Chapter 12
Anion Adsorption and Charge Transfer on Single-Crystal Electrodes 1
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Roberto Gómez, José M . Orts, and Juan M . Feliu
Departamento de Quimica Fisica, Universidad d'Alacant, Apartat 99, E-03080 Alacant, Spain
Anion adsorption on single crystal electrode surfaces plays a key role in the understanding of surface electrochemical processes. A detailed knowledge about the charge involved in the adsorption process as well as the surface coverage and charge state of the species forming the adlayer is required. In order to do so, a coupling of cyclic voltammetry, charge displacement and scanning tunneling microscopy is exploited. The necessity of charge correction for voltammetric stripping processes, derived from hydrogen and anion adsorption, is put in evidence.
Since their first publication in 1980 (7), the voltammetric profiles of clean, ordered platinum single crystals have been a subject of controversy, especially in the case of Pt(l 11). At first, the main discussion point was related to the surface state of the sample after the flame treatment. Did the obtained voltammetric profile correspond to the clean, atomically ordered surface expected for the crystallographic basal plane? or was the ordered surface obtained only after some voltammetric cycles involving oxygen adsorption (2)1 The answer to these questions was gotten by means of well-established Surface Science techniques, such as ex-situ LEED, either with clean samples (3) or with those prepared by I-CO replacement (4) and by insitu STM (5). These results proved that the flame treatment and subsequent water protection led to clean, well-ordered P t ( l l l ) surfaces, which gave a voltammetric response never observed before 1980. The following discussion point dealt with the interpretation of the voltammetric profiles. It is well known that two groups of adsorption states can be distinguished in the voltarnmogram of P t ( l l l ) in sulfuric acid media in spite of the existence of a single family of adsorption sites on this orientation. The one appearing at lower potentials could be already discerned in papers published in the late 70s and early 80s dealing with voltammetric profiles obtained after the transfer of UHV-characterized P t ( l l l ) samples to the electrochemical cell (6,7). The y
1
Corresponding author
© 1997 American Chemical Society
In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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differences in the measured charge density as well as in the voltammetric profiles among these studies were linked to surface contamination during transfer and to the so-called "electrochemical activation technique", which cleans, but also disorders, the surface. The second group of adsorption states, appearing at higher potentials, were termed "unusual adsorption states", since they had never been observed before the introduction of the flame treatment. On the other hand, these unusual states were observed with several working solutions containing different anions (7). The potential range in which these states appeared was found to depend on the nature and concentration of the electrolyte in a significant way. The charge densities measured for the unusual states were not strongly dependent on the nature of the anions in the test solution. Different processes were proposed as responsible for this voltammetric charge transfer, but they can be simply divided into two groups. In the first one (7), hydrogen is considered to be at the origin of the electronic transfer. Above 0.5 V in 0.5 M sulfuric acid (all potentials referred to the RHE scale), the P t ( l l l ) voltarnmogram presents a current density corresponding to a double layer capacity of 70 μΡ.αη" , including the contributions of specifically adsorbed anions; as the potential becomes less positive, the (bi)sulfate anions desorb competitively with hydrogen adsorption, the latter being responsible for the charge transfer recorded in the voltarnmogram. In perchloric acid, where specific adsorption of anions is expected to be absent, hydrogen adsorption would take place at higher potentials, involving energies unprecedented under U H V conditions, where water and the applied potential are absent. Another explanation (8) consisted in interpreting the voltammetric profile by considering that in 0.5 M sulfuric acid the bisulfate adlayer begins to desorb at 0.5 V in a process that involves the observed charge transfer in a relatively narrow potential range, thus giving rise to abnormally high current densities. In perchloric and hydrofluoric acids where specific adsorption of both anions is generally discarded, the species responsible for the unusual states was assumed to be -OH. In summary, the unusual states are due to an extremely reversible process involving either reductive adsorption/oxidative desorption of a cation (H ) or reductive desorption/oxidative adsorption of an anion, cyclic voltammetry being unable to distinguish between both of them. Different experimental results were published and interpreted favoring either the first or the second way of understanding the voltammograms, but none of them could unequivocally discriminate between both hypotheses on the adspecies undergoing the charge transfer. In this paper we would like to emphasize a different experimental approach (described in references 9-13) for investigating the nature and the amount of the species responsible for the charge transfer observed during voltammetric scans with platinum electrodes. It is based on the fact that CO is strongly adsorbed on platinum in a wide potential range. It also takes advantage of the fact that CO adsorption is electroneutral. Indeed, the strong adsorption of CO is effective in displacing from the surface most of the common adsorbates. The desorption of these species will supply a charge, released through the circuit, corresponding to a process opposite to that taking place during their adsorption. Insofar as in this technique only desorption processes account for the measured value and sign of the 2
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In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
SOLID-LIQUID ELECTROCHEMICAL INTERFACES
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1
Figure 1. Voltammetric profile of Pt(lll) in 0.5M H S0 . Sweep rate: 50 mV-s . Numerical values correspond to the displaced charge density at the highest and lowest potentials in charge displacement experiments. 2
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In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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displaced charge at a given potential, reductive desorption becomes distinguishable from reductive adsorption. This is a key point in the interpretation of the voltarnmogram.
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Experimental In order to illustrate the experimental procedure (9,72), we can start with the wellknown voltammetric profile of the P t ( l l l ) electrode in 0.5 M sulfuric acid (Figure 1) . The usual states range from 0.35 V down to the hydrogen evolution threshold, whereas the characteristic unusual states are observed at higher potentials, up to 0.50 V . The voltammetric charges involved in the usual-state potential range can be easily measured in this voltarnmogram, including or not the apparent double layer contribution. If we correct the apparent double layer charging current, the charge density determined for the usual states is around 160 μΟ/ϋπι . Let us describe briefly the charge displacement technique. The potential is fixed at a selected value, e.g. 0.08 V , where the background current is almost zero. Then, CO is introduced into the gaseous atmosphere of the cell near the meniscus. CO molecules get into the solution phase, particularly in the meniscus and reach the electrode surface. During the subsequent adsorption process, an oxidation transient current flows during a certain time interval, decreases and finally attains an almost zero value again. This indicates the end of the displacement of the previously adsorbed species. The displaced charge can be easily evaluated. The excess of CO molecules is removed from the solution by Ar bubbling for several minutes. Then the current corresponding to the fully blocked surface can be voltammetrically recorded and subsequently the involved capacitive charge be measured. Next, the potential window is opened, CO is stripped from the surface in a single sweep and the initial voltammetric profile is strictly recovered. The cleanliness of the working solution is thus verified, as well as the stability of the electrode surface structure during the overall experiment. After a new flame treatment the displacement of charge can be carried out at another potential, i.e. 0.5 V . In this case, a reduction transient current is recorded. The sum of the absolute values of both charges amounts, within the experimental error (±6 μ ^ π ι " ) , to the charge measured in the voltarnmogram between both displacement potentials. At 0.32 V the total displaced charge is nearly zero. This potential can be taken as the Potential of Zero Total Charge (PZTC) (14) The experiment can be carried out at any potential between 0.08 and 0.50 V. It is worth pointing out that the following relationship is held: 2
2
where Q/EJ and Q/EJ are the charge densities displaced at two different adsorption potentials, j is the voltammetric current density and ν is the sweep rate.
In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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SOLID-LIQUID ELECTROCHEMICAL INTERFACES
Results and Discussion In the light of the previous interpretations, and taking into account the new information coming from the charge displacement results for P t ( l l l ) in contact with H S 0 0.5 M , the voltammetric profile (Figure 1) can be read in the following way: at 0.5 V , i.e. more positive than any significant voltammetric feature, anions are adsorbed on the P t ( l l l ) surface. At 0.4 V the amount of adsorbed anions present on the surface is lower and, finally, at 0.32 V there is not a significant amount of anions adsorbed on the P t ( l l l ) electrode. At potentials lower than 0.32 V , hydrogen begins to adsorb, its coverage being governed by the imposed potential. The displaced charge now increases as die displacement potential becomes more negative. In any case, the maximum charge density displaced prior to the hydrogen evolution onset amounts to around 160 μΟοηι" , which means that the full hydrogen monolayer is not completed before hydrogen evolution. At this point the hydrogen coverage is about 2/3 of the full monolayer. Both displacement processes could be summarized as:
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2
4
2
Pt(l 11)-HS0 + CO + e -> Pt(l 1 l)-CO + H S 0 P t ( l l l ) - H + CO - » P t ( l l l ) - C O + IT + e 4
4
the first one occurring at E>0.32 V , and the second one at E
