Any increase in theconcentration of cyclohexanol causes a proportional almost linear enhancement of theanodic wave, Fig. 2C. It is worthto emphasis that the anodic formation of CuIII seems to be an irreversible process,Fig. 2 and the regeneration of thesurface happens througha chemical redox reaction. Theinvolvement of thecatalytically activeCuIIIspecies, in the early stages of the reverse cathodic sweepis supportedby theanodicresponse of cyclohexanol oxidation, Fig. 2 (curve c).
Fig. 3Aillustratescyclic voltammograms of 0.08M cyclo hexanol usingthe Cu/CuDMG electroderecorded at different potentialsweeprates. Fig.3Bindicates anodicpeakcurrents increasedlinearly with the square rootof thepotentialsweep rate. It indicatesa mass transfer-controllingprocess of oxidation
Cu 5.06 x 105 8.74 x 10-12 1.91 x 10-13 0.63 Cu/CuDMG 3.36 x 106 1.41 x 10-10 6.44 x 10-15 0.57 Ht 아iould be pointed out thatkis either k2(E) or k3(E) whichever is smaller which indicated in reactions^ of4 and 5. k evaluated from Eq.(4) by choronoamperometry technique. k/wasevaluatedfrom% = k2rO<Cm ref[20] where r is thetotal number of adsorption sitesperunit area of the electrode surface, 0's represent the fractional surface coverage 5s of copper of different valence states and Cm is thebulk concentration of cyclohexanol. "k%rwas evaluated from kz(E) =k；exp]쁱夺 I ref [20].
via diffusion. In additionthevalue of theelectron-transferco efficient for the reaction canbe obtainedfromthefollowing equation:22
EP 4] ln v + constant
2aF丿 (1)
withchronoamperometry in 0.1 MNaOH solution. For this order, 700mVpotentialstepsareapplied tothe Cu/CuDMG electrode and Cu electrodein 0.1 M NaOHcontaining differ
ent concentrationsofcyclohexanol. Thecorrespondingchro- noamperometric data are presented in Fig. 4A. The observations showedtwo noticeablepoints: (I) the current in thepresence of cyclohexanol ismuch higher than that recorded in the absence of cyclohexanol for both Cu/CuDMG and Cu electrodes. (II) Thecurrentdensities recordedonCu/CuDMGelectrode are significantlyhigher than that of Cuelectrode. (III) Whilethe cyclohexanol concentration isincreasing for bothofthetwo electrodesthecurrentincreases.
Therateconstants of thereactions of cyclohexanol and the ensuedintermediates with the redoxsites of the Cu/CuDMG electrode can be derived from the chronoamperograms accor
ding to:24
This isvalid for a totally irreversiblediffusion-controlled process.Using the dependency of anodic peak potential onthe neperian logarithm of thepotential sweeprate (Fig. 3C), the value of the electron-transfer coefficientwas obtained as 0.59.
Tafel slope is 41.99. Onthe basisof the slopes of thelinear dependency of theanodicpeakcurrents on thesquarerootof thepotential sweeprates (Fig. 3B), and the Randles-Sevcik equation:23
I = (2.99 x105)a1/2 n3/2 AC * D1/2 v1/2 (2)
Where Ip is thepeakcurrent, A isthe electrodesurface area, D is thediffusion coefficient and C* is the bulk concentration ofcyclohexanol. Thediffusion of coefficient for cyclohexanol was calculated to be 8.92 x 10-6 cm2 s-1. Plotting the current function(peak currentdivided by the square root of the potential sweeprate)againstthesquare rootof thepotentialsweep rate (Fig.3D) revealed negative slope confirming theelectrocatalytic nature of theprocess, too.
The redoxtransition of copperspeciespresent in the film is:
k1
Cu"<---CuIII + e (3)
k-1
and cyclohexanol is oxidized on themodifiedsurfaceviathe following reaction:
Cuin + (C6H1^)c^dis(Cyclohexanol)
北*E): intermediate + Cu"
Figure 4. A: Double step chronoamperograms of Cu and Cu/Cu DMG electrode (dashed lines) in 0.1 M NaOH solution with diff
erent cyclohexanol concentrations of 0 (a), 0.03 (b), 0.07 (c), 0.1 (d), 0.15 M (e), 0.2 (f), and 0.22 M (g). Potential steps were 700 mV for oxidation and 0.00 mV for reduction. B: Dependence of Icata/Id on t 0.5 derived from the data of chronoamperograms of a and f in part A for Cu/CuDMG electrode.
where the intermediate in the product is oxidized strongly through a similar electrooxidation process:
1 catal _ ^ 1/2 n%f(/)+ exp(了)
人］/2 (6)
where Icatal is the catalytic current in the presence of cyclo
hexanol, Id thelimitingcurrentin the absenceof cyclohexanol and X = k Cmt(k, Cand tarethecatalyticrateconstant, bulk concentration ofcyclohexanoland theelapsedtime, respec
tively)is theagumentoftheerror function. For X > 1.5, erf (X0.5) almostequalsunity andEq.(6) reducesto:
븡 = x1/2^1/2 = n ll2(kCt )1/2 (7)
Fromtheslope of the Icata〃d vs. squareroot of timeplot the value of k at a given concentration ofcyclohexanol is derived.
Fig. 4B indicates dependency of Icatal/Id ont 0.5 derivedfrom the data of chronoamperograms ofa and f in Fig. 4A for Cu/
CuDMGelectrode. The meanvalue of k in theconcentration range of 0.03- 0.22 Mwas found tobe 3.36 x 106 cm3mol-1 s-1.
It should be pointed out that k is either k 2(E) or k 3(E) which- hever is smaller. This resultwas compared with Cuelectrode that indicated in Table 1.
The pseudo-steady state polarization curvesof theanodic oxidation of cyclohexanol on Cu and Cu/CuDMGelectrodes in theabsence and presence of 0.08Mcyclohexanol are pre
sented in Fig. 5A. It is observed fromFig. 5Athatcurrent of cyclohexanol oxidation on Cu/CuDMG electrode is 2500 nA, more than threetimeshigher thanCuelectrode 1000nA; for both of thetwo electrodesanodic currentsin thepresence of cyclohexanolare much higher than those in the absence of cyclohexanol. These observations are consistentwith the results ofCVs.
Thepseudo-steady state polarizationcurves of the electro
oxidationofcyclohexanol on Cu/CuDMG electrode at a number of cyclohexanol concentrations are presentedin Fig. 5B. Poten- tiodynamic polarization curve for Cu/CuDMG electrode in 0.1 M NaOH in the presence of 0 - 0.25 Mcyclohexanolrecorded using a potential sweep rate of0.25 mV/ s. The rotation rate of the electrodeis maintainedat3000 rpm to avoidthe interference of the masstransfer in the kinetics measurements. Theoxidation process was found to beginatnearly600mVAg/AgCl and to reach aplateau at 710 mV Ag/AgCl. In the course of reaction thecoverage of Cu111increases and reaches a saturation (steady state) level and the oxidation current follows accordingly.
According toEq. (8):20
〔 끄齢I
if (8)
Theplots of the inverse currentagainstthe inverse cyclo
hexanol concentration shouldbelinear:20
if-1 =(FAk r )-1 + k1 + k-1
2FAkk r (9)
470
4000
3500- 4500-1 ■
270 170
<
E 370
-30
0.025 0.075 0.125 0.175 cm-1/mM-1
0 1E-09 2E-09 3E-09 3000
2500-
2000-
B
70
300
Exp(-nEF/RT)
4.7 3.7 2.7 1.7 0.7
2700 2200
1200 700
200 y=1E+12x+412.04 R2=0.9910 d) 1700
E/V
02 04 0.6 0.8
E / V
-0.2
0.47 0.52 0.57 1500-
1000
500
y=-27.335x+17.176 R2=0.9949
0
0 1.2 1.4
Figune 5. A: The pseudo-steady state polarization curves recorded on the Cu/CuDMG (a) and Cu (b) electrodes in the presence of 0.08 M cyclohexanol. B: Typical pseudo-steady state polarization curves of Cu/CuDMG electrode obtained in 0 (a), 0.03 (b), 0.07 (c), 0.1 (d), 0.15 M (e), 0.2 (f), 0.22 (g), and 0.25 M (g), cyclohexanol, respectively.
C: Plot of i-1 (from polarization curves in Fig. 5B) against Cm'1 at various potentials: (a) 570, (b) 590, (c) 616, (d) 635, (e) 629, (f) 644, (g) 658 and (h) 673 mV as curves (a-h). D: Plot of the slopes (of curves in A) vs. exp (-nFE/RT). E: Plot of the ln (intercepts) (of curves in A) vs. applied potentially.
Fig. 5C presents the i -1 versus Cm-1 dependencies where straight lines at various potentials havebeenobtained; both the intercepts and slopes of thestraight linesappearing in this figurearepotentialdependent.The slopesare plotted against exp(-nFE/RT) with n = 1and the graph is presented in Fig.
5D.Usingthis graph alongwith Eq. (9) reveals thatthe rate constant ofreaction 4,20 k 2「and the ratio ofk°-1/k°1 are 1.41x 10-10 cms-1and 4.82 x 1012, respectively. Fig. 5E presents the variationof theintercepts ofthelines in Fig. 5C with the applied potentialin a semi-log scale. Using this graph andEq. (9) the magnitudes ofk。/ and theanodic transfercoefficientof6.44 x 10-15mols-1 cm-2 and 0.57 have been obtained. From the above findingsthevalue of k。-/wasworked out tobe 3.104 x 10-2 mol s-1 cm-2. This result was comparedwith Cu electrode20 that isindicated in the Table 1.
and Its electrocatalytic activity compared with a Cu electrode. The results show thatthe Cu/CuDMG electrode exhibithigh electro catalytic activity towards cyclohexanol oxidation.Themagni tudes of the rate constants andanodictransfercoefficients of the electro-oxidation reaction havebeenobtained.
Acknowledgments. We gratefully acknowledge thesupport of this work by Arak University.
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