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Underpotential deposition of Zn2+ ions on platinum, palladium and gold at various pH values

Underpotential deposition of Zn2+ ions on platinum, palladium and gold at various pH values

61 Journal of Electroanalytical Chemistry, 373 (1994) 61-66 Underpotential deposition of Zn2+ ions on platinum, palladium and gold at various pH val...

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61

Journal of Electroanalytical Chemistry, 373 (1994) 61-66

Underpotential deposition of Zn2+ ions on platinum, palladium and gold at various pH values Md. A. Quaiyyum, Akiko Aramata *, Shinobu Moniwa, Satoshi Taguchi and Michio Enyo Catalysis Research Center, Hokkaido University, Sapporo 060 (Japan)

(Received 7 September 1993; in revised form 22 November 1993)

Abstract The underpotential deposition (UPD) of ZnzC ions on Pt, Pd and Au was observed in solutions at various pH values. The UPD peak potential on Pt and Pd shifted to more positive potentials with increase in Zn *+ ion concentration, the shift being 85-95% of the equilibrium shift of the Nernst equation for Zn 2++ 2e-= Zn. This suggests that the peak is due to UPD of Zn2+ ions and is associated with electron charge transfer of ca. 2. With Au, the shift of UPD peak potential with change in Zn*+ ion concentration varied with the change in pH and the electron transfer numbers were 1.5 and 2 at pH 5.0 and 6.0, respectively. The UPD shift for Zn2+-Pt and Zn*+-Pd systems was approximately the same, being ca. 1.0 V at pH 0.69-6.9, whereas that for Zn*+-Au was ca. 0.6 V. The half-width of the UPD peak increased with increase in pH; the interaction parameter of the Temkin adsorption isotherm was estimated as being repulsive for UPD of Zn*+ on Pt and Pd.

1. Introduction The underpotential deposition (UPD) of a metal, M, on a foreign metal substrate in the potential region positive to the equilibrium potential of M”+/M, has been studied extensively because of its theoretical and practical interest in relation to electrocatalysis, for example, in the design of an active electrode surface for methanol electro-oxidation. Kolb [l] summarized the UPD phenomena up to 1978 and Adzic [2] and Szabo [3] reviewed later UPD phenomena. For substrate metals employed in the present work, the UPDs of Bi3+ [4-61, Pb 2+ [4,5,7], Tl+ [4,5], Cd2+ [5] and Ge4+ [6,8] on Pt, those of Cu2+ 19,101, Tl+ [ll], Bi3’ [11,12], Ag+ [l&12], Cd2+ [11,13], Pb2+ [11,13] and Ge4+ [14] on Pd and those of Ag+ [15], Pb2+ [16-191, Bi3+ [17,20-221, Cu2+ and Cd2+ [21] on Au have been investigated. The UPD of Zn2+ on Ag and Au was reported from sulphate solution [l] and that on Ag, Cu and Au from alkaline solution [23,24]. Nicol and Philip [25] studied the UPD of Zn2+ ions on Ni in acidic solution. The UPD shift of Zn2+-Pt was expected to be as high as ca. 1.0 V, since an increase in the current

l

To whom correspondence

0022-0728/94/$7.00 SSDZ 0022-0728(94)03268-T

should be addressed.

of the cyclic voltammogram (CV> in the hydrogen adsorption region was observed in acidic solution [26]. This means that the presence of UPD of Zn2+ at positive potentials not only on Pt but also on the other noble metals can be found and such UPD of Zn2+ may give a new insight into electrocatalysis such as the decrease in the overvoltage of electrode processes and the creation of new processes. We have confirmed the presence of the UPD of Zn2+ ions on Pt [27] and reported its presence on Pd 1281. In this paper, we compare the UPD of Zn2+ ions on Pt, Pd and Au at various pH values. 2. Experimental A three-compartment Pyrex glass electrolysis cell was used [29]. Working electrodes were polycrystalline Pt, Pd and Au wires, and their roughness factors were 1.04-1.5 for Pt, as evaluated from the amount of adsorbed hydrogen up to a negative limit of E = 0.05 V (RHE), and 1.24 and 1.3 for Pd and Au, as evaluated from the amount of desorbed oxygen species in the negative sweep, where the upper limit potentials were determined as 1.45 V for Pd and 1.65 V (RHE) for Au according to refs. 30 and 31. An SCE or RHE (Pt-Pt wire in Hz-saturated solution) was used as the refer0 1994 - Elsevier Science S.A. All rights reserved

62

Md. A. Quaiyyumet al. / Underpotentialdepositionof Zn2 + ions on Pt, Pd and Au

ence electrode and Pt wire gauge as the counter electrode. The supporting electrolytes were 0.5 M HCIO,, 0.1 and 0.5 M H,SO,, 0.0125 M H,PO, + 0.0875 M NaH,PO, (pH 3.11, 0.1 M KH,PO, (pH 4.51, phosphate buffers (KH,PO, + K,HPO,, pH 5.0-7.4) and 0.1 M KOH (pH 12.7). ZnSO,, Zn(ClO,), or ZnO was added to the supporting electrolyte solution to give 1 x 10m6-1 x 10e3 M Zn*+ ion concentrations. All measurements were carried out at room temperature. The solutions were deaerated by bubbling Ar gas, and the Pd electrode was annealed before being inserted into each electrolyte. Cyclic voltammograms (CV) were recorded after steady CVs had been obtained. 3. Results and discussion During the cation adsorption study, addition of Zn’+ ions to an acidic solution increased the current in the CV in the hydrogen adsorption region on Pt 1261,but no such increase in the current was observed on Pd [131. Although a current increase in the UPD of Zn2+ ions may exist at Pd in the hydrogen adsorption-absorption region, the results in ref. 13 were confirmed in the present work. On increasing the pH to 3.1, we observed an increase in current density on Pd in the hydrogen adsorption-absorption region with addition of Zn*+ ions. Since the UPD of Zn*+ ions seems to overlap with hydrogen adsorption on both Pt and Pd at low pH, CVs were observed at higher pHs for various Zn*+ ion concentrations, in which the UPD peaks separated partially from the hydrogen wave. As typical examples, Figs. 1 and 2 show CVs on Pt and Pd, respectively, in 0.1 M KH,PO, (pH 4.6) with and without Zn*+ ions, where two peaks were observed from the double-layer region to the hydrogen region on addition of Zn*+ ions in both the negative and positive sweeps. The CVs at a sweep rate of 20 mV s-l on Au in 0.1 M KH,PO, (pH 4.6) with and without 4.7 X 10v4 M Zn*+ ions show characteristic curves as shown in Fig. 3(a), where a sharp reversible peak in the presence of Zn*+ ions appeared at 0.05 V (RHE) in both the positive and negative sweeps. The increase in current density and the appearance of the new peaks are ascribed to the UPD of Zn*+ ions on Pt [27], Pd [28] and Au. We employed the positive peak at the most positive potentials for analysis [l]. It is denoted a UPD peak, the peak potential of which is denoted E,. The E,s were observed at different Zn*+ ion concentrations ( < 1.0 x 10m3 M) on Pt, Pd and Au in 0.1 M KH,PO,, where the shift of Ep on Pt was 85-95% of the equilibrium potential shift from the Nernst equation for pH 3.4-6.0. The Znzf ion concentration dependences of E, on Pd (pH 4.5) and Au (pH 6.0) were 85% and 93%, respectively, of the equilib-

-30

I

I

I

I

I

200 400 600 Potential / mV vs. RI-IE

0

600

Fig. 1. Cyclic voltammograms at 20 mV s-l on Pt in 0.1 M KH,PO, or without (- - - - - -1 1 X 10e4 M Zn(ClO,),. (pH 4.6) with ( -_)

rium potential shift. These values on the three metal electrodes show that the charge-transfer process takes place with nearly two electrons under the above conditions. At pH 5.0, the Zn*+ ion concentration dependence of E, on Au shifted to the condition of an electron transfer number smaller than 2, i.e. ca. 1.5. The UPD of Zn*+ ions results in the inhibition of adsorption of so-called weaMy adsorbed hydrogen on

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-50 I

400

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600 1200 Potential / mV vs. RHE

Fig. 2. Cyclic voitammograms at 10 mV s-l on Pd in 0.1 M KH,PO, (pH 4.5) with () or without (- - - - - -) 1 X 10m4 M Zn(ClO,),.

Md. A. Quaiyyutn et al. / Underpotential deposition of Zn2 + ions on Pt, Pd and Au

63

Pt, as shown in Fig. 1, and in the lowering of the catalytic activity of Pt and Au for hydrogen evolution, as found in Figs. 1 and 3, respectively. The origin of the negative and positive peaks at 0.24 and 0.28 V, respectively, in Fig. 1 cannot be distinguished between Zn UPD and hydrogen adsorption at present. At pH 6.0, as shown in Fig. 3(b), the UPD of Zn2+ ions on Au causes a loss of the reversibility of the CV with peaks at 0.01 and 0.08 V (RHE) in the negative and positive sweeps, respectively, and these peak potentials are independent of sweep rate in the range l-20 mV s-l.

(a I

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20

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200

3

400

Potential / mV vs. RHE

(a)

101

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-200

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200

400

600

Potential / mV VS.RHE Fig. 3. Cyclic voltammograms at 20 mV s-* on Au (a) in 0.1 M KH,P04 (pH 4.5) with ()orwithout(------)4.7x10-4M Zn(ClO,), and (b) in phosphate buffer (pH 6.0) with f) or without (- - - - - -) 1 X 10e4 M Zn(ClO,),.

&+/mV

Fig. 4. Cyclic voltammograms (positive portion) (a) on Pt at 20 mV s-t in the presence of Zn*+ ions in phosphate buffer (pH 3.4) (. . . . .), 0.1 M KH,PO, (pH 4.6) () and phosphate buffers ofpH5.9(------)and6.9(.-.-.);(b)onPdatlOmVs-’inthe presence of Zn ‘+ ions in 0.1 M KH,PO, (pH 4.5) () and phosphatebuffersofpH5.5(......),6.0(------),6.6(.-.-.)and 6.9 (0).

The behaviour of the UPD of Zn*+ on Pt and Pd was observed at pH 5.5, 6.0, 6.6 and 6.9, where the UPD peak potential was shifted to more positive potentials with respect to RHE. Figures 4(a) and (b) show the CVs of the positive sweep at Pt and Pd, respectively, in phosphate buffer solutions of pH 3.4-6.9 with Zn*+ ions, where the potential scale is with respect to AE” = E - Eeq. All CVs were observed with a lower potential limit of 0.05 and 0.2 V (RHE) on Pt and Pd, respectively, where the E,s were found to change in various electrolytes. For the estimation of the halfwidth of the anodic peak at the most positive potential, AW,,,, current density increments of CVs, Aj, on addition of Zn*+ ions are plotted as Aj u-l against UPD shift potentials of AE” = E -E,, in the positive sweep on Pt and Pd, respectively, as shown in Figs. 5(a)

Md. A. Quaiyyum et al. / Underpotential deposition of Zn2 + ions on Pt, Pd and Au

64

and (b), where u is sweep rate. On both Pt and Pd, in alkaline medium (pH 12.7), three peaks in the potential range 0.4
Awl,27we

employ the Temkin approximation [32], in which the adsorption energy AHads is given by AH,,, = AH,“,, - re

(1)

where AH’,,, is AH,,, at coverage 0 = 0, and the interaction parameter, r, which represents the degree of interaction between the adsorbates, is assumed to be constant. According to ref. 32, we estimated r with n = 2 and the results are shown in Table 1 ranging from 12 to 26 and from 7 to 16 kJ mol-’ for Zn2+-Pt and Zn2+-Pd, respectively. As the r value estimated from the treatment in ref. 32 is dependent on the electron transfer number, n, comparison of the r values at different n values will not give specific meanings for the case of Au. In Table 1, the AE, values differ for the various phosphate solutions and range between 0.91 and 1.19 V for Pt and Pd and between 0.60 and 0.63 V for Au, where the AE, on Au is nearly identical with that in ref. 1. Trasatti [34] proposed the correction of E, values by AW,,,; the potential position of E, is a function of r values derived from AW,,,. Therefore, the shift of E, with r, de,, was evaluated from Conway and Gileadi’s treatment [321. The de, values were much smaller than those of AW,,,, and the former values were evaluated on the basis of eqn. (1). AE; = AE, + lAe,l and the values were plotted against pH together with AE, as shown in Fig. 7, where de, = 0 at r = 0. AE: and also A E, are likely to be independent of pH. Kolb et al. [35] studied the AEp of the UPD of various metals on different substrates and proposed an

TABLE 1. UPD of Zn*+ on Pt and Pd Substrate Pt

Electrolyte 0.1 M H,SO, Phosphate buffer 0.1 M KH2P04 Phosphate buffer

Pd

0.1 M KOH 0.5 M HClO, and HsS.0, Phosphate buffer

AE,, for pH 4.5-6.9, was calculated from for the respective pHs. [Zn2+] = 1 X 10m4 M for pH d 6.6, 2.5 X a It was difficult to determine AW,,, at formation [28], respectively. b At pH 3.1, the presence of UPD Zn2+

mot-’

I de, I/V

AE,/V 1.11

4%

3.4 4.6 6.0 6.9 12.7 =

1.06 1.05 1.04 1.06 1.19 None

0.09 0.11 0.13 0.19

4.5

0.98

0.09

8

0.022

5.5 6.0 6.6 6.9 12.7

0.93 0.93 0.91 0.97 1.18

0.10 0.11 0.12 0.13 0.14

10 12 14 15 17

0.026 0.030 0.035 0.037 0.045

/2V

_ 8 12 15 26

Ees of Zn2+/Zn

0.022 0.030 0.037 0.066

-

3.1 b

0.1 M KHaPO, Phosphate buffer (x KH,PO, +y K,HPO,)

0.1 M KOH

r/kJ

PH 0.69 a

and for pH 12.7 from Eeq of ZnOg-/Zn

with [Zn’+]. Ratio of x to y was changed

lo-’ M for pH 6.9 and 1.25 X 10e4 M for pH 12.7. pH 0.69 and 12.7 because the UPD peak was in the hydrogen region [27] and at the onset of oxide m the hydrogen adsorption-absorption

region was observed.

Md. A. Quaiyyutn et al. / Underpotential deposition of Zn2 + ions on Pt, Pd and Au

65

0.16-

(a)

AlFlmV

Fig. 6. Plot of half-width AW,,,

L&E”/ mV

(b)

Fig. 5. (a) Plots of A j u - ’ against potentials AE” = E - Eeq on Pt at pH 3.4 ( . . . . . .), 4.6 ( -_),5.9(------)and6.9(.-.-.).Ajis the anodic current density increment on addition of Zn2+ ions and u is the sweep rate of 20 mV s-l. The cathodic half of the most positive UPD desorption peak of Zn is assumed as (---. -_) for pH 4.6, from which the half-width (AW,,,) was estimated. (b) Plots of 5.5 (......I, 6.0 A j u-l against AE” on Pd at pH 4.5 ( -1, (------),6.6(.-.-.)and6.9(o).ThesweeprateuislOmVs-‘, except at pH 6.9 (5 mV s-l).

against pH.

de, and its change are small in comparison with those of AE,, de, does not give any significant change in true peak potential determination and consequently AE is used. The present AE, values for Zn*+-Pt, ZnP+-Pd, and Zn*+-Au do not fit into the relation in ref. 35, and give AE, = 0.88A+/F as shown in Fig. 8, where work function values were taken from Michaelson [36]. The slope of the above relation of AE, vs. AC#J becomes 1.08 when Trasatti’s work function values [37] are employed. The thermodynamic argument gives

empirical relationship between A E, and the difference in the work functions (Ac#J) of the substrate and the adsorbate metal as AE, = 0.5A4/F. As the value of

TABLE 2. UPD of Zn on Au Electrolyte

PH

A&/V

AW 12/V

0.1 M KH,PO, Phosphate buffer

4.6 5.0 5.4 6.0 6.5 6.9 1.4

0.62 0.61 0.62 0.63 0.60 0.60 0.61

0.05 0.06 0.08 0.08 0.09 0.10 0.11

AE,, for pH 4.6-7.4, was calculated from E,, of Zn*+/Zn at the respective [Zn2’]. [Zn”]= 1.OX1O-4 M for pH 6.5 and 6.9 and 4.7X10~3M,2X10~4M,1X10~5and2.5X10~5MforpH4.6,5.0, 6.0 and 7.4, respectively.

4

6

8

PH Fig. 7. Plot ofA Ei

(open symbols) and A E, (closed symbols) against PH. (0, n ) Pt; (A, A) Pd; (0) Au.

66

Md. A. Quaiyyum et al. / Underpotential deposition of Zn2 + ions on Pt, Pd and Au

References

Zn2+/Pd 0.8>

1 2 3 4 5 6 7 8 9 10 11

Fig. 8. Underpotential shift AE, versus Ad. AC#Jis the difference between the work functions of polycrystalline substrates and Zn: (A) from literature; ( A 1 this work.

AEp = A4 [34], which will be valid for the case that iomc contributions govern the total adsorbate-substrate bond rather than the covalent contribution [l]. The change m AW,,, with change in pH suggests that the UPD adsorption characteristics depend on the specific interaction between adsorbates and solvent water at the electrode interface, and also anion adsorption (e.g., OH- ions) is expected to interfere with the UPD of Zn2+ ions. The t value decreases in the order of the substrates Pt > Pd, as shown in Table 1. We reported previously [271 that, after each observation of CV on Pt with Zn2+ ions where the lower limit was 0.05 V (RHE), the Pt electrode was immersed in aqua regia for lo-15 min to regenerate a clean surface, and was freed from UPD Zn2+. However, such an effect was not observed with Pd and Au in this work. This suggests that with Pt there is a possibility of intermetallic compound or surface alloy formation, as stated in ref. 38, but the relatively large repulsive interaction parameter of UPD Zn2+ observed on Pt is not in agreement with simple phase formation of a binary system (work on single-crystal platinum shows no alloy formation [39]). The kinks or crystal defects may accommodate Zn as specific sites and keep it even when a high potential is applied. Therefore, the larger deviation of the slope from Kolb’s relationship is not likely to be correlated directly with alloy formation.

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Acknowledgement A Special Grant-in-Aid for Promotion of Education and Science in Hokkaido University and a Grant-in-Aid for Co-operative Research (A) (No. 04303007) from the Japanese Ministry of Education, Science and Culture are acknowledged.

37 38 39

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