J. Electroanal. Chem., 338 (1992) 367-372 Elsevier Sequoia S.A., Lausanne
JEC 02383PN Preliminary note
UPD of zinc on polycrystalline in various pH solutions
Akiko Aramata l, Md. A. Quaiyyum, Wilfred0 A. Balais, Takashi Atoguchi and Michio Enyo Catalysis Research Center, Hokkaido University, Sapporo 060 (Japan) (Received
18 May 1992; in revised
form 14 July 1992)
Underpotential deposition (UPD) of metal ions has been studied extensively over recent decades, since it can give wide changes in the electronic and electrocatalytic properties of the metal surface. Kolb summarized UPD phenomena up until 1978 [l], including Zn*+/Au and Zn*+/Ag, but not Zn*+/Pt (where Zn*+/M defines Zn*+ ion UPD on a substrate metal M). The UPD of metal ions on Pt has received much attention in view of an enhancement of electrocatalytic activity. In 1970, Mikuni and Takamura 121,who observed the cyclic voltammogram (CV) on Pt in the presence of Zn*+ ions, recognized an increase of the current due to the addition of Zn*+ ions in the hydrogen adsorption region in acidic solution. In the course of a study of multivalent cation adsorption on Pt , we found an increase of the current in the CV on Pt at the hydrogen adsorption region by the addition of Zn*+ ions in acidic solution, in agreement with ref. 2. Therefore, in this report we investigated, under various conditions, whether Zn*+ ion UPD occurs on Pt at potentials about 1 V more positive than the equilibrium potential of Zn*+/Zn. EXPERIMENTAL
A three-compartment glass electrolysis cell was used . The working electrode was a polycrystalline platinum wire of diameter 0.5 mm and length 20 mm. The
To whom correspondence
roughness factor of this electrode was approx. 1.04-1.31, estimated from the amount of adsorbed hydrogen at potentials of E > 0.05V (RHE). An SCE or an RHE (Pt-Pt wire in an H, saturated solution) was used as the reference electrode and a Pt wire gauge as the counter electrode. The supporting electrolytes employed were 0.1 M H,SO, (pH 0.69), 0.1 M KH,PO, (pH 4.61, and phosphate buffers of 0.025 M KH,PO, + 0.025 M K,HPO, (pH 6.9) and 0.2 M K,HPO, + 0.8 M KH,PO, (pH 5.9). ZnSO, or Zn(ClO,), was added to the supporting electrolyte solutions to give a Zn2+ ion concentration of 1.25 X 10e5 to 1 X 10e3 M. All . measurements were carried out at room temperature. The solutions were deaerated by bubbling He or Ar gas, which also stirred the solution. The CV curves were recorded after steady-state curves were obtained. After each observation of the CV in solutions with ZnZf ions, the Pt electrode tias washed with aqua regia to be freed from UPD Zn. RESULTS AND DISCUSSION
Figure 1 shows the CVs on Pt in an He-gas saturated 0.1 M H,SO, solution with and without Zn2+ ions at the sweep rate of 20 mV/s, which are similar to those of Mikuni and Takamura . On the addition of Zn2+ ions, the cathodic and anodic current densities in the hydrogen adsorption region were increased up to E L 0.15 V and then decreased at E < 0.15 V, in comparison with the CV in the solutions without Zn2+ ions. The changes of these CVs are ascribed to Zn deposition and dissolution on Pt, provided that hydrogen adsorption is not expected to take place on the UPD Zn surface. The increment of the amount of charge was 20 and 40 &/cm2 for negative and positive sweeps respectively, and the average increase of charge was about 15%. In order to separate the UPD of
Fig. 1. Cyclic voltammograms at 20 mV/s on Pt in 0.1 M H,SO, with (10e4 M ZnSO1.
) and without (- - - - - -)
Fig. 2. (a) Cyclic voltammograms at 20 mV/s on Pt in 0.1 M KH,PO, with (------- ) and without (- - - - - -) 10e4 M Zn(ClOJ,. (-. -. -1 is taken to represent the most positive UPD desorption peak of Zn, from which the half width (AW,,,) was estimated. (b) Cyclic voltammograms on Pt at 20 mV/s in phosphate buffer (pH 5.9) with () and without (- - - - - -1 10m4 M Zn(ClO,)a. (cl Cyclic voltammograms on Pt at 20 mV/s in phosphate buffer (pH 6.9) with () and without (- - - - - -) 10m4 M ZnSO,.
Zn*+ on Pt from hydrogen adsorption-desorption processes, we observed CVs at different pH values and in various Zn*+ ion solutions. Figures 2(a)-(c) show CVs on Pt with and without Zn*+ in 0.1 M KH,PO,, and phosphate buffers of pH 5.9 and 6.9 respectively. The increase in current was observed at E I 0.6 V (RHE) in Figs. 2(a) and (b) and at E I 0.8 V (RHE) in Fig. 2(c), and then’ hydrogen peaks in both hydrogen adsorption and desorption processes at E I = 0.2 V (RHE) were suppressed. The CVs of Figs. 1 and 2 show that both hydrogen adsorption and desorption peaks were least suppressed in the so-called weakly-adsorbed hydrogen potential region. Therefore, the observed CVs in the solution with Zn*+ ions are considered to show the presence of Zn*+ UPD on Pt. The anodic side peak was chosen as the UPD peak from which the UPD peak potential E, [l] and its shift AE, were determined with its half width AW,,, as shown in Fig. 2(a). The anodic CVs in various electrolytes of different pH values in the presence of Zn*+ ions are shown with respect to SCE in Fig. 3, where all CVs were observed
E/ V vs. SCE Fig. 3. Cyclic voltammograms of the anodic portion on Pt in the presence of 10m4 M Zn*+ in 0.1 M 0.1 M KH,PO, t-.-.-I, and phosphate buffers (pH 5.9) (----- -) and fpH 6.9) H,SO, ( -1, (-. .-. . .-I. Sweep rate, 20 mV/s.
with a lower potential limit of 0.050 V (RHE). The E,, AE, and AW,,, were found to change in various electrolytes which are listed in Table 1. It was difficult to estimate the Zn or H coverage on Pt in the solution with Zn2+ ions because Zn2+ UPD occurs from the double-layer region to the hydrogen adsorption region. As hydrogen evolution takes place at E < 0.05 V (RHE), the whole amount of UPD Zn down to E,, cannot be estimated. However, we found that the charge density was always higher by approx. 15-30% in the presence of Zn*+ ions than in their absence in electrolytes of pH < 6.9. This means that Zn deposits on Pt at least in the underpotential region of OS-l.0 V. In the case of the phosphate buffer (pH 6.9) in Fig. 2(c), it was difficult to estimate the amount of charge in the presence of Zn*+ ions, since Zn*+ ion deposition and dissolution take place over a wide potential range. We observed CVs in 0.1 M KH,PO, (pH 4.6) with different Zn*+ ion concentration from 1.25 x 10e5 M to 1 X 10e3 M, as shown in Fig. 4, where the E, value ranged from - 0.07 to - 0.02 V (SCE) for the E,, shift of 56 mV. Hence, the UPD shift (AE,) by various Zn*+ ion concentrations in 0.1 M KH,PO, is almost identical to that at 1.06 V, so we confirmed that UPD of Zn takes place on Pt. In Table 1, the difference between AE, values was 0.18 V among the observed solutions, and the half width of the peak AW,,, ranged from 0.08 to 0.34 V, where
TABLE 1 UPD of Zn on Pt Solutions
E, /V (RHE)
0.1 M HaSO, (pH 0.69) 0.1 M KH,PO, (pH 4.6) Phosphate buffer (pH 5.9) (0.8 M KH,PO, +0.2 M K,HPOJ Phosphate buffer (pH 6.9) (0.025 M KH,PO, +0.025 M KaHPO,)
0.27 0.44 0.40
1.11 1.05 0.93
0.08 0.14 0.14
Zn*+ ion concentration, 10e4 M from ZnfCIO,), Zn*+/Zn = -0.881 V (SCE) at [Zn*‘l= low4 M.
from &, of
0 0.2 t-/‘Jvs. SCE Fig. 4. Cyclic voltammograms of the anodic portion on Pt in 0.1 M K&PO, 1O-4 M ( -) and 1.25 x lo-’ M (- - - - - -) Zn(CIO,),.
with 10e3 M (-.-.-),
the cathodic half of AW,,, is taken to be identical to the anodic one at pH 0.69. The AW,,, in other electrolytes was also estimated in a similar manner. The change of AW,,, is interesting but its interpretation will require further work. We plotted AE, against the difference of work functions AI#J together with AE, values for the UPD of Zn*+ ions on different metals in Fig. 5, where the A4 values were from Michaelson  and the A E, values employed were those recalculated independently by us as 0.18, 0.21 and 0.54 V for Zn*+/Ag , Zn*+/Cu  and Zn*+/Au  from the E_, of ZnO;*/Zn in 1 M KOH respectively, 1.0 V for Zn*+/Pt  in 1 M HClO,, 0.59 V [l] for Zn*+/Au in 1 M Na,SO,, and 0.52 V for Zn*+/Au in 0.1 M Na,SO,, which was observed in the present work. We find A E, = 0.77 A4/F (Fig. 5), provided that AE, is made proportional to A4. This relation became AE, = 0.96 A+/F using Trasatti’s A+ values . These results are different from the empirical equation AE, = 0.5
Fig. 5. Underpotential shift AE, against A& A+ is the difference between the work functions of polycrystalline substrates and Zn: A, from literature; 0, this work.
A4/F proposed by Kolb [l]. In the case of Zn2’/Ag in Fig. 5, A+ is negative but becomes zero when Trasatti’s value of the work function is employed. The difference of AE, values for Zn*+/Pt by 0.18 V of Table 1 is likely to be correlated to the interaction between the anion, the cation, water and the electrode itself at the interface, but can be taken to remain within the deviation of the plot of Fig. 5, as in ref. 1. To summarize, the UPD shift AE, of Zn2+ ions on Pt was observed and varied with the electrolytes and pH. The variation of UPD shift, owing to the interaction of the anion, the cation, solvent water and the electrode metal at the interface of the electrode, is however taken to give a reasonably proportional relation of AE, to A& REFERENCES 1 2 3 4 5
D.M. Kolb, Adv. Electrochem. Electrochem. Eng., 11 (1978) 125. F. Mikuni and T. Takamura, Denki Kagaku, 38 (1970) 113. A. Aramata and M. Enyo, to be published. A. Katayama-Aramata and I. Toyoshima, J. Electroanal. Chem., 135 (1982) 111. H.B. Michaelson, in R.C. Weast (Ed.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1984-1985, p. E-76. 6 G. Adzic, J. McBreen, and G. Chu, J. Electrochem. Sot., 128 (1981) 1691. 7 M.G. Chu, J. McBreen, and G. Adzic, J. Electrochem. Sot., 128 (1981) 2281. 8 S. Trasatti, J. Electroanal. Chem., 33 (1971) 351.