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Influence of acetamidine on the electrosorption of UPD H at Pt single-crystal surfaces
Journal of Electroanalytical Chemistry 623 (2008) 102–108
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Influence of acetamidine on the electrosorption of UPD H at Pt single-crystal surfaces Brian E. Conway 1, Boguslaw Pierozynski * Chemistry Department, University of Ottawa, 10 Marie Curie Street, Ottawa, Ontario, Canada K1N 6N5
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Article history: Received 19 February 2008 Received in revised form 5 June 2008 Accepted 25 June 2008 Available online 4 July 2008 Keywords: Electrosorption Acetamidine ions H UPD Pt single-crystal surfaces
a b s t r a c t In our previously published paper we showed how guanidonium (G+) ion causes substantial, surface-specific displacements of the H underpotential deposition (UPD) voltammetric current profiles, at Pt(1 1 1), (1 0 0), (1 1 0) and (5 1 1) stepped surfaces, to less positive potentials. This behaviour was attributed to ion-pairing between adsorbed ions of the electrolyte (HSO4 , ClO4 or OH ) and the resonant G+ cation. Comparatively, the adsorption behaviour of N,N-dimethylguanidonium (DMG+) cation at Pt(1 1 1) surface, in contact with 0.5 M H2SO4, was also reported. The present work reports cyclic voltammetric results of comparative experiments, at the same (hkl) surfaces of Pt, in the presence of acetamidine (AA) molecule having structure related to that of guanidine (G). Acetamidine cations cause similar displacements of the voltammetric profiles for underpotential deposition and desorption of H as does G+, in ways characteristic of the Pt surface geometry. However, in some cases recorded voltammetric profiles are unique to acetamidine and as such they reflect differences between the resonant cations’ structures of G+ and AA+. In addition, current results strongly support the previously proposed mechanism of cation/anion interaction in the double-layer interphase. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction In previous work from this laboratory [1,2] we examined the electrosorption behaviour of guanidine (G), at single-crystal surfaces of Pt. This molecule contains an imine-type bond and in acidic solutions it becomes converted to the corresponding onium-type ion, in which the cation structure is stabilized by resonance between three canonical forms. The adsorption of resonant G+-onium cations was shown [1] to have major and surface-specific effects on the UPD of H at Pt single-crystal surfaces. However, no reactive chemisorption of the type previously found with nitriles [3,4] was observable. It was suggested in Ref. [1] that the voltammetric effects be associated with stabilization of anion (e.g. HSO4 , ClO4 ) adsorption through ion-pairing between the G+ cations and HSO4 or ClO4 4 anions, in the double-layer interphase. The overall effect was referred to as ‘‘anion mimetic”, since the resulting behaviour was similar to that caused by adsorption of anions, such as: Cl , Br or I (cf. Ref. [5]) at Pt, yet it was brought about by the resonant cation.
* Corresponding author. Address: Department of Chemistry, Faculty of Environmental Protection and Agriculture, University of Warmia and Mazury in Olsztyn, Plac Lodzki 4, 10-757 Olsztyn, Poland. Tel.: +48 89 523 4177; fax: +48 89 523 4801. E-mail addresses: [email protected], [email protected] (B. Pierozynski). 1 Deceased. 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2008.06.025
In the light of the previous work outlined above, the purpose of the present paper is to investigate how the surface structure of Pt single-crystals (and possibly anion of the supporting electrolyte) could influence the adsorption behaviour of two closely structure-related molecules, namely: guanidine (G) and acetamidine (AA). Acetamidine is an adsorbate related to both G and DMG, where one of the –NH2 groups on G is substituted by –CH3, thus partially breaking the resonance in the onium cation resulting from protonation. In G+ resonant cation, the C to N bond-order is formally 1.33 while in AA+ it is 1.5. Thus, it could be assumed (the pKa value for acetamidine appears unavailable) that the extent of AA protonation is somewhat limited, as compared to guanidine. It is also our intention to re-evaluate the proposed in Ref. [1] ion-pairing mechanism, in application to the effects of adsorption of the AA+ ion, in aqueous H2SO4, HClO4 and NaOH. It is expected that substitution of the +NH2 group with the uncharged CH3 group should significantly weaken the strength of ion-pairing between adsorbed acetamidine and e.g. bisulphate species. Conversely, considerably weakened ion-pairing (also dependent on the anion of the supporting electrolyte) should be reflected in less significant influence of AA+ on the electrosorption of UPD H and electrolyte anion, as compared to G+. Thus, in this paper we examine how the AA+ cation affects the cyclic voltammetry profiles for UPD of H at several single-crystal surfaces of Pt and how these effects compare with those previously observed for G+ (at comparable concentrations of the adsorbates).
B.E. Conway, B. Pierozynski / Journal of Electroanalytical Chemistry 623 (2008) 102–108
For convenience of the reader we display below in Fig. 1 the molecular structure of the AA+ cation, in relation to that for G+, emphasising the difference in possible canonical forms. 2. Experimental 2.1. Instrumental procedures Cyclic voltammetry experiments were conducted at 295 K, using an HA-501 potentiostat/galvanostat and an HB-104 function generator (Hokuto Denko). Voltammograms were recorded and collected on a Nicolet-310 digital oscilloscope. The voltammetric polarisations were conducted over controlled ranges of the working-electrode potential, usually in the H UPD regions or at more positive potentials, at the Pt electrode surfaces and at a sweep-rate, s, of 50 mV s 1 (cf. Ref. [1]). Experiments in which s was varied indicated a negligible role of diffusion processes in the response currents (see later in the text). All potential measurements or settings were referred to the potential of a hydrogen electrode in the same solution, in a three-compartment cell, in the usual way. The voltammograms resulting from these experiments were excellently reproducible from one solution to another of a given composition and concentration. 2.2. Procedures for preparation of well-ordered Pt single-crystal surfaces
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conditions [4,8]. Aqueous 0.5 M H2SO4 and 0.1 M HClO4 solutions were made up in the Milli-Q water from sulphuric and perchloric acids of the highest purity available (Sea Star Chemicals). Alkaline solutions were made up from AESAR, 99.996% NaOH pellets. All solutions were initially de-aerated with O2- free, high purity argon. Other details were the same as described in Ref. [1]. Acetamidine was only available in the form of its hydrochloride salt (Fluka, P97%). Therefore, in order to enable electrochemical studies in the absence of the otherwise strongly adsorbing Cl ion, its chemical form was changed to acetamidine sulphate by means of an exhaustive ion-exchange procedure employing a strongly basic, Dowex-1-chloride (50–100 mesh), ion-exchange resin (Aldrich). The resin was first converted to its sulphate form, until no turbidity could be detected in a nephelometric test with AgNO3. Dilute solutions of AA+ were made up in the above supporting electrolytes, at concentrations about 6 10 5 to 3 10 3 M. 3. Discussion of results 3.1. Adsorption behaviour of G+ and AA+ ions at polycrystalline Pt The adsorption behaviour of the guanidonium cation, at three concentrations of G+ in 0.5 M H2SO4, at a Pt poly-oriented singlecrystal, which exhibits mainly (1 1 0) facets and (1 0 0) (1 1 1) step sites, is illustrated by a series of voltammograms shown in Fig. 2a. A major effect is revealed as a shift of the current-potential profile
The techniques for preparation of Pt single-crystals, developed by Clavilier and Pineau [6], were employed. Single-crystal electrodes were made from 1 mm diameter, 99.9985% Pt wire (AESAR/Puratronic), which was melted at one end into a sphere ca. 3 mm in diameter. The resulting single-crystal sphere was oriented for cutting to the desired crystal face using the back von Laue X-ray diffraction method [7]. Other details of the procedure were reported in Ref. [1]. The principal investigations were addressed to the (1 1 1) surface; however, because of the importance of evaluating to what extent the adsorption effects with AA (in relation to those observed with G) were dependent on surface lattice geometry, comparative experiments were also conducted at (1 0 0), (1 1 0) and the (5 1 1) [i.e. 3(1 0 0) (1 1 1)] stepped planes. The (1 1 1) face and the polyoriented Pt sphere were flame annealed and cooled in air [6]. Other surfaces were prepared by flame annealing, followed by cooling in an H2 + Ar mixture [8]. The accuracy of single-crystal preparation was checked by comparison with the voltammograms of Clavilier et al. [9]; the agreements were excellent, as in previous work [4] from this laboratory. 2.3. Solutes and the water solvent All solutions were prepared from 18.2 MX cm Milli-Q ultrapure water (Millipore), as in other works involving high-purity
Fig. 1. Chemical structures of protonated forms of AA and G.
Fig. 2. (a) Cyclic voltammograms for polycrystalline Pt in 0.5 M H2SO4 at a sweeprate of 0.050 V s 1 and in the presence of G+, at the three concentrations indicated; voltammograms were recorded on the third cycle. (b) Cyclic voltammograms for polycrystalline Pt in 0.5 M H2SO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
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towards the H2 reversible potential (RHE) at the (1 0 0) (1 1 1) sites, especially observable at the highest concentration (3 10 3 M) of G+ in solution. A significant increase of response cur-
Fig. 3. Cyclic voltammograms for polycrystalline Pt in 0.1 M HClO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
rent-density in this potential region is also observed. Conversely, only small changes are detected over the potential ranges related to the (1 1 0) and (1 1 1) surface orientations. The corresponding cyclic voltammetric behaviour of the acetamidonium cation, at polycrystalline Pt and at two concentrations of AA+, is shown in Fig. 2b. Again, as for G+, the presence of AA+ leads to a significant shift of the voltammetric profile (being practically independent of the AA+ concentration) towards the RHE potential (by ca. 15 mV), over the range corresponding to the (1 0 0) (1 1 1) step sites. Moreover, a substantial increase of voltammetric charge (contrast to the behaviour of G+), over the potential range ca. 0.28–0.45 V, is observed. The reversible anodic and cathodic peaks, centred at ca. 0.25 V, are much sharper and the peaks’ current-densities are significantly higher than those recorded for the G+ cation in Fig. 2a. The adsorption behaviour of AA+ at polycrystalline Pt, in contact with 0.1 M HClO4, is shown in Fig. 3 below. In general, it resembles that of the G+ molecule-ion (compare with Fig. 9 in Ref. [1]). However, in the former case, the new peaks observable over the potential range ca. 0.20–0.35 V/RHE, are much sharper (especially the cathodic one) and the shift of the current-potential profile towards the RHE potential is more pronounced than in the case of G+.
Fig. 5. (a) The reaction for electrosorption of the AA+ cation on Pt. (b) The mechanism of ion-pairing between the AA+ and HSO4 ions, adsorbed at Pt(1 1 1) surface.
Fig. 4. (a) Cyclic voltammograms for Pt(1 1 1) in 0.5 M H2SO4 at a sweep-rate of 0.050 V s 1 and in the presence of G+, at the three concentrations indicated; voltammograms were recorded on the third cycle (Fig. 1a in Ref. [1]). (b) Cyclic voltammograms for Pt(1 1 1) in 0.5 M H2SO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
Fig. 6. Cyclic voltammograms for Pt(1 1 1) in 0.1 M HClO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the three concentrations indicated; voltammograms were recorded on the third cycle.
B.E. Conway, B. Pierozynski / Journal of Electroanalytical Chemistry 623 (2008) 102–108
3.2. Adsorption behaviour of AA+ at the Pt(1 1 1) surface 3.2.1. Behaviour in 0.5 M H2SO4 Fig. 4a and b present the adsorption behaviour of the G+ (Fig. 1a from Ref. [1]) and that of the AA+ ion (at concentrations indicated) at the Pt(1 1 1) plane, in contact with 0.5 M H2SO4, respectively. The former ‘‘guanidine” figure is shown here only as a ‘‘reference case”. Based on comparison of the voltammetric profiles of Figs. 4a and b, it can be concluded that the adsorption behaviour of the AA+ ion is significantly different from that of the G+. The ion-pairing effect for G+/HSO4 [1] leads to significant shift of the current-response profiles towards the H2 reversible potential, as well as to squeezing of the UPD H, which is manifested by appearance of two sharp peaks upon the cathodic sweep, at 0.1–0.3 V (denoted as D1 and D2 in Fig. 4a). However, no characteristic for G+ voltammogram squeezing and/or charge displacement effect is observed
Fig. 7. Cyclic voltammograms for Pt(1 1 1) in 0.1 M NaOH at a sweep-rate of 0.050 V s 1 and in the presence of AA+ (or AA), at the three concentrations indicated; voltammograms were recorded on the third cycle.
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in the voltammetric profiles for the AA+ ion (Fig. 4b). Also, these voltammograms are qualitatively similar to those observed in the presence of DMG+ (see Fig. 1b of Ref. [1]). This suggests that for AA+ significantly weaker chemisorptive interaction with HSO4 takes place than that for the G+ cation. Here, the ion-pairing between the AA+ species and HSO4 is substantially diminished because the CH3 group cannot carry a positive charge (as does = +NH2 in protonated G). The reaction for electrosorption of the AA+ cation and a proposed scheme for AA+/HSO4 electrostatic interaction are shown in Fig. 5a and b below, correspondingly. 3.2.2. Significance of the adsorption behaviour in aqueous HClO4 The main difference between the voltammograms at Pt(1 1 1) surface in aqueous H2SO4 and HClO4 is that the so-called ‘‘butterfly” adsorption region in the latter case becomes well-separated from the H UPD region and appears, reversibly, in the more positive potential range (ca. 0.55–0.85 V) than that in the former solution (e.g. compare G+-free voltammogram profiles of Figs. 3 and 1a in Ref. [1]). It is now well-established that the ‘‘butterfly” region in HClO4 corresponds to OH reversible adsorption and desorption processes [10–12]. In the presence of AA+ (see Fig. 6), there is a major displacement of the current-response region to less positive potentials, leading in this case to overlap with the H UPD region, with development of very sharp cathodic and also anodic current peaks, in the potential range 0.25–0.35 V. This behaviour is in contrast to that observed in the presence of G+ (see Fig. 3 in Ref. [1]), where most of the charge associated with the ‘‘butterfly” region was shifted to the potential range for double-layer charging and the UPD of H region was practically unaffected by the presence of G+. 3.2.3. Behaviour in 0.1 M NaOH At the (1 1 1) surface, the presence of AA in 0.1 M NaOH (Fig. 7) causes a major displacement of the ‘‘butterfly” region (ca. 0.6– 0.9 V) to substantially less positive (by ca. –0.50 V) potentials than it was observed for the G+ adsorbate (compare Fig. 7 with Fig. 4 of Ref. [1]). Now, an overlap with the H UPD region can be noticed, reminiscent of the G+ effect at this surface, in 0.5 M H2SO4. The resulting sharpened cathodic and anodic current responses in Fig. 7 are irreversible (contrast to G+), with a peak separation of ca. 0.08 V. However, note that these shifts are not associated with
Fig. 8. (a) Peak anodic current vs. sweep rate for Pt(1 1 1) in 0.1 M NaOH and in the presence of AA+, at the two concentrations indicated. (b) As in (a), but peak cathodic current.
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diffusion control as the peak currents are almost independent of potential sweep-rate (see Figs. 8a and b). The differences of peak potentials are probably associated with kinetic irreversibility in H deposition/ionization, due to irreversibility in the effects of AA on the displacement process involving adsorbed OH species. Similar effects were observed for other studied Pt surfaces in all three supporting electrolytes (as previously reported for G+ and DMG+ ions in Refs. [1] and [2]). 3.3. Adsorption behaviour of AA+ at the Pt(1 0 0), (1 1 0) and (5 1 1) surfaces In 0.5 M H2SO4, the voltammetric profile shifts, recorded in the presence of AA+ at the (1 0 0), (1 1 0) and (5 1 1) surfaces, are, respectively, very similar to those of the same surface geometries, observed in the presence of G+. Some, although not significant differences in the adsorption behaviour between G+ and AA+ cations can only be observed for the (1 0 0) facet (see Fig. 9 below and compare this figure with Fig. 8a from Ref. [1]), where in the latter case the displaced current-response profiles are not as sharp (especially the anodic one) as those recorded for G+. In 0.1 M HClO4, the adsorption behaviour of AA+ at the (1 0 0) and (5 1 1) Pt single-crystal surfaces is very similar to that of the same Pt surfaces, recorded in the presence of G+. However, at the (1 1 0) plane, the presence of AA+ (already at a concentration of 5 10 4 M) in the supporting electrolyte leads to the significant ‘‘squeezing” of the voltammetric profile and more than doubled response current-densities (as compared to the AA+-free solution), giving the voltammogram (Fig. 10) similar to that commonly obtained at the Pt(1 1 0) plane in 0.5 M H2SO4. Interestingly, G+ (at similar concentrations) showed practically no influence on the cyclic voltammetric profile of the (1 1 0) plane in HClO4 (see Fig. 10b in Ref. [1]). The effects of the presence of AA in 0.1 M NaOH solution, at the (1 0 0), (1 1 0) and (5 1 1) Pt surfaces, are quite similar to those observed with G. However, at the (1 0 0) surface, the observed sharp cathodic and anodic current peaks (Fig. 11) are widely separated (by ca. 0.15 V), which effect is more pronounced than that in the presence of G (compare Fig. 11 with Fig. 11a in Ref. [1]).
Fig. 9. Cyclic voltammograms for Pt(1 0 0) in 0.5 M H2SO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
Fig. 10. Cyclic voltammograms for Pt(1 1 0) in 0.1 M HClO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
3.4. An effect of the AA+ additive on measured total voltammetric charge-densities At several of the Pt surfaces, the resonant AA+ cation causes substantial changes (usually increases) in the apparent charges for H and anion deposition or ionization. Charges were evaluated (see Tables 1 and 2 below) by integration of the voltammogram profiles between 0.06 and 0.650 V vs. RHE, after corrections for the double-layer charging contribution (±3%). For the (1 1 1) plane in HClO4 and NaOH, an upper integration potential limit was 0.75 and 0.90 V, respectively. The absolute values of charge, q, are of less interest than the relative ones, i.e. the charge ratios (given as %) in the presence of the additive (at a given concentration) to that determined in the supporting electrolyte alone, over the
Fig. 11. Cyclic voltammograms for Pt(1 0 0) in 0.1 M NaOH at a sweep-rate of 0.050 V s 1 and in the presence of AA+ (or AA), at the three concentrations indicated; voltammograms were recorded on the third cycle.
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Table 1 Measured voltammetric charge-densities for Pt(1 0 0), (1 1 0) and (5 1 1) surfaces in 0.5 M H2SO4, 0.1 M HClO4 and 0.1 M NaOH, and in the presence of AA+ species, at the concentrations indicated Total voltammetric charge-densities, qa/lC cm
2
0.5 M H2SO4, Pt(1 0 0) 0.5 M H2SO4 216
[AA1+] = 6 10 261
Pt(1 1 0) 214
234
251
9.3/17.3
Pt(5 1 1) 243
278
281
14.4/15.6
0.1 M HClO4, Pt(1 0 0) 0.1 M HClO4 248
[AA1+] = 5 10 294
Pt(1 1 0) 220
249
246
13.2/11.8
Pt(5 1 1) 223
273
301
22.4/35.0
0.1 M NaOH, Pt(1 0 0) 0.1 M NaOH 266
[AA1+] = 5 10 296
Pt(1 1 0) 206
142
114
Pt(5 1 1) 222
272
294
4
4
4
[AA2+] = 1 10 269
M
[AA2+] = 3 10 302
M
[AA2+] = 3 10 317
M
3
3
3
M
M
M
Increase in qb)/% [AA1+[/[AA2+] 20.8/24.5
Increase in qb/% [AA1+]/[AA2+] 18.5/21.8
Increase in qb/% [AA1+]/[AA2+] 11.3/19.2 31.1/ 44.7 22.5/32.4
a
Charges calculated after correction for the double-layer charge contribution (±3%); usually third cycle was recorded for quantitative measurements (see also Table 2 below). b Increase above the total charge in AA+-free solutions; usually third cycle was recorded for quantitative measurements (see also Table 2).
Table 2 Measured voltammetric charge-densities for the Pt(1 1 1) surface in 0.5 M H2SO4, 0.1 M HClO4 and 0.1 M NaOH, and in the presence of AA+ species, at the concentrations indicated Total voltammetric charge-densities, qa/lC cm 0.5 M H2SO4 0.5 M H2SO4
[AA1+] = 6 10
241
258
0.1 M HClO4 0.1 M HClO4
[AA1+] = 5 10
244
225
0.1 M NaOH 0.1 M NaOH
[AA1+] = 6 10
278
290
4
M
2
[AA2+] = 1 10
3
M
Increase in qb/% [AA1+]/[AA2+] 7.0/ 1.2
3
M
Increase in qb/% [AA1+]/[AA2+] 7.8/5.3
238 4
M
[AA2+] = 3 10 257
4
M
[AA2+] = 3 10 298
3
M
Increase in qb/% [AA1+]/[AA2+] 4.3/7.2
same potential range. The changes of q were evaluated for two acetamidine concentrations. Usually, the higher adsorbate concentration, the larger increase of the total voltammetric charge was observed. As for the G+ ion [1], the most striking effects in the presence of AA+ were recorded for the (1 0 0) and (5 1 1) Pt planes (see Table 1). In the case of the (1 0 0) surface, 24.5 ± 3% in H2SO4, 21.8 ± 3% in HClO4 and 19.2 ± 3% in NaOH increases of the total charge, above those in AA+-free solutions, were recorded. The corresponding AA+ concentrations in the supporting electrolytes were 1 10 3 M for H2SO4 and 3 10 3 M for HClO4, and NaOH solutions. For the (5 1 1) plane, a maximum increase of 35.0, 32.4 and 15.6 ± 3% of the total charge was measured in HClO4, NaOH and H2SO4, respectively. On the other hand (contrary to the G+ effect), a substantial reduction (by ca. 45%, as compared to the AA-free solution) of the total measured voltammetric charge was recorded for the Pt(1 1 0) plane, in NaOH supporting electrolyte. For the (1 1 1) plane, only insignificant changes (usually several per cent)
of the total voltammetric charge-densities were measured in the presence of AA+, which is shown in Table 2. Since the initial q values for the Pt surfaces were in good agreement with those previously reported for smooth and annealed single-crystal surfaces, it seems that the effect of an additive is to increase the charge associated with co-adsorption of the AA+ cations and anions of the electrolyte. This would be consistent with our proposal [1,2] that the effect of the resonant cations is to stabilize states of the co-adsorbed anions of the electrolyte through electrostatic ion-pairing in the interphase. 4. Conclusions Acetamidonium cations, derived from acetamidine, are shown to have a major influence on the response-current profiles in cyclic voltammetry, for underpotential deposition and ionization of H at Pt. The above is usually expressed by displacement of the response-current profiles to less positive potentials, in the H UPD potential range. These effects are specific to the surface geometries of well-ordered Pt planes, as exemplified by results obtained at Pt(1 1 1), (1 0 0), (1 1 0) and the (5 1 1) stepped surface. The behaviour of AA+ cation is shown to be qualitatively and, in some cases, quantitatively similar to that for the resonant G+ ion (derived from guanidine), studied in previous papers. The effects observed with the AA+ cation, which has also resonant structures, support the mechanism and origin of the current-profile displacements, as due to ion association between the resonant cations and anions of the electrolyte, adsorbed with charge-transfer, e.g. HSO4 in H2SO4, ClO4 (or rather OH ) in perchloric acid and OH species in alkaline solution. The above is essential for explanation of the large voltammetric charge-density increases, as recorded for the Pt(1 0 0) and (5 1 1) planes. The ion-pairing between electrosorbed AA+ and HSO4 anion is less significant than that observed in the presence of G+. This is related to the fact that in the adsorbed state mutual interactions between the CH3 group of AA+ and the OH of bisulphate are much
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weaker than the corresponding effects for the +NH2 group of G+. This is particularly evident at the (1 1 1) surface, studied in H2SO4. The (1 1 1) plane has a threefold symmetry and is the most densely packed among the low-Miller index planes. Therefore, a trigonal coordination of the bisulphate anion is observed on this Pt crystallographic orientation. It can be assumed that at this surface, the ion-pairing between the AA+ and HSO4 ions is insufficient to counteract mutual repulsions between surface-adsorbed species; thus, it is unable to stabilize HSO4 desorption towards the H2 reversible potential (contrast to the G+ ion case). Understandably, at other low-Miller index planes, as well as at the (5 1 1) stepped surface, the voltammetric effects of AA+/anion interaction are analogous to those observed with G+. Nevertheless, in some cases (the best example is behaviour at the (1 1 0) plane in 0.1 M HClO4 – a case of ion-pairing between AA+ and OH species) the voltammetric profiles obtained in the presence of AA+ are significantly different from those recorded for G+. In summary, this work is a good example of how changes of the surface crystallographic orientation can lead to remarkable changes in electrosorp-
tion mechanism for adsorbates that are closely structure-related to each other, like AA and G. References [1] B. Pierozynski, S. Morin, B.E. Conway, J. Electroanal. Chem. 467 (1999) 30. [2] B. Pierozynski, A. Zolfaghari, B.E. Conway, Phys. Chem. Chem. Phys. 3 (2001) 469. [3] B.E. Conway, B.R. MacDougall, H.A. Kozlowska, J. Electroanal. Chem. 39 (1972) 289. [4] S. Morin, B.E. Conway, J. Electroanal. Chem. 376 (1994) 135. [5] M.W. Breiter, Electrochim. Acta 8 (1963) 925. [6] J. Clavilier, R. Pineau, CR Acad. Sci. Paris 260 (1965) 891. [7] A. Hamelin, in: B.E. Conway, R.E. White, J.O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, vol. 16, Plenum Press, New York, 1985, p. 1. [8] S. Mooto, N. Furuya, J. Electroanal. Chem. 172 (1984) 339. [9] J. Clavilier, A. Rodes, K. El Achi, M.A. Zamakhchari, J. Chim. Phys. 88 (1991) 1291. [10] K. Al Jaaf Golze, D.M. Kolb, D. Scherson, J. Electroanal. Chem. 200 (1986) 353. [11] E. Morallon, J.L. Vazquez, A. Aldaz, J. Electroanal. Chem. 334 (1992) 323. [12] A. Berna, V. Climent, J.M. Feliu, Electrochem. Commun. 9 (2007) 2789.
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Influence of acetamidine on the electrosorption of UPD H at Pt single-crystal surfaces Brian E. Conway 1, Boguslaw Pierozynski * Chemistry Department, University of Ottawa, 10 Marie Curie Street, Ottawa, Ontario, Canada K1N 6N5
a r t i c l e
i n f o
Article history: Received 19 February 2008 Received in revised form 5 June 2008 Accepted 25 June 2008 Available online 4 July 2008 Keywords: Electrosorption Acetamidine ions H UPD Pt single-crystal surfaces
a b s t r a c t In our previously published paper we showed how guanidonium (G+) ion causes substantial, surface-specific displacements of the H underpotential deposition (UPD) voltammetric current profiles, at Pt(1 1 1), (1 0 0), (1 1 0) and (5 1 1) stepped surfaces, to less positive potentials. This behaviour was attributed to ion-pairing between adsorbed ions of the electrolyte (HSO4 , ClO4 or OH ) and the resonant G+ cation. Comparatively, the adsorption behaviour of N,N-dimethylguanidonium (DMG+) cation at Pt(1 1 1) surface, in contact with 0.5 M H2SO4, was also reported. The present work reports cyclic voltammetric results of comparative experiments, at the same (hkl) surfaces of Pt, in the presence of acetamidine (AA) molecule having structure related to that of guanidine (G). Acetamidine cations cause similar displacements of the voltammetric profiles for underpotential deposition and desorption of H as does G+, in ways characteristic of the Pt surface geometry. However, in some cases recorded voltammetric profiles are unique to acetamidine and as such they reflect differences between the resonant cations’ structures of G+ and AA+. In addition, current results strongly support the previously proposed mechanism of cation/anion interaction in the double-layer interphase. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction In previous work from this laboratory [1,2] we examined the electrosorption behaviour of guanidine (G), at single-crystal surfaces of Pt. This molecule contains an imine-type bond and in acidic solutions it becomes converted to the corresponding onium-type ion, in which the cation structure is stabilized by resonance between three canonical forms. The adsorption of resonant G+-onium cations was shown [1] to have major and surface-specific effects on the UPD of H at Pt single-crystal surfaces. However, no reactive chemisorption of the type previously found with nitriles [3,4] was observable. It was suggested in Ref. [1] that the voltammetric effects be associated with stabilization of anion (e.g. HSO4 , ClO4 ) adsorption through ion-pairing between the G+ cations and HSO4 or ClO4 4 anions, in the double-layer interphase. The overall effect was referred to as ‘‘anion mimetic”, since the resulting behaviour was similar to that caused by adsorption of anions, such as: Cl , Br or I (cf. Ref. [5]) at Pt, yet it was brought about by the resonant cation.
* Corresponding author. Address: Department of Chemistry, Faculty of Environmental Protection and Agriculture, University of Warmia and Mazury in Olsztyn, Plac Lodzki 4, 10-757 Olsztyn, Poland. Tel.: +48 89 523 4177; fax: +48 89 523 4801. E-mail addresses: [email protected], [email protected] (B. Pierozynski). 1 Deceased. 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2008.06.025
In the light of the previous work outlined above, the purpose of the present paper is to investigate how the surface structure of Pt single-crystals (and possibly anion of the supporting electrolyte) could influence the adsorption behaviour of two closely structure-related molecules, namely: guanidine (G) and acetamidine (AA). Acetamidine is an adsorbate related to both G and DMG, where one of the –NH2 groups on G is substituted by –CH3, thus partially breaking the resonance in the onium cation resulting from protonation. In G+ resonant cation, the C to N bond-order is formally 1.33 while in AA+ it is 1.5. Thus, it could be assumed (the pKa value for acetamidine appears unavailable) that the extent of AA protonation is somewhat limited, as compared to guanidine. It is also our intention to re-evaluate the proposed in Ref. [1] ion-pairing mechanism, in application to the effects of adsorption of the AA+ ion, in aqueous H2SO4, HClO4 and NaOH. It is expected that substitution of the +NH2 group with the uncharged CH3 group should significantly weaken the strength of ion-pairing between adsorbed acetamidine and e.g. bisulphate species. Conversely, considerably weakened ion-pairing (also dependent on the anion of the supporting electrolyte) should be reflected in less significant influence of AA+ on the electrosorption of UPD H and electrolyte anion, as compared to G+. Thus, in this paper we examine how the AA+ cation affects the cyclic voltammetry profiles for UPD of H at several single-crystal surfaces of Pt and how these effects compare with those previously observed for G+ (at comparable concentrations of the adsorbates).
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For convenience of the reader we display below in Fig. 1 the molecular structure of the AA+ cation, in relation to that for G+, emphasising the difference in possible canonical forms. 2. Experimental 2.1. Instrumental procedures Cyclic voltammetry experiments were conducted at 295 K, using an HA-501 potentiostat/galvanostat and an HB-104 function generator (Hokuto Denko). Voltammograms were recorded and collected on a Nicolet-310 digital oscilloscope. The voltammetric polarisations were conducted over controlled ranges of the working-electrode potential, usually in the H UPD regions or at more positive potentials, at the Pt electrode surfaces and at a sweep-rate, s, of 50 mV s 1 (cf. Ref. [1]). Experiments in which s was varied indicated a negligible role of diffusion processes in the response currents (see later in the text). All potential measurements or settings were referred to the potential of a hydrogen electrode in the same solution, in a three-compartment cell, in the usual way. The voltammograms resulting from these experiments were excellently reproducible from one solution to another of a given composition and concentration. 2.2. Procedures for preparation of well-ordered Pt single-crystal surfaces
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conditions [4,8]. Aqueous 0.5 M H2SO4 and 0.1 M HClO4 solutions were made up in the Milli-Q water from sulphuric and perchloric acids of the highest purity available (Sea Star Chemicals). Alkaline solutions were made up from AESAR, 99.996% NaOH pellets. All solutions were initially de-aerated with O2- free, high purity argon. Other details were the same as described in Ref. [1]. Acetamidine was only available in the form of its hydrochloride salt (Fluka, P97%). Therefore, in order to enable electrochemical studies in the absence of the otherwise strongly adsorbing Cl ion, its chemical form was changed to acetamidine sulphate by means of an exhaustive ion-exchange procedure employing a strongly basic, Dowex-1-chloride (50–100 mesh), ion-exchange resin (Aldrich). The resin was first converted to its sulphate form, until no turbidity could be detected in a nephelometric test with AgNO3. Dilute solutions of AA+ were made up in the above supporting electrolytes, at concentrations about 6 10 5 to 3 10 3 M. 3. Discussion of results 3.1. Adsorption behaviour of G+ and AA+ ions at polycrystalline Pt The adsorption behaviour of the guanidonium cation, at three concentrations of G+ in 0.5 M H2SO4, at a Pt poly-oriented singlecrystal, which exhibits mainly (1 1 0) facets and (1 0 0) (1 1 1) step sites, is illustrated by a series of voltammograms shown in Fig. 2a. A major effect is revealed as a shift of the current-potential profile
The techniques for preparation of Pt single-crystals, developed by Clavilier and Pineau [6], were employed. Single-crystal electrodes were made from 1 mm diameter, 99.9985% Pt wire (AESAR/Puratronic), which was melted at one end into a sphere ca. 3 mm in diameter. The resulting single-crystal sphere was oriented for cutting to the desired crystal face using the back von Laue X-ray diffraction method [7]. Other details of the procedure were reported in Ref. [1]. The principal investigations were addressed to the (1 1 1) surface; however, because of the importance of evaluating to what extent the adsorption effects with AA (in relation to those observed with G) were dependent on surface lattice geometry, comparative experiments were also conducted at (1 0 0), (1 1 0) and the (5 1 1) [i.e. 3(1 0 0) (1 1 1)] stepped planes. The (1 1 1) face and the polyoriented Pt sphere were flame annealed and cooled in air [6]. Other surfaces were prepared by flame annealing, followed by cooling in an H2 + Ar mixture [8]. The accuracy of single-crystal preparation was checked by comparison with the voltammograms of Clavilier et al. [9]; the agreements were excellent, as in previous work [4] from this laboratory. 2.3. Solutes and the water solvent All solutions were prepared from 18.2 MX cm Milli-Q ultrapure water (Millipore), as in other works involving high-purity
Fig. 1. Chemical structures of protonated forms of AA and G.
Fig. 2. (a) Cyclic voltammograms for polycrystalline Pt in 0.5 M H2SO4 at a sweeprate of 0.050 V s 1 and in the presence of G+, at the three concentrations indicated; voltammograms were recorded on the third cycle. (b) Cyclic voltammograms for polycrystalline Pt in 0.5 M H2SO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
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towards the H2 reversible potential (RHE) at the (1 0 0) (1 1 1) sites, especially observable at the highest concentration (3 10 3 M) of G+ in solution. A significant increase of response cur-
Fig. 3. Cyclic voltammograms for polycrystalline Pt in 0.1 M HClO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
rent-density in this potential region is also observed. Conversely, only small changes are detected over the potential ranges related to the (1 1 0) and (1 1 1) surface orientations. The corresponding cyclic voltammetric behaviour of the acetamidonium cation, at polycrystalline Pt and at two concentrations of AA+, is shown in Fig. 2b. Again, as for G+, the presence of AA+ leads to a significant shift of the voltammetric profile (being practically independent of the AA+ concentration) towards the RHE potential (by ca. 15 mV), over the range corresponding to the (1 0 0) (1 1 1) step sites. Moreover, a substantial increase of voltammetric charge (contrast to the behaviour of G+), over the potential range ca. 0.28–0.45 V, is observed. The reversible anodic and cathodic peaks, centred at ca. 0.25 V, are much sharper and the peaks’ current-densities are significantly higher than those recorded for the G+ cation in Fig. 2a. The adsorption behaviour of AA+ at polycrystalline Pt, in contact with 0.1 M HClO4, is shown in Fig. 3 below. In general, it resembles that of the G+ molecule-ion (compare with Fig. 9 in Ref. [1]). However, in the former case, the new peaks observable over the potential range ca. 0.20–0.35 V/RHE, are much sharper (especially the cathodic one) and the shift of the current-potential profile towards the RHE potential is more pronounced than in the case of G+.
Fig. 5. (a) The reaction for electrosorption of the AA+ cation on Pt. (b) The mechanism of ion-pairing between the AA+ and HSO4 ions, adsorbed at Pt(1 1 1) surface.
Fig. 4. (a) Cyclic voltammograms for Pt(1 1 1) in 0.5 M H2SO4 at a sweep-rate of 0.050 V s 1 and in the presence of G+, at the three concentrations indicated; voltammograms were recorded on the third cycle (Fig. 1a in Ref. [1]). (b) Cyclic voltammograms for Pt(1 1 1) in 0.5 M H2SO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
Fig. 6. Cyclic voltammograms for Pt(1 1 1) in 0.1 M HClO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the three concentrations indicated; voltammograms were recorded on the third cycle.
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3.2. Adsorption behaviour of AA+ at the Pt(1 1 1) surface 3.2.1. Behaviour in 0.5 M H2SO4 Fig. 4a and b present the adsorption behaviour of the G+ (Fig. 1a from Ref. [1]) and that of the AA+ ion (at concentrations indicated) at the Pt(1 1 1) plane, in contact with 0.5 M H2SO4, respectively. The former ‘‘guanidine” figure is shown here only as a ‘‘reference case”. Based on comparison of the voltammetric profiles of Figs. 4a and b, it can be concluded that the adsorption behaviour of the AA+ ion is significantly different from that of the G+. The ion-pairing effect for G+/HSO4 [1] leads to significant shift of the current-response profiles towards the H2 reversible potential, as well as to squeezing of the UPD H, which is manifested by appearance of two sharp peaks upon the cathodic sweep, at 0.1–0.3 V (denoted as D1 and D2 in Fig. 4a). However, no characteristic for G+ voltammogram squeezing and/or charge displacement effect is observed
Fig. 7. Cyclic voltammograms for Pt(1 1 1) in 0.1 M NaOH at a sweep-rate of 0.050 V s 1 and in the presence of AA+ (or AA), at the three concentrations indicated; voltammograms were recorded on the third cycle.
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in the voltammetric profiles for the AA+ ion (Fig. 4b). Also, these voltammograms are qualitatively similar to those observed in the presence of DMG+ (see Fig. 1b of Ref. [1]). This suggests that for AA+ significantly weaker chemisorptive interaction with HSO4 takes place than that for the G+ cation. Here, the ion-pairing between the AA+ species and HSO4 is substantially diminished because the CH3 group cannot carry a positive charge (as does = +NH2 in protonated G). The reaction for electrosorption of the AA+ cation and a proposed scheme for AA+/HSO4 electrostatic interaction are shown in Fig. 5a and b below, correspondingly. 3.2.2. Significance of the adsorption behaviour in aqueous HClO4 The main difference between the voltammograms at Pt(1 1 1) surface in aqueous H2SO4 and HClO4 is that the so-called ‘‘butterfly” adsorption region in the latter case becomes well-separated from the H UPD region and appears, reversibly, in the more positive potential range (ca. 0.55–0.85 V) than that in the former solution (e.g. compare G+-free voltammogram profiles of Figs. 3 and 1a in Ref. [1]). It is now well-established that the ‘‘butterfly” region in HClO4 corresponds to OH reversible adsorption and desorption processes [10–12]. In the presence of AA+ (see Fig. 6), there is a major displacement of the current-response region to less positive potentials, leading in this case to overlap with the H UPD region, with development of very sharp cathodic and also anodic current peaks, in the potential range 0.25–0.35 V. This behaviour is in contrast to that observed in the presence of G+ (see Fig. 3 in Ref. [1]), where most of the charge associated with the ‘‘butterfly” region was shifted to the potential range for double-layer charging and the UPD of H region was practically unaffected by the presence of G+. 3.2.3. Behaviour in 0.1 M NaOH At the (1 1 1) surface, the presence of AA in 0.1 M NaOH (Fig. 7) causes a major displacement of the ‘‘butterfly” region (ca. 0.6– 0.9 V) to substantially less positive (by ca. –0.50 V) potentials than it was observed for the G+ adsorbate (compare Fig. 7 with Fig. 4 of Ref. [1]). Now, an overlap with the H UPD region can be noticed, reminiscent of the G+ effect at this surface, in 0.5 M H2SO4. The resulting sharpened cathodic and anodic current responses in Fig. 7 are irreversible (contrast to G+), with a peak separation of ca. 0.08 V. However, note that these shifts are not associated with
Fig. 8. (a) Peak anodic current vs. sweep rate for Pt(1 1 1) in 0.1 M NaOH and in the presence of AA+, at the two concentrations indicated. (b) As in (a), but peak cathodic current.
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diffusion control as the peak currents are almost independent of potential sweep-rate (see Figs. 8a and b). The differences of peak potentials are probably associated with kinetic irreversibility in H deposition/ionization, due to irreversibility in the effects of AA on the displacement process involving adsorbed OH species. Similar effects were observed for other studied Pt surfaces in all three supporting electrolytes (as previously reported for G+ and DMG+ ions in Refs. [1] and [2]). 3.3. Adsorption behaviour of AA+ at the Pt(1 0 0), (1 1 0) and (5 1 1) surfaces In 0.5 M H2SO4, the voltammetric profile shifts, recorded in the presence of AA+ at the (1 0 0), (1 1 0) and (5 1 1) surfaces, are, respectively, very similar to those of the same surface geometries, observed in the presence of G+. Some, although not significant differences in the adsorption behaviour between G+ and AA+ cations can only be observed for the (1 0 0) facet (see Fig. 9 below and compare this figure with Fig. 8a from Ref. [1]), where in the latter case the displaced current-response profiles are not as sharp (especially the anodic one) as those recorded for G+. In 0.1 M HClO4, the adsorption behaviour of AA+ at the (1 0 0) and (5 1 1) Pt single-crystal surfaces is very similar to that of the same Pt surfaces, recorded in the presence of G+. However, at the (1 1 0) plane, the presence of AA+ (already at a concentration of 5 10 4 M) in the supporting electrolyte leads to the significant ‘‘squeezing” of the voltammetric profile and more than doubled response current-densities (as compared to the AA+-free solution), giving the voltammogram (Fig. 10) similar to that commonly obtained at the Pt(1 1 0) plane in 0.5 M H2SO4. Interestingly, G+ (at similar concentrations) showed practically no influence on the cyclic voltammetric profile of the (1 1 0) plane in HClO4 (see Fig. 10b in Ref. [1]). The effects of the presence of AA in 0.1 M NaOH solution, at the (1 0 0), (1 1 0) and (5 1 1) Pt surfaces, are quite similar to those observed with G. However, at the (1 0 0) surface, the observed sharp cathodic and anodic current peaks (Fig. 11) are widely separated (by ca. 0.15 V), which effect is more pronounced than that in the presence of G (compare Fig. 11 with Fig. 11a in Ref. [1]).
Fig. 9. Cyclic voltammograms for Pt(1 0 0) in 0.5 M H2SO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
Fig. 10. Cyclic voltammograms for Pt(1 1 0) in 0.1 M HClO4 at a sweep-rate of 0.050 V s 1 and in the presence of AA+, at the two concentrations indicated; voltammograms were recorded on the third cycle.
3.4. An effect of the AA+ additive on measured total voltammetric charge-densities At several of the Pt surfaces, the resonant AA+ cation causes substantial changes (usually increases) in the apparent charges for H and anion deposition or ionization. Charges were evaluated (see Tables 1 and 2 below) by integration of the voltammogram profiles between 0.06 and 0.650 V vs. RHE, after corrections for the double-layer charging contribution (±3%). For the (1 1 1) plane in HClO4 and NaOH, an upper integration potential limit was 0.75 and 0.90 V, respectively. The absolute values of charge, q, are of less interest than the relative ones, i.e. the charge ratios (given as %) in the presence of the additive (at a given concentration) to that determined in the supporting electrolyte alone, over the
Fig. 11. Cyclic voltammograms for Pt(1 0 0) in 0.1 M NaOH at a sweep-rate of 0.050 V s 1 and in the presence of AA+ (or AA), at the three concentrations indicated; voltammograms were recorded on the third cycle.
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Table 1 Measured voltammetric charge-densities for Pt(1 0 0), (1 1 0) and (5 1 1) surfaces in 0.5 M H2SO4, 0.1 M HClO4 and 0.1 M NaOH, and in the presence of AA+ species, at the concentrations indicated Total voltammetric charge-densities, qa/lC cm
2
0.5 M H2SO4, Pt(1 0 0) 0.5 M H2SO4 216
[AA1+] = 6 10 261
Pt(1 1 0) 214
234
251
9.3/17.3
Pt(5 1 1) 243
278
281
14.4/15.6
0.1 M HClO4, Pt(1 0 0) 0.1 M HClO4 248
[AA1+] = 5 10 294
Pt(1 1 0) 220
249
246
13.2/11.8
Pt(5 1 1) 223
273
301
22.4/35.0
0.1 M NaOH, Pt(1 0 0) 0.1 M NaOH 266
[AA1+] = 5 10 296
Pt(1 1 0) 206
142
114
Pt(5 1 1) 222
272
294
4
4
4
[AA2+] = 1 10 269
M
[AA2+] = 3 10 302
M
[AA2+] = 3 10 317
M
3
3
3
M
M
M
Increase in qb)/% [AA1+[/[AA2+] 20.8/24.5
Increase in qb/% [AA1+]/[AA2+] 18.5/21.8
Increase in qb/% [AA1+]/[AA2+] 11.3/19.2 31.1/ 44.7 22.5/32.4
a
Charges calculated after correction for the double-layer charge contribution (±3%); usually third cycle was recorded for quantitative measurements (see also Table 2 below). b Increase above the total charge in AA+-free solutions; usually third cycle was recorded for quantitative measurements (see also Table 2).
Table 2 Measured voltammetric charge-densities for the Pt(1 1 1) surface in 0.5 M H2SO4, 0.1 M HClO4 and 0.1 M NaOH, and in the presence of AA+ species, at the concentrations indicated Total voltammetric charge-densities, qa/lC cm 0.5 M H2SO4 0.5 M H2SO4
[AA1+] = 6 10
241
258
0.1 M HClO4 0.1 M HClO4
[AA1+] = 5 10
244
225
0.1 M NaOH 0.1 M NaOH
[AA1+] = 6 10
278
290
4
M
2
[AA2+] = 1 10
3
M
Increase in qb/% [AA1+]/[AA2+] 7.0/ 1.2
3
M
Increase in qb/% [AA1+]/[AA2+] 7.8/5.3
238 4
M
[AA2+] = 3 10 257
4
M
[AA2+] = 3 10 298
3
M
Increase in qb/% [AA1+]/[AA2+] 4.3/7.2
same potential range. The changes of q were evaluated for two acetamidine concentrations. Usually, the higher adsorbate concentration, the larger increase of the total voltammetric charge was observed. As for the G+ ion [1], the most striking effects in the presence of AA+ were recorded for the (1 0 0) and (5 1 1) Pt planes (see Table 1). In the case of the (1 0 0) surface, 24.5 ± 3% in H2SO4, 21.8 ± 3% in HClO4 and 19.2 ± 3% in NaOH increases of the total charge, above those in AA+-free solutions, were recorded. The corresponding AA+ concentrations in the supporting electrolytes were 1 10 3 M for H2SO4 and 3 10 3 M for HClO4, and NaOH solutions. For the (5 1 1) plane, a maximum increase of 35.0, 32.4 and 15.6 ± 3% of the total charge was measured in HClO4, NaOH and H2SO4, respectively. On the other hand (contrary to the G+ effect), a substantial reduction (by ca. 45%, as compared to the AA-free solution) of the total measured voltammetric charge was recorded for the Pt(1 1 0) plane, in NaOH supporting electrolyte. For the (1 1 1) plane, only insignificant changes (usually several per cent)
of the total voltammetric charge-densities were measured in the presence of AA+, which is shown in Table 2. Since the initial q values for the Pt surfaces were in good agreement with those previously reported for smooth and annealed single-crystal surfaces, it seems that the effect of an additive is to increase the charge associated with co-adsorption of the AA+ cations and anions of the electrolyte. This would be consistent with our proposal [1,2] that the effect of the resonant cations is to stabilize states of the co-adsorbed anions of the electrolyte through electrostatic ion-pairing in the interphase. 4. Conclusions Acetamidonium cations, derived from acetamidine, are shown to have a major influence on the response-current profiles in cyclic voltammetry, for underpotential deposition and ionization of H at Pt. The above is usually expressed by displacement of the response-current profiles to less positive potentials, in the H UPD potential range. These effects are specific to the surface geometries of well-ordered Pt planes, as exemplified by results obtained at Pt(1 1 1), (1 0 0), (1 1 0) and the (5 1 1) stepped surface. The behaviour of AA+ cation is shown to be qualitatively and, in some cases, quantitatively similar to that for the resonant G+ ion (derived from guanidine), studied in previous papers. The effects observed with the AA+ cation, which has also resonant structures, support the mechanism and origin of the current-profile displacements, as due to ion association between the resonant cations and anions of the electrolyte, adsorbed with charge-transfer, e.g. HSO4 in H2SO4, ClO4 (or rather OH ) in perchloric acid and OH species in alkaline solution. The above is essential for explanation of the large voltammetric charge-density increases, as recorded for the Pt(1 0 0) and (5 1 1) planes. The ion-pairing between electrosorbed AA+ and HSO4 anion is less significant than that observed in the presence of G+. This is related to the fact that in the adsorbed state mutual interactions between the CH3 group of AA+ and the OH of bisulphate are much
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weaker than the corresponding effects for the +NH2 group of G+. This is particularly evident at the (1 1 1) surface, studied in H2SO4. The (1 1 1) plane has a threefold symmetry and is the most densely packed among the low-Miller index planes. Therefore, a trigonal coordination of the bisulphate anion is observed on this Pt crystallographic orientation. It can be assumed that at this surface, the ion-pairing between the AA+ and HSO4 ions is insufficient to counteract mutual repulsions between surface-adsorbed species; thus, it is unable to stabilize HSO4 desorption towards the H2 reversible potential (contrast to the G+ ion case). Understandably, at other low-Miller index planes, as well as at the (5 1 1) stepped surface, the voltammetric effects of AA+/anion interaction are analogous to those observed with G+. Nevertheless, in some cases (the best example is behaviour at the (1 1 0) plane in 0.1 M HClO4 – a case of ion-pairing between AA+ and OH species) the voltammetric profiles obtained in the presence of AA+ are significantly different from those recorded for G+. In summary, this work is a good example of how changes of the surface crystallographic orientation can lead to remarkable changes in electrosorp-
tion mechanism for adsorbates that are closely structure-related to each other, like AA and G. References [1] B. Pierozynski, S. Morin, B.E. Conway, J. Electroanal. Chem. 467 (1999) 30. [2] B. Pierozynski, A. Zolfaghari, B.E. Conway, Phys. Chem. Chem. Phys. 3 (2001) 469. [3] B.E. Conway, B.R. MacDougall, H.A. Kozlowska, J. Electroanal. Chem. 39 (1972) 289. [4] S. Morin, B.E. Conway, J. Electroanal. Chem. 376 (1994) 135. [5] M.W. Breiter, Electrochim. Acta 8 (1963) 925. [6] J. Clavilier, R. Pineau, CR Acad. Sci. Paris 260 (1965) 891. [7] A. Hamelin, in: B.E. Conway, R.E. White, J.O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, vol. 16, Plenum Press, New York, 1985, p. 1. [8] S. Mooto, N. Furuya, J. Electroanal. Chem. 172 (1984) 339. [9] J. Clavilier, A. Rodes, K. El Achi, M.A. Zamakhchari, J. Chim. Phys. 88 (1991) 1291. [10] K. Al Jaaf Golze, D.M. Kolb, D. Scherson, J. Electroanal. Chem. 200 (1986) 353. [11] E. Morallon, J.L. Vazquez, A. Aldaz, J. Electroanal. Chem. 334 (1992) 323. [12] A. Berna, V. Climent, J.M. Feliu, Electrochem. Commun. 9 (2007) 2789.