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desorption of acetate anions on a Pt(111) electrode and the effect of counter cations in acidic media
Journal of Electroanalytical Chemistry 467 (1999) 112 – 120
The study of the adsorption/desorption of acetate anions on a Pt(111) electrode and the effect of counter cations in acidic media Toshihiro Fukuda, Akiko Aramata * Catalysis Research Center and Graduate School of En6ironmental Earth Science, Hokkaido Uni6ersity, Sapporo 060 -0811, Japan Received 29 July 1998; received in revised form 9 March 1999; accepted 13 April 1999
Abstract The mechanism and kinetics of specific adsorption/desorption of acetate were studied at Pt(111) in acidic solution by the potential step method and also by cyclic voltammetry. A so-called anomalous wave in the cyclic voltammogram appeared with the addition of acetate ions into perchloric acid solution, and was shifted by − 60 mV with a 10-fold change of the acetate concentration. Such behavior shows that the anomalous wave is due to acetate ion adsorption/desorption by a one-electron transfer process. The acetate adsorption/desorption of the anomalous wave shifted with the increase of pH by − 60 mV per pH unit in low pH solutions at pH B2 with the effect of cations, whereas the pH dependence of the anomalous wave disappeared at pH \5. These pH dependent tendencies are discussed as that the adsorbed form of acetate is present on Pt(111) electrode over the pH range studied. The current–time ( j–t) curve of acetate adsorption/desorption showed different decay features in the adsorption and desorption directions and in different potential regions. In the case of a 0.2 M acetate solution of pH 5.0, random adsorption without interaction, analyzed as the Langmuir adsorption, took place at lower coverage, while random adsorption with repulsive interaction was observed at higher coverage. In contrast to adsorption, the desorption process did not take place by monotonic decay of the j–t curve at high coverage, and the desorption mechanism changed into random desorption without interaction in the medium coverage potential region. When the surface coverage was even lower, a humped j –t curve was observed again. Square-pulse potential step methods revealed the protuberant j–t curve for desorption of the adsorbed acetate within a few ms after the discharge process takes place by a random adsorption reaction. The addition of K + decreased the rate of acetate adsorption by 40% in the low coverage potential region. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Adsorption; Desorption; Pt(111) electrodes; Acetate anions
1. Introduction Electrochemical reaction is affected by the electric double layer at the electrode electrolyte interface in general, and often severely by anion specific adsorption at the interface. Models of the electric double layer including specific adsorption were proposed [1]; the double layer structure consisted of the outer and inner Helmholtz planes together with the Gouy – Chapman Dedicated to Dr Jean Clavilier on the occasion of his retirement from LEI CNRS and his contribution to Interfacial Electrochemistry. * Corresponding author. Fax: +81-11-7094748. E-mail address: [email protected] (A. Aramata)
diffuse layer. Electrochemical measurements with insitu and ex-situ techniques on noble metal single crystals have expanded our knowledge on the structure of the electrode interface and anion specific adsorption. In the case of Pt(111) in sulfuric acid solution, studies of cyclic voltammetry (CV) [2,3], chronocoulometry [4], radiotracer methods [5,6], and the technique of CO adsorption replacement [7] have revealed that the ‘anomalous wave’ of CV is due to an electron transfer process in the formation/desorption of the adsorbed sulfate species. On the other hand, in-situ FTIR [8,9] and STM [10], and ex-situ LEED, AES, and CEELS measurements [11] revealed the adsorption state, twodimensional structure, and components of these adsor-
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 1 6 6 - 7
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bates. In-situ STM has made it possible to observe the adsorbed layer directly, and shows that the adsorbed sulfate species forms an ordered structure of ( 3 × 7) with co-adsorbed H3O + or H2O at potentials more positive than the sharp spike of CV [10]. In view of the kinetics of anion specific adsorption, Dretschkow and Wandlowski reported the correlation of the formation of the ordered structure of the adsorbed sulfate with the length of terraces on Au(111) and on stepped surfaces around Au(111) in comparison with STM and potential step methods [12]. We have reported recently the kinetic study of the specific adsorption of phosphate anions on Pt(111) in acidic media by the potential step method [13]. The mechanism of phosphate adsorption processes was explained by random adsorption models with and without lateral interaction according to the phosphate surface coverage; the former random adsorption takes place at low coverage, and the latter random adsorption with lateral repulsive force was observed at high coverage. The feature of phosphate adsorption depends on pH; with the decrease of pH, the onset potential of phosphate adsorption shifts to more positive potentials and the rate constant of phosphate adsorption/desorption becomes greater. These pH dependences seem to be correlated with the relationship between the counter cation and the anion adlayer, since Clavilier et al. observed quite large cation effects in neutral solutions [14]. The acetate adsorption was observed by the addition of acetate to perchloric acid solution, and was nearly in the same potential region as the sulfate adsorption on Pt(111), as is confirmed by the CO displacement method, and the electron-transfer number was observed as ca. 1 [15]. Meanwhile, the adsorbed anion in perchloric acid solution was assigned to be an adsorbed hydroxyl species from the feature of its pH dependence [16]. In the present report, we report the kinetic study of specific adsorption/ desorption processes of acetate anion and the effect of the counter cation on a Pt(111) electrode by voltammetry and potential step methods in acetate solutions in the presence of K + and Na + . We chose the study of acetate adsorption in addition to phosphate adsorption since (a) acetate anion is in a single ionic state of CH3COO − in the solution which makes the discussion simpler, whereas many ionic states of phosphate ions, H2PO4− , HPO24 − and PO34 − , must be considered with the pH change, and (b) the similarity of the voltammetric features between the cases of acetate and phosphate adsorption relates the adsorption mechanism and cation effect in both solutions. Previous studies on the acetate adsorption at Pt(111), not only by cyclic voltammetry, but also by the radiotracer method [17], and FTIR measurement [18] give information about the potential dependence of surface excess and adsorption states. FTIR observation of adsorbed acetate on polycrystalline Pt and Au was reported in 1988 [19].
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2. Experimental A single crystal Pt(111) of area 0.38 cm2, prepared by the Clavilier method [20], was used as the working electrode, and Au(111) obtained by the same method was used as a reference for the evaluation of the duration of double layer charging-up in the absence of specific adsorption. Solutions were prepared with CH3COOK (Fluka Biochemica Microselect), CH3COOH (Merck Suprapur), HClO4 (Merck Suprapur), KClO4 (Merck Pro analysi), and NaClO4 (Wako reagent grade monohydrate), respectively. Milli-Q water was used throughout. A two-compartment cell was used. Pt mesh or Au foil as the counter electrode was put around the working electrode in the working electrode compartment. The reference electrode in the reference electrode compartment was a saturated calomel electrode (SCE) and it was electrically connected to the working electrode compartment through a closed glass stopcock and a Luggin capillary. Solutions were deaerated by ultrapure Ar prior to measurements and the measurements were done at room temperature with Ar gas flow over the solution. The measurement procedure of the potential step method was as follows [13]. After the cell was confirmed to be free from the pollution by checking the CV, the electrode potential was swept to the initial potential E0 and kept for 30 s. Then, a potential E was applied to the electrode, when the current density j was measured as a function of time t. The CVs and current-time ( j–t) relations observed were recorded on an EG&G potentiostat M263 with a 12 bit precision. The sampling rate was 110 ms in the potential step method.
3. Results and discussion
3.1. Cyclic 6oltammetry Fig. 1 shows the CVs at Pt(111) in a 0.2 M HClO4 solution with 0, 1, 3, and 11 mM CH3COOH at 20 mV s − 1. The addition of acetate anions gave the disappearance of the characteristic CV on Pt(111) in HClO4 around 0.3 to 0.55 V (SCE) and yielded a main large peak around 0.1–0.2 V which is accompanied by a small peak around 0.35–0.38 V. These features are in good agreement with those in Ref. [15]. The latter peak is irreversible in the positive and negative potential sweeps. These CV features are very similar to those in phosphate solutions [13,20,21], although the current density of the small peak around 0.35 V in acetate solution is larger than that in phosphate solution. The charge density of the main peak in a 1 mM CH3COOH+0.2 M HClO4 solution of Fig. 1 is ca. 111 mC cm − 2 from 0.08V to 0.46 V without subtracting the double layer current, being slightly larger than the ca.
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Fig. 1. Cyclic voltammograms at 20 mV s − 1 on Pt(111) in 0.2 M HClO4 at pH 0.8 with 0 mM (-·-·); (I) 1 mM (---); (II) 3 mM (···) and (III) 11 mM ( — ) CH3COOH.
99 mC cm − 2 of the corresponding wave in 20 mM H3PO4 +0.2 M HClO4 [13]. The main peak around 0.1 V shifted in the negative potential direction by 60 mV for a 10-fold increase of acetate concentration, which suggests from the Nernst equation that the peak can be considered as a one-electron transfer process of acetate adsorption/desorption. The small peak around 0.35 V also shifted in the negative potential direction with the increase of acetate concentration, although the shift of potentials was smaller than the shift of the main peak. Fig. 2 shows the capacitance vs. potential curves at 30, 60 and 300 mV s − 1 in a 20 mM CH3COOH+ 0.1 M HClO4 solution, where the capacitance in the ordinate is the current density divided by the sweep rate. An effect of the sweep rate appeared on the small peak around 0.3 V, but not on the main peak. The small peak shifted in the negative potential direction, and the peak height increased with the increase of the sweep
Fig. 2. Cyclic voltammograms on Pt(111) in 0.1 M HClO4 + 21 mM CH3COOH of pH 1.1 at (I) 30 (---); (II) 60 (···) and (III) 300 mV s − 1 ( —).
rate on the negative sweep, while the main peak of capacitance around 0.07 V is independent of the sweep rate. This suggests that the adsorption or desorption mechanisms are essentially different in the potential regions between the main peak and the small peak of the anomalous wave. Fig. 3(a) shows the pH dependence of CVs in 20 mM CH3COOH+0.02, 0.1, and 0.3 M HClO4 solutions of pH 1.9, 1.1 and 0.7, respectively. The main peaks shifted to the negative potential direction with the pH increase by −60 mV per pH unit in parallel with hydrogen adsorption/desorption waves. Such pH dependence of the anomalous wave is similar but not identical to the case of Pt(111) in phosphate solutions[16]. The small peak around 0.3 V also shifts with pH, although the shift is smaller than that of the main peak1. These pH effects on CV suggest that the H + cation takes part in the reaction of the adsorption/desorption of acetate anions in low pH solutions. With the acetic acid dissociation constant of 1.74×10 − 5 at 25°C, the acetate is in the form of CH3COOH at pHB 2.0, and therefore a reaction CH3COOHCH3COOads + H + + e − is suggested, which is in conformity with the result of Ref. [15]. At pH\5, the pH dependence on the main peak disappeared as shown Fig. 3(b), suggesting that the adsorption/desorption process proceeds as CH3COO − CH3COOads + e − Figs. 3(a and b) show that the adsorbed state of acetate is in the form of CH3COOads over the pH range studied. The pH effect on the small peak also disappeared at pH\ 5. This behavior means that the small peak is not due to simple OH − adsorption in acetate solution at pH\5. Although the indirect bonding of acetate through adsorbed water was suggested for the case on polycrystalline Pt from FTIR results in 1988 [19], the observation on a single crystal Pt suggested direct bonding of adsorbed acetate in 1994 [14]. Since acetate specific adsorption/desorption is taken to be a surface electron-transfer process, the direct bonding to the Pt substrate is likely to be the case, as discussed in Refs. [15,18]. We investigated the effect of addition of K + ions on the CV, as shown in Fig. 4, since Clavilier et al. reported the different features of CV of Pt(111) in phosphate buffer solution by the change of cation at pH 7–9 [21]. Fig. 4 shows the CVs in 20 mM CH3COOH+0.1 M HClO4 with 0, 25, and 60 mM KClO4. Although no remarkable change with cation addition was observed, the addition of K + gave the change in the negative side of the main peak from 1 In the case of sulfate adsorption, the pH effect on the main and the small peak is different. This difference is yet to be clarified.
T. Fukuda, A. Aramata / Journal of Electroanalytical Chemistry 467 (1999) 112–120
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Fig. 3. The effect of pH on CV of Pt(111) at 30 mV s − 1 in (a) 20 mM CH3COOH +0.02, 0.1, or 0.3 M HClO4 at pH (I) 0.7 ( — ); (II) 1.1 (---) and (III) 1.9 (···) and (b) 0.2 M acetate solution of CH3COOH+ CH3COOK of pH (I) 5.1 (---), (II) 5.6 (···), and (III) 6.0 ( — ), respectively.
−0.02 to +0.1 V; the onset potential of the main peak shifted slightly positive and the peak became sharp with the increase of K + concentration, resulting in a decrease of the current around 0 V and an increase of peak current around 0.07 V, while no change was observed in the positive side of the main peak beyond + 0.1 V. The effect of cation addition was nearly absent in the small peak region around 0.3 V, although the peak current on the negative sweep was increased slightly by K + addition. We also investigated the addition of Na + to the acetate solution and observed a similar tendency to the case of K + . These different responses to the K + addition may be due to the different adsorption states, as suggested in Ref. [18]. Iwasita et al. measured FTIR spectra of acetate adsorption on Pt(111) in 0.1 M HClO4 + 10 mM CH3COONa and suggested two adsorption states of acetate adsorption at different potentials [18], although the radiotracer method showed a monotonic increase of the acetate coverage in the region of the main CV peak [17].
present time scale of milli-seconds for the specific adsorption/desorption reaction is not likely to include any effect of the bulk dissociation reaction. Figs. 5(a–e) show the j–t curves in different potential regions of the anomalous wave, differing in the respective potential region. When the potential was stepped to more positive potentials for the acetate adsorption, the j– t curve showed two types of feature; the current diminished monotonically in a short period below 50 ms, as shown in Fig. 5(a), while a long tailing of the small current over 200 ms was observed, as shown in Fig. 5(b). In the case of the negative potential step for desorption of the adsorbed acetate species, the j–t curves showed three types; a monotonic current against t is found in Fig. 5(d), and a shoulder seems to be present, as shown in Figs. 5(c and e). In the case of the same potential scale from −0.19 to − 0.11 V of Fig. 5(a), for example, the changes of charge density by potential step and potential sweep of the CV are ca. 34 and 31.5 mC cm − 2,
3.2. Potential step method 3.2.1. The j–t cur6e of acetate adsorption/desorption The j–t curves in the potential step method were measured in a 0.14 M CH3COOK + 0.06 M CH3COOH solution of pH 5.0, where the diffusion effect on the current is negligible because of the high acetate concentration, and the main peak of the CV does not overlap with the hydrogen wave, as shown in the insets of Fig. 5. Although the state of acetate in the solution of pH 5.0 is in the form of CH3COOH or CH3COO − , only CH3COO − adsorption should be considered as we mentioned in the discussion of the pH dependence in Fig. 3(b). Moreover, the dissociation rate of CH3COOH is given as 7.8×105 s − 1 [22],2 and the 2
Recombination rate is 4.5× 1010 M − 1 s − 1.
Fig. 4. The effect of cations on CV at 30 mV s − 1 on Pt(111) in 20 mM CH3COOH +0.1 M HClO4 with (I) 0 mM ( — ); (II) 25 mM (···) and (III) 65 mM (---) KClO4.
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Fig. 5. The j– t curves for acetate adsorption/desorption at Pt(111) in 0.06 M CH3COOH +0.14 M CH3COOK of pH 5.0. Potential steps are indicated in the inset CVs: (a) from − 190 to − 110 mV; (b) from − 20 to +200 mV; (c) from 180 to 40 mV; (d) from 0 to −70 mV, and (e) from − 120 to −200 mV.
respectively, being in relatively good agreement with each other. The j–t curve previously observed in 0.2 M HClO4 showed that the double layer charging was completed within 4 ms or so in our present experimental set-up [13]. It was also confirmed in the presence of CH3COOH on Au(111) in 0.2 M CH3COOH + 0.05 M HClO4. The double layer charging current, measured by a potential step between 150 mV in the double layer region, is within 3 ms. Therefore the j– t relation B 5 ms was omitted in the kinetic analysis in the present work.
3.2.2. Description of current– time relations We assumed a random adsorption model for the adsorption/desorption kinetic analysis [13]. To simplify our discussion, we classified the random adsorption process into two categories with or without interaction between the adsorbates. 3.2.2.1. The Langmuir adsorption model. When the adsorption process is the rate-determining step with no interaction between adsorbates, the adsorption proceeds randomly on physically equivalent sites. This is
described as the Langmuir adsorption model [23]. After the electrode potential is stepped from E0 to E at t=0, the change of the coverage, u, with t is given at a certain E as a function of u as du = ka(1−u)− kdu dt
(1)
where ka and kd are constants at a given E, being functions of electrode potentials, and ka is also a function of acetate concentration in the solution. The differential equation (Eq. (1)) can be solved under the condition that u becomes an equilibrium value ueq when t becomes infinite at E, then the j –t curve is derived by differentiation of the solution of Eq. (1) with respect to t as du = neN(ueq − u0)(ka + kd) exp{−(ka +kd)t}, dt (2) where the e is the elementary electric charge, n the number of electrons transferred in the process, and N the surface excess of acetate at u= 1. Eq. (2) can be rewritten as
j neN
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j A1 exp(−A2t)
(3)
du = −kd exp(bdu), dt
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(5)
where A1 =neN(ueq −u0)(ka +kd) and A2 =(ka +kd). A2 represents the velocity of the adsorption/desorption process. From Eq. (2), a straight line of ln j versus t shows that the anion adsorption process can be described by the Langmuir adsorption model. The slope of ln j versus t gives the value of (ka +kd) and the individual values of ka and kd can be calculated as shown in our previous paper [13], if ueq is obtained. The potential dependence of the rate constants can be expressed by the Butler – Volmer relation; ka and kd, the components of A2 = (ka +kd), are given by
where ka, kd, ba, and bd are constants. Integration of Eq. (4) gives
ka = k %a exp(anFh/RT)
For desorption, the j –t curve from Eq. (5) is
and
j=
kd = k %d exp{− (1−a)nFh/RT}, where a is the electron transfer coefficient and h the overpotential after the potential was stepped and ka and kd depend on E but not E0.
3.2.2.2. The Elo6ich adsorption model. When there is interaction between adsorbates, various treatments have been proposed such as the Frumkin – Temkin adsorption model [24] and the empirical equation of the Elovich type [25]. Although the Frumkin – Temkin model is more acceptable than the Elovich equation as a physico-chemical treatment, a simple mathematical expression was not obtained for the analytical j– t relation, and we chose tentatively the Elovich equation for the present analysis of experimental results. In the case of the Elovich adsorption, the change of u against t is given by a function of u as du = ka exp(−bau) dt and the desorption rate is by
(4)
u=
1 ln{bakat+ exp(bau0)}, ba
(6)
where u0 is coverage at t= 0. Eq. (6) can give the j–t curve for adsorption as j neN
neN bd
du neN = dt ba
1 exp(bau0) t+ baka
1 . exp(bdu0) t+ bdkd
(7)
(8)
When plots of j − 1 versus t give a straight line, we accept that the adsorption is random with repulsive interaction.
3.2.3. Analysis of experimental current–time relations 3.2.3.1. Acetate specific adsorption process by a positi6e potential step. Figs. 6(a and b) show relations of ln j versus t and j − 1 versus t, respectively, for the j–t curves of Figs. 5(a and b), where specific acetate adsorption takes place by a positive potential step from E0 = − 0.19 V to E= −0.11 V (Fig. 5(a)), which is shown by a solid line, and from E0 = − 0.02 to E=0.2 V (Fig. 5(b)), which is shown by a broken line. Fig. 6(a) gives a better straight line in the j–t curve of Fig. 5(a), while Fig. 6(b) does in the j –t curve of Fig. 5(b). Hence, Fig. 6 shows that the acetate adsorption takes place according to the Langmuir adsorption at low coverage in the potential region of Fig. 5(a), and when the coverage u is increased in the potential region of Fig. 5(b), the repulsive interaction appears since the
Fig. 6. The plots of the j –t curve analysis for the acetate adsorption process. Solid line for Fig. 5(a) and broken line for Fig. 5(b). (a) The Langmuir adsorption by ln j vs. t and (b) the Elovich adsorption by j − 1 vs. t.
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Fig. 7. The plots of the j –t curve analysis for the desorption process. (a) The Langmuir desorption by ln j vs. t, (b) the Elovich desorption by j − 1 vs. t. Number in figure shows respective potential region; (I) broken line for potential region of Fig. 5(c); (II) solid line for that of Fig. 5(d); (III) dotted line for that of Fig. 5(e).
Elovich adsorption gives the best fitting. These features are very similar to the case of anion adsorption at Pt(111) in phosphate buffer solution of pH 4 [13].
3.2.3.2. Desorption of the adsorbed acetate by a negati6e potential step. Figs. 7(a and b) show the Langmuir plots of ln j versus t and the Elovich plots of j − 1 versus t, respectively, when the electrode potentials were stepped negatively for acetate desorption. We find that the solid line of Fig. 7(a) is taken to be straight, showing that, at medium coverage in the potential region of Fig. 5(d), the desorption process is described by random desorption without interaction between adsorbates by Eq. (3), and at lower coverage in Fig. 5(e) the desorption mechanism was not interpreted by the random desorption mechanisms of Eqs. (3) and (8) because of the humped j–t curve behavior, since the broken lines of Fig. 7 do not become straight in any plots of random adsorption. In the case of the measurement in 0.2 M CH3COOH+ 0.03 M HClO4 at pH 1.7, the main CV peak shifts negatively as shown in the inset of Fig. 8, and the potential difference between the main large peak and the small peak around 0.24 V increases, and the overlap between these peaks decreases. The j – t curve of Fig. 8 shows a humped feature, showing that the governing mechanism cannot be a random process. The disagreement of the j– t curve feature between adsorption and desorption processes shown in Figs. 5(a, d and e) suggests the presence of different adsorption states of the anion; the change of adsorption state arises after the adsorption takes place as a discharge process. We tried to find the time necessary for the ordered structure formation after the acetate adsorption by the square-pulse potential step method in a 0.06 M CH3COOH + 0.14 M CH3COOK solution. The j–t curves in Figs. 9(b and c) were observed when the potential was stepped from E0 = −0.19 V to E= − 0.11 V and kept for 66 or 8.8 ms at − 0.11 V when acetate adsorption proceeded. Then the potential was
switched to − 0.19 V, where the desorption of the adsorbed acetate occurred. The humped feature on the j–t curves is almost the same in Figs. 9(b and c), although the total charges are changed with the hold time at E0. Therefore, the change of the adsorption state is taken to occur in a shorter time than a few ms after the discharge of acetate of the Langmuir adsorption process.
3.2.3.3. Effect of K + ions on the adsorption process. As the CV feature changed due to the addition of K + in Fig. 4, the effect of K + ions on acetate adsorption at Pt(111) was observed by the potential step method in 0.1 M HClO4 + 20 mM acetate solutions with or without K + ions. Fig. 10(a) shows the ln j vs. t plots of j–t curves for a potential step from E0 = + 0.03 to E= + 0.09 V in 0.1 M HClO4 + 20 mM acetate solutions of pH 1.2 with 0, 25, and 65 mM K + ions, where the respective CVs are shown in the inset of Fig. 10(a).
Fig. 8. The j– t curve for the acetate desorption at Pt(111) in 0.2 M CH3COOH +0.03 M HClO4 solution of pH 1.7.
T. Fukuda, A. Aramata / Journal of Electroanalytical Chemistry 467 (1999) 112–120
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A2, which shows the rate of adsorption, is changed by E but not by the initial potential E0, when E0 varied from 0 to 0.04 V. In Fig. 10(b), the rate of adsorption decreased by 40% in the presence of 25 and 65 mM K + , being nearly independent of the change of K + concentration.
4. Conclusions
Fig. 9. The j – t curves observed by square potential steps between −190 and −110 mV as shown in the CV in (a). The length of the square pulse was changed; the holding time at −110 mV was 66 ms in (b) and 8.8 ms in (c).
The inset of Fig. 10(a) (or Fig. 4(a)) shows that the increase of K + concentrations causes the main peak of the CV to become a sharp feature, resulting in the increase of the peak height around 0.08 V. The plots in Fig. 10(a) are taken to be linear and to indicate that the acetate adsorption takes place according to the Langmuir model irrespective of the presence of K + ions in the solution. However, the slopes of the ln j − t lines in Fig. 10(a), i.e. A2 of Eq. (3), are changed by the addition of K + ions, as shown in Fig. 10(b), where the electrode potential was stepped from E0 =0.03 to E= 0.08, 0.09, 0.1, 0.11 and 0.12 V, and also E0 =0.01 and 0.04 V in the case of the K + free solution. The value of
The study of the adsorption/desorption mechanism of acetate anion at the Pt(111) surface was performed by potential step measurement. The CV consists of the main large peak at ca. − 80 mV versus SCE with a small asymmetric peak around 120 mV positive to the main peak in the case of 0.2 M acetate solution of pH 5. The acetate adsorption takes place by a surface one-electron transfer process and the main CV peak was shifted negatively with the increase of pH by ca. − 60 mV per pH unit at low pH although this pH dependence disappeared at pH\5. The addition of K + or Na + into the solution changed the onset potential of acetate adsorption, implying that the cation in solution gives effects on the acetate adsorption on Pt(111) to some extent even at pHB5. The adsorption/desorption mechanism of acetate on Pt(111) can be summarized as follows. In the adsorption process, a random adsorption process without interaction was observed at low coverage, and at high coverage repulsive interaction appeared. The addition of K + did not change the adsorption mechanisms, although the rate of the acetate adsorption became less on K + addition in the low acetate coverage potential region. In the desorption process, a humped j–t curve was observed at high coverage. In the potential region around the main CV peak, random desorption without interaction was observed, although the desorption process changed again to be humped when the surface coverage became even lower. These features suggest that the change of the adsorption state arises after the acetate adsorption takes place as a random discharge process. Results of the square-pulse potential step measurement suggested that such an ordered structure formation occurs within 10 − 3 s. The present observation of acetate adsorption seems to be supported by the observation of two adsorption states from FTIR measurement by Iwasita et al. [18], which are in conformity with the desorption mechanisms of the Langmuir type at the positive part of the main peak and the humped part at the negative region of the peak, respectively. They also proposed two possible adsorption models in which the two C–Os of acetate are oriented toward the Pt surface since the asymmetric stretching band was absent, as observed on a polycrystalline Pt electrode by Corrigan et al. [19]. They envisaged the formation of dimers or polymeric
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Fig. 10. (a) The effect of K + on the ln j vs. t plots for the acetate adsorption at Pt(111) in 20 mM CH3COOH +0.1 M HClO4 with (I) 0 (—), (II) 25 (···), and (III) 65 mM (---) KClO4. Potential step from E0 = +30 mV to E= + 90 mV was indicated in the inset. (b) Potential dependence of the current decay coefficient of the Langmuir fitting, A2: 0; 25 and á 65 mM K + , when the initial potential E0 =30 mV. In the case of K + free solution, A2 values are also shown for E0 = 0 mV and E0 =40 mV. A2 values are derived from fitting of the experimental curve to j =A1 exp(− A2t).
chains of adsorbates, which have C26 symmetry with the two oxygen atoms of acetate oriented toward the metal surface.
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[11] S. Thomas, Y.E. Sung, H.S. Kim, A. Wieckowski, J. Phys. Chem. 100 (1996) 11726. [12] Th. Dretschkow, Th. Wandlowski, Ber. Bunsenges. Phys. Chem. 101 (1997) 749. [13] T. Fukuda, A. Aramata, J. Electroanal. Chem. 440 (1997) 153. [14] J.M. Feliu, J. Valls, A. Aldaz, M.A. Climent, J. Clavilier, J. Electroanal. Chem. 345 (1993) 475. [15] J.M. Orts, R. Go´mez, J.M. Feliu, A. Aldaz, J. Clavilier, Electrochim. Acta 39 (1994) 1519. [16] S. Taguchi, A. Aramata, J. Electroanal. Chem. 457 (1998) 73. [17] L.M. Rice, E.K. Krauskopf, A. Wieckowski, J. Electroanal. Chem. 239 (1988) 413. [18] A. Rodes, E. Pastor, T. Iwasita, J. Electroanal. Chem. 376 (1994) 109. [19] D.S. Corrigan, E.K. Krauskopf, L.M. Rice, A. Wieckowski, M.J. Weaver, J. Phys. Chem. 92 (1988) 1596. [20] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [21] M.A. Climent, M.J. Valls, J.M. Feliu, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 326 (1992) 113. [22] M. Eigen, J. Schoen, Z. Phys. Chem. 3 (1955) 126. [23] I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361. [24] M. Temkin (English translation by D.M. Mohilner), Zhur. Fiz. Khim. 15 (1941) 296. [25] C. Aharoni, F.C. Tompkins, Adv. Catal. 21 (1970) 1.
The study of the adsorption/desorption of acetate anions on a Pt(111) electrode and the effect of counter cations in acidic media Toshihiro Fukuda, Akiko Aramata * Catalysis Research Center and Graduate School of En6ironmental Earth Science, Hokkaido Uni6ersity, Sapporo 060 -0811, Japan Received 29 July 1998; received in revised form 9 March 1999; accepted 13 April 1999
Abstract The mechanism and kinetics of specific adsorption/desorption of acetate were studied at Pt(111) in acidic solution by the potential step method and also by cyclic voltammetry. A so-called anomalous wave in the cyclic voltammogram appeared with the addition of acetate ions into perchloric acid solution, and was shifted by − 60 mV with a 10-fold change of the acetate concentration. Such behavior shows that the anomalous wave is due to acetate ion adsorption/desorption by a one-electron transfer process. The acetate adsorption/desorption of the anomalous wave shifted with the increase of pH by − 60 mV per pH unit in low pH solutions at pH B2 with the effect of cations, whereas the pH dependence of the anomalous wave disappeared at pH \5. These pH dependent tendencies are discussed as that the adsorbed form of acetate is present on Pt(111) electrode over the pH range studied. The current–time ( j–t) curve of acetate adsorption/desorption showed different decay features in the adsorption and desorption directions and in different potential regions. In the case of a 0.2 M acetate solution of pH 5.0, random adsorption without interaction, analyzed as the Langmuir adsorption, took place at lower coverage, while random adsorption with repulsive interaction was observed at higher coverage. In contrast to adsorption, the desorption process did not take place by monotonic decay of the j–t curve at high coverage, and the desorption mechanism changed into random desorption without interaction in the medium coverage potential region. When the surface coverage was even lower, a humped j –t curve was observed again. Square-pulse potential step methods revealed the protuberant j–t curve for desorption of the adsorbed acetate within a few ms after the discharge process takes place by a random adsorption reaction. The addition of K + decreased the rate of acetate adsorption by 40% in the low coverage potential region. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Adsorption; Desorption; Pt(111) electrodes; Acetate anions
1. Introduction Electrochemical reaction is affected by the electric double layer at the electrode electrolyte interface in general, and often severely by anion specific adsorption at the interface. Models of the electric double layer including specific adsorption were proposed [1]; the double layer structure consisted of the outer and inner Helmholtz planes together with the Gouy – Chapman Dedicated to Dr Jean Clavilier on the occasion of his retirement from LEI CNRS and his contribution to Interfacial Electrochemistry. * Corresponding author. Fax: +81-11-7094748. E-mail address: [email protected] (A. Aramata)
diffuse layer. Electrochemical measurements with insitu and ex-situ techniques on noble metal single crystals have expanded our knowledge on the structure of the electrode interface and anion specific adsorption. In the case of Pt(111) in sulfuric acid solution, studies of cyclic voltammetry (CV) [2,3], chronocoulometry [4], radiotracer methods [5,6], and the technique of CO adsorption replacement [7] have revealed that the ‘anomalous wave’ of CV is due to an electron transfer process in the formation/desorption of the adsorbed sulfate species. On the other hand, in-situ FTIR [8,9] and STM [10], and ex-situ LEED, AES, and CEELS measurements [11] revealed the adsorption state, twodimensional structure, and components of these adsor-
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 1 6 6 - 7
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bates. In-situ STM has made it possible to observe the adsorbed layer directly, and shows that the adsorbed sulfate species forms an ordered structure of ( 3 × 7) with co-adsorbed H3O + or H2O at potentials more positive than the sharp spike of CV [10]. In view of the kinetics of anion specific adsorption, Dretschkow and Wandlowski reported the correlation of the formation of the ordered structure of the adsorbed sulfate with the length of terraces on Au(111) and on stepped surfaces around Au(111) in comparison with STM and potential step methods [12]. We have reported recently the kinetic study of the specific adsorption of phosphate anions on Pt(111) in acidic media by the potential step method [13]. The mechanism of phosphate adsorption processes was explained by random adsorption models with and without lateral interaction according to the phosphate surface coverage; the former random adsorption takes place at low coverage, and the latter random adsorption with lateral repulsive force was observed at high coverage. The feature of phosphate adsorption depends on pH; with the decrease of pH, the onset potential of phosphate adsorption shifts to more positive potentials and the rate constant of phosphate adsorption/desorption becomes greater. These pH dependences seem to be correlated with the relationship between the counter cation and the anion adlayer, since Clavilier et al. observed quite large cation effects in neutral solutions [14]. The acetate adsorption was observed by the addition of acetate to perchloric acid solution, and was nearly in the same potential region as the sulfate adsorption on Pt(111), as is confirmed by the CO displacement method, and the electron-transfer number was observed as ca. 1 [15]. Meanwhile, the adsorbed anion in perchloric acid solution was assigned to be an adsorbed hydroxyl species from the feature of its pH dependence [16]. In the present report, we report the kinetic study of specific adsorption/ desorption processes of acetate anion and the effect of the counter cation on a Pt(111) electrode by voltammetry and potential step methods in acetate solutions in the presence of K + and Na + . We chose the study of acetate adsorption in addition to phosphate adsorption since (a) acetate anion is in a single ionic state of CH3COO − in the solution which makes the discussion simpler, whereas many ionic states of phosphate ions, H2PO4− , HPO24 − and PO34 − , must be considered with the pH change, and (b) the similarity of the voltammetric features between the cases of acetate and phosphate adsorption relates the adsorption mechanism and cation effect in both solutions. Previous studies on the acetate adsorption at Pt(111), not only by cyclic voltammetry, but also by the radiotracer method [17], and FTIR measurement [18] give information about the potential dependence of surface excess and adsorption states. FTIR observation of adsorbed acetate on polycrystalline Pt and Au was reported in 1988 [19].
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2. Experimental A single crystal Pt(111) of area 0.38 cm2, prepared by the Clavilier method [20], was used as the working electrode, and Au(111) obtained by the same method was used as a reference for the evaluation of the duration of double layer charging-up in the absence of specific adsorption. Solutions were prepared with CH3COOK (Fluka Biochemica Microselect), CH3COOH (Merck Suprapur), HClO4 (Merck Suprapur), KClO4 (Merck Pro analysi), and NaClO4 (Wako reagent grade monohydrate), respectively. Milli-Q water was used throughout. A two-compartment cell was used. Pt mesh or Au foil as the counter electrode was put around the working electrode in the working electrode compartment. The reference electrode in the reference electrode compartment was a saturated calomel electrode (SCE) and it was electrically connected to the working electrode compartment through a closed glass stopcock and a Luggin capillary. Solutions were deaerated by ultrapure Ar prior to measurements and the measurements were done at room temperature with Ar gas flow over the solution. The measurement procedure of the potential step method was as follows [13]. After the cell was confirmed to be free from the pollution by checking the CV, the electrode potential was swept to the initial potential E0 and kept for 30 s. Then, a potential E was applied to the electrode, when the current density j was measured as a function of time t. The CVs and current-time ( j–t) relations observed were recorded on an EG&G potentiostat M263 with a 12 bit precision. The sampling rate was 110 ms in the potential step method.
3. Results and discussion
3.1. Cyclic 6oltammetry Fig. 1 shows the CVs at Pt(111) in a 0.2 M HClO4 solution with 0, 1, 3, and 11 mM CH3COOH at 20 mV s − 1. The addition of acetate anions gave the disappearance of the characteristic CV on Pt(111) in HClO4 around 0.3 to 0.55 V (SCE) and yielded a main large peak around 0.1–0.2 V which is accompanied by a small peak around 0.35–0.38 V. These features are in good agreement with those in Ref. [15]. The latter peak is irreversible in the positive and negative potential sweeps. These CV features are very similar to those in phosphate solutions [13,20,21], although the current density of the small peak around 0.35 V in acetate solution is larger than that in phosphate solution. The charge density of the main peak in a 1 mM CH3COOH+0.2 M HClO4 solution of Fig. 1 is ca. 111 mC cm − 2 from 0.08V to 0.46 V without subtracting the double layer current, being slightly larger than the ca.
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Fig. 1. Cyclic voltammograms at 20 mV s − 1 on Pt(111) in 0.2 M HClO4 at pH 0.8 with 0 mM (-·-·); (I) 1 mM (---); (II) 3 mM (···) and (III) 11 mM ( — ) CH3COOH.
99 mC cm − 2 of the corresponding wave in 20 mM H3PO4 +0.2 M HClO4 [13]. The main peak around 0.1 V shifted in the negative potential direction by 60 mV for a 10-fold increase of acetate concentration, which suggests from the Nernst equation that the peak can be considered as a one-electron transfer process of acetate adsorption/desorption. The small peak around 0.35 V also shifted in the negative potential direction with the increase of acetate concentration, although the shift of potentials was smaller than the shift of the main peak. Fig. 2 shows the capacitance vs. potential curves at 30, 60 and 300 mV s − 1 in a 20 mM CH3COOH+ 0.1 M HClO4 solution, where the capacitance in the ordinate is the current density divided by the sweep rate. An effect of the sweep rate appeared on the small peak around 0.3 V, but not on the main peak. The small peak shifted in the negative potential direction, and the peak height increased with the increase of the sweep
Fig. 2. Cyclic voltammograms on Pt(111) in 0.1 M HClO4 + 21 mM CH3COOH of pH 1.1 at (I) 30 (---); (II) 60 (···) and (III) 300 mV s − 1 ( —).
rate on the negative sweep, while the main peak of capacitance around 0.07 V is independent of the sweep rate. This suggests that the adsorption or desorption mechanisms are essentially different in the potential regions between the main peak and the small peak of the anomalous wave. Fig. 3(a) shows the pH dependence of CVs in 20 mM CH3COOH+0.02, 0.1, and 0.3 M HClO4 solutions of pH 1.9, 1.1 and 0.7, respectively. The main peaks shifted to the negative potential direction with the pH increase by −60 mV per pH unit in parallel with hydrogen adsorption/desorption waves. Such pH dependence of the anomalous wave is similar but not identical to the case of Pt(111) in phosphate solutions[16]. The small peak around 0.3 V also shifts with pH, although the shift is smaller than that of the main peak1. These pH effects on CV suggest that the H + cation takes part in the reaction of the adsorption/desorption of acetate anions in low pH solutions. With the acetic acid dissociation constant of 1.74×10 − 5 at 25°C, the acetate is in the form of CH3COOH at pHB 2.0, and therefore a reaction CH3COOHCH3COOads + H + + e − is suggested, which is in conformity with the result of Ref. [15]. At pH\5, the pH dependence on the main peak disappeared as shown Fig. 3(b), suggesting that the adsorption/desorption process proceeds as CH3COO − CH3COOads + e − Figs. 3(a and b) show that the adsorbed state of acetate is in the form of CH3COOads over the pH range studied. The pH effect on the small peak also disappeared at pH\ 5. This behavior means that the small peak is not due to simple OH − adsorption in acetate solution at pH\5. Although the indirect bonding of acetate through adsorbed water was suggested for the case on polycrystalline Pt from FTIR results in 1988 [19], the observation on a single crystal Pt suggested direct bonding of adsorbed acetate in 1994 [14]. Since acetate specific adsorption/desorption is taken to be a surface electron-transfer process, the direct bonding to the Pt substrate is likely to be the case, as discussed in Refs. [15,18]. We investigated the effect of addition of K + ions on the CV, as shown in Fig. 4, since Clavilier et al. reported the different features of CV of Pt(111) in phosphate buffer solution by the change of cation at pH 7–9 [21]. Fig. 4 shows the CVs in 20 mM CH3COOH+0.1 M HClO4 with 0, 25, and 60 mM KClO4. Although no remarkable change with cation addition was observed, the addition of K + gave the change in the negative side of the main peak from 1 In the case of sulfate adsorption, the pH effect on the main and the small peak is different. This difference is yet to be clarified.
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Fig. 3. The effect of pH on CV of Pt(111) at 30 mV s − 1 in (a) 20 mM CH3COOH +0.02, 0.1, or 0.3 M HClO4 at pH (I) 0.7 ( — ); (II) 1.1 (---) and (III) 1.9 (···) and (b) 0.2 M acetate solution of CH3COOH+ CH3COOK of pH (I) 5.1 (---), (II) 5.6 (···), and (III) 6.0 ( — ), respectively.
−0.02 to +0.1 V; the onset potential of the main peak shifted slightly positive and the peak became sharp with the increase of K + concentration, resulting in a decrease of the current around 0 V and an increase of peak current around 0.07 V, while no change was observed in the positive side of the main peak beyond + 0.1 V. The effect of cation addition was nearly absent in the small peak region around 0.3 V, although the peak current on the negative sweep was increased slightly by K + addition. We also investigated the addition of Na + to the acetate solution and observed a similar tendency to the case of K + . These different responses to the K + addition may be due to the different adsorption states, as suggested in Ref. [18]. Iwasita et al. measured FTIR spectra of acetate adsorption on Pt(111) in 0.1 M HClO4 + 10 mM CH3COONa and suggested two adsorption states of acetate adsorption at different potentials [18], although the radiotracer method showed a monotonic increase of the acetate coverage in the region of the main CV peak [17].
present time scale of milli-seconds for the specific adsorption/desorption reaction is not likely to include any effect of the bulk dissociation reaction. Figs. 5(a–e) show the j–t curves in different potential regions of the anomalous wave, differing in the respective potential region. When the potential was stepped to more positive potentials for the acetate adsorption, the j– t curve showed two types of feature; the current diminished monotonically in a short period below 50 ms, as shown in Fig. 5(a), while a long tailing of the small current over 200 ms was observed, as shown in Fig. 5(b). In the case of the negative potential step for desorption of the adsorbed acetate species, the j–t curves showed three types; a monotonic current against t is found in Fig. 5(d), and a shoulder seems to be present, as shown in Figs. 5(c and e). In the case of the same potential scale from −0.19 to − 0.11 V of Fig. 5(a), for example, the changes of charge density by potential step and potential sweep of the CV are ca. 34 and 31.5 mC cm − 2,
3.2. Potential step method 3.2.1. The j–t cur6e of acetate adsorption/desorption The j–t curves in the potential step method were measured in a 0.14 M CH3COOK + 0.06 M CH3COOH solution of pH 5.0, where the diffusion effect on the current is negligible because of the high acetate concentration, and the main peak of the CV does not overlap with the hydrogen wave, as shown in the insets of Fig. 5. Although the state of acetate in the solution of pH 5.0 is in the form of CH3COOH or CH3COO − , only CH3COO − adsorption should be considered as we mentioned in the discussion of the pH dependence in Fig. 3(b). Moreover, the dissociation rate of CH3COOH is given as 7.8×105 s − 1 [22],2 and the 2
Recombination rate is 4.5× 1010 M − 1 s − 1.
Fig. 4. The effect of cations on CV at 30 mV s − 1 on Pt(111) in 20 mM CH3COOH +0.1 M HClO4 with (I) 0 mM ( — ); (II) 25 mM (···) and (III) 65 mM (---) KClO4.
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Fig. 5. The j– t curves for acetate adsorption/desorption at Pt(111) in 0.06 M CH3COOH +0.14 M CH3COOK of pH 5.0. Potential steps are indicated in the inset CVs: (a) from − 190 to − 110 mV; (b) from − 20 to +200 mV; (c) from 180 to 40 mV; (d) from 0 to −70 mV, and (e) from − 120 to −200 mV.
respectively, being in relatively good agreement with each other. The j–t curve previously observed in 0.2 M HClO4 showed that the double layer charging was completed within 4 ms or so in our present experimental set-up [13]. It was also confirmed in the presence of CH3COOH on Au(111) in 0.2 M CH3COOH + 0.05 M HClO4. The double layer charging current, measured by a potential step between 150 mV in the double layer region, is within 3 ms. Therefore the j– t relation B 5 ms was omitted in the kinetic analysis in the present work.
3.2.2. Description of current– time relations We assumed a random adsorption model for the adsorption/desorption kinetic analysis [13]. To simplify our discussion, we classified the random adsorption process into two categories with or without interaction between the adsorbates. 3.2.2.1. The Langmuir adsorption model. When the adsorption process is the rate-determining step with no interaction between adsorbates, the adsorption proceeds randomly on physically equivalent sites. This is
described as the Langmuir adsorption model [23]. After the electrode potential is stepped from E0 to E at t=0, the change of the coverage, u, with t is given at a certain E as a function of u as du = ka(1−u)− kdu dt
(1)
where ka and kd are constants at a given E, being functions of electrode potentials, and ka is also a function of acetate concentration in the solution. The differential equation (Eq. (1)) can be solved under the condition that u becomes an equilibrium value ueq when t becomes infinite at E, then the j –t curve is derived by differentiation of the solution of Eq. (1) with respect to t as du = neN(ueq − u0)(ka + kd) exp{−(ka +kd)t}, dt (2) where the e is the elementary electric charge, n the number of electrons transferred in the process, and N the surface excess of acetate at u= 1. Eq. (2) can be rewritten as
j neN
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j A1 exp(−A2t)
(3)
du = −kd exp(bdu), dt
117
(5)
where A1 =neN(ueq −u0)(ka +kd) and A2 =(ka +kd). A2 represents the velocity of the adsorption/desorption process. From Eq. (2), a straight line of ln j versus t shows that the anion adsorption process can be described by the Langmuir adsorption model. The slope of ln j versus t gives the value of (ka +kd) and the individual values of ka and kd can be calculated as shown in our previous paper [13], if ueq is obtained. The potential dependence of the rate constants can be expressed by the Butler – Volmer relation; ka and kd, the components of A2 = (ka +kd), are given by
where ka, kd, ba, and bd are constants. Integration of Eq. (4) gives
ka = k %a exp(anFh/RT)
For desorption, the j –t curve from Eq. (5) is
and
j=
kd = k %d exp{− (1−a)nFh/RT}, where a is the electron transfer coefficient and h the overpotential after the potential was stepped and ka and kd depend on E but not E0.
3.2.2.2. The Elo6ich adsorption model. When there is interaction between adsorbates, various treatments have been proposed such as the Frumkin – Temkin adsorption model [24] and the empirical equation of the Elovich type [25]. Although the Frumkin – Temkin model is more acceptable than the Elovich equation as a physico-chemical treatment, a simple mathematical expression was not obtained for the analytical j– t relation, and we chose tentatively the Elovich equation for the present analysis of experimental results. In the case of the Elovich adsorption, the change of u against t is given by a function of u as du = ka exp(−bau) dt and the desorption rate is by
(4)
u=
1 ln{bakat+ exp(bau0)}, ba
(6)
where u0 is coverage at t= 0. Eq. (6) can give the j–t curve for adsorption as j neN
neN bd
du neN = dt ba
1 exp(bau0) t+ baka
1 . exp(bdu0) t+ bdkd
(7)
(8)
When plots of j − 1 versus t give a straight line, we accept that the adsorption is random with repulsive interaction.
3.2.3. Analysis of experimental current–time relations 3.2.3.1. Acetate specific adsorption process by a positi6e potential step. Figs. 6(a and b) show relations of ln j versus t and j − 1 versus t, respectively, for the j–t curves of Figs. 5(a and b), where specific acetate adsorption takes place by a positive potential step from E0 = − 0.19 V to E= −0.11 V (Fig. 5(a)), which is shown by a solid line, and from E0 = − 0.02 to E=0.2 V (Fig. 5(b)), which is shown by a broken line. Fig. 6(a) gives a better straight line in the j–t curve of Fig. 5(a), while Fig. 6(b) does in the j –t curve of Fig. 5(b). Hence, Fig. 6 shows that the acetate adsorption takes place according to the Langmuir adsorption at low coverage in the potential region of Fig. 5(a), and when the coverage u is increased in the potential region of Fig. 5(b), the repulsive interaction appears since the
Fig. 6. The plots of the j –t curve analysis for the acetate adsorption process. Solid line for Fig. 5(a) and broken line for Fig. 5(b). (a) The Langmuir adsorption by ln j vs. t and (b) the Elovich adsorption by j − 1 vs. t.
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Fig. 7. The plots of the j –t curve analysis for the desorption process. (a) The Langmuir desorption by ln j vs. t, (b) the Elovich desorption by j − 1 vs. t. Number in figure shows respective potential region; (I) broken line for potential region of Fig. 5(c); (II) solid line for that of Fig. 5(d); (III) dotted line for that of Fig. 5(e).
Elovich adsorption gives the best fitting. These features are very similar to the case of anion adsorption at Pt(111) in phosphate buffer solution of pH 4 [13].
3.2.3.2. Desorption of the adsorbed acetate by a negati6e potential step. Figs. 7(a and b) show the Langmuir plots of ln j versus t and the Elovich plots of j − 1 versus t, respectively, when the electrode potentials were stepped negatively for acetate desorption. We find that the solid line of Fig. 7(a) is taken to be straight, showing that, at medium coverage in the potential region of Fig. 5(d), the desorption process is described by random desorption without interaction between adsorbates by Eq. (3), and at lower coverage in Fig. 5(e) the desorption mechanism was not interpreted by the random desorption mechanisms of Eqs. (3) and (8) because of the humped j–t curve behavior, since the broken lines of Fig. 7 do not become straight in any plots of random adsorption. In the case of the measurement in 0.2 M CH3COOH+ 0.03 M HClO4 at pH 1.7, the main CV peak shifts negatively as shown in the inset of Fig. 8, and the potential difference between the main large peak and the small peak around 0.24 V increases, and the overlap between these peaks decreases. The j – t curve of Fig. 8 shows a humped feature, showing that the governing mechanism cannot be a random process. The disagreement of the j– t curve feature between adsorption and desorption processes shown in Figs. 5(a, d and e) suggests the presence of different adsorption states of the anion; the change of adsorption state arises after the adsorption takes place as a discharge process. We tried to find the time necessary for the ordered structure formation after the acetate adsorption by the square-pulse potential step method in a 0.06 M CH3COOH + 0.14 M CH3COOK solution. The j–t curves in Figs. 9(b and c) were observed when the potential was stepped from E0 = −0.19 V to E= − 0.11 V and kept for 66 or 8.8 ms at − 0.11 V when acetate adsorption proceeded. Then the potential was
switched to − 0.19 V, where the desorption of the adsorbed acetate occurred. The humped feature on the j–t curves is almost the same in Figs. 9(b and c), although the total charges are changed with the hold time at E0. Therefore, the change of the adsorption state is taken to occur in a shorter time than a few ms after the discharge of acetate of the Langmuir adsorption process.
3.2.3.3. Effect of K + ions on the adsorption process. As the CV feature changed due to the addition of K + in Fig. 4, the effect of K + ions on acetate adsorption at Pt(111) was observed by the potential step method in 0.1 M HClO4 + 20 mM acetate solutions with or without K + ions. Fig. 10(a) shows the ln j vs. t plots of j–t curves for a potential step from E0 = + 0.03 to E= + 0.09 V in 0.1 M HClO4 + 20 mM acetate solutions of pH 1.2 with 0, 25, and 65 mM K + ions, where the respective CVs are shown in the inset of Fig. 10(a).
Fig. 8. The j– t curve for the acetate desorption at Pt(111) in 0.2 M CH3COOH +0.03 M HClO4 solution of pH 1.7.
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A2, which shows the rate of adsorption, is changed by E but not by the initial potential E0, when E0 varied from 0 to 0.04 V. In Fig. 10(b), the rate of adsorption decreased by 40% in the presence of 25 and 65 mM K + , being nearly independent of the change of K + concentration.
4. Conclusions
Fig. 9. The j – t curves observed by square potential steps between −190 and −110 mV as shown in the CV in (a). The length of the square pulse was changed; the holding time at −110 mV was 66 ms in (b) and 8.8 ms in (c).
The inset of Fig. 10(a) (or Fig. 4(a)) shows that the increase of K + concentrations causes the main peak of the CV to become a sharp feature, resulting in the increase of the peak height around 0.08 V. The plots in Fig. 10(a) are taken to be linear and to indicate that the acetate adsorption takes place according to the Langmuir model irrespective of the presence of K + ions in the solution. However, the slopes of the ln j − t lines in Fig. 10(a), i.e. A2 of Eq. (3), are changed by the addition of K + ions, as shown in Fig. 10(b), where the electrode potential was stepped from E0 =0.03 to E= 0.08, 0.09, 0.1, 0.11 and 0.12 V, and also E0 =0.01 and 0.04 V in the case of the K + free solution. The value of
The study of the adsorption/desorption mechanism of acetate anion at the Pt(111) surface was performed by potential step measurement. The CV consists of the main large peak at ca. − 80 mV versus SCE with a small asymmetric peak around 120 mV positive to the main peak in the case of 0.2 M acetate solution of pH 5. The acetate adsorption takes place by a surface one-electron transfer process and the main CV peak was shifted negatively with the increase of pH by ca. − 60 mV per pH unit at low pH although this pH dependence disappeared at pH\5. The addition of K + or Na + into the solution changed the onset potential of acetate adsorption, implying that the cation in solution gives effects on the acetate adsorption on Pt(111) to some extent even at pHB5. The adsorption/desorption mechanism of acetate on Pt(111) can be summarized as follows. In the adsorption process, a random adsorption process without interaction was observed at low coverage, and at high coverage repulsive interaction appeared. The addition of K + did not change the adsorption mechanisms, although the rate of the acetate adsorption became less on K + addition in the low acetate coverage potential region. In the desorption process, a humped j–t curve was observed at high coverage. In the potential region around the main CV peak, random desorption without interaction was observed, although the desorption process changed again to be humped when the surface coverage became even lower. These features suggest that the change of the adsorption state arises after the acetate adsorption takes place as a random discharge process. Results of the square-pulse potential step measurement suggested that such an ordered structure formation occurs within 10 − 3 s. The present observation of acetate adsorption seems to be supported by the observation of two adsorption states from FTIR measurement by Iwasita et al. [18], which are in conformity with the desorption mechanisms of the Langmuir type at the positive part of the main peak and the humped part at the negative region of the peak, respectively. They also proposed two possible adsorption models in which the two C–Os of acetate are oriented toward the Pt surface since the asymmetric stretching band was absent, as observed on a polycrystalline Pt electrode by Corrigan et al. [19]. They envisaged the formation of dimers or polymeric
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Fig. 10. (a) The effect of K + on the ln j vs. t plots for the acetate adsorption at Pt(111) in 20 mM CH3COOH +0.1 M HClO4 with (I) 0 (—), (II) 25 (···), and (III) 65 mM (---) KClO4. Potential step from E0 = +30 mV to E= + 90 mV was indicated in the inset. (b) Potential dependence of the current decay coefficient of the Langmuir fitting, A2: 0; 25 and á 65 mM K + , when the initial potential E0 =30 mV. In the case of K + free solution, A2 values are also shown for E0 = 0 mV and E0 =40 mV. A2 values are derived from fitting of the experimental curve to j =A1 exp(− A2t).
chains of adsorbates, which have C26 symmetry with the two oxygen atoms of acetate oriented toward the metal surface.
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