Adsorption of acetonitrile on polycrystalline Ag electrodes: comparison with Hg electrodes

187

Z Electroanal. Chem., 349 (1993) 187-195 Elsevier Sequoia S.A., Lausanne

JEC 02420

Adsorption of acetonitrile on polycrystalline Ag electrodes: comparison with Hg electrodes * L.M. Doubova IPELP of the CNI~ Corso Stati Uniti 4, 35020 Camin (Pd) (Italy)

S. Trasatti ** Department of Physical Chemistry and Electrochemistry, University of Milan, Via Venezian 2L 20133 Milan (Italy)

S. Valcher "G. Ciamician" Department of Chemistry, University of Bologna, Via Selmi 2, 44126 Bologna (Italy) (Received 20 July 1992)

Abstract

The adsorption of acetonitrile (ACN) from aqueous solution was investigated with a polycrystalline Ag electrode using 0.05 mol dm -3 KCIO4 as a supporting electrolyte. Charge-potential data were obtained both by integration of double-layer capacitance curves recorded by means of a lock-in amplifier, and by direct determination of the charge with potential steps. ACN was found to adsorb on Ag with qualitatively the same orientation as on Hg, but the adsorption Gibbs energy is lower, as expected from correlations involving other interfacial parameters.

INTRODUCTION

Lucien Gierst has devoted much work to the study of adsorption of organic substances on Hg [1,2], especially compounds of the quinoline group [3,4]. Recently, the work on quinoline has been extended to investigate the effect of organic adsorption on electron transfer [5], the mechanism of which depends on the structure of the surface film, a topic already dealt with by Gierst long ago [6]. * Dedicated to Professor Lucien Gierst on the occasion of his 70th birthday. ** To whom correspondenc~should be addressed. \

0022-0728/93/$06.00 © 1993 - Elsevier Sequoia S.A. All rights reserved

188

The structure of adsorbed layers and the Gibbs energy of adsorption AadG° of Organic compounds at the electrode-solution interface are known to be a function of the nature of the metallic phase [7]. In particular AadG° has been found to decrease linearly with the "interfacial parameter" X, which measures the modification of the electron work function of the metal at the interface with the solution [8]. In such plots, Ag has been placed in two conflicting positions depending on the data available in the literature. Thus, if the data of adsorption of organic substances refer to aqueous solution of sulphates [9], Ag turns out to be a stronger adsorbent than Hg, being the first in a group of sp-metals. If the data of adsorption refer to aqueous solutions of fluorides, Ag is shifted to the last position of the sequence, turning out to adsorb substances less than every other metal investigated thus far [10]. This apparent inconsistency has been attributed [11] to the effect of specific adsorption of sulphate ions on the Ag surface. The above ambiguous picture has prompted this work, the purpose of which has been to verify whether Ag electrodes, in the absence of ion-specific adsorption, do adsorb neutral substances less than other sp-metals, in particular Hg. Since data for sp-metals refer as a rule to polycrystaUine surfaces, polycrystalline Ag has been used as a first approach. Acetonitrile (ACN), never investigated with Ag electrodes, has been chosen for two reasons: (1) there is already a detailed investigation available for Hg [12], and (2) since plans have been made to study the Ag-ACN interface, knowledge of the behaviour of the organic molecule at the electrode-aqueous-solution interface is a prerequirement. EXPERIMENTAL

Electrode Polycrystalline Ag rods of 3 mm diameter were cut perpendicularly to the main axis and the exposed section was carefully ground to a mirror finish using emery paper and alumina. The surface was finally etched with CrO3 as described elsewhere [13]. The cell contained a glassy carbon counter-electrode in the form of a rod under the working electrode, contact with the solution being realized using the hanging meniscus arrangement [14]. A saturated calomel reference electrode (SCE) was contained in a separate compartment. Solutions The supporting electrolyte solution contained 0.05 mol dm -3 KC104, the anion of which is known to be negligibly adsorbed on Ag electrodes [15]. Solutions, prepared from suprapure KCIO 4 (Fluka) and Millipore Q water, were deaerated before each run. They were kept constantly under a n N 2 atmosphere and stirred slowly during the measurements. ACN was purified according to specifications in the literature [16]. Eight concentrations were used between 0.02 and 1.3 mol dm -3.

189

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4

9 . 1 3 x I0 -r,

b

" i

f

5.0

3.69

2.00x10-9~

2.38 1.07

.%

-0.24

50~ I s

Fig. 1. Typical chronoeoulometric curves for polycrystalline Ag in 0.05 moi d m -3 KCIO 4 aqueous solution, initial potential - 1.2 V(SCE); final potential (1) - 1.1, (2) - 1.0, (3) - 0 . 9 , (4) - 0 . 8 , (5) - 0 . 7 , (6) - 0.6, (7) - 0.5 and (8) - 0.4 V(SCE).

Experimental techniques Capacitance curves were recorded by means of a lock-in amplifier (Dynatrac 3 Ithaco) coupled with a 551 AMEL potentiostat and a 566 AMEL function generator. Electrodes were cycled between - 1 . 4 and 0.4 V(SCE) at 10 mV s -1 with a superimposed a.c. signal of 10 mV and 164 Hz. The frequency was imposed by the response of the instrumentation. However, in view of the semiquantitative scope of this work the effect of frequency was found to have no significant effect. Since the above experimental arrangement has been shown to produce possible non-equilibrium conditions [17], charge-potential curves were also measured directly by means of potential steps (chronocoulometry) carried out with a PAR 173 potentiostat. In the presence of the adsorbing substance, the potential was initially held for up to 10 s at - 1 . 4 V(SCE), assuming complete desorption of the adsorbate at this potential, then stepped to the selected value. Charge-time data were then extrapolated to t = 0, Examples of charge-time curves for different potential steps are shown in Fig. 1 in a tridimensional plot. The fairly horizontal

190 plateaux indicate that no spurious faradaic processes took place during the charge determination.

RESULTS A N D DISCUSSION

Verification of cleanliness The occurrence of the appropriate experimental conditions were verified by recording a capacitance curve for A g ( l l l ) in the base solution. The charge-potential curve was then obtained by integration from - 1.4 V(SCE). The resulting curve is collated in Fig. 2 with the corresponding curve derived from Valette's data in 0.04 mol dm -3 NaCIO 4 [18]. It can be seen that the two curves overlap excellently between - 1 . 4 and - 0 . 6 V(SCE). Only at E > - 0 . 6 V are small deviations observed, but this potential is beyond the range of interest in this work. Figure 2 also shows that the data obtained at 164 Hz in this work are not affected by frequency effects: Valette's data were in fact collected at 20-320 Hz.

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1 20.00

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-30.00

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1.5

-1.1

-0,6

-0.2

E I V (vs. SCE)

Fig. 2. Charge-potential curves from the integration of differential capacitance data for A g ( l l l ) : (1) 0.04 mol dm -3 NaC10 4 solution [18]; (2) 0.05 mol dm -3 KCIO 4 solution (this work).

191

500

1

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1

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i

,

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,

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,

,

,

,

I

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,

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,

'

,

'

i

,

--0.70

E/V

,

,

,

'

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'

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i

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Fig. 3. D i f f e r e n t i a l c a p a c i t a n c e - p o t e n t i a l curves o f polycrystalline A g in 0.05 m o l dm -3 KC10 4 solution c o n t a i n i n g acetonitrile ( A C N ) at various concentrations: (1) 0, (2) 0.0244, (3) 0.0971, (4) 0.193, (5) 0.312, (6) 0.430, (7) 0.545, (8) 0.995 and (9) 1.27 mol dm-3.

Capacitance-potential curves

Figure 3 shows a family of capacitance curves in the presence of ACN. The curve of the base solution shows a minimum at -0.985 + 0.005 V(SCE) which is identified with the potential of zero charge. This value is very close to E~,=0 for the (110) face [13]. A very small effect of perchlorate adsorption is probably present. ACN adsorption results in a general depression of the capacitance curve. The curves appear to merge at - 1 . 4 V, thus indicating probable complete dcsorption of ACN, but no well developed desorption peak is visible on the negative side of E~= 0. This may be a result of non-equilibrium capacitance data, or of the heterogeneity of the electrode surface.

192 40.0

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-1.25

-1.05

-0.85

-0.65

-0.45

E / V (vS. SCE)

Fig. 4. Charge-potential curves, obtained by integration of the capacitance data in Fig. 3, for adsorption of ACN on polycrystalline Ag from 0.05 mol d m - 3 K C I O 4 aqueous solution. The inset shows charge-potential curves from potential step experiments. ACN concentration as in Fig. 3.

Charge-potential curves

Figure 4 shows the family of charge-potential curves obtained by integration of the data in Fig. 3 for the "blank" solution from E ~ 0 and for the ACN solutions back from - 1.4 V. As a result of the absence of a desorption peak, the curves with ACN lie below the "blank" solution and no charge in maximum adsorption (intersection point) can be identified. As they are, the data indicate a modest shift of the potential o f zero charge to more positive potential values. Although potential step experiments should ensure equilibrium conditions, the inset to Fig. 4 shows that the qualitative picture does not change dramatically. Comparison with capacitance data reveals that the charge deviates progressively as the potential is made more positive, but the curves with ACN remain close to the "blank" throughout the potential range. No charge of maximum adsorption is

193 100

75

z~

1 >

50 II

25

A Z~

•

2

~

3

,x

0 -~'~"~ z~

0

0.5

1

1.5

c / mol dm -3 Fig. 5. Shift of the potential of zero charge of (1) Hg and (2,3) polycrystalline Ag electrodes upon adsorption of ACN from 0.05 mol dm -3 KCIO4 solution: (2) from capacitance data; (3) from potential step experiments.

evident, but the shift in adsorption potential is smaller. However, potential step data are not unambiguous in view of the uncertain coverage of the electrode with adsorbate at the most negative potential. Actually, complete desorption at - 1.4 V (the initial potential of the steps) is inferred from capacitance data, which is not unambiguously proven as remarked above. Comparison with H g

ACN adsorbs on Hg with a shift of E~= 0 to more positive values [12]. Figure 5 shows a quantitative comparison with polycrystalline Ag. Capacitance data still show a positive but smaller shift of E,= 0, while coulometric measurements give scattered data with a very modest negative shift at the higher concentrations. On the whole, the data for Ag point to a small value of AE,~=0, probably close to zero. Since capacitance data in the region of the desorption peak are probably non-equilibrium values, no double integration can be carried out to obtain surface pressure. A semiquantitative comparison with Hg has therefore been carried out on the basis of the raw capacitance data. If the Frumkin-Damaskin model is assumed [19], the coverage with adsorbate can be expressed approximately by: 0 = (Co - C o ) / ( C o - C1)

(1)

where C o is the capacitance of the base solution, C 1 that at saturation coverage, and C e the experimental capacitance at the given ACN concentration. Since C 1 has been obtained by extrapolation for Hg while it is unknown for Ag, the following expedient has been used. C o at the potential of zero charge is

194

0.6

0.3 0 0

-1.6 -1.4 -1.2

-1

-0.8 -0.6 -0.4 -0.2

0

0.2

Iog(c / mol dm "s) Fig. 6. Adsorption isotherms for ACN on (1) Hg and (2) polycrystalline Ag electrodes.

about 1.30 times higher for Ag than for Hg [12]. Since capacitances differ on different metals [20], mainly because of their different surface electronic structures [21], it is assumed here that capacitances are systematically higher for Ag than for Hg at least at the potential of zero charge. Thus, C 1 = 18/xF cm -2 has been taken for Ag, consistently with C 1= 14/zF cm -2 for Hg. Figure 6 shows the isotherms of ACN adsorption on Ag as well as on Hg. It is evident that Ag adsorbs ACN much less than Fig. The difference in AadG° at E,,= 0 can be estimated, from the distance on the log c axis, to amount to ca. 3 kJ mol-1. It is interesting that this value agrees semiquantitatively with the A(AadG°) value estimated in the case of alkyl alcohols [10,22]. CONCLUSIONS

ACN is adsorbed on polycrystalline Ag electrodes qualitatively with the same Orientation as on Hg electrodes. However, adsorption is weaker on Ag than on Hg, and this fits in with the correlation, found for sp-metals, between the standard Gibbs energy of adsorption and the "interracial parameter" X derived from the relationship between the electron work function and the potential of zero charge [10,22]. The very small value of AE~= 0 on Ag points to the absence of substantial preferential orientation of ACN molecules. In this respect, it is interesting that ACN has been found to adsorb flat on the (331) face of Ag from the gas phase [23] with a negligible normal component of the molecular dipole, although solvent-adsorbate interactions are non-operating under similar circumstances. If the shift in adsorption potential on Hg is attributed mostly to the preferential orientation of adsorbed solvent molecules, the smaller shift observed with Ag leads us to conclude that solvent molecules are probably less preferentially oriented on

195

Ag than on Hg. At the same time, the lower Gibbs energy of adsorption of ACN on Ag than on Hg might be attributed to stronger adsorption of water molecules (i.e. higher hydrophilicity of Ag), which results in higher capacitances with sp-metals other than Hg. However, one of us has shown that the higher capacitance of Ag may not necessarily be related to solvent effects, but possibly to the high surface electronic concentration [24]. The present results are thus consistent with the latter conclusion. The small value of AE~= 0 (close to zero) rules out the possibility that water molecules can be strongly adsorbed with appreciable preferential orientation. Nevertheless, AadG° is also small because of the presence of a large electronic dipole at the Ag surface. It is probable that the main factor governing AE,,z0 on Ag is the variation of the surface electronic dipole 8%M with coverage rather than any contributions from the molecular dipoles of the adsorbate and of the solvent. ACKNOWLEDGMENTS

The authors wish to thank the C.N.R. and the M.U.R.S.T. for financial support for this work. REFERENCES 1 L. Gierst, in E. Yeager (Ed.), Transactions of the Symposium on Electrode Processes, Wiley, New York, 1961, p. 294. 2 L. Gierst and C. Pecass¢, in O.J. Hills (Ed.), Polarography, Macmillan, London, 1964, p. 305. 3 CI. Buess-Herman, L. Gierst, N. Vanlaethem-Meur~e and G. Quarin, J. Electroanal. Chem., 123 (1981) 1. 4 G. Quarin, CI. Buess-Herman and L. Gierst, J. Electroanal. Chem., 148 (1983) 97. 5 J. Lipkowski, CI. Buess-Herman, J.P. Lambert and L. Gierst, J Electroanal. Chem., 202 (1986) 169. 6 L. Gierst, D. Bermane and P. Corbusier, Ric. Sci., 29 (suppl.) (1959) 75. 7 S. Trasatti, Electrochim. Acta, 28 (1983) 1083. 8 S. Trasatti, Electrochim. Acta, 36 (1991) 1659. 9 S. Trasatti, Croat. Chem. Acta, 60 (1987) 357. 10 S. Trasatti, Electrochim. Acta, 37 (1992) 2137. 11 A. Poppy, O. Velev and T. Vitanov, J. Electroanal. Chem., 256 (1988) 405. 12 A. De Battisti and S. Trasatti, J. Electroanal. Chem., 48 (1973) 213. 13 M. Bacchetta, S. Trasatti, L Doubova and A. Hamelin, J. Electroanal. Chem., 200 (1986) 389. 14 D. Dickertmann, F.D. Koppitz and J.W. Sehultze, Electrochim. Acta, 21 (1976) 967. 15 G. Valette, J. Electroanal. Chem., 122 (1981) 285. 16 J.F. O'Donnel, J.T. Ayres and C.K. Mann, Anal. Chem., 37 (1965) 1161. 17 J. Lipkowski, C. Nguyen van Huong, C. Hinnen, R. Parsons and J. Chevalet, J. Electroanal. Chem., 143 (1983) 375. 18 G. Valette, J. Electroanal. Chem., 269 (1989) 191. 19 A.N. Frumkin, B.B. Damaskin and A.A. Survila, J. Electroanal. Chem., 16 (1968) 493. 20 A. Frumidn, B. Damaskin, N. Grigoryev and I. Bagotskaya, Eiectrochim. Acta, 19 (1974) 69. 21 E. Leiva and W. Schmickler, J. Electroanal. Chem., 205 (1986) 323. 22 S.Trasatti, in R. Guidelli (Ed.), Electrified Interfaces in Chemistry, Physics and Biology, Kluwer, Dordrecht, 1992, p. 245. 23 K. Bange, R. McIntyre, J.K. Sass and N.V. Richardson, J. Electroanal. Chem., 178 (1984) 351. 24 S. Trasatti, J. Electroanal. Chem., 138 (1982) 449.