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Pt(1 1 0) in view of the measured nickel formal partial charge number upon underpotential deposition on platinum surfaces in sulphate media
Electrochimica Acta 53 (2007) 369–376
Understanding CO-stripping mechanism from NiUPD/Pt(1 1 0) in view of the measured nickel formal partial charge number upon underpotential deposition on platinum surfaces in sulphate media Marian Chatenet ∗ , Yvonne Soldo-Olivier, Eric Chaˆınet, Ren´e Faure Laboratoire d’Electrochimie et de Physicochimie des Mat´eriaux et des Interfaces, LEPMI, UMR 5631 CNRS-INPG-UJF, BP 75, 38402 Saint-Martin d’H`eres Cedex, France Received 19 October 2006; received in revised form 3 January 2007; accepted 17 January 2007 Available online 30 January 2007
Abstract We recently showed nickel-underpotential deposition (Ni-UPD) occurs on polycrystalline or single crystal platinum electrodes in acidic media. Whereas the decoupling of the nickel and hydrogen adsorption/desorption peaks is difficult for low pH, these processes can be better separated for higher pH values, typically pH > 3. However, even for platinum single crystals, high pH solutions do not enable to sufficiently separate nickel from hydrogen phenomena. As a result, electrochemistry alone cannot yield important information about Ni-UPD, such as the formal partial charge number (valency of electrosorption) and the role of the sulphate or hydrogen sulphate anions. So, we decided to couple cyclic voltammetry to electrochemical quartz crystal microbalance (EQCM). EQCM measurements enable to decorrelate the simultaneous hydrogen and nickel adsorption/desorption peaks, which we could not attempt solely with electrochemistry. The coupling between gravimetric and electrochemical measurements allows us to detect the contribution of the anions and thus to isolate that of nickel: nickel coverage can then be determined. Nearly 4/5 NiUPD monolayer (θ Ni ≈ 0.8) over platinum is reached at nickel equilibrium potential for high pH solutions (5.5). The QCM and electrochemistry coupling further allows the determination of nickel formal partial charge number: ιNi,EQCM = 1.3 ± 0.13. Direct electrochemistry measurements (Swathirajan and Bruckenstein method) yield: ιNi,Pt(poly) = 1.5 ± 0.17. These two values are close, which validates the electrochemical method for the nickel/platinum system. In consequence, we used Swathirajan and Bruckenstein method for Pt(1 1 0)-(1 × 2) crystal and found: ιNi,Pt(1 1 0) ≈ 1.4 ± 0.1. Whatever the system (NiUPD /Pt(poly) or NiUPD /Pt(1 1 0)-(1 × 2)) or the experimental technique, nickel formal partial charge number is lower than nickel cation charge: ιNi < zNi = 2. In consequence, upon underpotential deposition on platinum surfaces, nickel cations discharge and then undergo additional charge exchange processes, such as anion (or water) adsorption, resulting in apparent partial nickel cation discharge. Moreover, NiUPD /Pt(1 1 0) surface displays high activity towards COad oxidation reaction. We explain such positive effect by the possible existence of a bifunctional mechanism in which oxygenated-species-covered NiUPD adatoms provide the oxygen atom to COad · · ·Pt species, enabling its facile oxidation. © 2007 Elsevier Ltd. All rights reserved. Keywords: Nickel-underpotential deposition; Pt(1 1 0); EQCM; Formal partial charge number; CO-stripping
1. Introduction We recently published [1] that nickel-underpotential deposition (UPD) on platinum, already monitored for bulk polycrystalline platinum [2–6], is favoured for the Pt(1 1 0) orientation, which in electrolytic medium is reconstructed into Pt(1 1 0)-(1 × 2) [1,7]. As nickel adsorption/desorption occurs
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in the hydrogen-UPD zone in acidic solutions, we attempted to higher the medium pH in order to isolate nickel-related phenomena and to further understand nickel-underpotential deposition mechanism. However, for pH > 3, the voltammograms exhibit many features, such as unclear hydrogen evolution region following interfacial pH changes upon platinum polarization negative to hydrogen equilibrium potential or anionic solution composition changes (HSO4 − predominate for pH < 2, while SO4 2− predominate for pH > 2), which are difficult to characterize [8]. In another approach, we attempted to use CO as a probe to evidence the presence of NiUPD and (if possible) to determine
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ιNi,Pt(QCM) = νNi,Pt(QCM) = 1.3 ± 0.13. Let we note at that point that the Pt(QCM) electrodes could be considered as polycrystalline platinum [14]. This value, calculated from the QCM and electrochemistry coupling, was also confirmed by a pure electrochemical method introduced by Swathirajan and Bruckenstein [16]: ιNi,Pt(poly) = 1.5 ± 0.17. The object of the present paper is to take benefit of the EQCM results to validate the use of Swathirajan and Bruckenstein pure electrochemical method for nickel formal partial charge number determination for Pt(1 1 0) single crystal. Then, the remarkable activity of NiUPD /Pt(1 1 0)-(1 × 2) surface, already evidenced in reference [1] (see Fig. 1), will be further investigated and discussed knowing nickel formal partial charge number value upon UPD over Pt(1 1 0)-(1 × 2). 2. Experimental procedures Fig. 1. COad , NiUPD and (COad + NiUPD )-stripping voltammograms following 3 min CO-adsorption and 15 min N2 flushing or 18 min Ni-deposition at −0.005 V vs. NHE; Pt(1 1 0)-(1 × 2) in 0.1 M H2 SO4 + 10−3 M NiSO4 ; S = 0.099 cm2 ; sweep rate 50 mV s−1 . Replotted from the data of Fig. 8 in Chatenet et al. [1], with permission from Elsevier.
nickel coverage. The voltammograms typically obtained for CO-stripping, Ni-stripping and (CO + Ni)-stripping (Fig. 1) helped us to draw the following basic conclusions [1]: • nickel-UPD occurs in the presence of CO and thus nickel exhibits high affinity for Pt(1 1 0)-(1 × 2) surfaces; • CO adsorbs on the nickel submonolayer and COad species protect NiUPD from dissolution; • COad -stripping is significantly favoured in the presence of NiUPD compared to Pt(1 1 0)-(1 × 2) surfaces. However, interesting, such experiments could neither provide a clear knowledge about NiUPD coverage (since CO adsorbs both on Pt and NiUPD /Pt, which is forecast from experiments on bulk nickel [9–12] or nickel alloys [13]) nor about its mechanism. Thus, we tried to use a coupled technique, the electrochemical quartz crystal microbalance (EQCM), so as to isolate the different processes linked to nickel-underpotential deposition on platinum, and further get more insight into its mechanism [14]. This technique has already been used in the past to study UPD of various metals and their interaction with anions [15]. The coupling between gravimetric measurements and electrochemistry enabled to subtract the contributions of hydrogen/anions and then to isolate that of nickel. From the resulting nickel coverage determination, we calculated nickel formal partial charge number (assimilated to the electrosorption valency)1 for the Pt(QCM) electrode, 1 The term valency of electrosorption was subject to criticism, as summarized by Trasatti and Parsons [28]. They concluded that the term formal partial charge number (ιB ) is more correct on a conceptual basis than that of valency of electrosorption (νB ), even if both correspond to the same definition. Therefore, we
Our experiments on Pt(1 1 0) were conducted using a classical four-electrode Pyrex cell, paying close attention to the glassware cleanness, as already thoroughly described in reference [1]. The EQCM experiments were conducted using a 5 MHz EQCM polycrystalline platinum crystals (Ti/Pt polished, geometric area = 1.37 cm2 , MAXTEK), in a devoted QCM cell, using a computer-controlled PM-710 plating monitor (MAXTEK) [14]. The sensitivity of our QCM apparatus was 0.1 Hz, which translates into 2.4 ng. In order to guaranty the EQCM platinum/quartz crystals cleanness, we did wash the platinum/quartz crystal in pure water (18.2 M cm–3 ppb TOC, Millipore Elix® + Gradient® ) before each experiment. The Pt(QCM) holder and electrode were cleaned according to the procedure described in [14]. After mounting in the electrochemical cell, the Pt(QCM) electrode underwent ca. 300 voltammetric cycles from the hydrogen evolution region to the platinum oxide formation region (typically between 0.005 and 1.25 V versus NHE), in order to stabilize its electrochemical response and further clean the platinum surface. Such procedure also favours the rearrangement of the superficial platinum atoms, until the voltammetry does not vary anymore [17,18]. This corresponds to stable platinum surface and gravimetric signal. The voltammetry experiments were controlled by a potentiostat PAR 263 (EG&G) connected to a saturated calomel electrode (SCE: +0.241 V versus NHE); all electrode potentials are nevertheless expressed versus the normal hydrogen electrode (NHE) potential. All the experiments were carried out under inert atmosphere (Ar N45, Air-Liquide) at room temperature. COad -stripping experiments were conducted after CO-adsorption at fixed potential: −5 mV versus NHE under pure CO bubbling (CO N47-Alphagaz) followed by sufficient argon flushing to remove any trace of dissolved CO in the electrolyte [1]. The electrochemical techniques employed are those thoroughly described in references [1,14].
will assimilate these two values as identical in the manuscript and use only formal partial charge number; we point out that most papers of the literature only refer to valency of electrosorption.
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Fig. 2. First three cycles for (COad + NiUPD )-stripping voltammograms following 3 min CO-adsorption and 15 min N2 flushing at −0.005 V vs. NHE; Pt(1 1 0)-(1 × 2) in 0.1 M H2 SO4 + 10−3 M NiSO4 ; S = 0.099 cm2 ; potential scan rate 50 mV s−1 ; first cycle from −0.005 to 0.545 V vs. NHE (bold grey); second (bold black), third (thin black) and reference (cross, the same as in Fig. 1; top) scans from −0.005 to 0.995 V vs. NHE.
All solutions were prepared using pure water. The sulphuric acid, sodium sulphate and nickel sulphate were all purchased from Merck in the Suprapur® quality. 3. Results 3.1. COad -stripping for NiUPD -covered Pt(1 1 0)-(1 × 2) Fig. 1 shows the first cycle of a typical COad -stripping voltammogram for a Pt(1 1 0)-(1 × 2) electrode in 0.1 M H2 SO4 (lower black curve). The Pt-surface is blocked in the hydrogen desorption zone, while a well defined COad -stripping peak is centred at 0.665 V versus NHE (peak 1). The small anodic feature at 0.39 V versus NHE (labelled peak 1 ) can be ascribed to the oxidation of part of the COad monolayer (ca. 10%), namely the weakly adsorbed CO [19], which is usual when CO is adsorbed in the HUPD region [19–21]. When nickel is present and no CO is bubbled in the solution during the waiting time at the initial potential (middle dark grey curve), the voltammogram still displays some H-desorption at ca. 50 mV versus NHE. In addition, another anodic peak is centred at ca. 0.28 V versus NHE; it can be ascribed to NiUPD -stripping. Such features have been fully described in [1] and show that under these experimental conditions, NiUPD coverage is below unity: θ Ni < 1. The COad -stripping voltammogram in the presence of nickel (higher grey curve) shows 2 stripping peaks (labeled peak 2 and peak 1) at ca. 0.410 and 0.645 V versus NHE, respectively. The latter resembles that for the COad -stripping from Pt(1 1 0)-(1 × 2) in the absence of nickel (see lower curve, peak 1), both in peak and onset positions and in coulometry: it should correspond to the stripping of COad /Pt(1 1 0)-(1 × 2). On the contrary, peak 2 should be related to the presence of both NiUPD and COad . The voltammograms from Fig. 2 clearly exhibit that peaks 1 and 2 are indeed corresponding to very different phenomena. When the potential is reversed at 0.545 V versus NHE (positive to (COad + NiUPD )-stripping: peak 2), the following complete voltammogram (from −0.005 to 0.995 V versus NHE) only exhibits peak 1. Moreover, after the first (COad + NiUPD )-layer
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Fig. 3. H or (H and Ni)-adsorption/desorption voltammograms on Pt(1 1 0)(1 × 2) in 0.1 M Na2 SO4 + 10−3 M H2 SO4 (pH ≈ 3) in the absence (black) or presence (grey) of 10−3 M NiSO4 for θ Ni ≈ 1 (bold) or θ Ni > 1 (thin); SPt = 0.099 cm2 ; sweep rate = 2 mV s−1 .
stripping cycle (peak 2), peak 1 potential, shape and coulometry are barely changed compared to the reference voltammogram (see Fig. 2, bold black and cross traces). In addition, the main difference between the COad -stripping voltammogram from Pt(1 1 0)-(1 × 2) (Fig. 1, lower black trace) and the second cycle of (COad + Niad )-stripping (Fig. 2, bold black trace) is the absence of anodic feature (peak 1 ) monitored at 0.39 V versus NHE for COad -stripping from Pt(1 1 0)-(1 × 2) when CO is adsorbed in the hydrogen region. Such findings agree with the fact that after the first cycle of Fig. 2, the Pt(1 1 0)-(1 × 2) electrode behaves as if it was only covered by a full COad layer adsorbed positive to hydrogen region; this peculiar result will be discussed in Section 4. After COad and NiUPD stripping, the Pt(1 1 0)-(1 × 2) surface is recovered (Fig. 2, third cycle). 3.2. Ni-UPD on Pt(1 1 0)-(1 × 2) for pH ≥ 3 As thoroughly discussed in references [1,14] separating the hydrogen and nickel-UPD peaks is easier for high pH. It also enables to reach higher nickel coverage, as the potential can be swept down to nickel reversible (Nernst) potential without dramatic masking from hydrogen adsorption/desorption and evolution/oxidation peaks/waves (nickel Nernst potential is ca. −0.32 V versus NHE in the present conditions). These are indeed hindered by harsh proton diffusion limitation when [H+ ] < 10−3 M [14]. Fig. 3 reveals the possibility to reach the full nickel layer (UPD, grey bold trace) and even nickel overpotential (bulk) deposition (OPD, grey thin trace) when the potential is swept below nickel Nernst potential. Ni-deposition peaks are not well resolved (despite NiUPD onset positive to HUPD onset), following either overlap with H-adsorption peak and HER wave or Ni2+ diffusion limitation. On the contrary, both NiUPD and NiOPD -stripping peaks are well defined at ca. 0.2 and −0.125 V versus NHE, respectively. Fig. 3 also clearly shows that NiUPD -stripping peak does not change for various nickel coverage (provided θ Ni ≥ 1). From such voltammograms, it is not possible to determine nickel formal partial charge number, because neither nickel adsorption site, nor the net charge exchange with the surface and/or the solution are known.
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and: mNi tot = − 133θNi − 2.3(1 − θNi )θH − 219θSO4 /Ni + 219θSO4 (2 )
Fig. 4. (H and Ni)-adsorption/desorption voltammograms (primary Y-axis) and corresponding gravimetric signals (secondary Y-axis) for Pt(QCM) in 0.1 M Na2 SO4 (pH ≈ 5.5) in the absence (black) or presence (grey) of 6 × 10−4 M NiSO4 ; sweep rate = 2 mV s−1 .
3.3. Using the QCM to monitor Ni-UPD on polycrystalline platinum at pH > 3 Typical examples of EQCM results for the study of Ni-UPD in sodium sulphate solution are presented in Fig. 4. The voltammograms are not easily understood, as nickel and hydrogen related peaks overlap. Such overlap is more severe for polycrystalline platinum, as the hydrogen region is wider than for Pt(1 1 0), due to the contribution of other platinum orientations. However, the gravimetric/voltammetric coupling enables the determination of nickel and hydrogen/anions contributions, as detailed in reference [14]. First, we determine in supporting electrolyte without NiSO4 , the a priori unknown proton (θ H ) and anion (θSO4 ) coverage over platinum, using the charge balance: Qtot = 220θH + 440θSO4
(1)
where Qtot = 220 C cm−2 Pt is the measured charge related to the positive scan. We assume 220 and 440 C cm−2 Pt for 1 ML Had and 1 ML SO4,ad , respectively, expressing the full discharge of the ions upon specific adsorption [22–24]. The mass conservation yields: mtot = 219θSO4 − 2.3θH
(2)
where mtot = 36 ng cm−2 Pt is the measured mass variation during the positive scan, assuming 2.3 and 219 ng cm−2 Pt for 1 ML Had and 1 ML SO4,ad , respectively. The EQCM data thus yield: θ H = 0.66 and θSO4 = 0.17, in agreement with the bisulphate and proton saturation coverage measured by Funtikov et al. [25], Herrero et al. [24] or Mostany et al. [23] for Pt(1 1 1). In the presence of NiSO4 , the system becomes: QNi tot = 440θNi − 440θSO4 /Ni + 220(1 − θNi )θH + 440θSO4 (1 )
−2 Pt and mNi = −126 ng cm−2 Pt are where QNi tot = 345 mC cm tot the measured charge and mass variations related to the positive scan, and θ Ni and θSO4 /Ni are the NiUPD and the anion (bisulphate) over NiUPD coverage, respectively. Eqs. (1 ) and (2 ) yield: θ Ni ≈ 0.80 and θSO4 /Ni ≈ 0.26. These calculations assume (i) H and Ni coadsorption over platinum at low potentials, (ii) Had coverage on the NiUPD -free zones of Pt equal to θ H (1 − θ Ni ), (iii) possible bisulphate adsorption over NiUPD and (iv) mainly bisulphate adsorption over platinum at high potentials (as shown by Funtikov et al. [25] and Woodard et al. [26]). Such coverage values yield nickel formal partial charge number of ca. 1.4, which expresses that bisulphate anions concomitantly adsorb over Niad during their UPD over platinum. We point out here that the calculation was undertaken for potentials corresponding to NiUPD desorption, monitored in the range [0.03–0.4] V versus NHE on the grey voltammogram plotted in nickel-containing solution. For greater accuracy and determination of the experimental error, the calculation of θ Ni was repeated for several identical experiments, attempted when the deposition of nickel did not reach the overpotential deposition, but remained at constant coverage (θ Ni = 0.75, see Table 1 in reference [14]). Then, using the NiUPD -stripping peak coulometry, we determined the average nickel formal partial charge number in the present experimental conditions: ιNi,Pt(QCM) = 1.3 ± 0.13 [14]. This average value agrees with that determined from a direct electrochemical method: ιNi,Pt(poly) = 1.5 ± 0.17 [14], first published by Swathirajan and Bruckenstein [16], as described below.
3.4. Determination of nickel formal partial charge number for Pt(1 1 0)-(1 × 2) substrate The method of Swathirajan and Bruckenstein [16] consists of plotting the difference between nickel-stripping peak potential (at given nickel coverage) and nickel Nernst potential, according to Eq. (3) [2,16]: 1 RT 1 2+ Ep (ML) − ENernst (Ni ) = K + ln[Ni2+ ] − F ιNi zNi (3) where Ep (ML) is the monolayer peak potential (determined as the average of nickel deposition peak potential and nickelstripping peak potential, following Blum et al. conclusions [27]), ENernst (Ni2+ ) the nickel reversible potential, K a constant, zNi = 2 the nickel ion charge, ιNi the nickel formal partial charge number, [Ni2+ ] the nickel ion concentration, assimilated as nickel ion activity and R, T and F have their usual meanings. Note that in their paper, Swathirajan and Bruckenstein were referring to the term valency of electrosorption, which we assimilate to the formal partial charge number, following the conclusions from Trasatti and Parsons [28], Lang and Horanyi [29–31] or de Levie [32].
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Fig. 5. Evolution of the averaged NiUPD -peak potential measured at constant coverage (θ Ni ≈ 0.8) on Pt(1 1 0)-(1 × 2) in 0.1 M Na2 SO4 (pH ≈ 5.5) containing increasing amounts of NiSO4 ; SPt = 0.099 cm2 ; sweep rate = 2 mV s−1 .
Using Eq. (3) for Pt(1 1 0)-(1 × 2) at pH 5.5 (for which the NiUPD desorption peaks are well defined) for three experiments, we find: νNi,Pt(1 1 0) = 1.4 ± 0.1 (Fig. 5). Such nickel formal partial charge number value is, within the error bars, identical to that for nickel/polycrystalline platinum system, as stated in the previous section. Note that Ep (ML) determination suffers non negligible experimental error (±7 mV averaged on the three experiments), which induces uncertainty for the slope determined from Fig. 5. Thus, the resulting uncertainty for ιNi,Pt(1 1 0) is non negligible: ±0.5. This value exceeds the good experimental reproducibility (±0.1), calculated from three identical experiments; its smallness shows the reproducibility of our protocol. Nevertheless, we wanted to validate Swathirajan and Bruckenstein experimental method using the more precise EQCM technique on the first hand. Now, despite the electrochemical technique lower accuracy than for EQCM, the results for Pt(1 1 0)-(1 × 2) strongly attest that nickel adatoms undergo anion or water adsorption after their underpotential deposition (and Ni2+ discharge) over platinum single crystal. 4. Discussion The literature does not provide a clear knowledge about nickel formal partial charge number corresponding to nickel adsorption over platinum in acidic medium. Whereas El-Shafei found ιNi = 2 [2], Franaszczuck and Sobkowski measured: ιNi = 1 [3], both paper dealing with polycrystalline platinum electrodes. The nickel formal partial charge number values we measured from EQCM and electrochemistry for Ni-UPD onto Pt(poly) in sulphate media are close, which validates both techniques. Thus, we decided to use the electrochemical method (from Swathirajan and Bruckenstein [16]) to determine nickel formal partial charge number on Pt(1 1 0)-(1 × 2) in sulphate media: ιNi,Pt(1 1 0) = 1.4 ± 0.1. This latter value is, within the error bars, equal to that for Pt(poly). However, we cannot rule out that single crystalline and polycrystalline surfaces could yield different formal partial charge number, for example following their different potential of zero charge or work function values (see for example [33,34]). We nevertheless emphasize that such measured ιNi values are consistent with the fact that Ni2+ discharge is apparently
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incomplete upon nickel UPD on platinum surfaces in sulphate medium. Difference between one element formal partial charge number and its ionic charge is not common for metal UPD over metal supports. In such cases, it is usually close to the charge of the ion deposited, expressing the total discharge of this ion upon adsorption [35]. However, since nickel is usually oxidized superficially in acidic medium, one cannot exclude the possibility for nickel adatoms to undergo water (OHad or Oad ) adsorption right after their adsorption on platinum. Second, nickel potential of zero charge is much lower than that of platinum [34,36] and it could lead to further anion adsorption after its own adsorption on platinum. However, Nakamura et al. showed that at pH 5.5, nickel oxide formation on Ni(1 1 1) surfaces starts above 0 V versus NHE [37], positive to Ni-UPD onset. So, the most likely explanation for nickel formal partial charge number lower than 2 is anion coadsorption (anion adsorption on NiUPD /Pt), as similarly monitored for the CuUPD (Au) system [38]. Additional in situ experiments, like X-ray absorption spectroscopy (XAS) or electrochemical scanning tunnelling microscopy (ESTM) could enable further discussion. Nevertheless, we think all our results are in line with the fact that NiUPD atoms are more oxophilic than the underlying Pt atoms. As a result, NiUPD /Pt layer is covered by anions at electrode potentials for which the platinum substrate is not, either for polycrystalline or single crystal electrodes. In the presence of COad at low potentials (in the hydrogen region), NiUPD /Pt(1 1 0)-(1 × 2) is probably free of water/anions adsorbate, following its potential of zero charge positive shift (or CO-induced displacement), as documented in references [39–41]. Upon potential increase starting from Ni and CO deposition potential (−0.005 V versus NHE), NiUPD adatoms may nucleate water (OHad species) at potentials negative to platinum (Eq. (4)), as expected from the determined formal partial charge number for nickel: NiUPD + H2 O → NiUPD · · ·OHad + H+ + e−
(4)
Then the fraction of the OHad which is located at the edge of the partial NiUPD layer will react with COad species adsorbed on the closest platinum atom, according to Eq. (5): NiUPD · · ·OHad + Pt· · ·COad → NiUPD + Pt + CO2 + H+ + e−
(5)
NiUPD /Pt(1 1 0)-(1 × 2) adatoms thus act as an oxygen provider for COad species over Pt atoms. In a way, such mechanism resembles the well known bifunctional mechanism [42] and occurs under peak 2 in the (COad + NiUPD )-stripping voltammograms of Figs. 1 and 2. Now, one may wonder why the NiUPD · · ·OHad species would not react with NiUPD · · ·COad species which are also adsorbed over the NiUPD /Pt(1 1 0)-(1 × 2) adlayer (see Fig. 1 and reference [1]). If it were the case, the COad and underlying NiUPD adatoms would be stripped concomitantly from the Pt surface as soon as COad species are stripped, thus freeing the underlying Pt(1 1 0) atoms. As a result, we should recover the fraction of the platinum surface which was covered by NiUPD atoms free of COad . Thus, we should only monitor a fraction (in term of coulometry) of peak 1 in the (COad + NiUPD )stripping voltammograms. Conversely, Figs. 1 and 2 show that
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Fig. 6. Reaction scheme for NiUPD and COad -stripping from Pt(1 1 0)-(1 × 2) surface; OHad first nucleates on NiUPD (step 1), combines with the COad species from the nearest Pt site, yielding CO2 (step 2), then COad diffuses from NiUPD to the freed Pt site (step 3), freeing one NiUPD site; NiUPD is not stable at such electrode potential and strips out (step 4); COad diffusion process/NiUPD -stripping can start again (step 5 and step 4) until the full NiUPD layer is stripped, and the (almost) full COad layer is formed onto the Pt(1 1 0)-(1 × 2) surface.
peak 1 coulometry (and potential) basically correspond to the stripping of a full CO-adlayer over Pt(1 1 0)-(1 × 2) surface, exhibiting that the surface created after peak 2 resembles a COad covered Pt(1 1 0)-(1 × 2) surface. Moreover, should Ni· · ·OHad species react with Ni· · ·COad species, the coulometry for peak 2 would be roughly twice bigger (corresponding to COad and NiUPD concomitant stripping, as pointed out by Karimi Shervedani and Lasia for CO-stripping from Ni surfaces in alkaline medium [13]). As stated above, the presence of peak 1 for (COad + NiUPD )-stripping experiment, the coulometry of which is close to that for COad -stripping voltammograms in the absence of nickel in solution, is a strong evidence that after peak 2, the
CO-adlayer has been rebuilt over the Pt(1 1 0)-(1 × 2) surface freed from any NiUPD atoms. The fast COad diffusion coefficient on nickel substrates, as monitored in UHV conditions for nickel crystals [43–47] or for Ni/W(1 1 0) surfaces [48], is consistent with such scenario. In addition, COad displays a greater stability over platinum surface, at least for singles crystals in the (1 1 1) orientation in UHV conditions [48]. The conjunction of these two findings is consistent with the possibility for COad to simply diffuse (superficially) from NiUPD sites to the neighbouring free Pt(1 1 0) sites. Doing so, they would (i) replenish the COadlayer over Pt(1 1 0)-(1 × 2) and (ii) free NiUPD surface from its COad protection. As this occurs at potentials greater than
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ca. 0.25 V versus NHE (positive to the NiUPD -stripping peak potential, as seen on Fig. 1; middle grey curve), NiUPD atoms are not longer stable over the Pt(1 1 0)-(1 × 2) substrate and are then instantaneously stripped (in peak 2) upon COad diffusion down to the Pt sites. Let we point out that (COad + NiUPD ) oxidation has already been monitored on a single peak (as in peak 2) for polycrystalline nickel surfaces in alkaline media by Karimi Shervedani and Lasia [13], which goes in line with our data. Moreover, the CO-adlayer over Pt(1 1 0)-(1 × 2) replenishment originating from such COad surface diffusion occurs at potential positive to the hydrogen region on Pt(1 1 0)-(1 × 2) (see Fig. 1). In consequence, the COad -stripping of such replenished COadlayer over Pt(1 1 0)-(1 × 2) (free of any NiUPD atoms) occurs only in peak 1, without the small anodic feature (peak 1 ) at 0.39 V versus NHE (Fig. 2), which is linked to COad -stripping when CO is adsorbed in the hydrogen region [19]. To summarize, as the potential increases, (i) NiUPD atoms nucleate water (OHad ), which (ii) reacts with Pt· · ·COad species, (iii) thus freeing Pt sites and (iv) yielding COad surface diffusion from NiUPD to Pt sites. The freed NiUPD sites are unstable at the considered electrode potential and (v) strip off, (vi) freeing another Pt site. The three processes (iv) CO-surface diffusion, (v) nickel-stripping and (vi) Pt-free site formation can then continue until all the NiUPD atoms are stripped from the Pt(1 1 0)-(1 × 2) surface and the CO-adlayer is replenished over Pt surface, as depicted schematically in Fig. 6.2 Once this process is complete and if the potential has not been swept up to water nucleation potential for Pt(1 1 0)-(1 × 2), the resulting surface resembles a COad -covered Pt(1 1 0)-(1 × 2) surface for which CO-adlayer has been built at potentials positive to the hydrogen region. The voltammogram of the second cycle in Fig. 2 is totally consistent with such scenario. 5. Conclusion Using the EQCM technique enabled to determine nickel formal partial charge number: ιNi (which we considered equivalent to its valency of electrosorption: νNi ) from direct gravimetric/electrochemical coupling measurement for polycrystalline platinum electrodes in sulphate media: ιNi,Pt(QCM) = 1.3 ± 0.13. On this basis, we did validate the use of Swathirajan and Bruckenstein pure electrochemical method for ιNi determination for QCM polycrystalline platinum electrodes: ιNi,Pt(poly) = 1.5 ± 0.17, and applied it for Pt(1 1 0)-(1 × 2) electrodes. Nickel formal partial charge number was then determined to be ca. ιNi,Pt(1 1 0) ≈ 1.4 ± 0.1 for Ni-underpotential deposition over Pt(1 1 0)-(1 × 2) in sulphate media. This value compares well with that for Pt(poly). Such low nickel formal partial charge number, inferior compared to the charge of nickel cations 2 The scheme of Fig. 6 does not take into account the real arrangement of the Pt(1 1 0)-(1 × 2) surface (missing row-configuration); we do neither know where NiUPD atom stand (e.g. in the missing row?) nor whether the NiUPD domains are 2D or 1D. More knowledge about Ni-adsorption site could be provided by additional in situ physical experiments (i.e. XAS or electrochemical STM), which is beyond the scope of the present paper.
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in solution, signs (bisulphate) anion adsorption over nickel adatoms, and shows NiUPD adatoms remain partially oxidized after adsorption over Pt(1 1 0)-(1 × 2) in sulphate media. In the presence of COad , which displaces the anions from the nickel adlayer, NiUPD /Pt(1 1 0)-(1 × 2) surfaces nucleate water at lower potentials than Pt(1 1 0)-(1 × 2), resulting in more facile COad -stripping, probably following oxygen back-donation to COad species from (partially oxidized) NiUPD adatoms. The mechanism on such surfaces resembles the well-known bifunctional mechanism, but differs in a way that COad species repopulate the Pt sites in the course of the stripping, inducing NiUPD adatoms stripping. Acknowledgements The authors are deeply thankful to Fr´ed´eric Maillard for his helpful advices and Guy Espagnac for elaborating the Pyrex electrochemical cells. References [1] M. Chatenet, R. Faure, Y. Soldo-Olivier, J. Electroanal. Chem. 580 (2005) 275. [2] A.A. El-Shafei, J. Electroanal. Chem. 447 (1998) 81. [3] K. Franaszczuck, J. Sobkowski, Surf. Sci. 204 (1998) 530. [4] H.R. Bojkov, H.R. Tzvetkova, S. Rashkov, Bulg. Bull. Electrochem. 7 (1991) 578. [5] S. Kania, R. Holze, Surf. Sci. 408 (1998) 252. [6] M.D. Obradovic, B.N. Grgur, L.M. S Vracar, International Society of Electrochemistry 55th Annual Meeting, Thessaloniki, 2004. [7] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, Surf. Sci. 384 (1997) L805. [8] M. Chatenet, R. Faure, Y. Soldo-Olivier, Proceedings of the Electrochemical Society—Seventh international Symposium on Electrode Processess 206th Meeting Abstract #2282, Honolulu, 2004. [9] K. Wang, G.S. Chottiner, D.A. Scherson, J. Phys. Chem. 96 (1992) 6742. [10] M. Zhao, K. Wang, D.A. Scherson, J. Phys. Chem. 97 (1993) 4488. [11] C.F. Zinola, E.J. Vasini, U. Mu¨eller, H. Baltruschat, A.J. Arvia, J. Electroanal. Chem. 415 (1996) 165. [12] A. Cuesta, C. Gutierrez, J. Phys. Chem. 100 (1996) 12600. [13] R. Karimi Shervedani, A. Lasia, J. Appl. Electrochem. 29 (1999) 979. [14] M. Chatenet, Y. Soldo-Olivier, E. Chainet, R. Faure, Electrochem. Commun. 9 (2007) 1463. [15] M. Hepel, in: A. Wieckowski (Ed.), Interfacial Electrochemistry—Theory, Experiment and Application, Marcel Dekker Inc., New York, 1999, p. 599. [16] S. Swathirajan, S. Bruckenstein, Electrochim. Acta 28 (1983) 865. [17] F. Gloaguen, J.-M. L´eger, C. Lamy, J. Electroanal. Chem. 467 (1999) 186. [18] F. Vigier, F. Gloaguen, J.-M. L´eger, C. Lamy, Electrochim. Acta 46 (2001) 4331. [19] N.M. Markovic, P.N. Ross, Surf. Sci. Rep. 45 (2002) 121. [20] A. Lopez-Cudero, A. Cuesta, C. Gutierrez, Electrochem. Commun. 6 (2004) 395. [21] A. Lopez-Cudero, A. Cuesta, C. Gutierrez, J. Electroanal. Chem. 579 (2005) 1. [22] R.J. Behm, Personal Communication, 2006. [23] J. Mostany, E. Herrero, J.M. Feliu, J. Lipkowski, J. Electroanal. Chem. 558 (2003) 19. [24] E. Herrero, J. Mostany, J.M. Feliu, J. Lipkowski, J. Electroanal. Chem. 534 (2002) 79. [25] A.M. Funtikov, U. Stimming, R. Vogel, J. Electroanal. Chem. 428 (1997) 147. [26] F.E. Woodard, C.L. Scortichini, C.N. Reilley, J. Electroanal. Chem. 151 (1983) 109. [27] L. Blum, N. Marzari, R. Car, J. Phys. Chem. B 108 (2004) 19670.
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Understanding CO-stripping mechanism from NiUPD/Pt(1 1 0) in view of the measured nickel formal partial charge number upon underpotential deposition on platinum surfaces in sulphate media Marian Chatenet ∗ , Yvonne Soldo-Olivier, Eric Chaˆınet, Ren´e Faure Laboratoire d’Electrochimie et de Physicochimie des Mat´eriaux et des Interfaces, LEPMI, UMR 5631 CNRS-INPG-UJF, BP 75, 38402 Saint-Martin d’H`eres Cedex, France Received 19 October 2006; received in revised form 3 January 2007; accepted 17 January 2007 Available online 30 January 2007
Abstract We recently showed nickel-underpotential deposition (Ni-UPD) occurs on polycrystalline or single crystal platinum electrodes in acidic media. Whereas the decoupling of the nickel and hydrogen adsorption/desorption peaks is difficult for low pH, these processes can be better separated for higher pH values, typically pH > 3. However, even for platinum single crystals, high pH solutions do not enable to sufficiently separate nickel from hydrogen phenomena. As a result, electrochemistry alone cannot yield important information about Ni-UPD, such as the formal partial charge number (valency of electrosorption) and the role of the sulphate or hydrogen sulphate anions. So, we decided to couple cyclic voltammetry to electrochemical quartz crystal microbalance (EQCM). EQCM measurements enable to decorrelate the simultaneous hydrogen and nickel adsorption/desorption peaks, which we could not attempt solely with electrochemistry. The coupling between gravimetric and electrochemical measurements allows us to detect the contribution of the anions and thus to isolate that of nickel: nickel coverage can then be determined. Nearly 4/5 NiUPD monolayer (θ Ni ≈ 0.8) over platinum is reached at nickel equilibrium potential for high pH solutions (5.5). The QCM and electrochemistry coupling further allows the determination of nickel formal partial charge number: ιNi,EQCM = 1.3 ± 0.13. Direct electrochemistry measurements (Swathirajan and Bruckenstein method) yield: ιNi,Pt(poly) = 1.5 ± 0.17. These two values are close, which validates the electrochemical method for the nickel/platinum system. In consequence, we used Swathirajan and Bruckenstein method for Pt(1 1 0)-(1 × 2) crystal and found: ιNi,Pt(1 1 0) ≈ 1.4 ± 0.1. Whatever the system (NiUPD /Pt(poly) or NiUPD /Pt(1 1 0)-(1 × 2)) or the experimental technique, nickel formal partial charge number is lower than nickel cation charge: ιNi < zNi = 2. In consequence, upon underpotential deposition on platinum surfaces, nickel cations discharge and then undergo additional charge exchange processes, such as anion (or water) adsorption, resulting in apparent partial nickel cation discharge. Moreover, NiUPD /Pt(1 1 0) surface displays high activity towards COad oxidation reaction. We explain such positive effect by the possible existence of a bifunctional mechanism in which oxygenated-species-covered NiUPD adatoms provide the oxygen atom to COad · · ·Pt species, enabling its facile oxidation. © 2007 Elsevier Ltd. All rights reserved. Keywords: Nickel-underpotential deposition; Pt(1 1 0); EQCM; Formal partial charge number; CO-stripping
1. Introduction We recently published [1] that nickel-underpotential deposition (UPD) on platinum, already monitored for bulk polycrystalline platinum [2–6], is favoured for the Pt(1 1 0) orientation, which in electrolytic medium is reconstructed into Pt(1 1 0)-(1 × 2) [1,7]. As nickel adsorption/desorption occurs
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in the hydrogen-UPD zone in acidic solutions, we attempted to higher the medium pH in order to isolate nickel-related phenomena and to further understand nickel-underpotential deposition mechanism. However, for pH > 3, the voltammograms exhibit many features, such as unclear hydrogen evolution region following interfacial pH changes upon platinum polarization negative to hydrogen equilibrium potential or anionic solution composition changes (HSO4 − predominate for pH < 2, while SO4 2− predominate for pH > 2), which are difficult to characterize [8]. In another approach, we attempted to use CO as a probe to evidence the presence of NiUPD and (if possible) to determine
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ιNi,Pt(QCM) = νNi,Pt(QCM) = 1.3 ± 0.13. Let we note at that point that the Pt(QCM) electrodes could be considered as polycrystalline platinum [14]. This value, calculated from the QCM and electrochemistry coupling, was also confirmed by a pure electrochemical method introduced by Swathirajan and Bruckenstein [16]: ιNi,Pt(poly) = 1.5 ± 0.17. The object of the present paper is to take benefit of the EQCM results to validate the use of Swathirajan and Bruckenstein pure electrochemical method for nickel formal partial charge number determination for Pt(1 1 0) single crystal. Then, the remarkable activity of NiUPD /Pt(1 1 0)-(1 × 2) surface, already evidenced in reference [1] (see Fig. 1), will be further investigated and discussed knowing nickel formal partial charge number value upon UPD over Pt(1 1 0)-(1 × 2). 2. Experimental procedures Fig. 1. COad , NiUPD and (COad + NiUPD )-stripping voltammograms following 3 min CO-adsorption and 15 min N2 flushing or 18 min Ni-deposition at −0.005 V vs. NHE; Pt(1 1 0)-(1 × 2) in 0.1 M H2 SO4 + 10−3 M NiSO4 ; S = 0.099 cm2 ; sweep rate 50 mV s−1 . Replotted from the data of Fig. 8 in Chatenet et al. [1], with permission from Elsevier.
nickel coverage. The voltammograms typically obtained for CO-stripping, Ni-stripping and (CO + Ni)-stripping (Fig. 1) helped us to draw the following basic conclusions [1]: • nickel-UPD occurs in the presence of CO and thus nickel exhibits high affinity for Pt(1 1 0)-(1 × 2) surfaces; • CO adsorbs on the nickel submonolayer and COad species protect NiUPD from dissolution; • COad -stripping is significantly favoured in the presence of NiUPD compared to Pt(1 1 0)-(1 × 2) surfaces. However, interesting, such experiments could neither provide a clear knowledge about NiUPD coverage (since CO adsorbs both on Pt and NiUPD /Pt, which is forecast from experiments on bulk nickel [9–12] or nickel alloys [13]) nor about its mechanism. Thus, we tried to use a coupled technique, the electrochemical quartz crystal microbalance (EQCM), so as to isolate the different processes linked to nickel-underpotential deposition on platinum, and further get more insight into its mechanism [14]. This technique has already been used in the past to study UPD of various metals and their interaction with anions [15]. The coupling between gravimetric measurements and electrochemistry enabled to subtract the contributions of hydrogen/anions and then to isolate that of nickel. From the resulting nickel coverage determination, we calculated nickel formal partial charge number (assimilated to the electrosorption valency)1 for the Pt(QCM) electrode, 1 The term valency of electrosorption was subject to criticism, as summarized by Trasatti and Parsons [28]. They concluded that the term formal partial charge number (ιB ) is more correct on a conceptual basis than that of valency of electrosorption (νB ), even if both correspond to the same definition. Therefore, we
Our experiments on Pt(1 1 0) were conducted using a classical four-electrode Pyrex cell, paying close attention to the glassware cleanness, as already thoroughly described in reference [1]. The EQCM experiments were conducted using a 5 MHz EQCM polycrystalline platinum crystals (Ti/Pt polished, geometric area = 1.37 cm2 , MAXTEK), in a devoted QCM cell, using a computer-controlled PM-710 plating monitor (MAXTEK) [14]. The sensitivity of our QCM apparatus was 0.1 Hz, which translates into 2.4 ng. In order to guaranty the EQCM platinum/quartz crystals cleanness, we did wash the platinum/quartz crystal in pure water (18.2 M cm–3 ppb TOC, Millipore Elix® + Gradient® ) before each experiment. The Pt(QCM) holder and electrode were cleaned according to the procedure described in [14]. After mounting in the electrochemical cell, the Pt(QCM) electrode underwent ca. 300 voltammetric cycles from the hydrogen evolution region to the platinum oxide formation region (typically between 0.005 and 1.25 V versus NHE), in order to stabilize its electrochemical response and further clean the platinum surface. Such procedure also favours the rearrangement of the superficial platinum atoms, until the voltammetry does not vary anymore [17,18]. This corresponds to stable platinum surface and gravimetric signal. The voltammetry experiments were controlled by a potentiostat PAR 263 (EG&G) connected to a saturated calomel electrode (SCE: +0.241 V versus NHE); all electrode potentials are nevertheless expressed versus the normal hydrogen electrode (NHE) potential. All the experiments were carried out under inert atmosphere (Ar N45, Air-Liquide) at room temperature. COad -stripping experiments were conducted after CO-adsorption at fixed potential: −5 mV versus NHE under pure CO bubbling (CO N47-Alphagaz) followed by sufficient argon flushing to remove any trace of dissolved CO in the electrolyte [1]. The electrochemical techniques employed are those thoroughly described in references [1,14].
will assimilate these two values as identical in the manuscript and use only formal partial charge number; we point out that most papers of the literature only refer to valency of electrosorption.
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Fig. 2. First three cycles for (COad + NiUPD )-stripping voltammograms following 3 min CO-adsorption and 15 min N2 flushing at −0.005 V vs. NHE; Pt(1 1 0)-(1 × 2) in 0.1 M H2 SO4 + 10−3 M NiSO4 ; S = 0.099 cm2 ; potential scan rate 50 mV s−1 ; first cycle from −0.005 to 0.545 V vs. NHE (bold grey); second (bold black), third (thin black) and reference (cross, the same as in Fig. 1; top) scans from −0.005 to 0.995 V vs. NHE.
All solutions were prepared using pure water. The sulphuric acid, sodium sulphate and nickel sulphate were all purchased from Merck in the Suprapur® quality. 3. Results 3.1. COad -stripping for NiUPD -covered Pt(1 1 0)-(1 × 2) Fig. 1 shows the first cycle of a typical COad -stripping voltammogram for a Pt(1 1 0)-(1 × 2) electrode in 0.1 M H2 SO4 (lower black curve). The Pt-surface is blocked in the hydrogen desorption zone, while a well defined COad -stripping peak is centred at 0.665 V versus NHE (peak 1). The small anodic feature at 0.39 V versus NHE (labelled peak 1 ) can be ascribed to the oxidation of part of the COad monolayer (ca. 10%), namely the weakly adsorbed CO [19], which is usual when CO is adsorbed in the HUPD region [19–21]. When nickel is present and no CO is bubbled in the solution during the waiting time at the initial potential (middle dark grey curve), the voltammogram still displays some H-desorption at ca. 50 mV versus NHE. In addition, another anodic peak is centred at ca. 0.28 V versus NHE; it can be ascribed to NiUPD -stripping. Such features have been fully described in [1] and show that under these experimental conditions, NiUPD coverage is below unity: θ Ni < 1. The COad -stripping voltammogram in the presence of nickel (higher grey curve) shows 2 stripping peaks (labeled peak 2 and peak 1) at ca. 0.410 and 0.645 V versus NHE, respectively. The latter resembles that for the COad -stripping from Pt(1 1 0)-(1 × 2) in the absence of nickel (see lower curve, peak 1), both in peak and onset positions and in coulometry: it should correspond to the stripping of COad /Pt(1 1 0)-(1 × 2). On the contrary, peak 2 should be related to the presence of both NiUPD and COad . The voltammograms from Fig. 2 clearly exhibit that peaks 1 and 2 are indeed corresponding to very different phenomena. When the potential is reversed at 0.545 V versus NHE (positive to (COad + NiUPD )-stripping: peak 2), the following complete voltammogram (from −0.005 to 0.995 V versus NHE) only exhibits peak 1. Moreover, after the first (COad + NiUPD )-layer
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Fig. 3. H or (H and Ni)-adsorption/desorption voltammograms on Pt(1 1 0)(1 × 2) in 0.1 M Na2 SO4 + 10−3 M H2 SO4 (pH ≈ 3) in the absence (black) or presence (grey) of 10−3 M NiSO4 for θ Ni ≈ 1 (bold) or θ Ni > 1 (thin); SPt = 0.099 cm2 ; sweep rate = 2 mV s−1 .
stripping cycle (peak 2), peak 1 potential, shape and coulometry are barely changed compared to the reference voltammogram (see Fig. 2, bold black and cross traces). In addition, the main difference between the COad -stripping voltammogram from Pt(1 1 0)-(1 × 2) (Fig. 1, lower black trace) and the second cycle of (COad + Niad )-stripping (Fig. 2, bold black trace) is the absence of anodic feature (peak 1 ) monitored at 0.39 V versus NHE for COad -stripping from Pt(1 1 0)-(1 × 2) when CO is adsorbed in the hydrogen region. Such findings agree with the fact that after the first cycle of Fig. 2, the Pt(1 1 0)-(1 × 2) electrode behaves as if it was only covered by a full COad layer adsorbed positive to hydrogen region; this peculiar result will be discussed in Section 4. After COad and NiUPD stripping, the Pt(1 1 0)-(1 × 2) surface is recovered (Fig. 2, third cycle). 3.2. Ni-UPD on Pt(1 1 0)-(1 × 2) for pH ≥ 3 As thoroughly discussed in references [1,14] separating the hydrogen and nickel-UPD peaks is easier for high pH. It also enables to reach higher nickel coverage, as the potential can be swept down to nickel reversible (Nernst) potential without dramatic masking from hydrogen adsorption/desorption and evolution/oxidation peaks/waves (nickel Nernst potential is ca. −0.32 V versus NHE in the present conditions). These are indeed hindered by harsh proton diffusion limitation when [H+ ] < 10−3 M [14]. Fig. 3 reveals the possibility to reach the full nickel layer (UPD, grey bold trace) and even nickel overpotential (bulk) deposition (OPD, grey thin trace) when the potential is swept below nickel Nernst potential. Ni-deposition peaks are not well resolved (despite NiUPD onset positive to HUPD onset), following either overlap with H-adsorption peak and HER wave or Ni2+ diffusion limitation. On the contrary, both NiUPD and NiOPD -stripping peaks are well defined at ca. 0.2 and −0.125 V versus NHE, respectively. Fig. 3 also clearly shows that NiUPD -stripping peak does not change for various nickel coverage (provided θ Ni ≥ 1). From such voltammograms, it is not possible to determine nickel formal partial charge number, because neither nickel adsorption site, nor the net charge exchange with the surface and/or the solution are known.
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and: mNi tot = − 133θNi − 2.3(1 − θNi )θH − 219θSO4 /Ni + 219θSO4 (2 )
Fig. 4. (H and Ni)-adsorption/desorption voltammograms (primary Y-axis) and corresponding gravimetric signals (secondary Y-axis) for Pt(QCM) in 0.1 M Na2 SO4 (pH ≈ 5.5) in the absence (black) or presence (grey) of 6 × 10−4 M NiSO4 ; sweep rate = 2 mV s−1 .
3.3. Using the QCM to monitor Ni-UPD on polycrystalline platinum at pH > 3 Typical examples of EQCM results for the study of Ni-UPD in sodium sulphate solution are presented in Fig. 4. The voltammograms are not easily understood, as nickel and hydrogen related peaks overlap. Such overlap is more severe for polycrystalline platinum, as the hydrogen region is wider than for Pt(1 1 0), due to the contribution of other platinum orientations. However, the gravimetric/voltammetric coupling enables the determination of nickel and hydrogen/anions contributions, as detailed in reference [14]. First, we determine in supporting electrolyte without NiSO4 , the a priori unknown proton (θ H ) and anion (θSO4 ) coverage over platinum, using the charge balance: Qtot = 220θH + 440θSO4
(1)
where Qtot = 220 C cm−2 Pt is the measured charge related to the positive scan. We assume 220 and 440 C cm−2 Pt for 1 ML Had and 1 ML SO4,ad , respectively, expressing the full discharge of the ions upon specific adsorption [22–24]. The mass conservation yields: mtot = 219θSO4 − 2.3θH
(2)
where mtot = 36 ng cm−2 Pt is the measured mass variation during the positive scan, assuming 2.3 and 219 ng cm−2 Pt for 1 ML Had and 1 ML SO4,ad , respectively. The EQCM data thus yield: θ H = 0.66 and θSO4 = 0.17, in agreement with the bisulphate and proton saturation coverage measured by Funtikov et al. [25], Herrero et al. [24] or Mostany et al. [23] for Pt(1 1 1). In the presence of NiSO4 , the system becomes: QNi tot = 440θNi − 440θSO4 /Ni + 220(1 − θNi )θH + 440θSO4 (1 )
−2 Pt and mNi = −126 ng cm−2 Pt are where QNi tot = 345 mC cm tot the measured charge and mass variations related to the positive scan, and θ Ni and θSO4 /Ni are the NiUPD and the anion (bisulphate) over NiUPD coverage, respectively. Eqs. (1 ) and (2 ) yield: θ Ni ≈ 0.80 and θSO4 /Ni ≈ 0.26. These calculations assume (i) H and Ni coadsorption over platinum at low potentials, (ii) Had coverage on the NiUPD -free zones of Pt equal to θ H (1 − θ Ni ), (iii) possible bisulphate adsorption over NiUPD and (iv) mainly bisulphate adsorption over platinum at high potentials (as shown by Funtikov et al. [25] and Woodard et al. [26]). Such coverage values yield nickel formal partial charge number of ca. 1.4, which expresses that bisulphate anions concomitantly adsorb over Niad during their UPD over platinum. We point out here that the calculation was undertaken for potentials corresponding to NiUPD desorption, monitored in the range [0.03–0.4] V versus NHE on the grey voltammogram plotted in nickel-containing solution. For greater accuracy and determination of the experimental error, the calculation of θ Ni was repeated for several identical experiments, attempted when the deposition of nickel did not reach the overpotential deposition, but remained at constant coverage (θ Ni = 0.75, see Table 1 in reference [14]). Then, using the NiUPD -stripping peak coulometry, we determined the average nickel formal partial charge number in the present experimental conditions: ιNi,Pt(QCM) = 1.3 ± 0.13 [14]. This average value agrees with that determined from a direct electrochemical method: ιNi,Pt(poly) = 1.5 ± 0.17 [14], first published by Swathirajan and Bruckenstein [16], as described below.
3.4. Determination of nickel formal partial charge number for Pt(1 1 0)-(1 × 2) substrate The method of Swathirajan and Bruckenstein [16] consists of plotting the difference between nickel-stripping peak potential (at given nickel coverage) and nickel Nernst potential, according to Eq. (3) [2,16]: 1 RT 1 2+ Ep (ML) − ENernst (Ni ) = K + ln[Ni2+ ] − F ιNi zNi (3) where Ep (ML) is the monolayer peak potential (determined as the average of nickel deposition peak potential and nickelstripping peak potential, following Blum et al. conclusions [27]), ENernst (Ni2+ ) the nickel reversible potential, K a constant, zNi = 2 the nickel ion charge, ιNi the nickel formal partial charge number, [Ni2+ ] the nickel ion concentration, assimilated as nickel ion activity and R, T and F have their usual meanings. Note that in their paper, Swathirajan and Bruckenstein were referring to the term valency of electrosorption, which we assimilate to the formal partial charge number, following the conclusions from Trasatti and Parsons [28], Lang and Horanyi [29–31] or de Levie [32].
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Fig. 5. Evolution of the averaged NiUPD -peak potential measured at constant coverage (θ Ni ≈ 0.8) on Pt(1 1 0)-(1 × 2) in 0.1 M Na2 SO4 (pH ≈ 5.5) containing increasing amounts of NiSO4 ; SPt = 0.099 cm2 ; sweep rate = 2 mV s−1 .
Using Eq. (3) for Pt(1 1 0)-(1 × 2) at pH 5.5 (for which the NiUPD desorption peaks are well defined) for three experiments, we find: νNi,Pt(1 1 0) = 1.4 ± 0.1 (Fig. 5). Such nickel formal partial charge number value is, within the error bars, identical to that for nickel/polycrystalline platinum system, as stated in the previous section. Note that Ep (ML) determination suffers non negligible experimental error (±7 mV averaged on the three experiments), which induces uncertainty for the slope determined from Fig. 5. Thus, the resulting uncertainty for ιNi,Pt(1 1 0) is non negligible: ±0.5. This value exceeds the good experimental reproducibility (±0.1), calculated from three identical experiments; its smallness shows the reproducibility of our protocol. Nevertheless, we wanted to validate Swathirajan and Bruckenstein experimental method using the more precise EQCM technique on the first hand. Now, despite the electrochemical technique lower accuracy than for EQCM, the results for Pt(1 1 0)-(1 × 2) strongly attest that nickel adatoms undergo anion or water adsorption after their underpotential deposition (and Ni2+ discharge) over platinum single crystal. 4. Discussion The literature does not provide a clear knowledge about nickel formal partial charge number corresponding to nickel adsorption over platinum in acidic medium. Whereas El-Shafei found ιNi = 2 [2], Franaszczuck and Sobkowski measured: ιNi = 1 [3], both paper dealing with polycrystalline platinum electrodes. The nickel formal partial charge number values we measured from EQCM and electrochemistry for Ni-UPD onto Pt(poly) in sulphate media are close, which validates both techniques. Thus, we decided to use the electrochemical method (from Swathirajan and Bruckenstein [16]) to determine nickel formal partial charge number on Pt(1 1 0)-(1 × 2) in sulphate media: ιNi,Pt(1 1 0) = 1.4 ± 0.1. This latter value is, within the error bars, equal to that for Pt(poly). However, we cannot rule out that single crystalline and polycrystalline surfaces could yield different formal partial charge number, for example following their different potential of zero charge or work function values (see for example [33,34]). We nevertheless emphasize that such measured ιNi values are consistent with the fact that Ni2+ discharge is apparently
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incomplete upon nickel UPD on platinum surfaces in sulphate medium. Difference between one element formal partial charge number and its ionic charge is not common for metal UPD over metal supports. In such cases, it is usually close to the charge of the ion deposited, expressing the total discharge of this ion upon adsorption [35]. However, since nickel is usually oxidized superficially in acidic medium, one cannot exclude the possibility for nickel adatoms to undergo water (OHad or Oad ) adsorption right after their adsorption on platinum. Second, nickel potential of zero charge is much lower than that of platinum [34,36] and it could lead to further anion adsorption after its own adsorption on platinum. However, Nakamura et al. showed that at pH 5.5, nickel oxide formation on Ni(1 1 1) surfaces starts above 0 V versus NHE [37], positive to Ni-UPD onset. So, the most likely explanation for nickel formal partial charge number lower than 2 is anion coadsorption (anion adsorption on NiUPD /Pt), as similarly monitored for the CuUPD (Au) system [38]. Additional in situ experiments, like X-ray absorption spectroscopy (XAS) or electrochemical scanning tunnelling microscopy (ESTM) could enable further discussion. Nevertheless, we think all our results are in line with the fact that NiUPD atoms are more oxophilic than the underlying Pt atoms. As a result, NiUPD /Pt layer is covered by anions at electrode potentials for which the platinum substrate is not, either for polycrystalline or single crystal electrodes. In the presence of COad at low potentials (in the hydrogen region), NiUPD /Pt(1 1 0)-(1 × 2) is probably free of water/anions adsorbate, following its potential of zero charge positive shift (or CO-induced displacement), as documented in references [39–41]. Upon potential increase starting from Ni and CO deposition potential (−0.005 V versus NHE), NiUPD adatoms may nucleate water (OHad species) at potentials negative to platinum (Eq. (4)), as expected from the determined formal partial charge number for nickel: NiUPD + H2 O → NiUPD · · ·OHad + H+ + e−
(4)
Then the fraction of the OHad which is located at the edge of the partial NiUPD layer will react with COad species adsorbed on the closest platinum atom, according to Eq. (5): NiUPD · · ·OHad + Pt· · ·COad → NiUPD + Pt + CO2 + H+ + e−
(5)
NiUPD /Pt(1 1 0)-(1 × 2) adatoms thus act as an oxygen provider for COad species over Pt atoms. In a way, such mechanism resembles the well known bifunctional mechanism [42] and occurs under peak 2 in the (COad + NiUPD )-stripping voltammograms of Figs. 1 and 2. Now, one may wonder why the NiUPD · · ·OHad species would not react with NiUPD · · ·COad species which are also adsorbed over the NiUPD /Pt(1 1 0)-(1 × 2) adlayer (see Fig. 1 and reference [1]). If it were the case, the COad and underlying NiUPD adatoms would be stripped concomitantly from the Pt surface as soon as COad species are stripped, thus freeing the underlying Pt(1 1 0) atoms. As a result, we should recover the fraction of the platinum surface which was covered by NiUPD atoms free of COad . Thus, we should only monitor a fraction (in term of coulometry) of peak 1 in the (COad + NiUPD )stripping voltammograms. Conversely, Figs. 1 and 2 show that
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Fig. 6. Reaction scheme for NiUPD and COad -stripping from Pt(1 1 0)-(1 × 2) surface; OHad first nucleates on NiUPD (step 1), combines with the COad species from the nearest Pt site, yielding CO2 (step 2), then COad diffuses from NiUPD to the freed Pt site (step 3), freeing one NiUPD site; NiUPD is not stable at such electrode potential and strips out (step 4); COad diffusion process/NiUPD -stripping can start again (step 5 and step 4) until the full NiUPD layer is stripped, and the (almost) full COad layer is formed onto the Pt(1 1 0)-(1 × 2) surface.
peak 1 coulometry (and potential) basically correspond to the stripping of a full CO-adlayer over Pt(1 1 0)-(1 × 2) surface, exhibiting that the surface created after peak 2 resembles a COad covered Pt(1 1 0)-(1 × 2) surface. Moreover, should Ni· · ·OHad species react with Ni· · ·COad species, the coulometry for peak 2 would be roughly twice bigger (corresponding to COad and NiUPD concomitant stripping, as pointed out by Karimi Shervedani and Lasia for CO-stripping from Ni surfaces in alkaline medium [13]). As stated above, the presence of peak 1 for (COad + NiUPD )-stripping experiment, the coulometry of which is close to that for COad -stripping voltammograms in the absence of nickel in solution, is a strong evidence that after peak 2, the
CO-adlayer has been rebuilt over the Pt(1 1 0)-(1 × 2) surface freed from any NiUPD atoms. The fast COad diffusion coefficient on nickel substrates, as monitored in UHV conditions for nickel crystals [43–47] or for Ni/W(1 1 0) surfaces [48], is consistent with such scenario. In addition, COad displays a greater stability over platinum surface, at least for singles crystals in the (1 1 1) orientation in UHV conditions [48]. The conjunction of these two findings is consistent with the possibility for COad to simply diffuse (superficially) from NiUPD sites to the neighbouring free Pt(1 1 0) sites. Doing so, they would (i) replenish the COadlayer over Pt(1 1 0)-(1 × 2) and (ii) free NiUPD surface from its COad protection. As this occurs at potentials greater than
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ca. 0.25 V versus NHE (positive to the NiUPD -stripping peak potential, as seen on Fig. 1; middle grey curve), NiUPD atoms are not longer stable over the Pt(1 1 0)-(1 × 2) substrate and are then instantaneously stripped (in peak 2) upon COad diffusion down to the Pt sites. Let we point out that (COad + NiUPD ) oxidation has already been monitored on a single peak (as in peak 2) for polycrystalline nickel surfaces in alkaline media by Karimi Shervedani and Lasia [13], which goes in line with our data. Moreover, the CO-adlayer over Pt(1 1 0)-(1 × 2) replenishment originating from such COad surface diffusion occurs at potential positive to the hydrogen region on Pt(1 1 0)-(1 × 2) (see Fig. 1). In consequence, the COad -stripping of such replenished COadlayer over Pt(1 1 0)-(1 × 2) (free of any NiUPD atoms) occurs only in peak 1, without the small anodic feature (peak 1 ) at 0.39 V versus NHE (Fig. 2), which is linked to COad -stripping when CO is adsorbed in the hydrogen region [19]. To summarize, as the potential increases, (i) NiUPD atoms nucleate water (OHad ), which (ii) reacts with Pt· · ·COad species, (iii) thus freeing Pt sites and (iv) yielding COad surface diffusion from NiUPD to Pt sites. The freed NiUPD sites are unstable at the considered electrode potential and (v) strip off, (vi) freeing another Pt site. The three processes (iv) CO-surface diffusion, (v) nickel-stripping and (vi) Pt-free site formation can then continue until all the NiUPD atoms are stripped from the Pt(1 1 0)-(1 × 2) surface and the CO-adlayer is replenished over Pt surface, as depicted schematically in Fig. 6.2 Once this process is complete and if the potential has not been swept up to water nucleation potential for Pt(1 1 0)-(1 × 2), the resulting surface resembles a COad -covered Pt(1 1 0)-(1 × 2) surface for which CO-adlayer has been built at potentials positive to the hydrogen region. The voltammogram of the second cycle in Fig. 2 is totally consistent with such scenario. 5. Conclusion Using the EQCM technique enabled to determine nickel formal partial charge number: ιNi (which we considered equivalent to its valency of electrosorption: νNi ) from direct gravimetric/electrochemical coupling measurement for polycrystalline platinum electrodes in sulphate media: ιNi,Pt(QCM) = 1.3 ± 0.13. On this basis, we did validate the use of Swathirajan and Bruckenstein pure electrochemical method for ιNi determination for QCM polycrystalline platinum electrodes: ιNi,Pt(poly) = 1.5 ± 0.17, and applied it for Pt(1 1 0)-(1 × 2) electrodes. Nickel formal partial charge number was then determined to be ca. ιNi,Pt(1 1 0) ≈ 1.4 ± 0.1 for Ni-underpotential deposition over Pt(1 1 0)-(1 × 2) in sulphate media. This value compares well with that for Pt(poly). Such low nickel formal partial charge number, inferior compared to the charge of nickel cations 2 The scheme of Fig. 6 does not take into account the real arrangement of the Pt(1 1 0)-(1 × 2) surface (missing row-configuration); we do neither know where NiUPD atom stand (e.g. in the missing row?) nor whether the NiUPD domains are 2D or 1D. More knowledge about Ni-adsorption site could be provided by additional in situ physical experiments (i.e. XAS or electrochemical STM), which is beyond the scope of the present paper.
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in solution, signs (bisulphate) anion adsorption over nickel adatoms, and shows NiUPD adatoms remain partially oxidized after adsorption over Pt(1 1 0)-(1 × 2) in sulphate media. In the presence of COad , which displaces the anions from the nickel adlayer, NiUPD /Pt(1 1 0)-(1 × 2) surfaces nucleate water at lower potentials than Pt(1 1 0)-(1 × 2), resulting in more facile COad -stripping, probably following oxygen back-donation to COad species from (partially oxidized) NiUPD adatoms. The mechanism on such surfaces resembles the well-known bifunctional mechanism, but differs in a way that COad species repopulate the Pt sites in the course of the stripping, inducing NiUPD adatoms stripping. Acknowledgements The authors are deeply thankful to Fr´ed´eric Maillard for his helpful advices and Guy Espagnac for elaborating the Pyrex electrochemical cells. References [1] M. Chatenet, R. Faure, Y. Soldo-Olivier, J. Electroanal. Chem. 580 (2005) 275. [2] A.A. El-Shafei, J. Electroanal. Chem. 447 (1998) 81. [3] K. Franaszczuck, J. Sobkowski, Surf. Sci. 204 (1998) 530. [4] H.R. Bojkov, H.R. Tzvetkova, S. Rashkov, Bulg. Bull. Electrochem. 7 (1991) 578. [5] S. Kania, R. Holze, Surf. Sci. 408 (1998) 252. [6] M.D. Obradovic, B.N. Grgur, L.M. S Vracar, International Society of Electrochemistry 55th Annual Meeting, Thessaloniki, 2004. [7] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, Surf. Sci. 384 (1997) L805. [8] M. Chatenet, R. Faure, Y. Soldo-Olivier, Proceedings of the Electrochemical Society—Seventh international Symposium on Electrode Processess 206th Meeting Abstract #2282, Honolulu, 2004. [9] K. Wang, G.S. Chottiner, D.A. Scherson, J. Phys. Chem. 96 (1992) 6742. [10] M. Zhao, K. Wang, D.A. Scherson, J. Phys. Chem. 97 (1993) 4488. [11] C.F. Zinola, E.J. Vasini, U. Mu¨eller, H. Baltruschat, A.J. Arvia, J. Electroanal. Chem. 415 (1996) 165. [12] A. Cuesta, C. Gutierrez, J. Phys. Chem. 100 (1996) 12600. [13] R. Karimi Shervedani, A. Lasia, J. Appl. Electrochem. 29 (1999) 979. [14] M. Chatenet, Y. Soldo-Olivier, E. Chainet, R. Faure, Electrochem. Commun. 9 (2007) 1463. [15] M. Hepel, in: A. Wieckowski (Ed.), Interfacial Electrochemistry—Theory, Experiment and Application, Marcel Dekker Inc., New York, 1999, p. 599. [16] S. Swathirajan, S. Bruckenstein, Electrochim. Acta 28 (1983) 865. [17] F. Gloaguen, J.-M. L´eger, C. Lamy, J. Electroanal. Chem. 467 (1999) 186. [18] F. Vigier, F. Gloaguen, J.-M. L´eger, C. Lamy, Electrochim. Acta 46 (2001) 4331. [19] N.M. Markovic, P.N. Ross, Surf. Sci. Rep. 45 (2002) 121. [20] A. Lopez-Cudero, A. Cuesta, C. Gutierrez, Electrochem. Commun. 6 (2004) 395. [21] A. Lopez-Cudero, A. Cuesta, C. Gutierrez, J. Electroanal. Chem. 579 (2005) 1. [22] R.J. Behm, Personal Communication, 2006. [23] J. Mostany, E. Herrero, J.M. Feliu, J. Lipkowski, J. Electroanal. Chem. 558 (2003) 19. [24] E. Herrero, J. Mostany, J.M. Feliu, J. Lipkowski, J. Electroanal. Chem. 534 (2002) 79. [25] A.M. Funtikov, U. Stimming, R. Vogel, J. Electroanal. Chem. 428 (1997) 147. [26] F.E. Woodard, C.L. Scortichini, C.N. Reilley, J. Electroanal. Chem. 151 (1983) 109. [27] L. Blum, N. Marzari, R. Car, J. Phys. Chem. B 108 (2004) 19670.
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