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Langmuir 1997, 13, 2572-2574
Notes Simple Electrocatalytic Proof of the Intact Emersion of the Electrochemical Double Layer A. Couto, M. C. Pe´rez, A. Rinco´n, and C. Gutie´rrez* Instituto de Quı´mica Fı´sica “Rocasolano”, C.S.I.C., C. Serrano, 119, 28006-Madrid, Spain Received October 17, 1996. In Final Form: February 5, 1997X
1. Introduction It is now well established that when an electrode is emersed from the electrolyte under potential control at a potential in the double-layer region, it retains its electrochemical double layer. This was first demonstrated in 1978 by means of conductance and XPS measurements of very thin (tens of nanometers) Au and SnO2 film electrodes.1 The XPS results have been confirmed and extended.2,3 Furthermore, work function measurements showed that, as was to be expected, in the potential region of ideal polarizability the slope of the work function vs electrode potential plot was a straight line of unity slope, since the electrode potential is simply the electrochemical potential of the electron in the electrode, or Fermi level, and the work function is the potential difference between the Fermi and vacuum levels. This linear increase of the work function with electrode potential was observed both with the Kelvin probe technique4 and by measuring the photoelectric work function in UHV.5 The electrooxidation of CO on Pt in acidic solutions critically depends on the potential (admission potential) at which the Pt electrode is held while CO is first bubbled in the electrolytic cell.6 If this admission potential is lower than 0.2 V vs the reversible hydrogen electrode (RHE), then diffusion-limited electrooxidation of dissolved CO takes place already at 0.50 V, while if the admission potential is higher than 0.3 V, electrooxidation of CO first takes place at 0.86 V.7 This unique influence on the electrocatalytic activity of Pt of the potential at which it is held while the CO reactant is first introduced in the cell suggested to us a simple electrochemical procedure of clearly demonstrating the intact emersion of the double layer. 2. Experimental Procedure A conventional three-electrode electrochemical cell was used. A 6-mm diameter Pt disk electrode was glued with epoxy adhesive to a glass tube which fitted snugly in a cylindrical hole in a machined Teflon stopper that closed the cell, and which had conical apertures for a saturated calomel electrode, an auxiliary Pt electrode, and entrance and exit of gases. Ultrapure water * Corresponding author. Phone: 34-1-5619400 ext 1327. Fax: 34-1-5642431. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Hansen, W. N.; Wang, C. L.; Humphryes, T. W. J. Electroanal. Chem. 1978, 90, 137. (2) Scherson, D.; Krause, S.; Kolb, D. M. J. Electroanal. Chem. 1984, 176, 363. (3) Haupt, S.; Collisi, U.; Speckmann, H. D.; Strehblow, H.-H. J. Electroanal. Chem. 1985, 194, 179. (4) Hansen, W. N.; Kolb, D. M. J. Electroanal. Chem. 1979, 100, 493. (5) Neff, H.; Ko¨tz, R. J. Electroanal. Chem. 1983, 151, 305. (6) Kita, H.; Shimazu, K.; Kunimatsu, K. J. Electroanal. Chem. 1988, 241,163. (7) Caram, J. A.; Gutie´rrez, C. J. Electroanal. Chem. 1991, 305, 259.
S0743-7463(96)01012-8 CCC: $14.00
Figure 1. (a, top) Cyclic voltammogram (CV) at 20 mV s-1 of a Pt electrode. The Pt electrode had been emersed from the 0.5 M HClO4 electrolyte at 0.07 V and kept above the electrolyte for 10 min, during which time CO gas was bubbled through the electrolyte. After this the Pt electrode was immersed in the electrolyte with the potentiostat already set at 0.07 V, and a CV was immediately recorded. (b, bottom) Same as part a, but after emersion 15 min were allowed to pass before bubbling CO in the electrolyte. was from a Milli-RO + Milli-Q apparatus from Millipore. The electrolyte was 0.5 M HClO4. All potentials are referred to the RHE. A PC-controlled potentiostat, model µAutolab from Eco Chemie, was used.
3. Results and Discussion 3.1. Influence of the Emersion Potential on the Electrocatalytic Activity of Pt for Dissolved CO Electrooxidation. The emersion experiments were as follows: the polished Pt electrode was introduced into the electrolyte, this was deoxygenated by N2 bubbling under strong magnetic stirring, the Pt electrode was activated by potential cycling between 0.07 and 1.46 V at 1 V s-1 until the typical cyclic voltammogram (CV) of clean Pt was obtained, the sweep rate was decreased to 20 mV s-1, the sweep was stopped at a given potential during a positive sweep, the Pt electrode was emersed from the electrolyte (without interrupting the potential control), but kept within the cell, for a given time, then CO was bubbled in the electrolyte for 10 min, after which the Pt © 1997 American Chemical Society
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Langmuir, Vol. 13, No. 9, 1997 2573
Figure 2. (a, top) Cyclic voltammogram at 20 mV s-1 of a Pt electrode. The procedure was the same as in the experiment of Figure 1a, but the emersion potential was increased to 0.17 V. (b, bottom) Same as part a, but the emersion potential was further increased to 0.27 V.
electrode was immersed into the electrolyte with the control potential of the potentiostat already fixed at 0.07 V, and immediately several potential cycles at 20 mV s-1 between 0.07 and 1.46 V were carried out. While the Pt electrode is emersed and CO is being bubbled through the electrolyte, the CO gas will adsorb on the Pt electrode, displacing the chemisorbed hydrogen (as H2), since the heat of adsorption of CO on Pt (47 kcal mol-1) is higher than that of H2 (32 kcal mol-1).8 If initially, i.e. before contact with CO gas, the electrical double layer of the emersed Pt electrode retains the same composition and structure it had in the electrolyte, the electrocatalytic characteristics of the layer of chemisorbed CO produced by interaction of the emersed Pt electrode with CO gas should show the same dramatic influence of the emersion potential as that observed for the CO admission potential of an immersed Pt electrode in conventional electrochemical experiments. The first CV obtained with an emersion potential of 0.07 V shows an anodic peak at 0.63 V of electrooxidation of dissolved CO (Figure 1a), in exactly the same way as in a conventional experiment without electrode emersion, which clearly shows that the double layer on Pt was emersed intact. The only difference with a conventional (8) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons, Inc.: New York, 1994; pp 310 and 312.
Figure 3. (a, top) Cyclic voltammogram (CV) at 20 mV s-1 of a Pt electrode. The Pt electrode had been emersed from the 0.5 M HClO4 electrolyte at 0.07 V, CO gas was bubbled above the electrolyte for 5 min, and then N2 gas was bubbled through the electrolyte with strong magnetic stirring in order to eliminate the dissolved CO. After this the Pt electrode was immersed in the electrolyte with the potentiostat already set at 0.07 V, and a CV was immediately recorded. (b, middle) Same as part a, but the emersion potential was 0.17 V. (c, bottom) Same as part a, but the emersion potential was 0.27 V.
experiment is that, besides a peak of chemisorbed CO at 0.74 V (designated by us as CO(2)7), another peak, also of chemisorbed CO, appeared at 0.86 V (designated by us as CO(3)7). This is not surprising, since we have found that for admission potentials lower than 0.2 V chemisorbed CO can give rise to a CO(2) peak, to a CO(3) peak, or to
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a mixture of them, depending critically on minor experimental variables.9 The emersed double layer remains unaltered for a few minutes only, as far as our experimental procedure can determine. So, after a 15 min interval of electrode emersion (plus 10 min of CO bubbling with the electrode still emersed) a subsequent CV shows the same behavior as a Pt electrode, without emersion, for a CO admission potential higher than 0.3 V (Figure 1b): there is no peak of dissolved CO electrooxidation at low potentials, and chemisorbed CO originates a single peak of chemisorbed CO(3) at 0.85 V.7 The influence of the emersion potential on the catalytic activity of the Pt electrode can be clearly seen in Figure 2. When the emersion potential is increased to 0.17 V, a subsequent CV shows that the peak of dissolved CO electrooxidation has been shifted positively so much that it overlaps the peak of chemisorbed CO(2) (Figure 2a). A further increase to 0.27 V brings about the complete disappearance of the electrooxidation of dissolved CO at low potentials, with only the peak of chemisorbed CO(3) appearing, which is the well-known behaviour of Pt in conventional experiments without electrolyte emersion7 (Figure 2b). 3.2. Influence of the Emersion Potential on the Behavior of Chemisorbed CO on Pt. We have postulated10 that electrooxidation at low potentials of dissolved CO on Pt does not take place on all the Pt surface but only on a small fraction of it, since in parallel experiments in which dissolved CO is eliminated by N2 bubbling before running a CV, a hump, designated by us as CO(1),7 precedes the main peak of chemisorbed CO, the charge of the hump being about 15% of the total charge of chemisorbed CO. However, both the current density in steady-state experiments and the peak current density in CVs at sweep rates up to 10 V s-1 are in good agreement
with those calculated assuming that all the Pt surface is active. We have explained this apparent disagreement by assuming that the CO-free Pt sites are arranged in patches that act as microelectrodes whose diffusion layers overlap, simulating a wholly active surface.9 In this series of experiments the Pt electrode was emersed, CO was passed above the electrolyte for 5 min, then N2 was bubbled in the electrolyte for 5 min with strong magnetic stirring for eliminating the dissolved CO, and finally the electrode was introduced into the electrolyte. With an emersion potential of 0.07 V, the typical hump of chemisorbed CO(1) precedes the main peak of chemisorbed CO (Figure 3a). When the emersion potential is increased to 0.17 V, the hump of CO(1) is still seen, although somewhat reduced (Figure 3b). However, when the emersion potential is increased to 0.27 V, the hump disappears completely (Figure 3c), this being the reason why no electrooxidation of dissolved CO at low potentials is observed, since all the Pt surface is covered by the chemisorbed CO(3), which is first oxidized at 0.86 V (in CO-saturated solution; in the absence of CO in solution the peak of CO(3) is slightly shifted negatively to 0.73 V, and actually it is not possible to distinguish between the voltammetric peaks CO(2) and CO(3)).7
(9) Couto, A.; Pe´rez, M. C.; Rinco´n, A.; Gutie´rrez, C. J. Phys. Chem. 1996, 100, 19538. (10) Wieckowski, A.; Rubel, M.; Gutie´rrez, C. J. Electroanal. Chem. 1995, 382, 97.
Acknowledgment. This work was carried out with the help of the Spanish DGICYT under Project PB93-046.
4. Conclusions Taking advantage of the critical dependence of the electrocatalytic activity of Pt for the electrooxidation of dissolved CO on the potential at which the Pt electrode is held while CO is first introduced in the electrochemical cell, we have been able to show by cyclic voltammetry that when the Pt electrode is emersed from the electrolyte under potential control, it retains its electrochemical double layer.
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