In situ probing of the adsorption of Pb2+ on oxidised platinum electrodes using an electrochemical quartz crystal microbalance

359

J. Electroanal. Chem., 338 (1992) 359-365 Elsevier Sequoia S.A., Lausanne

JEC 02381PN Preliminary note

In situ probing of the adsorption of Pb2+ on oxidised platinum electrodes using an electrochemical quartz crystal microbalance C. Paul Wilde Department (Received

l

and Meijie Zhang

of Chemistry,

lJnicersi@ of Ottawa,

140 Louis Pasteur Pric., Ottawa, Ont. KIN 6N5 (Canada)

13 July 1992)

INTRODUCTION

The influence of adsorbed anions on the oxidation of electrodes such as Pt and Au is well known, and the various ways in which this influence is manifested (competitive adsorption, changes in the inner-layer field, etc.) have been clearly outlined [l]. Cation adsorption has been less extensively studied, although ions such as Ca2+, Sr2+ and Ba2+ have been shown to interact with oxidised platinum in alkaline media, leading to an increase of oxygen overvoltage [2]. More recently, reflectivity/ modulated reflectivity studies have revealed the adsorption of cations such as Bi3+, Pb2+, Cd2+ and Tl+ on the oxidised surfaces of gold and platinum electrodes in acidic media [3,4]. Adsorption at such high potentials in acidic media has fundamental significance in extending the understanding of the underpotential deposition process (UPD) associated with these cations, because adsorption can precede discharge during the UPD process [5,6]. Studies of cation adsorption at oxidised Pt electrodes are more difficult than at gold because for the latter, the potential range where underpotential deposits develop is well removed from that where electrode surface oxidation occurs. However, this is not the case for Pt, and for Bi 3+, Pb2+ and Tl+ the initial stages of development of the UPD deposit overlap with the oxide reduction peak. Similarly, on a positive scan the removal of the under-potential deposit obscures the early stages of electrode surface oxidation. This complication forced previous workers to employ modulated reflectivity in a study of cation adsorption at Pt in

l

To whom correspondence

should be addressed.

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acid media since there is no potential at which the reflectivity can be normalised to unity [4]. The electrochemical quartz crystal microbalance (EQCM) has recently proven to be a powerful technique for probing mass changes at electrode surfaces [7-91 and provides, under appropriate circumstances, a simple method for the study of adsorption processes. In a recent paper on lead UPD, we noted the adsorption of Pb2+ on Pt electrodes [6]. We report here a more complete investigation of the adsorption of Pb*+ on oxidised Pt electrodes in acidic solution as a function of the electrode potential and of the concentrations of Pb*+ and HClO,. This adsorption is revealed simply through its effect on the resonant frequency of a quartz crystal oscillator. The data shown here were obtained simply through the addition of aliquots of Pb*+ to the background electrolyte. As discussed below, the mass is not necessarily the only factor which influences frequency in this situation. For example, any solution density or viscosity change caused by the addition of Pb*+ might also affect the frequency. However, separate experiments show that these effects can be neglected under almost all the conditions used here. The relatively large frequency response resulting from the adsorption of Pb*+ on the electrode surface is easily detectable and reliable. EXPERIMENTAL

10 MHz AT-cut quartz crystals with gold electrodes deposited by the manufacturer (International Crystal Manufacturing Co., Oklahoma City, OK) were employed in a configuration described in the literature [7]. The measured frequency is the difference between the working crystal and a reference crystal, and is converted to a voltage for display on either a Kipp and Zonen BD91 or Philips PM 8272XYY’ chart recorder. The frequency difference was converted to mass as described by Bruckenstein and Shay [7]. A standard three-electrode glass cell was employed. The counter electrode was a Pt wire kept in a separate compartment fitted with a glass frit, and a saturated calomel electrode (SCE), connected to the main cell by a Luggin capillary, was used as the reference electrode. All stated potentials are referred to the SCE. Potential control was accomplished by means of an Oxford Electrodes (Abingdon, UK) potentiostat. Chemicals were obtained from Aldrich (Pb(ClO,),, 98%) and BDH (HCIO,, AnalaR and H,PtCl,, analytical reagent), and the solutions were prepared using water from a Millipore Milli-Q purification system. All solutions were carefully deoxygenated and bubbled with nitrogen during measurements. The Pt electrode was prepared by plating 20 k 3 pg of platinum on the gold electrode using 2 mmol dme3 H,PtCl, in 0.2 mmol dme3 H,SO,. The detailed plating procedure has been described elsewhere [6]. All electrodes were examined by voltammetry in 0.1 mol dme3 HClO, before adsorption experiments. Such experiments were carried out by adding appropriate amounts of a solution of 0.052 mol dm -3 Pb(ClO,), to the 0.1 mol dme3 HCIO, with the electrode potential poised in the range where the electrode surface is oxidised. In order to establish

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that observed mass changes result from Pb2+ adsorption rather than solution viscosity and density effects, the same experiments were also performed on a gold electrode which was covered with a very thin insulating PTFE layer (a frequency change of 2-3 kHz was observed as a result of film deposition) using a Fluoroglide@ Spray (Norton Co.>. All experiments were carried out at room temperature (22 f 1°C>. RESULTS

AND DISCUSSION

Typical cyclic voltammograms for Pb2+ are shown in Fig. 1 and illustrate the overlap between the UPD process and the oxidation/reduction of the electrode surface. Development of the UPD deposit begins at about 0.5 V, a potential at which reduction of the oxidised Pt surface is not yet complete. On the positive scan, the peak for removal of the UPD deposit arises at potentials where the first stages of electrode surface oxidation occur in the background electrolyte. Above 0.8 V, the presence of Pb2+ in solution makes little difference to the voltammogram. Thus, in contrast to the situation for Pb2+ on Au (where there is no complication from the overlap of UPD and electrode oxidation processes [3]) voltammetry offers no clear evidence for cation adsorption on the oxidised electrode surface. However, such adsorption is revealed in simple injection experi-

M-

0.8 E/V

(SCE)

Fig. 1. Cyclic voltammograms for 0.1 mol dm -3 HCIO, containing (2) and 2 mmol dm-3 (3) Pb*+. Scan rate 20 mV/s.

0 mmol dmm3 cl), 0.8 mmol dm-3

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Fig. 2. Mass-time transients for the successive injection of different concentrations of Pb2+ ions: curve (11, an electrode covered with a Fluoroglide layer in 0.01 mol dmA3 HCIO,; curve (2), as curve (1) except in 0.1 mol dme3 HC104; curve (3), an oxidised platinum electrode in 0.01 mol dme3 HCIO, with the electrode potential held at 0.8 V (SCE). Note that the vertical bar for the mass response corresponds to 20 ng for curves (1) and (2) whereas for curve (3) the bar represents 32 ng. Mass responses are absolute mass changes (not normalised for surface area) and are displaced vertically for reasons of clarity.

ments (described above) with the potential chosen so that neither UPD nor oxygen evolution processes can occur. The results of several such experiments are shown in Fig. 2. The upper trace (curve (3)) shows the variation of the frequency difference (converted to mass as described earlier) with time, as the concentration of Pb2+ is increased from 0 to 2 mmol dme3 at 0.8 V in 0.01 mol dme3 HCIO,. However, before discussing these results further, it is important to eliminate the possibility that phenomena other than that of adsorption of Pb2+ give rise to these mass changes. Two simple theoretical treatments of the QCM when immersed in solution indicate that the resonant frequency is proportional to the product (~~n~)r/~, where ps is the density of the solution and 7, is the viscosity [7,10,11]. Experiments such as cyclic voltammetry, are normally performed in a solution of unchanging composition and constant temperature so these effects can be neglected. However, when solution composition is changed this is not necessarily the case. Several observations support the suggestion that the mass changes seen in Fig. 2 are indeed a result of cation adsorption. Firstly, in a study of Pb UPD at Ag [5], the addition of 0.8 mmol dm-3 Pb2+ to 0.1 mol dmp3 HClO, was seen to produce no mass change at all, whereas when 0.4 mmol dmp3 Pb2+ was added to 0.1 mol dmp3 NaOH a large mass change was observed. These results were attributed to the different electrostatic interactions between the ions (Pb2+ in acid, HPbO; in alkaline solution) and the positively-charged Ag surface at potentials positive of the UPD region [S]. Secondly, the changes observed here are potential dependent,

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01 -42

’

-3.8

-3.4

-3.0 log( t &mol

-2.6

I -2.2

dm”)

Fig. 3. Normalised mass change (AM/ng cm-*) resulting from injections of Pbzf HCIO, at different potentials as a function of log (cp,,z+/mol dme3).

into 0.1 mol drnm3

increasing with potential between 0.8 V and 1.1 V (Fig. 3). Thirdly, the addition of concentrated KClO, solution to obtain a final concentration of 2 mmol dmW3 (in an attempt to produce roughly similar viscosity and density changes) gave no mass change. Finally, in order to further establish that the observed effects are a result of Pb *+ adsorption, a separate gold electrode which was covered with an insulating PTFE layer (formed by spraying the electrode with Fluoroglide@) was employed for injection experiments to try to eliminate effects from adsorption in the mass response. Typical results are depicted in Fig. 2, curves (1) and (2). Mass changes are very small (note that the mass scale is more sensitive for these two traces than for trace (3)). Only in 0.01 mol dmp3 HClO, are any changes detected and even then the addition of 2 mmol dm- 3 Pb*+ led to a mass increase of just 5 ng. Thus we believe that this is further evidence for the attribution of the observed mass changes to adsorption of the cation. Small corrections were applied to adsorption data obtained in 0.01 mol dme3 HCIO, as necessary. Figure 3 shows the results of injection experiments in which the mass change per unit area of the electrode surface (AM) is plotted as a function of the logarithm of the concentration of Pb*+ (log crsz+) for several different potentials in 0.1 mol dmp3 HClO,. A linear relationship is seen to exist between the normalised mass change (AM) and log(c ,,z+/mol dmp3), at least over the concentrations used here. AdZi6 and MarkoviC found a similar relationship between the relative change of reflectivity (AR/R,) and log cpbz+ in their experiments on Au electrodes in HCIO, [3]. Experiments such as that shown in Fig. 2, curve (31, were also performed for several concentrations of background electrolyte. The effect of perchloric acid concentration (cucIo,) on the observed mass changes is shown in Fig. 4 for an electrode potential of 1.0 V. At lower concentrations of HClO, larger mass changes are found. There is a linear relationship between AM and log cucIo,. Again, this parallels previously stated results for Bi3+ adsorption at gold where

364

0’ -2

1 1

ml.7

-1. 3

-09 log (c ,,,,,,lmol

-05

-01

dm”)

Fig. 4. Normalised mass change (AM/ng cm-*) as a function of log(~~~~,,~/rnol dmm3) at a potential of 1.0 V (SCE): 0, 0.1 mmol dm-3 Pbzf; W, 0.4 mmol dmw3 Pb*+; A, 0.8 mmol dm-3 PbZC; +, 2 mmol dmv3 Pb2+.

(AR/R,) was found to decrease linearly with log cuc,o, [4]. Evidently there is some competition between cations and anions for sites on the oxidised electrode surface. These EQCM results clearly show that Pb2+ is adsorbed on the oxidised surface of Pt electrodes in HClO, and that the extent of adsorption increases with potentral and cpt,z+, and decreases with cHCIO. The experiments are simpler and more direct than the modulated reflectivity \echnique used previously 141. Although direct comparisons cannot be made, it is pleasing that the linear relationships of Figs. 3 and 4 match literature results for the behaviour of AR/R, for Pb2+ on Au [3] and Bi3+ on Au [4] respectively. While the presence of adsorbed Pb2+ on the oxidised Pt surface is established, the exact influence of these ions on the formation/removal of the oxide is not. Little information can be gleaned from the cyclic voltammetry for two reasons. Firstly, the presence of lead makes little difference to the CV above 0.8 V (interestingly it was reported that for Pb*+ at gold, scanning into the UPD region resulted in changes in the CV and reflectivity responses arising from Pb*+ adsorption being diminished [3]), and secondly, the overlap between oxide and UPD regions obscures any effects of cation adsorption on the initial stages of oxide development (clearly visible at gold [3,4]). Stabilisation effects of the cation (resulting in a shift of the reduction peak to more negative potentials) cannot be seen for the same reason. However, it seems likely that Pb2+ interacts with the dipoles of MOH or MO species as suggested previously 141.The increased cation adsorption with potential might suggest that the interaction is stronger with MO rather than MOH species, but no firm conclusions can be drawn without further work. Finally, the effect of the background electrolyte seems to indicate that there is some competition for adsorption between anions and cations. As a consequence of this, although it is possible to convert AM data to coverage

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[6] (using a value for the monolayer coverage of lead of 260pC/cm* [12-141with the assumption that the mass change results solely from Pb*+ adsorption) our previous data treated in this way may have underestimated the coverage if anions are displaced as Pb*+ adsorbs. Further work is in progress on various aspects of this system and on the adsorption of Bi3+ at oxidised Pt electrodes, and this will be reported in a future paper.. ACKNOWLEDGEMENTS

We are pleased to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERCC) and of the University of Ottawa. MZ thanks the Government of Ontario for the award of an Ontario Government Scholarship (O.G.S.). REFERENCES 1 H. Angerstein-Kozlowska, B.E. Conway, B. Barnett and J. Mozota, J. Electroanal Chem., 100 (1979) 417. 2 A. Kozawa, J. Electroanal. Chem., 8 (1964) 20. 3 R.R. Ad% and N.M. Markovic, J. Electroanal. Chem., 102 (19791 263. 4 R.R. Ad% and N.M. Markovic, Electrochim. Acta, 30 (19851 1473. 5 M. Hepel, K. Kanige and S. Bruckenstein, J. Electroanal. Chem., 266 (19891 409. 6 C.P. Wilde and M. Zhang, J. Electroanal. Chem., 327 (1992) 307. 7 S. Bruckenstein and M. Shay, Electrochim. Acta, 30 (1985) 1295. 8 D.A. Buttry, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 17, Marcel Dekker, New York, 1990. 9 R. Schumacher, Angew. Chem. Intl. Edn. Engl., 29 (19901329. 10 K.K. Kanazawa and J.G. Gordon II, Anal. Chem., 57 (19851 1770. 11 K.K. Kanazawa and J.G. Gordon II, Anal. Chim. Acta, 175 (1985) 99. 12 A.R. Nisbett and A.J. Bard, J. Electroanal. Chem., 6 (1963) 332. 13 M. Shabrang, H. Mizota and S. Bruckenstein, J. Electrochem. Sot., 131 (1984) 306. 14 M.M. Nicholson, J. Am. Chem. Sot., 79 (195717.