ARTICLE pubs.acs.org/Langmuir
Large Amplitude Fourier Transformed AC Voltammetric Investigation of the Active State Electrochemistry of a Copper/Aqueous Base Interface and Implications for Electrocatalysis Muhammad J. A. Shiddiky,†,z Anthony P. O’Mullane,†,‡ Jie Zhang,† L. Declan Burke,§ and Alan M. Bond*,† †
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia School of Applied Sciences, RMIT University, GPO Box 2476 V, Melbourne, Victoria 3001, Australia § Department of Chemistry, University College Cork, Cork, Ireland ‡
bS Supporting Information ABSTRACT: The higher harmonic components available from large-amplitude Fourier-transformed alternating current (FT-ac) voltammetry enable the surface active state of a copper electrode in basic media to be probed in much more detail than possible with previously used dc methods. In particular, the absence of capacitance background current allows low-level Faradaic current contributions of fast electron-transfer processes to be detected; these are usually completely undetectable under conditions of dc cyclic voltammetry. Under high harmonic FT-ac voltammetric conditions, copper electrodes exhibit well-defined and reversible premonolayer oxidation responses at potentials within the double layer region in basic 1.0 M NaOH media. This process is attributed to oxidation of copper adatoms (Cu*) of low bulk metal lattice coordination numbers to surface-bonded, reactive hydrated oxide species. Of further interest is the observation that cathodic polarization in 1.0 M NaOH significantly enhances the current detected in each of the fundamental to sixth FT-ac harmonic components in the Cu*/Cu hydrous oxide electron-transfer process which enables the underlying electron transfer processes in the higher harmonics to be studied under conditions where the dc capacitance response is suppressed; the results support the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis. The underlying quasi-reversible interfacial Cu*/Cu hydrous oxide process present under these conditions is shown to mediate the reduction of nitrate at a copper electrode, while the mediator for the hydrazine oxidation reaction appears to involve a different mediator or active state redox couple. Use of FT-ac voltammetry offers prospects for new insights into the nature of active sites and electrocatalysis at the electrode/solution interface of Group 11 metals in aqueous media.
’ INTRODUCTION The importance of surface active sites in heterogeneous catalytic processes at metal surfaces was postulated by Taylor in 1925.1 This concept has been extended to surface electrocatalysis, and the role of activated chemisorption in this area was discussed by Pletcher in 1984.2 However, activated chemisorption is evidently not the sole factor involved in surface catalysis, as the Group 11 metals (Cu, Ag, and Au), which are very weak chemisorbers, often display marked catalytic and electrocatalytic properties. While such behavior is highlighted by recent works on supported metal electrodes,3 copper is known to be highly active as a catalyst for both the watergas shift reaction4 and the selective oxidation of methanol to methanal;5 it also displays unusual electrocatalytic activity for nitrate reduction to ammonia6 in acid solution and CO2 reduction to hydrocarbons7 in base. In recent years, a new interpretation of the electrocatalytic behavior of the Group 11 metals was proposed by Burke et al., known as the incipient hydrous oxide adatom mediator (IHOAM) model.8,9 According to the IHOAM model, a Group r 2011 American Chemical Society
11 metal electrode contains two types of surface atoms, namely, low energy (or high coordination) and high energy (or low coordination) atoms. The low energy atoms are of high coverage, well embedded, and their oxidation is assumed to occur in the normal monolayer oxide formation region, the onset of which is frequently accompanied by loss of electrocatalytic activity. The high energy surface atoms are of very low coverage but are very active with respect to electrocatalysis.10 They undergo oxidation, usually in a quasi-reversible manner, at low potentials within the so-called double layer region; the anodic process involved is known as “premonolayer oxidation” and is assumed to involve a metal adatom (M*ads)/metal hydrous oxide transition. In the case of electrocatalysis, this transition is assumed to be associated with a quasi-reversible electron transfer process which occurs at a potential, Eunstable, where both the oxidized and reduced states of Received: May 12, 2011 Revised: June 23, 2011 Published: July 01, 2011 10302
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the interfacial couple are thermodynamically unstable. The interfacial metal hydrous oxide species may function as a mediator for oxidation of a dissolved reductant (when the applied potential E > Eunstable), whereas M*ads may act as a mediator for reduction of an oxidant (when E < Eunstable). The IHOAM model of electrocatalysis has been applied to gold (Au),11,12 platinum,13,14 palladium,15 copper,16,17 silver,18 cobalt,19 and nickel20 electrodes. According to the model relevant to this study, at a copper electrode copper adatoms (Cu*) undergo oxidation at unexpectedly low potentials, typically within the so-called double layer region, to form surface-bonded copper(I) hydrous oxide species,13 viz. h i þ e ð1Þ Cu þ H2 O ¼ CuðH2 OÞn þ ads
+
where Cu ads is assumed to be an adsorbed Cu(I) hydrated ion. However, active site behavior of metal surfaces is notoriously difficult to investigate, as the coverage of such sites is usually quite low; less than 1% of surface metal atoms may be involved and these active atoms are outside (and are only weakly bonded to) the stable surface lattice. For example, only about 0.1% of surface metal atoms are estimated to be adatom species in the case of a silver electrode in basic solution.21 In the case of copper, the adatom coverage in the double layer region was proposed to be even lower.17 Furthermore, Ertl22 pointed out earlier that active site atoms have thermodynamic, kinetic, and coordination properties different from similar, but stable, atoms at the same surface. Detection of a premonolayer, redox, or pseudocapacitive transition at a metal electrode surface requires a highly sensitive in situ technique that can directly monitor such a process within the double layer region and, ideally, distinguish between such a response and that associated with conventional double layer, charging/discharging, behavior. Bard et al. used scanning electrochemical microscopy to demonstrate the formation of incipient oxides on a gold electrode in the double layer region at neutral pH.23 They estimated that the coverage of the incipient oxide in the case of a gold electrode was of the order of ca. 0.2 monolayers, which is consistent with previous contact electroresistance measurements.24 Bond and colleagues recently showed that large-amplitude Fourier-transformed alternating current (FT-ac) voltammetry revealed the presence of significant Faradaic processes at potentials within the double layer region of gold electrodes in acidic and basic media, i.e., at potentials where the current is widely assumed to be solely capacitive in nature.12 The important feature of this technique is that it achieves almost complete suppression of background capacitive currents in the higher harmonics.2527 Consequently, according to this approach, the currents observed in the double layer region are undoubtedly Faradaic in nature and were attributed to quasireversible electron transfer kinetics.12 In the present work, the large-amplitude FT-ac voltammetric technique12,2533 is used to explore the surface active site behavior of a copper electrode in alkaline media. In both the positive and negative sweeps, the activity according to the FT-ac technique is distributed over much of the double layer region with several broad responses centered at ca. 1350, 1100, and 900 mV and one sharp peak at 550 mV (vs Ag/AgCl) (these values correspond, in terms of the RHE scale, to ca. 0.34, 0.09, +0.11, and +0.46 V (RHE), respectively). Among these, the premonolayer oxidation process at 1100 mV (vs Ag/AgCl), attributed to a Cu*/Cu hydrous oxide transition, is quite interesting from an electrocatalysis viewpoint: this transition cannot
be detected by dc cyclic voltammetry without the application of prolonged cathodic polarization. Furthermore, the effect of cathodic polarization of a copper electrode in 1.0 M NaOH solution on the extent of Cu*/Cu hydrous oxide transitions was established as was the electrocatalytic activity of these processes toward the reduction of nitrate and hydrazine in 1.0 M NaOH solution. The data observed are discussed in terms of the IHOAM model of electrocatalysis.
’ EXPERIMENTAL SECTION Reagents. Aqueous 1.0 M NaOH (AnalaR, BDH) solutions were prepared with deionized water (resistivity of 18.2 MΩ cm) purified by use of a Milli-Q filtering system (Millipore). Sodium nitrate (AnalaR, BDH) and hydrazine (Sigma Aldrich) were used as received. All other chemicals were obtained from either Sigma Aldrich or BDH and also used as received, unless otherwise stated. Apparatus and Procedures. A 1.6-mm-diameter copper electrode (Bioanalytical Systems (BAS), West Lafayette, IN, USA) was used as the working electrode. The electrode was cleaned before use in electrochemical experiments by brief immersion (30 s) in a solution containing 5% (v/v) H2O2 + 10% (v/v) H2SO4 followed by polishing with an aqueous 0.3 μm alumina slurry on a polishing cloth (Microcloth, Buehler), sonicating in deionized water for 5 min, and drying under a stream of nitrogen gas. Nitrogen gas was bubbled through the aqueous electrolyte solution (1.0 M NaOH) for at least 15 min prior to voltammetric experiments in order to remove oxygen. A nitrogen atmosphere was maintained above the solution throughout the course of the experiments, except when cathodic polarization was used, in which case the electrolyte solution was continuously degassed with nitrogen. The three-electrode electrochemical cell consisted of the copper working electrode, a Pt wire counter electrode, and an Ag/AgCl (3 M NaCl) reference electrode. The potentials are reported vs Ag/AgCl, which are measured, or RHE where the values are obtained using the relationship ERHE = EAg/AgCl + 0.0591 pH + 0.1976 V). A detailed description of the large-amplitude FT-ac instrumentation is available elsewhere.25 Sine waves of frequencies f = 20 to 35 Hz and amplitudes of ΔE = 80 to 120 mV were used as ac perturbations. Some conventional dc cyclic voltammetric experiments were carried out with the FT instrumentation by using a zero amplitude perturbation. Conventional experiments employing cyclic voltammetry also were undertaken using a BAS model 100B electrochemical workstation. The electrocatalytic response at the copper electrode was measured via large-amplitude FT-ac cyclic voltammetry in 1.0 M NaOH solution containing designated concentrations of NaNO3 or N2H4.
’ RESULTS AND DISCUSSION DC Cyclic Voltammetry at a Copper Electrode in 1.0 M NaOH. Initially, the conventional dc voltammetric behavior of
the copper electrode in 1.0 M NaOH solution was recorded between the hydrogen (HER) and oxygen evolution reactions (OER) (Figure 1) and results compared with literature data. On the anodic sweep, two major peaks (A1 and A2) were observed, which have been previously assigned to the following redox processes. Cu0 is first oxidized at ca. 445 mV (or 560 mV (vs RHE)), to Cu2O (peak A1), according to 2Cu þ H2 O f Cu2 O þ 2Hþ þ 2e 10303
ð2Þ
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Figure 1. DC cyclic voltammogram (υ = 83.82 mV s1) obtained with a copper electrode in aqueous 1.0 M NaOH.
The small current magnitude response of this peak has been attributed to the formation of the compact, poorly conducting, and high passivating Cu2O.13,16 The second peak (A2) at ca. 130 mV appears to be an overlap of two processes in which Cu2O and Cu metal are converted to a mixture of CuO and Cu(OH)2,34 according to Cu þ H2 O ¼ CuO þ 2Hþ þ 2e
ð3Þ
Cu þ 2H2 O ¼ CuðOHÞ2 þ 2Hþ þ 2e
ð4Þ
Finally, an increase in anodic current is evident above 500 mV or ca. 1500 mV (RHE) which is assumed to reflect a combination of Cu (III), e.g., CuO.OH, formation and oxygen gas evolution; such behavior of a copper electrode in 1.0 M NaOH has been discussed in detail.35 The response in the reverse sweep is complicated by the fact that C2 (the counterpart of A2) is relatively small. This has been attributed16 to the formation of a Cu2O barrier layer at the CuO [Cu(OH)2]/Cu interface, i.e., the product of Cu(II) oxide reduction is Cu2O, which acts as a barrier film, severely inhibiting further reduction of the Cu(II) oxide deposit. Cu2O oxide undergoes reduction just below 700 mV, giving rise to peak C1. As the Cu2O reduction approaches completion, the reduction of the residual Cu(II) oxide is no longer strongly impeded, and this reaction goes rapidly to completion, yielding the relatively sharp peak C0. The final shoulder in the negative sweep, i.e., C0 1, extending from ca. 1000 to 1350 mV is assumed to be due to reduction of a Cu(I) hydrous oxide deposit. It has been demonstrated previously that such a deposit, which is markedly different from Cu2O, may be produced in multilayer form on copper in base by repetitive potential cycling using appropriate potential sweep limits.16 Finally, the increase in cathodic response below ca. 1400 mV is assumed to be due to hydrogen gas evolution (the overpotential for the latter reaction in this case is quite high, ca. 400 mV; the absence of any indication of a Hads response is also evident). Large Amplitude FT-ac Voltammetry at a Copper Electrode in 1.0 M NaOH. FT-ac cyclic voltammograms obtained at unactivated and activated copper electrodes in a 1.0 M NaOH employing a sine wave of frequency f = 20.25 Hz and amplitude ΔE = 80 mV superimposed on to the dc waveform (potential range = 1550 to 700 mV, scan rate υ = 83.82 mV s1) are provided in Figure 2 (fundamental to fifth ac harmonics) and Supporting Information Figure S1 (sixth ac harmonic). The fundamental
harmonic response showed peaks in the forward sweep at ca. 500 mV (Cu2O formation), ca. 200 mV (CuO/Cu(OH)2 formation), and 600 mV (CuO.OH formation and O2 gas evolution) (Figure 2a). In the reverse sweep, there are overlap processes at potentials above 0 mV, one at ca. 600 mV (possibly associated with O2 gas evolution) and another at ca. 400 mV which is assumed to be due to a Cu(III)/Cu(II) oxide/oxide transition. Again, as in the dc case, the behavior in the reverse sweep below ca. 400 mV is complicated, three peaks being evident over the range ca. 500 to 1000 mV. In the higher harmonics, there are indications of redox transitions within the double layer region which, according to the forward sweep in Figures 2 and S1 in Supporting Information, extend from ca. 1400 to 500 mV. Thus, in Figures 2 and S1 the response (not a fully resolved process) at 1350 mV (vs Ag/AgCl), ca. 0.35 V (vs RHE), is apparently not due to adsorbed H but is due to a surface active state of copper. It was demonstrated earlier36 that in the case of nitrate reduction at copper in base (at 60 °C) three regions of significant current increase (reflecting the existence of three distinct copper surface mediator systems) occurred in the double layer region of the negative sweep, at ca. 0.30, 0.00, and 0.35 V (vs RHE), respectively. There is clear evidence in the inset of the forward (positive) sweep in Figure S1 for redox transitions, one at ca. 0.35 V (RHE) and the other at ca. 0.10 V (RHE). There is also a major response in the positive sweep, Figure S1, commencing at ca. 0.7 V (vs Ag/AgCl) or ca. 0.3 V (vs RHE); there is evidently (as outlined earlier35) a further active state response in this region, but this response is complicated by the proximity of the conventional Cu2O formation process which commences at ca. 0.5 V (RHE)—the presence of the latter oxide tends to deactivate the surface. The existence of several surface active states in the double layer region (three in the case of copper) is not unusual; the multiplicity of such states was pointed out earlier in the case of both gold11 and silver.37 Clearly, the responses within the double layer region for both forward and reverse sweeps remain significant even in the sixth harmonics (Figure S1), which implies that the rate of the electron transfer is reasonably fast. The copper adatoms involved in the premonolayer oxidation process, which is the vital element of this state from a catalytic viewpoint, are assumed to exist in a more active form than those responsible for the well-known conventional oxide formation/ removal processes that occur at more positive potentials in Figure 2. In the forward sweeps in Figure 2, especially in the higher harmonics, a relatively sharp response is evident just 10304
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Figure 2. Fundamental to fifth harmonics (ae) components derived from large-amplitude FT-ac voltammetry with an unactivated copper electrode (black) and an activated electrode (prepolarized at 1100 mV 5 min) (red) in aqueous 1.0 M NaOH. Conditions employed: f = 20.25 Hz, ΔE = 80 mV, υ = 83.82 mV s1, initial potential = 1500 mV, potential range = 1500 to 700 mV.
below 500 mV. It is unlikely that this is due to the main Cu/ Cu2O reaction, as the oxide involved in the latter is compact, poorly conducting, and passivating in character: evidently there is an initial, more reactive, oxide species such as CuOHads involved, but this is rapidly superseded (with increasing potential) by less reactive Cu2O species. There are further, relatively small, ac responses in the forward sweep in Figure 2e at ca. 200 and 600 mV; evidently, these correspond to Cu(0)/Cu(II)
and Cu(1)/Cu(II) oxide transitions at ca. 200 mV, and Cu(II)/Cu(III) oxide, plus oxygen gas evolution, responses at 600 mV. In the reverse sweeps in Figure 2, the current magnitude of the ac response at ca. 500 mV is relatively low; as discussed above, the copper/solution interface is deactivated by a passivating Cu2O deposit in this region. The inner Cu2O layer undergoes reduction at ca. 750 mV (see peak C1 in Figure 1) and it is only below this value, ca. 900 mV (peak C0, Figure 1), that most of 10305
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Figure 3. Fundamental to fifth harmonics (ae) components derived from large-amplitude FT-ac voltammetry for a copper electrode in aqueous 1.0 M NaOH with (red) and without (black) 50 mM NaNO3. Conditions employed: f = 20.25 Hz, ΔE = 80 mV, υ = 83.82 mV s1, initial potential = 1500 mV.
the copper surface becomes oxide-free (note the large FT-ac peak at ca. 900 mV in the reverse sweeps in Figure 2). To exclude oxide film formation, the FT-ac response for the copper electrode was examined solely over the potential range of 1550 to 750 mV. Under this restricted potential range, the fundamental to sixth harmonics for the forward and reverse scan directions are very similar (data not shown) and have the characteristic of electrochemical reversibility, based on comparison with theoretical predictions.38,39 Furthermore, these
data also demonstrate that the product formed from the electron transfer step is stable on the voltammetric time scale. The ac data in Figures 2 and S1 (black line) clearly reveals the effectiveness of the FT-ac technique in discriminating between the Faradaic and capacitance currents and hence enhancement of the feasibility of detection of the premonolayer Cu*/Cu hydrous oxide electron-transfer process even without the introduction of electrochemical or thermal pretreatments to activate the electrode surfaces. 10306
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Figure 4. Fundamental to fifth harmonics (ae) components derived from large-amplitude FT-ac voltammetry for a copper electrode in aqueous 1.0 M NaOH containing 50 mM NaNO3. Conditions employed: f = 20.25 Hz, ΔE = 80 mV, υ = 83.82 mV s1, initial potential = 700 mV.
Cathodic polarization in the HER region or thermal pretreatment is known to disrupt the outer layers of copper electrode,13,16 and hence enhance the amount of Cu* at the interface which should be detected by a marked increase in the premonolayer oxidation/reduction response. Figure S2 in Supporting Information
(red line) shows dc cyclic voltammograms obtained in 1.0 M NaOH before and after the copper electrode had been subjected to cathodic polarization at 1100 mV for 5 min. No clear evidence of a premonolayer response in the 1400 to 750 mV region was obtained, even after activation, in the dc response 10307
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Figure 5. DC voltammograms (υ = 83.82 mV s1) for a copper electrode in aqueous 1.0 M NaOH with (red) and without (black) 50 mM hydrazine.
(Figure S2). The ac approach is clearly more sensitive; thus, according to the ac data shown in both Figure 2 and Figure S1 the premonolayer responses are seen to be significantly enhanced following cathodic polarization of the copper electrode. In the forward sweep, the feature at ca. 600 mV (close to the onset of Cu2O deposition) is quite dramatic and is significantly enhanced by polarization pretreatment. The more obvious premonolayer oxidation responses are located at ca. 950, 1100, and 1350 mV (see the insets in Figure S1); however, the redox process involved at ca. 950 mV did not appear to play a major role in electrocatalysis; it seems that there is a third mediator system (as discussed later) present at ca. 600 mV (it is important in the case of copper to distinguish between active metal/oxide-based, e.g., Cu*/CuOHads, and oxide/oxide-based, e.g., CuO/CuO. OH, mediators; the latter are less prone to display active state behavior. The FT-ac and dc responses obtained under similar experimental conditions are quite different. The dc responses are a measure of the net Faradaic current (or charge) associated with a given reaction, whereas the FT-ac response, in simple terms, emphasizes the degree of reversibility of the electrode reaction. In the reverse sweep in Figures 2 and S1 (red lines), there is a major ac activity below ca. 600 mV, and especially below 1000 mV. The second to sixth ac harmonic components also demonstrate that the premonolayer oxidation processes at ca. 1100 mV is more significant that that obtained for the nonactivated copper electrode. Furthermore, the electrochemical response obtained at ca. 1100 mV in the forward sweep is much higher than that obtained in the reverse sweep. This behavior in the higher harmonics at the activated copper electrodes bears a distinct similarity to that reported at a gold electrode.12 The dependence of the time of cathodic polarization on the copper electrode response in 1.0 M NaOH was also investigated (Figure S3 in the Supporting Information). Clearly, the fundamental to higher ac harmonic component currents increase substantially with increasing polarization time. This enhancement in the higher harmonics, particularly in the fifth harmonic, is very pronounced when 10 or 20 min polarization times are used. It should be noted that the cathodic polarization performed in this study is milder than that employed in previous studies,16,13 i.e., prolonged (>10 min) vigorous hydrogen evolution is not involved during the activation process, as it can be seen that the onset for significant HER currents occurs at < 1350 mV. This shows again the excellent sensitivity of the FT-ac technique in detecting these types of premonolayer oxidation/reduction transitions.
According to previous studies on copper electrodes,13,14,16 during the activation process the outer layer of the metal lattices are disrupted via a hydrogen embrittlement process. The entry of hydrogen into the metal generates lattice strain which results in the expulsion of copper atoms from the bulk of the metal to the surface40 and hence their conversion from a low to a high energy state with a low lattice coordination number. The resulting activated Cu* state is readily oxidized as seen by the ac current in the potential region of ca. 1100 mV. The enhancement is particularly pronounced in the higher harmonics (fourth, fifth, sixth), where the response now mimics behavior close to that expected for fully reversible surface confined electron-transfer processes, described theoretically in ref 12. The important aspect of large-amplitude FT-ac voltammetry is that when a large amplitude sinusoidal perturbation is used, the higher harmonics components are extremely effective in discriminating against background capacitive current and hence in emphasizing Faradaic currents of processes involving fast electron-transfer kinetics in the double layer region. As can be seen in Figures 1, 2, and S1, the background capacitive current dominates the dc cyclic voltammetric data. The fundamental harmonic data also contain significant background current, but its influence is essentially absent in fourth and higher harmonics. Therefore, the nature of the fourth and higher harmonics, in the potential region often regarded as purely capacitive, supports the conclusion that the origin of the premonolayer responses in the double layer region especially below 1100 mV is Faradaic in nature, and that the electron-transfer kinetics are fast. It is worth noting in the case of FT-ac responses for premonolayer processes in both the forward and reverse sweeps that the activity tends to be distributed over much of the double layer region (1400 to 500 mV) with several broad (at ca. 1350, 1100, and 900 mV) and one sharp (at ca. 550 mV) maxima. As discussed later, nitrate reduction at copper in base occurs below 900 mV; the mediator involved is evidently active copper (Cu*), but in the present case, the rate of reaction over the range 1400 to 900 mV varies almost linearly with potential, i.e., current plateaus are poorly defined as the redox potential of the mediator system (Cu*/Cu) (hydr., ads) occurs over a range of potentials, rather than at a discrete value. In earlier work, carried out at slightly elevated temperature (60 °C),41 three regions of current increase, followed by short plateaus, were clearly observed in the double layer region of the negative sweep for nitrate reduction at copper in base (these features were less well-defined, but still evident, in the positive sweep). 10308
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Figure 6. Fundamental to fifth harmonics (ae) components derived from large amplitude FT-ac voltammetry for a copper electrode in aqueous 1.0 M NaOH with (red) and without (black) 5.0 mM hydrazine. Conditions employed: f = 20.25 Hz, ΔE = 80 mV, υ = 83.82 mV s1.
Large Amplitude FT-ac Voltammetry at a Copper Electrode in 0.1 M NaOH in the Presence of Nitrate. To assess the
role of the Cu*/Cu hydrous oxide transition as an electrocatalytic mediator, the reduction of nitrate at a copper electrode in 1.0 M NaOH solution was studied by dc and large-amplitude FT-ac
voltammetry. This reaction is believed to be an example of a chemically irreversible electrocatalytic process.42 Figure S4 in Supporting Information provides dc cyclic voltammograms obtained at a copper electrode in 1.0 M NaOH containing 50 mM NaNO3 when the potential is scanned between 1550 and 700 mV, 10309
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Langmuir starting from 1550 mV in the anodic direction. The voltammetric behavior also is shown over the potential range of 700 and 1550 mV (Figure S5 in Supporting Information), when the scan starts from 700 mV and initially occurs in the negative potential direction. For comparison, dc voltammograms obtained in the absence of nitrate are included in these figures. Irrespective of the initial potential or initial scan direction, a substantial increase in the dc cathodic current is observed within the potential region usually associated with the double layer region. This electrocatalytic reaction corresponds to a complex reduction of nitrate to intermediates or final products such as nitrite, ammonia, nitrous oxide, and nitrogen dioxide.43 The ratedetermining step has been shown to be the one electron transfer to nitrate ions adsorbed onto the copper electrode surface to generate nitrite ions.44 Under the present conditions, catalytic reduction commences in the negative sweep direction at a potential more negative that for the C0 peak, e.g., ca. 820 mV. However, dc voltammetry cannot be used to ascertain if there is any underlying interfacial redox cycling of the Cu*/Cu hydrous oxide active state that accommodates the catalytic current detected over the potential range of 850 to 1500 mV. Figure 3 provides the fundamental to fifth FT-ac harmonic components detected when the potential is scanned over the range of 1550 to 700 mV for a copper electrode in 1.0 M NaOH solution in the presence and absence of 50 mM NaNO3. A similar electrocatalytic experiment was also carried out at a copper electrode in 1.0 M NaOH solution containing 50 mM NaNO3 by scanning the potential from 700 to 1550 mV, but commencing the scan from 700 mV in the cathodic direction (Figure 4). In the presence of nitrate, the fundamental to fifth harmonic components all show evidence of two well-defined processes at ca. 1275 and 1100 mV that are similar to those present when a copper electrode in the absence of nitrate is activated in 1.0 M NaOH (Figure S6 in Supporting Information). Notably, the potential of the process at ca. 1100 coincides with that for the Cu*/Cu hydrous oxide transition observed with both unactivated and activated copper electrodes. The enhanced current is attributed to the oxidized copper hydrous oxide state being rapidly reduced back to the active copper adatoms state, facilitating a reversible underlying electron transfer process. The response at 1100 mV in the fifth harmonic (Figures 3e and 4e) now mimics that expected for an electrochemically quasi-reversible electron transfer process.12 DC and Large Amplitude FT-ac Voltammetry at a Copper Electrode in 0.1 M NaOH in the Presence of Hydrazine. Figure 5 provides dc cyclic voltammograms at a copper electrode in 1.0 M NaOH solution containing 50 mM hydrazine (initial anodic sweep: 1550 to 700 mV). In this case, the initial oxidation commences at ca. 770 mV, which is approximately the same potential when nitrate reduction commences under similar conditions (Figure S5b). The rate in the initial stages is sluggish in both cases and there is an ac response at ca. 0.77 V (Figure 6e and Figure S6a). This initial anodic response apparently mediated by the Cu*/CuOHads couple decays rapidly at potentials of about 0.5 V (Figure 5a), as the conventional Cu2O oxide species is formed at the surface. With more anodic potentials, vigorous oxidation of hydrazine recommences at ca. 0.0 V (Ag/AgCl) or ca. 1.0 V (RHE) (Figure 5a). This onset potential seems too low for CuO.OH involvement. However, participation of the latter as an oxidant cannot be excluded at higher potentials. The reaction sequence in eqs 5 and 6 to give
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the overall eq 7 gives a mechanism which may explain the mediation.
On the cathodic sweep, two processes are detected at ca. 520 and 820 mV. Their magnitudes are lower than found with unactivated copper or activated copper electrode in 1.0 NaOH in the absence of hydrazine (Figure 5). The decrease in current magnitude is a function of the hydrazine concentration (Figure S7 in Supporting Information), and occurs because hydrazine reduces much of the oxide chemically rather than electrochemically during the cathodic sweep. Given that hydrazine is a very strong, reactive, reducing agent, it could in principle undergo mediated oxidation at potentials as low as 1100 mV V (Ag/ AgCl). However, oxidation commences sluggishly only at ca. 770 mV. Nitrate reduction is quite vigorous at E < 1100 mV, so the question arises as to why hydrazine oxidation is sluggish in this potential region? A significant difference may be that the mediator for nitrate reduction is a cationic Cu+ads(hydr.) species and hence this reaction is favored by a strong Coulombic interaction (Cu+—NO3). In contrast, hydrazine is a neutral molecule in base and the absence of strong Coulombic interaction with the mediator system plausibly results in very sluggish oxidation behavior at these negative potentials.
’ CONCLUSIONS The higher harmonic components derived from the largeamplitude FT-ac voltametric technique readily reveal the presence of well-defined important Faradaic processes at potentials within the so-called purely capacitive double layer region of the copper electrode in aqueous 1.0 M NaOH solution. The origin of the Faradaic current may be explained in terms of the IHOAM model in which low coordination Cu* on the metal electrode surface in the double layer region, present at low coverage, is oxidized to Cu hydrous oxide species. These pre-monolayer Faradaic processes are distributed over much of the double layer region with several broad (at 1350, 1100, and 900 mV) and one sharp (at 550 mV) maxima. The Cu*/Cu hydrous oxide transition occurs at 1100 mV provides a redox mediator capability in the electrocatalytic reduction of nitrate by providing a process that is electrochemically detectable by FT-ac measurements. The mediator for this catalytic process appears to be a cationic Cu+ads(hydr.) species. However, in the absence of strong Coulombic affects, this Faradaic couple is unable to mediate the oxidation of neutral and strongly reducing hydrazine whose oxidation appears to be mediated by another Cu*/Cu hydrous oxide transition at more positive potentials. This work demonstrates that the FT-ac technique is useful in studying the mechanism of electrocatalytic reactions that occur within the double layer region of metals such as copper. ’ ASSOCIATED CONTENT
bS
Supporting Information. Large amplitude FT-ac sixth harmonics, dc cyclic voltammograms for unactivated and activated copper electrodes in 1.0 M NaOH, FT-ac harmonic components for unactivated and activated copper electrodes as
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: +61 3 99054597. Present Addresses z
Australian Institute of Bioengineering and Nanotechnology, The University of Queensland, St Lucia, Qld 4072, Australia.
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