Langmuir 1994,10,3929-3932
3929
Site Selection in Electrode Reactions: BenzoquinoneLHydroquinone Redox at Submonolayer Sulfur-CoatedIridium Surfaces Woldegabr Temesghen, Jiann-Jong Jeng, Arnaldo Carrasquillo, Jr., and Manuel P. Soriaga*pt Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received May 31,1994@ The electrode kinetics of the benzoquinonehydroquinone redox reaction in the unadsorbed state has been studied at a smooth polycrystalline Ir electrode pretreated with sulfur at monolayer and submonolayer coverages. Kinetic measurements, performed at pH 4 buffered aqueous solutions, were based upon thinlayer electrochemical methods. The electrode-transfer rate was found to increase in the order: oxide cm s-l) < benzoquinone coated (k" coated (k" cm s-l) < sulfur coated (k" cm s-l). Even at sulfur coverages as low as 0.1 monolayer, the rate already approached Nernstian behavior; this suggests that the electrode-transfer reaction occurred preferentially at the chemisorbed-sulfur sites. Such sulfur-site selectivity runs counter to the conventional view that chemisorbed sulfur functions as a poison to essentially all electrode reactions.
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Introduction We have long held an interest in the influence ofneutral species chemisorbed in monolayer (or lower) quantities at electrode surfaces on the redox kinetics of unadsorbed, uncharged electroactive materials.1,2 In a recent study,lb we examined the effect of adsorbedzerovalent iodine atoms on the electrode kinetics of the benzoquinonehydroquinone [BQ(,jHQ(,)] reaction in pH 4 bufYered aqueous solutions where the rate is supposed to be i r r e ~ e r s i b l e ; ~ smooth polycrystalline Pt and Ir electrodes, known to be surface-active toward both iodine and the organic reactants, were employed. It was found that, in the presence of iodine at saturation coverages, the BQ(,jHQ(,,) redox rate approached quasi-Nernstian behavior. Of greater significance was the observation that such kinetic enhancement was also enforced even if the surface coverage of iodine was decreased to a tenth of a monolayer. Since (i)thepartially I-coated electrode contains a n appreciable amount of reactant-derived intermediates and (ii) the redox rate at organic-pretreated electrode was slow, the results indicated that the BQ(aqjHQ(aq) reaction took place preferentially at surface sites covered by the iodine atoms. We feel that these findings are of fundamental importance to electrochemical surface science since, while the catalysis of the electrode kinetics of neutral, solution-phase compounds by metal adatoms is now well-known,4 rate enhancement by nonmetallic adatoms is neither as widely documented nor as sufficiently understood. We have since extended our work to determine the effect of zerovalent sulfur, adsorbed at monolayer and submonolayer amounts, on the electron-transfer kinetics of the BQ(,jHQ(,,, redox reaction. The initial expectation, based on the conventional view that adsorbed sulfur
* To whom correspondence should be addressed.
t Residential Young Investigator. @
Abstract published inAduanceACSAbstracts, October 1,1994.
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poisions all electrochemical reactions, is that the rate will be retarded. However, such an expectation is not borne out by the results of the present study. Experimental Section A smooth polycrystalline iridium thin-layer electrode was utilized in the present study. The fabrication of the thin-layer electrochemicalcell5 and the preparation of the Ir electrodeswere as previously described; a special procedure is necessary for Ir because of its extreme hardness and propensity to form hydrous oxide layers.7 Patterned after a procedure developed for P t , S sulfur-coated electrodes were prepared by immersion of the clean surface to dilute NazS solutionsbuffered at pH 10 with Na2HP04 and NaH2Pod. For saturation (monolayer)coverages, the electrode was simply exposed to 10 mM NazS solution for 180 s; based upon studies carried out with the platinum-group metals,gJo chemisorption under these conditions occurs oxidatively to yield zeroualent sulfur. Excess (unadsorbed)sulfide was removed by washingthe TLEcell with pure supportingelectrolyte. Fractional coverages of sulfur were prepared by exposing the clean electrode to a single aliquot of NazS, the concentration of which (Cs) determines the fractional coverage of sulfur 0s
where Ts is the interfacial packing density of sulfur, rS,mthe monolayer (saturation) packing density,A the active area of the electrode surface, V the volume of the thin-layer cell (3.92pL), and C"s the concentrationat which a single aliquot would result in a full monolayer of sulfur. C"Sis given by
Ts,m(1.4nmol cm-2) was calculated using the van der Waals' (5)(a)Hubbard, A.T. Crit. Reu.Ana1. Chem. 1973,3,201.(b) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984,177,
(l)(a)Beny,G.M.;Bravo,B.G.;Bothwell,M.E.;Cali,G.J.;Harris, 89.
J. E.; Mebrahtu, T.; Michelhaugh, S. L.; Rodriguez, J. F.; Soriaga, M. P. Langmuir 1989,5,707.(b) Michelhaugh, S.L.; Carrasquillo, A,, Jr.; Soriaga, M. P. J. Electroanal. Chem. 1991,319,387. (2)(a) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1986,185,331.(b)White, J. H.; Soriaga,M. P.; Hubbard, A. T. J. Phys. Chem. 1986,89,3227. (3)(a)Vetter, K.J. 2.Electrochem. 1962 56,797. (b) Vetter, K. J. Electrochemical Kinetics;Academic Press: New York, 1967. (c) Laviron, E. J . Electroanal. Chem. 1984,164,213. (4) Adzic, R. In Modern Aspects of Electrochemistry. XXI; White, R. E.; Bockris, J. O'M., Conway, B. E., Eds.; Plenum Press: New York, 1990.
(6) Rodriguez, J. F.; Bothwell, M. E.; Harris, J. E.; Soriaga, M. P. J. Phys. Chem. 1988,92,2702. (7)(a)Rand, D.A. J.; Woods, R. Electroanal. Chem. 1974,55,375. (b) Breiter, M. W. J.Electroanal. Chem. 1983,157,327.(c) Motoo, S.;
Furuya, N. J. Electroanal. Chem. 19&4,167,309. (d) Pickup, P. G.; Birss, V. I. J. Electroanal. Chem. 1987,220,83. (8)Svetlicic, V.; Clavilier, J.; Zuric, V.; Chevalet, J.; Elachi, K. Electroanal. Chem. 1993,344, 145. (9)Batina, N.; McCargar, J. W.; Laguren-Davidson, L.; Lin, C.-H.; Hubbard, A. T. Langmuir 1989,5,123. (10)Mebrahtu, T.;Bothwell, M. E.; Harris, J. E.; Cali, G. J.;Soriaga, M. P. J. Electroanal. Chem. 1991,300,487.
0743-746319412410-3929$04.50/00 1994 American Chemical Society
3930 Langmuir, Vol. 20,No. 11, 1994
Letters
radius of sulfur and the metallic radius of iridium11 assuming hexagonally close-packed interfacial layers.12 The active area of Ir (1.25 cm2) was measured using the iodine-chemisorption method.'* The sulfurcoveragesexpected from eq 1were verified by measurement of the electrolytic charge for the oxidative desorption of the adsorbed sulfurg
S(ads) + 4H,O
-
BQ/HQ Redox at Pretreated It 20 15 1 10
502-
+ 8H' + 6e-
(3)
Ts was obtainedfrom this anodic oxidationchargeusing Faraday's law
. . . ( . . . ( . . . , . . . , . . . , . . . ( . . .
:
5:
4
0:
h
-5 :
where (Q - Q& is the background-corrected charge and F is Faraday's constant. The sulfur coverages reported here were those actually determined from eq 4 . Pretreatment of the Ir surface with reactant-derived species was accomplished by immersion of the clean electrode in 1mM HQ solution buffered at pH 4 with NaOH and H3P04. No work has yet been done to determine the orientation of the adsorbed organic intermediates under reaction conditions. Based upon earlier studies with Pt,l0it is most likely that the chemisorption of HQ on Ir will occur oxidatively. Hence, for this work, the HQ-derived surface complex is simply modeled as flat-oriented BQ. Oxide pretreatment involved a 60-s excursion into the potential region just prior to the oxygen evolution region. Uniquely for Ir, the oxide layer thusly formed is maintained as long as potentials close to the hydrogen evolution region are HQ(,) a~oided;~ that is, at the potential region where the BQc,) reaction takes place, the anodized Ir electrodewill remain oxided. The electrode kinetics of the BQ(aqjHQ(aq) redox couple was studied using thin-layer cyclic voltammetry, along the lines described in earlier work.l3 Particular attention was given to measurement of the separation in potential (AE)between the and cathodic (193,) peaks, AE = BEp - JCP. The anodic (ap) electrochemical rate constant k", for appreciably irreversible reactions (k" 10-3 cm 6-1) is related to AE according to13
-
where T i s the temperature, r the potential-sweep rate, and R the gas constant. Although thin-layerelectrochemicaltechniques are not suited for fast electrode reactions (k" =- 10-3 cm s-l), the need to minimize surface contamination by impurities other than the desired adsorbates necessitated the use of thin-layer cells. Moreover, for the single purpose ofinvestigatingrelative changes in the redox kinetics brought about by changes in surface composition, thin-layer electrochemicalmethods are adequate.l3 In this study, only order-of-magnitude rate constants k" were evaluated; for detailed comparisons, AE values were used. The potentiostat (CV-27;Bioanalytical Systems,West Lafayette, IN) employed in this work does not include circuitry for ohmic drop compensation; hence, AE values reported here were corrected for ohmic drop with reference to the Fe3+/Fe2+redox couple; although this reaction is strictly not Nernstian ( K O = 5 x 10-3 cm s-'),l4its hE value, under thin-layer conditions, is essentially zero in the absence of an ohmic drop. Applied electrodepotentials were measured against a Ag/AgCl (1M C1-) reference electrode.
Results and Discussion Early studies3 on the electrode kinetics of the BQ(aq)/ HQ(,,, couple suggested a pH-dependent rate, with a minimum at pH 7. Later work, disputing this claim, showed that the redox kinetics was actually pH-inde(11)Dean, J.A.,Ed. Lange's Handbook of Chemistry; McGraw Hill: New York. 1985. -(12)Rodriguez, J. F.;Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987,233,283. (13)(a) Lane, R. F.; Hubbard, A. T. J.Phys. Chem. 1973,77,1401, 1412. (b) Lai, C.N.; Hubbard, A. T. Znorg. Chem. 1972,11, 2081. (14)Jordan, J.; Stalici, N. R. Handbook of Analytical Chemistry; Meites, L., Ed.; McGraw-Hill: New York, 1963. v
~
Oxide-Coated Ir
-10 1
-15
t
-20 -0.1
/%
:
S-Coated Ir
BQ-Co&d Ir
-0.0
01
0.2
0.3
0.4
0.5
0.6
E N vs. AglAgCl Figure 1. Thin-layer cyclic current-potential curves for a smooth polycrystalline iridium electrodein an aqueous solution containing 1mMhydroquinone(HQ)in 1M NazSOr phosphatebuffered a t pH 4: (solid curve) the Ir surface contains a full monolayer of adsorbed sulfur, (dashed curve) the Ir electrode contains surface oxides, (dotted curve) the Ir surface contains HQ-derived surface intermediates. The volume of the thinlayer cell, V = 3.92 pL;the area of the electrode,A = 1.25 cm2; temperature, T = 298 K potential sweep rate, r = 2.0 mV s-l. pendent.15 Our own investigations's2 established that if species derived from HQ(,,, or BQ(,, are present in an irreversibly adsorbed state on the surface, a minimum in the redox kinetics does occur at pH 4; however, in the presence of adsorbed iodine, even at submonolayer coverages, the rate becomes independent of pH. Preferential or site-selective electron transfer was throught to take place at the adsorbed-iodine sites. Figure 1 shows thin-layer cyclic current-potential curves for pretreated Ir in a pH 4 buffered solution containing 1M NazSOr and 1mM HQcaq,;no ohmic drop corrections have yet been applied to data in this figure. The dashed curve represents the voltammogram for the oxide-pretreated surface, the dotted curve denotes the potentiodynamic scan for the BQ-preadsorbed electrode, while the solid curve denotes the voltammetric curve for the sulfur-precoated iridium. The redox peaks correspond to the benzoquinonehydroquinone oxidation-reduction reaction: 0
OH
0
OH
It is clear from Figure 1that the separation in peakpotential separation AE ( E&, - JZp) between the anodic and cathodic (&,) peaks, is largest for the oxidecoated surface and smallest for the sulfur-covered electrode; that is, at pH 4, the electron-transfer rate is slowest for Ir precoated with an oxide layer and fastest for Ir pretreated with sulfur. The ohmic drop-corrected AE values and the corresponding order-of-magnitude rate constants are as follows: oxide coated, AE = 0.40 f 0.04 V (k" cm sf); BQ coated, AE = 0.24 f.0.02V (k" cm s-l); S coated, AE = 0.04 f 0.01 V (k" cm SI).
(aE,)
- -
-
(15)Loshkarev, M. A.; Tomilov, B. 1. Zh. Fiz. Khim. 1960,34,1735; 1962,36,132.
Langmuir, Vol. 10, No. 11, 1994 3931
Letters
BQMQ Redox at S-Coated Ir
Anodic Oxidation of Pretreated Ir 20
. 1MH2S04
1MNa2S04
15:
: pH4Buffered 10 :
Coadsorbed S (0.17 ML) and BQ
6 .
. . . , . . . , . . . , . . . , . . .
5:
f >
5 >
4-
0: -5 :
2-
0.0
No S (BQ-Coated)
-10 L
0.2
0.4
0.6
0.8
1.o
E N vs. AglAgCl Figure 2. Thin-layer current-potential curves in the anodic oxidation region in 1 M HzSO4 for a smooth polycrystalline iridium electrode pretreated as follows: (solid curve) coated with a full monolayer of sulfur, (closed circles) coated with a full monolayer of HQ-derived organic intermediates, (open circles)coated with a coadsorbed layer of sulfur (0s = 0.17) and BQ, (dashed curve) untreated Ir electrode. The preparation of the mixed layer was as described in the Experimental Section. All other experimental parameters were as in Figure 1.
The observation here t h a t the BQ(aq)/HQ(aq) redox kinetics is considerably enhanced on the S-treated surface is similar to the effect previously reported when Pt and Ir were coated with a full monolayer of iodine.lb The present result, however, is unexpected based on the traditional view that chemisorbed sulfur functions as a poision for essentially all electrochemical reactions. The data in Figure 1 were obtained for full-coverage pretreatments. It is important to know whether or not the rate enhancement by the adsorbed sulfur is primarily due to prevention of the formation of the organic adlayer that retards the BQ/HQ redox rate. Insights into this question can be obtained by observing the dependence of the electron-transfer rate on the fractional coverage of sulfur; a simple blocking (adsorption-prevention) mechanism would be manifested by a monotonic decrease in AE as 0s is decreased. In this regard, it has to be realized that, in the absence of surface oxides, the sites left vacant by sulfur are occupied by chemisorbed BQ. Electrochemical evidence for this can be gleaned from Figure 2 which shows thin-layer current-potential curves for the anodic oxidation of Ir containing (i) a full monolayer of S, (ii) a full monolayer of BQ, and (iii)a submonolayer of S (0s = 0.17) exposed to dilute HQ prior to oxidation; for reference, the voltammogram for an untreated Ir electrode is also given. The broad peaks observed for the pretreated Ir surfaces correspond to the oxidative desorption of the adsorbed sulfur (to S02-) or BQ (probably to COz). The electrolytic charge measured for oxidation of the submonolayer S-coated Ir after exposure to aqueous HQ was 180% larger than that obtained when the layer contained only submonolayer S; this excess oxidative charge serves as the electrochemical evidence for the presence of coadsorbed BQ. Figure 3 compares the thin-layer cyclic voltammograms for a sulfur-free (BQ-coated) Ir electrode with those for Ir coated with (i) a full monolayer of S (0s rs/rs,m= 1.0) and (ii) a coadsorbed layer of S (0s = 0.17) and BQ. It can be seen t h a t the AE values for the fully and partially
0.0
0.2
0.1
0.4
0.3
0.5
E N vs. AgIAgCI Figure 3. Thin-layer cyclic current-potential curves for a smooth polycrystalline iridium electrode in an aqueous solution containing 1mMhydroquinone(HQ)in 1M NazSOd phosphatebuffered at pH 4: (solid curve) the Ir surface contains a full monolayer of adsorbed sulfur, (dotted curve) the Ir electrode contains 0.17 monolayer of sulfur (and coadsorbed HQ-derived species), (dashed curve) the S-free Ir surface contains a full monolayer ofBQ. Experimental parameters were as in Figure 1.
BQ/HQ Kinetics at Pretreated If
pH 4-Buffered
Fully Oxided Electrode
0'5 301 \
$ C B Q - C o a t e d Ir (No S)
0.201
0.00'
-0.1
'
'
Co-adsorbed S and BQ
'
"
0.1
'
"
0.3
'
'
"
0.5
'
'
'
"
0.7
'
"
0.9
'
'
'
I 1 .I
Fractional Sulfur Coverage Figure 4. AE vs 0s plots for a smooth polycrystalline Ir electrode in an aqueous solution of HQ in 1M NazSO4 phosphatewhere the JCp is the anodic bufferd at pH 4. AE = BEp - SP, peak potential and SPis the cathodic peak potential for the BQca )/HQ(aq) redox couple. The AE values have been corrected for okmic drop using a Fe3+/Fe2+reference couple. All other experimental parameters were as in Figure 1.
S-coated electrodes are almost identical; that is, even on a surface containing a small fraction of S (along with a n appreciable amount of adsorbed organic compound), the electrode kinetics of the BQ(aq)/HQ(aq) couple is already quasi-Nernstian. The influence of sulfur coverage on the electrode kinetics of the BQ(aqjHQ(aq) couple is further showcased in Figure 4 which provides AE vs 0s plots on Ir. The dramatic rate enhancement, from complete irreversibility to quasi-
3932 Langmuir, Vol. 10,No. 11, 1994
Scheme 1. Site Selectivity of BQ/HQ Redox on Ir with Coadsorbed S and BQ n
n
ti
'n
n
n
reversibility is already enforced at s u l h r coverages as low as 10% of a monolayer. Since the BQcaqfiQcaq, reaction at an electrode covered
Letters
with surface oxide or with diphenol-derived intermediates is slow, the case may be made that the fast kinetics observed at 8s
