Chemically modified electrodes and electrocatalysis

J. Electroanal. Chem., 150 (1983) 645-664

645

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

CHEMICALLY M O D I F I E D E L E C T R O D E S AND ELECTROCATALYSIS

JERZY ZAK and THEODORE KUWANA Department of Chemistry, The Ohio State University, Columbus, OH 43210 (U.S.A.)

(Received 31st December 1982)

INTRODUCTION Since its inception in the mid-seventies, research on chemically modified electrodes (CMEs) continues to attract a high level of interest and activity. While many research groups have contributed to the current level of technology and operational understanding of CMEs [1-4], the focus of our research from the very beginning has been with electrocatalytic CMEs. It is in this context that CMEs will be discussed. Almost all catalytic CMEs have relied on the immobilization of redox mediators onto the electrode surface. The function of the mediator is to facilitate th7 chargetransfer between the electrode and the species in solution. In most cases the reaction sequence can be described by an ec catalytic regeneration mechanism as follows (illustrated for a reduction): ks, M

Mox + n M e - ~ M R

E~a'

4

~1)

kf

N M R + So× ~ N M o x +

SR

(2)

where M represents the mediator (i.e. the catalyst) and S, the solution species. The driving force for reaction (2) depends on the respective values of E~' and E~' where E~' is the reversible redox potential for reaction (3): k~,s

Sox + ns e-

sR

(3)

The stoichiometric multiplier, N, is given by n s / n M. In this ec mecbanism the thermodynamic and the kinetic requirements to experimentally observe a catalytic response are relatively well understood [5-8]. Recently, another type of electrocatalysis that relies on the dispersion of alumina particles on a carbon surface has been discovered in our laboratory. In this catalysig, it appears that the electroactive species itself is adsorbed (on alumina), is electrolyzed and then undergoes "catalytic" reaction with the solution species. A tentative suggestion is that the catalysis is associated with a proton transfer step involving alumina [9], An ideal ec catalytic case would be one in which both the forward axed back 0022-0728/83/$03.00

© 1983 ElsevierSequoia S.A.

646

directions of reaction (2) are catalyzed through the electrogeneration of M R o r Mox. The identity or very near identity of E M and E s would be required as well as favourable values of ks, M, kf and k b. For the case of product reversibility, which will be illustrated shortly for catechol, the catalysis using dispersed alumina approaches the ideal ec situation since species M and S are identical except for M being adsorbed. The discussion to follow will focus on the electrochemical characteristics of the " m e d i a t e d " ec catalysis, with selected experimental examples, and on recent results of mediated catalysis using alumina dispersed on glassy carbon surfaces. T H E ec C A T A L Y T I C R E G E N E R A T I O N M E C H A N I S M

The homogeneous case One major advantage of this mechanism is that it is possible to verify experimentally the reaction sequence and its governing parameters homogeneously prior to the immobilization of the m e d i a t o r / c a t a l y s t onto the electrode surface. Thermodynamically, Sox should be reduced prior to Mox at E~', which is more negative in potential than E~'. However, in the presence of a large overpotential due to the heterogeneous rate constant, ks, s, being much less in magnitude than ks, M, the reduction of Mox proceeds prior to So~, and hence the turnover of Sox to S R via reaction (2). The cyclic voltammetric (CV) current-potential (i-E) curves for a simple ec catalytic mechanism have been digitally simulated [7]. The results are illustrated graphically in Fig. 1. The simulation parameters are: n s = n M ----- 1; D s = D M = 1.0 × 10 -5 c m 2 / s ; c s = c M = 1.0 × 10 - 3 mol/1; ks, M = 1.0 c m / s ; ks. s = 5 X 10 -9 c m / s ; kf varied from 0.0 to 2 × 10 4 M - 1 s - l ; scan rate, v, = 100 m V / s ; and E~' = - 2 0 0 mV and is referenced to E~' which is assigned a value of 0.00 V. As expected the peak current, ip, S, of the irreversible wave for the electrochemical conversion of species S at E - - 0 . 6 V, decreases in magnitude while the ec coupled wave increases as the rate constant, k f , for reaction (2) increases in value. The anodic CV wave for the reversal of reaction (1) i~s observed because of the depletion of Sox and accumulation of M R in reaction (diffusion) layer due to the time required for the potential excursion to and from ca. - 0 . 8 V; i.e. the negative scan limit. If the CV scan was stopped at ca. - 0 . 2 V prior to the irreversible Sox reduction wave, the reverse i-E wave for the anodic oxidation of M R will be much smaller and perhaps barely perceptible above background (except the case where k f = 0). Cyclic voltammetry can serve as an extremely useful diagnostic tool. The governing parameters and their consequences on the CV i-E behavior of both the homogeneous and the heterogeneous ec catalytic reaction sequence are quite clear, thanks to the extensive computational work reported by Savrant and co-workers [5,6]. The peak potential of the catalytic w a v e , Ep,cat, is closely related to the E~' value. How closely the Ep,ca t tracks the E~' depends on experimental parameters such as concentrations and kinetic rates of reactions (1-3). Nonetheless, Ep,ca t w i l l be within about 50 mV of E~' when ks. M >~ 10 - 3 c m / s and k f > ~ 10 4 M - 1 s - 1 in the

647

normal CV scan range of ca. 100 m V / s . As the value of ks, M decreases, E p , c a t m o v e s toward negative potentials and k f primarily affects the current function; ie. ip,cat a s already seen in Fig. 1. It is interesting to note that the maximum value of the peak current, ip, cat' is attained when the Ep,cat coincides with the Ep, M. That is, the maximum peak current occurs at the potential determined by the reduction of Mo~ in the absence of Sox. The ip,cat under such circumstances is given by [7] ip,cat = 2.67 X 105(1.09) .~a:b~M__Mn'/Z,,'/2,~ ... + 'p.M"

(4)

where ip, M is the peak current given by the Randles-Sevcik equation [10] for a reversible electrode reaction of species Mox in the absence of Sox. The other symbols in eqn. (4) have their usual significance. For the catalytic portion of the current, n s does not attenuate the peak current to the three-halves power as in the

I

I

I

I

I

I

to

t-- -1 Z >OC OC

Z Itl

0

2

Fig. 1. Computer-simulated current-potential curves of the ec catalytic m e c h a n i s m plotted as normalized current (i,~¢/i~,¢ o) vs. normalized potential (E - E ~ ) [7]. Kinetic parameters for curve A were: ks, M = 0, ks.s=5X~10-~cm s -1, k f = 0 , for curves B; ks, M = l . 0 cm s -1, ks, s = 5 × 1 0 -9, kt values; (1) 0.0, (2) 2 × 102, (3) 6 × 102, (4) 2X 103, (5) 2× 104 M -1 s - k Other parameters in text.

648

Randles-Sevcik equation, but instead, the stoichiometric multiplier, N, appears. As the values of ks, re and k f continue to increase, an interesting phenomenon occurs. The CV i - E waves separates into a double wave. The "prewave" is the catalytic wave (reaction 2) which appears because So, is depleted near the electrode surface in a manner similar to a diffusion controlled process prior to the depletion of Mox. The separation between the waves becomes more pronounced as the scan rate is decreased. The width of the prewave is also narrower than the usual Nernstian controlled, reversible electrode reaction due to the depletion of Sox within a narrower range of potential. Figure 2 shows the appearance of a double wave as the scan rate is decreased for the catalytic reduction of oxygen [11] via the electroreduction of water-soluble tetrakis(N-methyl-4-pyridyl)iron(III) (abbreviated as FeTMPyP). The heterogeneous case

The homogeneous ec catalysis is translated to a heterogeneous case by the immobilization of the mediator/catalyst, M, onto an electrode surface. Three main methods of M immobilization are currently practiced: (a) strong adsorption, (b) covalent linkage, and (c) incorporation into a polymeric matrix. From a synthetic standpoint, the covalent linkage method appears to be the most complex one due to the need to functionalize the electrode surface prior to reacting and hence linking the

o 3 4

-20

~-

5

0

% L) -60

-80 I

-Q2

l

I

I

O.O Q2 0.4 POTENTIAL/V

Fig. 2. Current-potential curves for reduction of 2.4 x 10-5 M 02 by 5.3 x 10 - 4 M Fe TMPyP in 0.05 M H2SO 4 as a function of scan rate (V s - l): (1) 0.005, (2) 0.010, (3) 0.025, (4) 0.050, (5) 0.100, (6) 0.200, (7) 0.400 [1 lb].

649

redox mediator. The choice of which method to utilize for immobilization depends on several factors including the purpose and use of the immobilization. There is much activity presently with extended 3-D polymers that "carry" redox sites. The immobilization of mediators in these polymers may be via ionic-site interactions [12] similar to ion-exchanger or may be through covalent bonds [13] possibly involving pendant ligands coordinating metal ions and complexes [14]. There are several experimental results that validate the ec catalytic mechanism. For example, the catalytic CME oxidation of nicotinamide adenine dinucleotide (NADH) is illustrated in Fig. 3 using three different modes of immobilization. Cases (A), (B) and (C) are the CV i - E curves for catechol (1,2-dihydroxybenzene) that had been immobilized via strong adsorption [15], covalent linkage [16] and polymeric incorporation [17], respectively, onto carbonaceous surfaces. The experimental details are given in the figure caption. In the case of the covalently linked 3,4-dihydroxybenzylamine (3,4-DHBA), it is possible to examine the solution electrochem-

C

I

2

I

I

l

t~ a: u w -> b-

1

I

I

I

I

hJ A

a::

2

1

I

I

I

I

-0.2

0.0

0.2

0.4

E / V(vs SCE )

Fig. 3. Catalytic oxydation of N A D H . Traces (1): CV curves for modified electrodes in supporting electrolyte + phosphate buffer at pH 7.0; traces (2): CV curves for these electrodes in solutions of NADH, (A) graphite electrode with adsorbed P S C H 2 in 2 m M solution of N A D H [15]; (B) GC electrode with vinyl-polymerized eugenol in solution 0.2 m M N A D H [17]; (C) GC electrodes with covalently linked 3,4-DHBA in 0.2 m M solution of N A D H [16].

650 TABLE 1 Mediated catalysis using chemically modified electrodes Mediator/immob. mode/electrode a

Soln. cond.

Substance catalyzed b

Benzidine/covalent, amide/PG 3,4-DHBA/covalent, amide/PG,GC Pentachloroiridite/polymeric, vinylpyridine/GC Ferrocyanide/polymeric, ionic/Pt Ferrocene/polymeric, vinyl/PG Eugenol/polymeric, vinyl/PG

pH 2.5/aq. pH 7/aq.

(A) Ascorbic acid (O)

FeP d/covalent, amide/PG,GC FeP/covalent, amide/PG,GC CoP e/covalent, amide/Pt,GC CoP/covalent, amide/GC CoP/covalent, amide/GC

TBAPF/DMF TBAPF/DMF TEAP/Me 2 SO TEAP/Me 2 SO TEAP/Me 2SO

DABPy f/covalent/Pt,Au - F c P g/covalent, silane/Pt

pH 7/aq. p n 7/aq.

(C) Cytochrome c

3,4-DHBA/covalent, amide/PG,GC Dopamine/polymeric, amide/GC Eugenol/polymeric, vinyl/GC NSCH 2 i/adsorbed/GR ASCH 2 i/adsorbed/GR

pH pH pH pH pH

(D) NADH h (O)

PSCH 2 i/ads°rbed/GR MB + i/adsorbed/GR

pH 7.0/aq. pH 7.0/aq.

CoTPyPJ/adsorbed/ GC

0.05 M H2SO4/a q. 0.5 CFaCO2H/a q. 0.05 M H2SO4/a q. 0.05 M H2SO4/a q. pH 3/aq. at. H + or OH Vaq. pH 7/aq.

(E) Oxygen (R)

Cofacial CoP/adsorbed/PG FeTAPP J/polymeric, amide/GC FeTEPyPJ/polymeric, ester/GC Co(II)P k/adsorbed/PG Various MP//in-matrix/carbon NQ "/covalent, silane/Pt,W Co(II)P k/adsorbed/PG

0.1 M NaOH/aq.

(F)

1 M H2SO4/aq. pH 3.2/aq. pH 2.5/aq. pH 7/aq.

7/aq. 6.35/aq. 7/aq. 7.0/aq. 7.0/aq.

(B) Alkyl halides (R) hexachloroethane diphenylmethyl bromide PhCHBrCH 2 Br PhCHBrCHBrPh CH 2BrCHBrCH 3 (O) (R, O)

H202 (O)

'~ PG = pyrolytic graphite, GC = glassy (vitreous) carbon, GR = graphite rod (spectrographic), Pt = platinum, Au = gold. b (O) = oxidation, (R) = reduction. c At/= decrease in overpotential between catalyzed and uncatalyzed electrolysis; in many cases estimated from data presented. d FeP meso-tetra(p-aminophenyl)porphyrinatoiron(IlI). e CoP meso-tetra(p-aminophenyl)porphyrinatocobalt(III). f DABPy = N,N'-dialkyl-4,4'-bipyridinium. g - F c P = 2,3,4,5-tetramethyl-l-(dichlorosilylmethyl)-[2]-ferrocenophane. n NADH = reduced form of nicotinamide adenine dinucleotide.

651

~cat/

F/nmol cm

2

Kinetic p a r a m e t e r

Comments

Ref.

V c

-

- -

0.06

--

- 0.25 - 0.1 - 0.25 0.3

-1.8 4

k c H F > 0.1 c m / s k c n 1 0 3 - 6 × 104 M -1 s - 1 --

-

0.2

18 16

- -

- 0.3

0.2

1.0 -0.6 -0.6 - 0.9 -

0.25 0.4 0.22 0.2

1.5

1.5 0.545 0.25 0.25

- -

0.06 0.12 0.2 to 0.6 1.3 0.7

k ( h o m o g e n e o u s ) 7 . 7 x 104 M

> 0.5 >~ 0.8 0.5 - 0.5

0.02-0.03 0.2 0.5-1.1 0.01-0.016

19 20 20 20

k c H F = 4 . 0 × 10 3 c m / s k c l a F = 1.4× 10 3 c m / s k c H F = 1.5 × 10 4 c m / s

khe t 2.4-8.7 × 1 0 - 4 c m / s (R) 1 . 0 - 4 . 4 × 10 - 4 c m / s (O)

0.4 0.77

Cyt c not ( R or O) at Pt 1 s-i

1 × 106 M - l s ! 3 × slower t h a n NSCH 2 desorption of A S C H 2 occurs

1-1202 p r o d u c t H20 product H20 product H202 product H202 product

k = 2 . 0 × 105 M - I s - I

H202 product

khe t > 0.013 c m / s ls-latpH

12

21,22 23 16 13k 17 24 24

15 25

k c n = 6 × 103 M - 1 s l at F ° = 0.3 n m / c m 2

k=6.0×104M

14b 12f 12f 17

19

- -

2-10

0.5 - 0.6

-

k c n d e p e n d s on F

26a 27 26b 26b 28 29 30 28

i N S C H 2 = 4-[2-(naphthyl)vinyl]catechol; A S C H 2 = 4-[2-(9,10-ethanoanthracen-9-yl)vinyl]catechol; P S C H 2 = 4-[2-(1-pyrenyl)vinyl)catechol; M B + = M e d o l a ' s Blue, 7 - d i m e t h y l a m i n - l , 2 - b e n z o p h e n o xazinium. J C o T P P = c o b a l t tetrakis(pyridyl)porphyrin. F e T A P P = iron t e t r a - ( o - a m i n o p h e n y l ) p o r p h y r i n . F e T E P y P = iron tetra[N-(2-hydroxyl)pyridyl]porphyrin. k Co(II)P, see ref. 28 for structure; p r o b a b l y not an ec reaction sequence. t Various m e t a l p o r p h y r i n s i m b e d d e d in c a r b o n matrices a n d heat-treated; good stability reported a n d m e c h a n i s t i c sequence a p p e a r s to fit ec catalysis scheme. " NQ = naphthaquinone.

652

istry in the absence and presence of NADH prior to immobilization. The close correspondence between the solution and CME oxidation of NADH gives credence to the ec catalytic mechanism. In the other two cases, it was not possible to study the solution behavior. Nonetheless, it may be seen from the CV i - E curves shown in Fig. 3 that the potential for NADH catalysis is closely related to the potential at which catechol is oxidized to the quinone. The reaction sequence proposed in the case of the amide-linked 3,4-DHBA was:

e step:

0 ~_~OH

I

c step: II + NADH + H + ~ I + NAD

(5) II (6)

Degrand and Miller [13k] have also studied the catalyzed oxidation of NADH using a carbon electrode modified by the incorporation of dopamine into a thin polymeric film of poly(methacryloyl chloride). Their results closely correlated with the previous example; ie. the catechol/quinone CME of reactions (5) and (6). Although it was suggested that polymeric immobilization might provide greater stability, recent studies on the question of stability indicated that the loss rate of catalytic activity was not due to the loss of the mediator from the surface but due to an internal reaction between the quinone and the dihydro- form of the mediator [17]. Therefore, the loss rate of activity between various catechol/quinone CMEs did not differ significantly. Table 1 summarizes several examples of CME ec catalysis where the mediator is immobilized onto the electrode surface by one of the previously described modes. From the entries in Table 1, it can be seen that the change in the overpotential, ,/, due to catalysis varies from 200-400 mV up to ca. 1 V. This latter value, for the reduction of alkyl halides by immobilized iron or cobalt porphyrin [19,20] is impressive. Another most promising CME application is the mediation of biological redox components that normally undergo very slow electron transfer with "bare" electrodes. For example, Wrighton and co-workers [23] have recently shown that both the forward(reduction) and back(oxidation) reactions for cytochrome c can be accelerated at a Pt electrode via a covalently-linked ferrocene derivative. The close correspondence between the E~a' and the E c°'y t c and the favourable rates of electron exchange between the immobilized M and the cytochrome c make possible the use of this mediated CME as a potentiometric sensor. Also along bioelectrochemical lines, Yacynych and co-workers [31,32] have immobilized enzymes through a cyanuric chloride linkage to carbon. They have reported the observation of enhanced electron transfer with the enzyme by virture of the immobilization. The potential for coupling enzymatic charge-transfer sequences by proper design of CMEs is an exciting area that certainly merits further effort.

653 ELECTROCATALYSISVIA ALUMINA PARTICLESDISPERSED ON CARBON In a recent communication [9] we described preliminary results on the electrocatalytic oxidation of catechol and ascorbic acid (abbr. A H 2) via their adsorption onto 1 /~m a-alumina particles dispersed on glassy carbon surfaces. The results are significant from the viewpoint that glassy carbon [33-38] and other carbonaceous materials are being widely used for electrochemical purposes, especially for surface modification studies. Furthermore, the most common pretreatment method involves polishing the surface with small particles of metal oxides, for example, alumina. Thus, it is possible that residues of these particles on carbon, and perhaps on other electrode materials as well, may profoundly influence the observed electrochemistry. As a matter of fact, our earlier recommended use of an ultrasonic bath as a final cleaning step after polishing [36], was predicated on the assumption that any alumina remaining on the surface could be deleterious to the electrochemistry. Alumina is, after all, a non-conductor. The removal of alumina from the G C surface by ultrasonication has been verified by scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). As part of our on-going electrode surface modification/characterization studies [39,40], we have observed that surface pretreatment procedures could greatly influence the nature of the electrode reaction. For example, the overpotential for the oxidation of ascorbic acid, as observed by CV, was often lowered by 200-300 mV. Initial suspicion focused on the possibility of GC surface functionalities or impurities being responsible for the response variations. However, the most consistent correlation appeared to be associated with the presence of alumina on the surface due to its incomplete removal during the cleaning step. This finding about alumina led us to intentionally disperse onto G C surfaces various sizes and types of particles such as those used as cleaning compounds onto GC surfaces. Of these particle dispersed electrodes (PDEs) the most effective catalytic electrode appeared to be the one with 1 /~m particles of a-alumina. a-Alumina GC electrodes

Examination of both the ultrasonically cleaned GC and the alumina "repolished" or GCA electrodes by scanning electron microscopy (SEM) clearly showed respectively the absence and the presence of alumina on the surfaces, as may be seen in the SEM photomicrographs of Fig. 4. The alumina particles are ca. 1 /~m in diameter and are dispersed as single and multi-particle clusters. A typical "catalytic" electrode has ca. 30% of its surface covered by alumina. A clean electrode was prepared by polishing the G C surface successively with emery paper and then with a-alumina (Buehler: 1.0, 0.3 and 0.05/~m) on an optical fiat. After polishing, the electrode was cleaned in an ultrasonic bath with frequent changing of the bath water (triply distilled). The GCA electrode was prepared by repolishing the cleaned GC surface lightly with 1 /~m a-alumina in a dilute water slurry. After repolishing, the surface was rinsed with water and then used im-

654

655

4

m

4~rn

Fig. 4. SEM photomicrographs of a glassy carbon surface (GC-10 Tokai). (A) polished and cleaned ultrasonically; (B) polished with alumina and rinsed only with water; (C) as (B), but greater magnification (see bars in figure).

mediately. Analysis of the G C A surface by XPS showed the presence of alumina by the appearance of the 2P and 2S AI peaks at 73 and 118 eV, respectively. When the optical flat was used as a support base for the polishing, the presence of silicon on the G C surface was indicated by XPS. Thus, it is important in the repolishing step that only a light pressure be applied for dispersing the alumina on the surface.

Catalysis." catechol, ascorbic acid and oxalic acid Typical CV i - E curves for the oxidation of catechol, ascorbic acid and oxalic acid on a G C versus a G C A electrode are shown in Fig. 5. As may be seen in the figure, the oxidation of all three compounds occurs at a less positive potential on the G C A than on the G C electrode, clearly indicating catalysis. As described previously for catechol [9], the shape of the i - E wave depended on the concentration of catechol in the solution. For example at low concentrations below ca. 0.5 m M , the anodic and cathodic waves were essentially mirror images of each other characteristic of a reversible electrode reaction for a surface confined, adsorbed species. The separation of the peak potentials, Ep = IEpa-//pc[, was typically 10-15 mV and the full width

656 i

i

I

F--Y~

I

I

[

[

~ 4O,uA 1

2

l

V

°i

B

© I

~ [

I

I

POTENTIAL / V

I

I

I

I

I

I

t

vs. Ag/AgCI

Fig. 5. Current-potential curves for ascorbic acid (A), catechol (B), oxalic acid (C). Traces (1) are obtained on the glassy carbon electrodes with alumina, traces (2) on the bare glassy carbon electrodes. The solutions contained 2 m M of redox species in 0.1 M phosphate buffer at pH 2.0. The geometric

surfacearea of the electrodewas 0.071 cm2, scan rate 0.064 V s- ].

at half-height of either wave was ca. 60 mV at a scan rate of 100 m V / s . If CV scans were run immediately after a G C A electrode was immersed into a catechol solution at low concentrations, the i - E wave increased in height upon successive scans until a steady state maximum was attained. This maximum was assumed to correspond to a state of saturation coverage of catechol on the alumina. The peak maximum, ip. . . . depended linearly on the scan rate as expected for an adsorbed species and on the amount of alumina dispersed on the surface. As the catechol concentration increased in the solution the i - E wave shape correspondingly changed until at concentrations above ca. 2 m M , the wave was characteristic of a reversible, diffusion-controlled electrode reaction for a solution species. Thus, it is evident that the current is a convolution of two components; one due to the adsorbed catechol and the other due to solution catechol diffusing to and reacting with the GCA. The extent of adsorption and catalysis were further determined by examining the integrated charge, Q, under the anodic and cathodic CV i - E waves as a function of catechol concentration. In Fig. 6 the values of Qa and Qc versus the inverse of the square-root of scan rate are plotted as a function of catechol concentration for a G C and a G C A electrode. The GC without alumina exhibited the usual current depen-

657 0

2

4

0.5 x 110-3M I 200 -0 zx, ,.Ox,O-3M ~ rl I 2.0 x , O - ' M f "

6

J

8

'

' /

J

I[-

120

4O

LU 0

rr -r

40

120 Qc 200 0

Fig. 6. The electrical charge of the i - E wave as a function of the inverse of the square-root of the scan rate for glassy carbon with ( ) and without (. . . . . . ) alumina in catechol solutions. Supporting electrolyte: 0.1 M phosphate buffer at pH 2.0. Charge determined by integration of current between potentials of +0.120 to +0.590 V for Qc uncatalyzed; +0.325 to +0.780 V for Qa uncatalysed; and +0.360 to +0.640 V for Qc and Qa catalysed.

dence characteristic of electrolysis of a diffusing species with the extrapolated zero intercept on the charge axis for both Qa and Qc (corrected for b a c k g r o u n d charging current). F o r G C A , the extrapolated charge plots for all three concentrations of catechol intersected at an identical but non-zero value. The charge from the extrapolated intercept was assumed to correspond to the saturation coverage of catechol on alumina. The close correspondence of the slopes for the Q, and Qc vs. v - 1/2 pl0t s at each concentration between the G C and G C A electrodes suggested that the diffusing catechol was being electrolyzed within the same potential range as the adsorbed catechol. Since the saturation coverage of catechol was dependent on the a m o u n t of alumina dispersed on the surface, the extent of catalysis was also dependent on the alumina level. Thus, when the alumina coverage was low, the uncatalyzed wave due to the electrolysis of catechol on the bare carbon became evident on the wings of the a d s o r b e d / c a t a l y z e d wave. As suggested earlier, the G C A catalysis with catechol approaches the ideal ec case where both the forward and back reactions are catalyzed. The situation for A H 2 and oxalic acid is quite different as a consequence of p r o d u c t irreversibility. Thus, only the oxidative CV i - E waves are observed (see Figs. 5B and C, traces labelled No. 1).

658

The irreversibility, however, provides an opportunity to further probe the character of the alumina catalysis. For example, if a G C A electrode is exposed to air in the laboratory for long periods of time, the peak potential for the electrooxidation is shifted to more positive potentials and in some cases, up to + 800 mV. However, if the polishing and transfer of the electrode are performed in a controlled atmosphere (eg. nitrogen) in a glove bag, the experimental results are reproducible and the i - E wave does not depend on the time the electrode is kept out of the solution. Our conclusion is that the G C A electrode can be easily contaminated and the experimental results shown in Fig. 7 support this contention. Traces B, C, and D in Fig. 7 correspond to a G C A electrode that had been exposed for 1 min to benzene vapor, methanol vapor, and air. The values of the peak potential are + 750, + 600 and + 370 mV for the above contaminated electrodes compared to a value of + 340 mV obtained for an electrode stored in the nitrogen atmosphere (trace A). The results indicate that the alumina surface can be readily deactivated, probably by adsorption of benzene, methanol or atmospheric contaminants on the alumina. Alumina is, of course, a well known chromatographic adsorbent [41]. Due to its large porosity, it is possible that a solution containing redox species could penetrate its interior channels as well as any free space between the alumina and the G C surface. Thus, the electroactivity of the adsorbed species may be initiated at the interface where alumina and the G C surfaces are in intimate contact. The specific chemical and catalytic properties of alumina [42] must also be considered in attempting to understand the present catalytic phenomenon. The specificity of alumina as a catalyst was further tested by examining the CV i - E waves of A H 2 oxidation at three other abrasives that have been commonly used

~A[

lO,uA

O.'2

O.'4 0.'6 POTE NTiAL/V

0'8

Fig. 7. Effect of the surface contamination on the voltammetric wave of ascorbic acid. The electrode was exposed for 1 min to benzene vapors (B), methanol vapors (D) and to the air (C). Trace (A) was obtained for the electrode stored in nitrogen atmosphere. Solution of 3 m M ascorbic acid in 0.05 M H2SO4, scan rate 0.050 V s-1.

659 as polishing materials. The data in terms of the cyclic voltammetric Ep and relative peak current values for this oxidation are listed in Table 2. The Ep values under the column labelled G C are those for a freshly polished (using the abrasive listed) and then ultrasonically cleaned surfaces. The P D E s were prepared by lightly repolishing the cleaned ones in the abrasive followed by successive rinsing with distilled water as previously described for G C A . All of the cleaned G C electrodes gave essentially identical values of Ep at + 454 + 5 mV. This value will be considered characteristic of A H 2 oxidation at GC. The largest decrease in the overpotential (160 mV) and increase in the relative current (119%) were observed for alumina c o m p a r e d to 2 0 - 3 0 m V and ca. 30-50% for KRS-5 and Garnet, respectively. Ceric oxide P D E showed essentially no difference from a cleaned G C electrode. Although d i a m o n d is an inert and inactive material, the commercially prepared d i a m o n d " d u s t " used for polishing purposes was received as a suspension and contained organics that were not easily removed from the G C surface. Thus, polishing with this d i a m o n d resulted in a d i a m o n d - P D E with a large overpotential for A H 2 oxidation, even after ultrasonication. The organic contaminants could be removed and the G C surface restored only by following the usual p o l i s h / c l e a n procedure with alumina. In order to understand the reason for the increase in the current response for the P D E electrodes, a much wider variety of abrasive materials including several metal oxides and an clean d i a m o n d sample (commercial abrasive particles) need to be examined. In particular, the correlations between catalysis and the chemical composition a n d / o r particle s i z e / p o r o s i t y of dispersed particles on G C or other carbon surfaces must be carefully evaluated. The a d s o r p t i o n / c a t a l y s i s of G C A was next evaluated by examining the CV and chronocoulometric behavior as a function of A H 2 concentration. At low concentrations, the oxidative i - E wave was characteristic of that for a surface-confined species with a full width at half peak height equal to 45 mV and independent of scan rate between the range of 5 0 - 4 0 0 m V / s . The width did not b r o a d e n with scan rate as would be expected for an irreversible process [43,44]. The ip values varied linearly with scan rate when the A H 2 concentration was below ca. 0.25 m M . As the A H 2

TABLE 2 Peak position and relative increase of voltammetric peak current for different polishing compounds a Polishing compound Cerium oxide (Harrick Garnet (Harrick) KRS-5 (Harrick) Alumina 1/zm (Buehler)

before polishing

after polishing

( AI / I o ) × 100

0.450 0.460 0.460 0.450

0.420, 0.425, 0.430 0.440, 0.390, 0.400 0.420, 0.415, 0.428 0.290, 0.288, 0.292

3 52 32 119

Ep/V

'~ The electrode was polished on the polishing cloth (Buehler). The solution was 0.05 .H2SO4 +2 mM ascorbic acid. Scan rate 0.05 V/s, potentials vs. Ag/AgCI.

660

concentration increased, the i - E wave shape changed gradually to appear like that for a reversible, diffusion controlled electrode reaction (see trace B, Fig. 5). Similar to what has been observed for catechol, the i - E wave is a convolution of the current due to the adsorbed and the diffusing A H 2. The plots of the anodic charge, Qa, v e r s u s t 1/2 for the chronocoulometric experiments are shown in Fig. 8 for the GC and G C A electrodes. The A H 2 concentration varied from 0.1 to 2.0 m M (pH 2.0) and the applied potential was stepped from + 200 to + 600 mV. For the cleaned GC electrodes the extrapolation of the plots for all four concentrations intersected the charge axis at the same value as that of the double layer charge, which was obtained in the absence of ascorbic acid. This identity of charge means that A H 2 is not adsorbed on the clean GC surface. For A H 2 at a G C A electrode, the extrapolated Qa vs. t ~/2 plots for the concentrations of 0.5, 1.0 and 2.0 m M intersected the Q axis at the identical value of 33/~C which is ca. 10 × greater than that for a cleaned GC electrode. However, the double-layer charge was also much greater. This increase in Qdl for G C A (12 /~C compared to 2.5 /~C for GC) is surprising since alumina is considered to be a

15(

10(

zk w ~9 c~ < 3U 5(

Qd~ . Qat 0

o.2

o.4 T 1/2 /

0.6 s 1/2

0.8

1.0

Fig. 8. Potential step chronocoulometry of ascorbic acid on glassy carbon electrodes without ( . . . . . . ) and with ( ) alumina. The potential ranges respectively: E i = + 0 . 2 0 0 V, Ef = + 0 . 6 0 0 V, and +0.100, +0.400. The electrolyte was 0.1 M phosphate buffer at p H 2.0, the geometric surface area of the electrode 0.071 cm 2.

661 non-conductor. There is a possibility that there is a finite amount of carbon adhering to the alumina surface from the dispersion process. Irrespective of the reason for the increase in QoI, the difference between the charges allows a calculation of the coverage assuming a two electron process for the oxidation of adsorbed A H 2. The value of coverage is 1.4 × 10 -9 m o l / c m 2, assuming that the surface area is equal to the geometric area of GC. Two other electrochemical problems were briefly explored. The first was whether G C A would catalyze the oxidation of oxalic acid which had been previously examined by Anson and Schultz [45] at a Pt electrode. They reported that the formation of platinum oxide or the preferential adsorption of other substances such as amyl alcohol or chloride ion could inhibit oxalic acid adsorption, and hence its oxidation. The second problem was whether 1,4-dihydroxybenzene (1,4-DHB) could be catalytically oxidized at GCA and thus allow the electrochemical separation/ analysis of a mixture of A H 2, catechol and 1,4-DHB. As may be seen in Fig. 5C, traces 1 and 2, the oxidation of oxalic acid at p H 2.0 occurs with a shift in peak potential of ca. 300 mV at a GCA compared to a GC electrode. The peak height also increased by ca. 1.4 times indicating that indeed, the oxidation was catalyzed. The ip versus v 1/2 was linear for this catalytic wave indicating that the peak current was governed by the diffusion of oxalic acid. Because oxalic acid has pKa values at 1.27 and 4.27 [46], the pH dependence of the peak potential was examined as a function of the solution pH. At pH values of less than unity, the Ep value was ca. + 1200 mV. Between pH 1 and 2, there was a change in Ep to less positive values of nearly 200 mV. In the pH range 2-5, the Ep changed by 30 m V / p H and then from pH 5 to 8, it became a constant at + 850 inV. Thus, the breaks in the Ep vs. p H plot occur near the p K a values. Upon successive CV scanning between the potentials of + 200 and + 1200 mV, Ep moved to more positive potentials and ip slowly decreased. A steady state was attained at an Ev value of ca. + 1160 mV which is still some 140 mV less positive than at the bare GC electrode. Although speculative, the decrease in catalytic activity may be associated with product adsorption from the oxalic acid oxidation. The extent of oxalic acid and product adsorption is currently being investigated. The presence of three well:separated anodic waves in Fig. 9 (solid curve) for the oxidation of a mixture containing AH 2, 1,4-DHB and catechol clearly demonstrates the analytical utility of the GCA electrode compared to the bare GC (dotted curve). The absence of one cathodic wave one scan reversal is due to the product irreversibility of A H 2. It is also significant that 1,4-DHB was catalyzed since the other compounds, AH2, catechol, and oxalic acid, can be considered as all possessing "1,2" position/sites of interaction with alumina. The scope of catalysis in terms of compound types is being investigated and will be reported separately. Perhaps it is not surprising that alumina dispersed on a carbon surface adsorbs and catalyzes charge transfer processes when one considers the behavior of mediators immobilized in polymeric matrices on electrodes. Certainly in the case of catechol, there is ample evidence [15,24] that adsorption enhances the reversibility of the electrode reaction on graphitic or carbonaceous materials. The oxidation of the

662

]20, A

/ /

\

\

/\

',

/' ~ : ~ - - ~

1

\, '/

I

0

I

I

O.4 POTENTIAL

I

O.8 / V

Fig. 9. V o l t a m m o g r a m s of the solution containing 1 m M of ascorbic acid, 1 m M of 1,4-hydroquinone and 1 m M of catechol in 0.1 M p h o s p h a t e buffer at p H 2.0. Trace ( ) for the G C electrode with alumina, trace ( . . . . . . ) for bare GC. Scan rate 0.100 V s ~.

adsorbed and diffusing catechol on GCA can occur through electron exchange; that is, the oxidized catechol in the quinone form as initiated at or near the G C / a l u m i n a interface, reacts with the dihydro form and provides a pathway for charge-transfer similar to the mediator in the polymer. The mechanisms for the cases of A H 2 and oxalic acid are less clear since product irreversibility, irrespective of the electrode material or solution conditions, is always observed with these two compounds. The possibility that alumina particles may be partially covered by a thin layer of carbon has not been completely dismissed. However, alumina particles that had been previously exposed to catechol have been dispersed on GC and have exhibited the characteristic i - E waves for adsorbed catechol. Also, fl- and "~-alumina have been examined. To date, the most effective alumina appears to be the 1 /~m a-alumina purchased from Buehler Co. (Evanston, IL, U.S.A.; label indicates that the primary source of the alumina is Union Carbide Co.). Pressed disks [47] incorporating alumina in carbon matrices have been used with only limited success. SEM indicated a very inhomogeneous surface with these disks and the extent of carbon on the alumina particles remains a question. As recently pointed out [48]¢'... it should be recognized that some of the best electrocatalysts employ inner-sphere pathways to achieve their potency.." Thus, the example in Table 1 utilizing mediators in the ec catalytic mode, certainly supports this viewpoint. The opportunity to elicit electrocatalysis via modification of conduc-

663

tive surfaces appears only limited by one's imagination and willingness to experiment. ACKNOWLEDGEMENTS

This work was supported by NSF Grant CHE-8110013 and by the OSU Materials Research Laboratory. Portions of this paper were written during a summer sabbatical leave to the Solar Energy Research Institute, Golden, CO, U.S.A. Helpful discussions with D. Karweik are hereby acknowledged. REFERENCES 1 2 3 4 5

6 7

8 9 10 11 12

13

14

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