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COMMENTS Comment on “Role of the Anion in the Underpotential Deposition of Cadmium on a Rh(111) Electrode: Probed by Voltammetry and in Situ Scanning Tunneling Microscopy” Gyo _zo _ G. La´ng*,† and Gyo¨rgy Hora´nyi‡ Department of Physical Chemistry, Institute of Chemistry, Eo¨tVo¨s Lora´ nd UniVersity, H-1117 Budapest, Pa´ zmany P. s. 1/A, and Research Institute of Materials and EnVironmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Post Office Box 17, Budapest H-1525, Hungary ReceiVed: September 10, 2005 In an interesting paper by Yang et al.,1 an attempt was made to investigate and to clarify the role of anions in the underpotential deposition (UPD) of Cd on a Rh(111) electrode. Using voltammetry and in situ scanning tunneling microscopy (STM) in H2SO4 and HCl solutions, the authors arrived at the conclusion that (bi)sulfate and chloride anions are co-deposited with Cd adatoms on Rh(111). This result seems to be in good agreement with the statement made in the Introduction of ref 1 on the basis of a survey of the relevant literature: “Coadsorption of anion, such as (bi)sulfate and halide, appears ubiquitously in UPD”. Results of radiotracer experiments2,3 also support this conclusion, and in this respect, the authors of the present paper completely agree with the opinion of Yang et al.1 However, considering the very fact that in ref 1 the extent of the UPD of cadmium ions was determined by voltammetric measurements from the charge involved in the UPD process, the discussion of a very interesting contradiction seems to be inevitable. This contradiction may be formulated on the basis of the following considerations: (i) When a coadsorption of positively and negatively charged species occurs, the net electrical charge involved cannot be characteristic neither for the adsorption of the positively charged species nor for the negatively charged species alone. (ii) Thus any conclusion concerning the extent of the UPD of a metal ion on the basis of data obtained from voltammetric data should be drawn only after the determination or estimation of the extent of coadsorbed anions. In the following, we try to demonstrate how the anion adsorption could influence the charge balance in the course of the UPD of cadmium ions. Let us examine the processes occurring during a typical UPD experiment from the point of view of an observer located in the bulk solution phase! The electrolyte solution in the cell with two (or three) electrodes (Figure 1) contains two electrolytes, for example, H2SO4 or HCl (i.e., H +, bisulfate or sulfate ions, or chloride ions) and Cd(ClO4)2 (i.e., Cd2+ and ClO4- ions). The potential of the working electrode (right-hand electrode) is swept from E (t * Corresponding author. Tel.: +36 1 209-0555/1107. Fax: +36 1 3722592. E-mail:
[email protected]. † Eo ¨ tvo¨s Lora´nd University. ‡ Chemical Research Center.
Figure 1. Schematic picture representing the “measurable” electronic current (I) and the corresponding ionic fluxes and currents in the cell at the working electrode and in the outer circuit during an electrosorption process. 1, working electrode; 2, counter electrode; 3, wires of the outer circuit; 4, potentiostat; 5, ammeter; τ1, thickness of the interfacial layer at the working electrode (a, boundary of this layer at the solution side); τ2, thickness of the interfacial layer at the counter electrode (b, boundary of this layer at the solution side); J /+, flux of the positively charged ions (Cd2+); J /-, flux of the coadsorbing anions (HSO4-, Cl-); I+, partial current corresponding to J /+; I-, partial current corresponding to J /- (obviously, I+ - |I-| ) I).
) 0) to E at the beginning of the experiment, for example, with the help of a potentiostat or otherwise. The charge involved in the electrochemical process is determined by integrating the current versus the potential (or the current vs time) curves. However, the current can be measured outside of a galvanic cell only, in the “outer circuit” (Figure 1), where the leads connected to the electrodes are purely electronic conductors. The instantaneous current (I) flowing through the ammeter (the “measurable” current) is equal with that flowing through the cell, because the electrochemical cell and the outer circuit elements form a series circuit. The current in all parts of a series circuit has the same magnitude. In electrolyte solutions, the electrical current is carried by ions. It means that the circuit can be closed only if ions are moving in the cell from one electrode to the other, that is, in the bulk phase of the solution in the cell, and accordingly, identical amounts of charge are transferred at the electrodes (corresponding to both the so-called “Faradaic” and “non-Faradaic” current). The nature of coupling between current and adsorption at the working electrode can be best understood on the basis of a surface layer or interfacial layer model. In this model, the region of space comprising and adjoining the phase boundary within which the properties of matter are significantly different from the values in the adjoining bulk phases is called the surface layer or interfacial layer, as shown schematically in Figure 1. In addition, it may be expedient to be more explicit and to define a surface or interfacial layer of finite thickness (τ) bounded by two appropriately chosen surfaces parallel to the phase boundary, one in each of the adjacent homogeneous bulk phases (in Figure 1, τ1 andτ2 are the interfacial layer thicknesses at the working and the counter electrodes, respectively).
10.1021/jp0551216 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/02/2006
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J. Phys. Chem. B, Vol. 110, No. 7, 2006 3445
Figure 2. Schematic pictures representing the movement of ions and electrons at the surface during the UPD of cadmium. , Cadmium ion; Q, anion; x, other positively charged ion (e.g., H+ ); e-, electron. a1-d1: initial states with adsorbed ions, and the movement of ionic components and electrons during the UPD process. a2-d2: final states. a1 and a2: adsorption of cadmium ions only; the charge calculated from the voltammograms (6e-) by integrating the measured current corresponds to the transfer of cadmium ions from the solution to the surface (6e-). b1 and b2: The adsorption of cadmium ions is connected with the simultaneous desorption of anions. The “calculated” charge (6e-) is greater than that corresponding to the adsorbed cadmium ions (4e-). c1, c2, d1, and d2: The adsorption of cadmium ions is connected with the simultaneous adsorption of anions. The “calculated” charge (6e-) is smaller than that corresponding to the adsorbed cadmium ions (10e- for case c and 8e- for case d).
Figure 3. Curves A1 and A2: potential dependence of the sulfate adsorption on polycrystalline rhodium covered with underpotentially deposited cadmium; A1, negative going plot; A2, positive going plot; cH2SO4 ) 2.4 × 10-5 mol‚dm-3; cCd(ClO4)2 ) 5.0 × 10-4 mol‚dm-3. Curves B1-B4: effect of the sulfuric acid concentration on sulfate adsorption on polycrystalline rhodium at various potentials; B1, cH2SO4 ) 1.4 × 10-5 mol‚dm-3; B2, cH2SO4 ) 4 × 10-5 mol‚dm-3; B3, cH2SO4 ) 8.0 × 10-5 mol‚dm-3; B4, cH2SO4 ) 2.4 × 10-4 mol‚dm-3. Curves C1-C3: effect of the Cd2+ concentration on the potential dependence of the sulfate adsorption on (“rhodized”) rhodium; cH2SO4 ) 10-3 mol‚dm-3; perchloric acid supporting electrolyte; cHClO4 ) 1 mol‚dm-3; C1, cCd2+ ) 0; C2, cCd2+ ) 10-3 mol‚dm-3; C3, cCd2+ ) 5 × 10-3 mol‚dm-3. Curves D1 and D2: effect of Cd2+ ions on the potential dependence of the sulfate adsorption at low supporting electrolyte concentration (cHClO4 ) 5 × 10-2 mol‚dm-3); cH2SO4 ) 10-3 mol‚dm-3; D1, cCd2+ ) 0; D2, cCd2+ ) 2 × 10-4 mol‚dm-3.
According to the above model, at boundary “b” (at the counter electrode) fluxes of ions (Ji) can be observed, which corresponds to the current I (Ab is the surface area of boundary “b”):
I)
∑i zi FAb Ji
(1)
In eq 1, index i stands for the ions participating in the conduction process, for example, H+, bisulfate or sulfate ions, Cd2+, and ClO4- ions, and so on, depending on the system under test; however, for the sake of simplicity and without loss of generality, we can also assume that a “special” counter electrode is used, and the current at boundary “b” is transported solely by Cd2+ ions. On the other hand, taking into account the statement in ref 1 that coadsorption of anions occurs in the course of the UPD of cadmium ions, we encounter some difficulties in the interpreta-
tion of the overall process at the working electrode. The source of these problems is related to the simple fact that in the course of the adsorption process the possibility of charge transport (transfer) by coadsorbing anions cannot be excluded. The overall charge transport at boundary “a” in Figure 1 with consideration of the coadsorption of anions in various extent is visualized by very simple schemes shown in Figure 2. It should be stressed that these schemes reflect the various charge and mass balances only and do not correspond to any real mechanism. In all cases (a-d), the passing of the same electric charge (measured in the “outer circuit”) corresponding to the current flowing through the cell is assumed. It can be seen from the schemes that there is only a single case (Figure 2, case a) when the amount of this charge can be unambiguously attributed to the UPD of cadmium ions. However, this case assumes that, before and after the UPD
3446 J. Phys. Chem. B, Vol. 110, No. 7, 2006 process, the extent of anion adsorption is the same (i.e., the surface concentration of adsorbed anions remains constant; no induced or enhanced adsorption of anions occurs). In all other cases (Figure 2, b-d), the ratio of adsorbed Cd2+ ions and anions is different, consequently, without the knowledge of the extent of anion adsorption, no reliable conclusions concerning the amount of deposited cadmium can be drawn from the voltammetric measurements. On the other hand, there are evidences in the literature proving by radiotracer technique that Cd adatoms on rhodium induce enhanced adsorption of sulfate or chloride ions. However, the extent of the anion adsorption depends on the electrode potential. This can be demonstrated by combining some illustrative data already reported in the literature2,3 (Figure 3). Taking into account these results, we can conclude that “case a” in Figure 2 is possible but highly improbable. Strong support for the above statement is obtained from experiments in which the underpotential deposition of Cd on Cu(111) was studied.4-6 According to Stuhlmann et al.,5 from voltammograms recorded with different scan rates, a charge transfer of 1.65 ( 0.11 e/Cd atom was calculated on the basis of the adsorption/desorption charge. However, the authors emphasized that “This calculation is only correct if the Cl coVerage of the surface does not change in the course of the Cd layer formation. Our data suggest, howeVer, that not only does the adsorbed Cl stay on the surface when the Cd layer forms, but the Cl coVerage eVen increases ... Assuming complete discharge of the coadsorbed Cl, the charge of the additional chloride ions which are adsorbed aboVe the amount already present on the Cd free surface also contributes to the charge under the Cd adsorption peak. Refining the calculation with this effect we obtain (1.82 ( 0.11)e/Cd ion or (1.94 ( 0.11)e/ Cd ion, depending on which structure for the Cd free surface is correct. ... Of course, the charge redistribution within the Cd-Cl bilayer is not really known and if the Cl retains a partial charge the Values for Cd change accordingly.” It should be also noted that the formation of ion pairs or ion associates in solutions of cadmium salts is a well-known feature.7,8 This effect can increase the directly measured or apparent transference number of the constituent Cd2+, because
Comments the complex ion or ion associate can carry anions in the reverse direction to the normal motion of that ion. When the state of cadmium ions in the solution phase is considered, it is not very surprising that cation-anion complexes can be formed in the adsorbed state as well. When these considerations are summarized, it can be stated that strong counterarguments can be formulated against the interpretation of the experimental data in ref 1. Finally, a critical remark should be made in connection with the use of cadmium-perchlorate (sometimes in relatively high concentration) in the experiments. It is known from the literature9-12 that the reduction of perchlorate ions takes place on rhodium, leading to the formation of chloride ions, and presumably, this process should be taken into consideration for the calculation of charge involved in the overall process. Acknowledgment. Financial support from the Hungarian Scientific Research Fund is acknowledged. (Grants OTKA T037588, T045888, T060191). References and Notes (1) Yang, L. Y. O.; Bensliman, F.; Shue, C. H.; Yang, Y. C.; Zang, Z. H.; Wang, L.; Yau, S. L.; Yoshimoto, S.; Itaya, K. J. Phys. Chem. B 2005, 109, 14917. (2) Zelenay, P.; Hora´nyi, G.; Rhee, C. K.; Wieckowski, A. J. Electroanal. Chem. 1991, 300, 499. (3) Hora´nyi, G.; Rizmayer, E. M. J. Electroanal. Chem. 1986, 201, 187. (4) Stuhlmann, C.; Hoffschulz, H.; Wandelt, K. In Interfacial Electrochemistry, Theory, Experiment and Application; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 805. (5) Stuhlmann, C.; Park, Z.; Bach, C.; Wandelt, K. Electrochim. Acta 1998, 44, 993. (6) Hommrich, J.; Hu¨mann, S.; Wandelt, K. Faraday Discuss. 2002, 121, 129. (7) MacInnes, D. A. The Principles of Electrochemistry; Dover Publications, Inc.: New York, 1961; p 89. (8) Rudolph, W.; Irmer, G. J. Solution Chem. 1994, 23, 663. (9) Clavilier, J.; Wasberg, M.; Petit, M.; Klein, L. H. J. Electroanal. Chem. 1994, 374, 123. (10) Rhee, C. K.; Wasberg, M.; Zelenay, P.; Wieckowski, A. Catal. Lett. 1991, 10, 149. (11) Ahmadi, A.; Evans, R. W.; Attard, G. J. Electroanal. Chem. 1994, 374, 123. (12) La´ng, G. G.; Hora´nyi, G. J. Electroanal. Chem. 2003, 552, 197.
