Piezoelectric response to changes in surface stress of a palladium electrode in perchloric acid solution containing chloride ions

185

J. Efectrounui. Chem., 347 (1993) 185-194 Elsevier Sequoia S.A., Lausanne

JEC 02417

Piezoelectric response to changes in surface stress of a palladium electrode in perchloric acid solution containing chloride ions Masahiro Seo and Masaki Aomi Faculty of Engineering, Hokkdo

University, Kita-

Jo, Nhhi-8 Chome, fita-ku, Sapporo 060 (Japan)

(Received 26 May 1992; in revised form 29 July 1992)

Abstract

The interfacial properties of a palladium electrode in 0.1 mol dm-3 perchloric acid solution containing chloride ions were investigated from the piezoelectric response of changes in surface stress induced by potential modulation. It was found from the piezoelectric signals that the potential of electrocapillary maximum E,, or potential of zero charge (pzc) is present in the potential region of hydrogen absorption into palladium. Tbe pzc shifted linearly toward the negative direction with an increasing logarithm of chloride ion concentration, i.e. the Esin-Markov relation held between the pzc and chloride ion concentration, indicating specific adsorption of chloride ions on the palladium surface. Furthermore, the piezoelectric signals indicated that sign reversal of the surface charge took place in the potential range of oxide formation and reduction. The sign-reversal of the surface charge was attributed to structural changes of the palladium surface associated with oxide formation and reduction. The potentials at which the sign-reversal of the surface charge took place during cathodic reduction of oxide shifted toward the positive direction with increasing chloride ion concentration. The effects of chloride ions on the sign-reversal of the surface charge for palladium are discussed and compared with those for platinum.

INTRODUCTION

The measurement of surface stress or surface tension of a solid metal electrode, if it is possible, would provide much useful information on the interfacial properties of the electrode. The measurement of surface stress or surface tension on a solid electrode, however, is more difficult than measurement on a liquid metal electrode. Gokhshtein [l] first developed a piezoelectric technique which is capable of detecting sensitively a small change in surface stress of solid electrodes. Afterwards, Malpas et al. [21 employed a simplified electrode design for piezoelectric detection. We also applied this technique to interfacial studies of noble metal 0022-0728/93/.$06.00 8 1993 - Elsevier Sequoia S.A. All rights reserved

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electrodes and succeeded in measuring the potential of zero charge (pzc) of the electrodes [3-71. Furthermore, it was found that the sign reversal of surface charge took place in the potential region of oxide formation and reduction for platinum [3] and palladium [7]. The sign reversal of surface charge was associated with surface reconstruction or place exchange due to oxide formation and reduction. A strong effect of chloride ions on the sign reversal of surface charge was observed for platinum in perchloric acid solutions and explained in terms of a displacement or substitution of adsorbed chloride ions with surface 0 or OH species [6]. In this study, piezoelectric detection of changes in surface stress of a palladium electrode in perchloric acid solution containing chloride ions was carried out to investigate the effect of chloride ions on the potential of zero charge or sign-reversal of surface charge. EXPERIMENTAL

The electrode design and the technique used for piezoelectric detection of changes in surface stress are almost the same as those reported previously [3-71. The working electrode was constructed by attaching a polycrystalline palladium foil (20 pm x 5 mm x 20 mm) via a thin polyimide film (7.5 pm) for electric isolation to a piezoelectric ceramic element (0.5 x 5 x 20 mm3) with strain gauge cement and by doubly coating with epoxy cement and silicone sealant for electric isolation from solution. A potentiostat connected with a function generator was used to supply a linear potential sweep (33.3 mV s-l) to the electrode. A small sinusoidal voltage of 5 mV (rms) at 320 Hz was superimposed simultaneously on the linear potential sweep from an oscillator in the potentiostat to induce an alternating change in surface stress of the electrode, producing a corresponding response from the piezoelectric element. The electrical signal from the piezoelectric element was synchronously detected at the same frequency as that of the superimposed voltage modulation using a lock-in amplifier. The principle for analysis of changes in surface stress from the piezoelectric signals has been described elsewhere [31. The amplitude I A I of the piezoelectric signal is proportional to a derivative of surface stress with respect to electrode potential, dy/dE, and the phase angle 4 involves a component of the change in sign of d y/d E. According to the Gibbs adsorption isotherm [8], d y/d E is equal to the surface charge density of the electrode u under a constant chemical potential p of electrolytes which participate in adsorption. If an electrocapillary curve can be measured for a solid electrode, then the slope of the curve corresponds to the surface charge density u, the absolute value of which should be proportional to the amplitude, I A I of the piezoelectric signals and the sign of which should reflect the phase angle 4. The phase angle 4, however, involves additional components resulting from the experimental instruments and the mechanical properties of the electrode system. In the case where the contribution of additional components to 4 is kept constant, the change (from plus to minus or vice versa) in sign of the surface charge density u would give rise to the relative change of 180” in 4. In

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principle, at the potential of the electrocapillary maximum, i.e. the potential of zero charge, I A 1 takes a minimum value (strictly zero value) and #J changes by 180”. The electrolytes used in this experiment were 0.1 mol dme3 perchloric acid solutions containing chloride ions of 10-6-10-2 mol dmP3, which were prepared from guaranteed reagent grade chemicals of HClO, and NaCl with ultrapure water supplied through a super Q Millipore filter system. Before introduction to an electrochemical cell, the electrolytes were deaerated with prepurified nitrogen gas in solution reservoirs. The electrode potential was measured with an Ag/AgCl electrode in a saturated KC1 solution and referred to a standard hydrogen electrode (SHE). RESULTS AND DISCUSSION

Piezoekctric signal curves and cyclic voltammograms Figure 1 shows the piezoelectric signal curve and cyclic voltammogram of a palladium electrode in 0.1 mol dme3 perchloric acid solution without chloride ions. The piezoelectric signal curve and cyclic voltammogram in perchloric acid

I

0

0.4

E / V

0.8

1.2

vs.SHE

Fig. 1. Piezoelectric signal curve and cyclic voltammogram of palladium in 0.1 mol dmm3 HCIO, solution without chloride ions (I A I amplitude, 4 phase angle).

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solution were similar to those in sulphuric acid solution previously reported [7]. The minima of I A I with the large relative change in 4, although I A I does not take zero value and #I does not always change by MO”, appear at characteristic potentials in the potential regions of both hydrogen absorption-desorption and oxide formation-reduction. According to the previous definition [7], the characteristic potentials in the hydrogen absorption-desorption region are defined by Ea,H (in the positive potential scan> and by E,, (in the negative potential scan), whereas those in the oxide formation-reduction region are defined by E,, (in the positive potential scan) and by E,, (in the negative potential scan>. Roth the deviations of I A 1 from zero and of A4 from 180” are significantly large at E,, and EC,, rather than at E,, and E,,. These large deviations at E,, and E,, may be associated with the uptake of hydrogen into palladium. Recent studies [9,10] on hydrogen absorption into palladium electrodes using a quartz crystal microbalance have revealed that a large stress is created in the electrode owing to lattice expansion caused by hydrogen absorption. The hydrogen absorption would respond to the potential modulation to some extent and give some perturbation of the stress created in the electrode. This perturbation of the stress in the electrode probably overlaps with the change in surface stress originating from the surface charge density, and eventually induces the deviations of I A I and A&. The above speculation will be supported by the fact that I A I and Ad at E,, and E,, approach zero and 180” respectively in the higher pH solution, where hydrogen absorption becomes insignificant as seen in a previous report [7]. Even if some deviations of I A I and A4 from the ideal case are present, the potential of the electrocapillary maximum can be evaluated from the piezoelectric signals within experimental error as far as the potential dynamic method is used in this study. The potential dynamic method frequently provides some hysteresis [3,4,7] of the potential of the electrocapillary maximum in the positive and negative potential scans, which makes the evaluation obscure. The mean values of E,, and E,, in Fig. 1, therefore, would correspond to the potential of the electrocapillary maximum, i.e. the potential of zero charge. However, as reported previously [7], E,, and EC,0 would correspond to the potential of sign reversal of surface charge associated with oxide formation and reduction. The large hysteresis between E,, and EC,, results from the irreversibility of the oxide formation and reduction processes. Figure 2 shows the piezoelectric signal curve and cyclic voltammogram of a palladium electrode in 0.1 mol dmd3 perchloric acid solution containing 1O-2 mol dme3 chloride ions. It is seen that E,, and E,, shift significantly toward the negative potential direction owing to the addition of chloride ions. Conversely, E c,. shifts toward the positive potential direction but the shift in e,, is not significant compared with that in EC.,.

Ea,H Ec,n, Ea,o,andE,o 7

as functions

of chloride ion concentration

In Fig. 3(a), Ea,H and E,, are plotted vs. the logarithm of chloride ion concentration. As seen from gig. 3(a), the linear relation holds between E,., or

189

0

0.8

0.4

E / V

1.2

vs.SHE

Fig. 2. Piezoelectric signal curve and cyclic voltammogram solution with lo-* mol dmm3 chloride ions.

of palladium in 0.1 mol dmm3 HCIO,

E,, and the logarithm of chloride ion concentration. expressed as follows,

This linear relation can be

E&V

= -0.22 - 0.047 log( [Cl-]/M)

(I)

E&V

= -0.26 - 0.045 log( [Cl-]/M)

(2)

In eqns. (1) and (2), the uncertainties of the slopes and intercepts are less than 5%. The linear relation between electrode potential and logarithm of electrolyte concentration or activity at a constant surface charge density is well known as the Esin and Markov effect of specific adsorption 181.If the electrode potential E, in equilibrium is measured vs. a reference electrode of constant electrolyte concentration in a cell with a liquid junction, as in this experiment, the following equation is derived from the thermodynamic correlations of the Gibbs adsorption isotherm: (=,/a

log a)uM = (2.303RT/I

z IF)[(6a_/6aM),

+ l/2]

(3)

where a is the activity of the electrolyte, ur,, is the surface charge density of the metal side, u_ is the surface charge density of anions on the solution side, and I z I is the charge number of z-z electrolyte. The value of (2.303RT/ I z I F) is 59.1 mV at 298 K for NaCl solution ( I z I = 1). In the absence of specific adsorption of anions, (&7_/SaM), = - l/2 is given at a potential of zero charge (cr,,, = 01, and hence (6E,/6 log a&a = 0 is obtained from eqn. (3!, indicating that the potential of zero charge is independent of anion activity or concentration. However, (SE,/6 log c),~,~ = -46 mV is obtained in this experiment if the mean value of

190

Fig. 3. (a) E,,,

and EC,“, and (b) E,,, and Ec,O, vs. logarithm of chloride ion concentration,

log[CI- 1.

and E,, is taken as the potential of zero charge, E, (uM = 01, in equilibrium. Here, 6 log c can be regarded as equivalent to S log a, because the concentration of NaCl used in this experiment is low (c = 10m5 to 10-T mol dme3). Thus, the value of (6~_/6cr,), = - 1.3 at the potential of zero charge is calculated from eqn. (3), indicating that the specific adsorption of chloride ions takes place predominantly on palladium. The equilibrium potential [ll] for the formation of palladium hydride, PdH, (x = 0.5) in 0.1 mol dmm3 HClO, solution (pH 1.1) without chloride ions is 0 V (SHE), which is almost equal to the potential of zero charge. The hydrogen adsorption, absorption or hydride formation would be influenced by the specific adsorption of chloride ions which shifts the potential of zero charge toward the negative direction. Probably, the competitive adsorption of chloride ions. and protons or hydrogen atoms retards the subsequent hydride formation. Further discussion of the roles of the specific adsorption of chloride ions in hydrogen absorption or hydride formation, however, is difficult because of the limited number of experimental results.

E,,

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A

0.2

0.6

E / V

LO

1.4

vs. SHE

Fig. 4. Piezoelectric signal curve and cyclic voltammogram of palladium in the narrow potential range of oxide formation and reduction in 0.1 mol dmm3 HClO, solution without chloride ions.

In Fig. 3(b), E,, and EC,, are plotted vs. the logarithm of chloride ion concentration. The dependence of chloride ion concentration on E,, and E,,, is described as follows: E,,o/V

= 1.1 + 0.006 log( [Cl-]/M)

(4)

E,,o/V

= 0.80 + 0.022 log( [Cl-]/M)

(5)

In eqns. (4) and (51, the uncertainties of the slopes and intercepts are also less than 5%. Figures 4 and 5 show the piezoelectric signal curves and cyclic voltammograms measured in the narrow potential region of oxide formation and reduction in 0.1 mol dmm3 HClO, without and with 10e2 mol dmW3 chloride ions. The scales of current density in the cyclic voltammograms of Figs. 4 and 5 are magnified to observe the electrochemical behaviour associated with anodic dissolution, oxide formation, and reduction. In the absence of chloride ions, E,, appears prior to the cathodic current peak of oxide reduction. The addition of 10e2 mol dme3 chloride ions not only causes the appearance of an anodic current peak at about 0.9 V (SHE), but also promotes the deviation of E,,, in the positive direction from the cathodic current peak. The appearance of an anodic current peak due to the addition of chloride ions means that palladium dissolves anodically as PdCl,

192

E / V

vs. SHE

Fig. 5. Piezoelectric signal curve and cyclic voltammogram of palladium in the narrow potential range of oxide formation and reduction in 0.1 mol drne3 HCIO, solution with lo-’ mol dmV3 chloride ions.

(intermediate species) into solution through forming adsorbed complex ions, PdCl; with adsorbed chloride ions as follows: Pd + 2Cl-(ad) (PdCl;)(ad)

--) (PdCl;)(ad)

+ e-

(6)

--) PdCl, + e-

(7)

PdCl, + 2C1-+ PdCl;-

(8)

According to Genesca and Duran [12], it is considered that the reaction of eqn. (7) is the rate-determining step of the palladium dissolution process, although the stable dissolved species in solution would be PdCl:- [13]. After the anodic dissolution peak, f&, appears at which the sign reversal of surface charge from plus to minus takes place associated with PdO formation. The discharge reactions of water molecules prior to PdO formation would proceed in competition with the reactions of eqns. (6), (7), and (8). Pd + H,O + PdOH( ad) + H++ ePdOH(ad)

--$ PdO(ad)

+ H++ e-

Pd f H,O --) PdO( ad) + 2H++ 2e-

(9) or

(10)

193 The

reactions of eqns. (9) and (10) dominate over the reactions of eqns. (6), (7), and (8) with increasing potential, suppressing the anodic dissolution, and attain monolayer coverage of PdO(ad). Subsequently, the oxide (Pdo(ox)) formation proceeds at E,,, accompanying the sign reversal of surface charge as reported previously [7]: PdO( ad) + PdO( ox)

(11) Therefore, E,, is not influenced significantly by the addition of chloride ions. However, as shown in Fig. 5, the cathodic reduction of PdO is influenced significantly by the addition of chloride ions. In particular, EC,, appears prior to the onset of cathodic current flow, suggesting that the addition of chloride ions promotes the reverse reaction of eqn. (11). The subsequent reduction would also proceed with the reverse reactions of eqns. (10) and (9). Conversely, in the absence of chloride ions, the direct cathodic reduction of PdO(ox) to metallic palladium as represented by eqn. (12) would proceed like that of platinum in both the absence and presence of chloride ions: PdO(ox) + 2H++ 2e-+

Pd + H,O

(12)

Differences in piezoelectric behaviour associated with oxide formation and reduction between palladium and platinum in the presence of chloride ions It was previously reported [61 that E,,O for platinum shifts towards the more negative direction with increasing chloride ion concentration, which is different from the behaviour for palladium. The discharge reaction of adsorbed water molecules on platinum is retarded by the strong adsorption of chloride ions and takes place only on a small fraction of surface sites via a displacement or substitution step of adsorbed chloride ions. Therefore the shift in E,, for platinum was ascribed to the promotion of surface rearrangement or place exchange due to lateral repulsive forces between adsorbed chloride ions and OH or 0 species to form PtO on the localized sites. In the case of palladium, however, the effect of chloride ions on E,, is minor because adsorbed chloride ions are easily replaced by electrosorbed OH or 0 species after anodic dissolution of palladium as PdCl,. However, E,, for palladium as well as for platinum shifts towards the positive direction with increasing chloride ion concentration. In the case of platinum, EC,O always appears after the cathodic current peak of oxide reduction, even at high concentrations of chloride ions, suggesting that PtO is reduced directly to metallic platinum without passing the reversed route of PtO(ox) = PtO(ad). In contrast with platinum, palladium tends to take the reversed route for cathodic reduction of oxide in the presence of chloride ions. The adsorption of chloride ions or dissolved species such as PdCl, or PdCli- on PdO may make this feasible. At present, however, it is difficult to explain why palladium tends to take the reversed route, because there is no information on the difference in adsorption state of chloride

194

ions between PdO and PtO. We need to carry out further experiments on chloride ion adsorption on oxygenated palladium and platinum surfaces associated with sign reversal of surface charge. CONCLUSIONS

The following conclusions were drawn from the piezoelectric response to changes in surface stress of palladium electrodes in perchloric acid solution containing chloride ions. (1) It was found from the piezoelectric signals that the potential of electrocapillary maximum or potential of zero charge (pzc) is present in the potential region of hydrogen absorption into palladium. (2) The potential of zero charge (pzc) shifted linearly toward the negative direction with increasing logarithm of chloride ion concentration, i.e. the EsinMarkov relation held between the pzc and chloride ion concentration, from which the degree of specific adsorption of chloride ions on palladium was evaluated. (3) The piezoelectric signals indicated that the sign reversal of surface charge took place in the potential range of oxide formation and reduction, similar to platinum. The sign reversal of surface charge was attributed to structural changes in the palladium surface associated with oxide formation and reduction. (4) The potential at which the sign reversal of surface charge occurred during cathodic reduction of oxide (PdO) shifted toward the positive direction with increasing chloride ion concentration. In particular, at high chloride ion concentration, the sign reversal of surface charge took place prior to the cathodic reduction, different from platinum. It was suggested that adsorption of chloride ions tends to induce structural changes from the oxide to an adsorbed oxygen layer, PdO(ad), without direct cathodic reduction to metallic palladium. REFERENCES 1 2 3 4 5 6 7 8 9 10 11

A.Y. Gokhshtein, Surface Tension of Solids and Adsorption, Nauka, Moscow, 1976. R.E. Malpas, R.A. Fredlein and A.J. Bard, J. Electroanal. Chem., 98 (1979) 171. M. Seo, T. Makino and N. Sato, J. Electrochem. Sot., 133 (1986) 1138. M. Seo, X.C. Jiang and N. Sato, J. Electrochem. Sot., 134 (1987) 3094. X.C. Jiang, M. Seo and N. Sato, J. Electrochem. Sot., 137 (1990) 3804. X.C. Jiang, M. Seo and N. Sato, J. Electrochem. Sot., 138 (1991) 137. M. Seo and M. Aomi, J. Electrochem. Sot., 139 (1992) 1087. P. Delahay, Double Layer and Electrode Kinetics, Wiley, New York, 1965, p. 17, p. 53. G.T. Cheek and W.E. O’Grady, J. Electroanal. Chem., 277 (1990) 341. L. Grasjo and M. Seo, J. Electroanal. Chem., 2% (19%) 233. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, Houston, TX, 1974, p. 358. 12 J. Genesca and R. Duran, Electrochim. Acta, 32 (1987) 541. 13 J.F. Llopis, and F. Colom, in A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, Vol. VI, Marcel Dekker, New York, 1976, Chapter 7.