Super-Nernstian Shifts of Interfacial Proton-Coupled Electron Transfers: Origin and Effect of Noncovalent Interactions

Subscriber access provided by Library, Special Collections and Museums, University of Aberdeen The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Article Super-Nernstian Shifts of Interfacial Proton-Coupled Electron Transfers: Origin and Effect of Non-Covalent Interactions Christopher Wildi, Gema Cabello, Martin Eduardo Zoloff Michoff, Patricio Vélez Romero, Ezequiel Leiva, Juan Jose Calvente, Rafael Andreu, and Angel Cuesta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04560 • Publication Date (Web): 06 Jul 2015 Downloaded from http://pubs.acs.org on July 7, 2015 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. 1 Super-Nernstian Shifts of Interfacial Proton-Coupled Electron Transfers: Origin and Effect of Non- Covalent Interactions Christopher Wildi, † Gema Cabello, ‡ Martín E. Zoloff Michoff, § Patricio Vélez, # Ezequiel P. M. Leiva, # Juan José Calvente, ┴ Rafael Andreu, ┴ and Angel Cuesta* ,† † Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, UK ‡ Department of Chemistry, Universidade Federal de São Carlos (UFSCAR), 13565-905, São Carlos, SP, Brazil § Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 44801 Bochum, Germany # Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, INFIQC, Córdoba, Argentina ┴ Departamento de Química Física, Universidad de Sevilla, 41012 Sevilla, Spain ABSTRACT: Anions whose specific adsorption involves a proton-coupled electron transfer (PCET) include adsorbed OH (OH ad ), which plays an enormously relevant role in many fuel-cell reactions. OH ad formation has often been found to happen in alkaline solutions at potentials more negative than expected from a Nerstian shift, and this has been proposed as the reason for the Page 1 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 often easier oxidation of organic molecules in alkaline media, as compared to acids. Noncovalent interactions with electrolyte cations have also been shown to affect the stability of OH ad . Using cyanide-modified Pt(111) as a model, we show here that interfacial PCETs will show a super-Nernstian shift if (i) less than one electron per proton is transferred, and (ii) the plane of proton-electron transfer and that of the metal surface do not coincide. We also show that electrolyte cations have a double effect: they provoke an additional shift by blocking the site of transfer, but decrease the super-Nernstian contribution by separating the plane of transfer from the outer Helmholtz plane (OHP). 1. INTRODUCTION All chemical reactions involve the transfer of either ions, electrons, or both. The most common of the latter are proton-coupled electron transfers (PCET). Excluding that of coal, all the combustion reactions on which modern energy-consuming economies are largely based involve several PCETs. When performed electrochemically, the PCET steps of these reactions often correspond to the formation on the electrode surface of H, OH, or O adlayers, or of surface oxides, whose stability is crucial in determining the activity of the electrode as an electrocatalyst. Since PCETs nominally involve the transfer of the same number of electrons and protons, the equilibrium potential of any such reaction should always shift -0.059 V per pH unit. However, it has been known for over three decades that the oxidation of metal surfaces in alkaline solutions occurs at potentials more negative than expected from a purely Nernstian shift. 1-3 Although, as far as we know, no satisfactory explanation for this has been yet given, this apparent stabilization of surface oxygenated species with increasing pH beyond that expected from Nernst’s equation Page 2 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 can significantly affect the electrocatalytic activity of the electrode surface, 4 and the magnitude of this effect has been found to be cation-dependent. 5 Very recently, 6 we have employed cyanide-modified Pt(111) electrodes (Pt(111) electrodes chemically modified by spontaneous and irreversible adsorption of a cyanide adlayer, 7,8 which have proven to be excellent model surfaces for the study of atomic-ensemble effects in electrocatalysis 9-13 ) to study the interaction between specifically adsorbed anions and alkalimetal cations at electrode-electrolyte interfaces, and developed a simple model which we later showed can be generalized to completely different systems. 14 Here, we use cyanide-modified Pt(111) electrodes to offer an explanation for the occurrence of the above-mentioned super- Nernstian shifts, as well as for the cation dependence of the magnitude of those shifts. 2. EXPERIMENTAL Buffer solutions of pH 2, 4, 6, 8, 10 and 11 were prepared using H 3 PO 4 (Aldrich, 85% wt. in H 2 O), NaH 2 PO 4 (Sigma-Aldrich, puriss. p.a., ≥ 99.0%), Na 2 HPO 4 (Sigma-Aldrich, puriss. p.a., 98.5-101%), and Na 3 PO 4 (Sigma-Aldrich, puriss. p.a., ≥ 98.0%). When necessary, NaClO 4 (Sigma-Aldrich for HPLC, ≥ 99.0%) was added to the buffer solutions in order to keep the concentration of Na + and the ionic strength of the solution constant, and equal to 0.2 M. (See the Supporting Information for exact composition of buffer solutions.) The pH values of the buffer solutions were accurately determined by measuring the open-circuit potential of a platinum wire in a hydrogen-saturated buffer solution vs. a home-made Ag/AgCl (KCl sat ) reference electrode. The experiments examining the pH effect on the CV of cyanidemodified Pt(111) electrodes were performed using this home-made Ag/AgCl (KCl sat ) electrode as reference, and the measured potentials were then converted either to the RHE or the SHE Page 3 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 scale. CO adlayers for the CO-stripping experiments were formed by bubbling CO through the solution at E d = 0.1 V, the solution being then purged with N 2 . A home-made RHE was used as reference in the CO-stripping experiments. Other experimental details have been reported previously, 8,9,12,13 and can be found in the Supporting Information. All calculations were performed using the DFT code SIESTA. 15 Exchange and correlation effects were taken into account using the generalized gradient approximation within the Perdew- Burke-Ernzerhof (PBE). 16 Norm-conserving pseudopotentials (with relativistic corrections in the case of Pt) generated according to the scheme of Troullier and Martins 17,18 were used. All calculations were performed with spin polarization and using a double-z basis set with polarization and an energy shift, used to confine the electrons in the pseudopotentials, of 5 meV. For the energy cutoff that defines the fineness of the real space grid used to represent the charge density, a value of 150 Ry (~ 2 keV) was employed. We took care to test the convergence of our results with respect to all these parameters. More details are provided in the Supporting Information. 3. RESULTS AND DISCUSSION Fig. 1(A) shows the effect of increasing the pH on the cyclic voltammograms (CVs) of cyanidemodified Pt(111) electrodes. All the CVs show two distinct regions (appearing at E < 0.6 V and at E > 0.8 V at pH 1), attributed to hydrogen and OH adsorption, respectively, 7,8,11,19 separated by a double-layer region. Would hydrogen and OH adsorption shift -0.059 V per pH unit, as should be expected, they would appear at exactly the same potential in the reversible hydrogen electrode (RHE) scale used in Fig. 1(A), whatever the solution pH. However, both regions clearly shift negatively with increasing pH, the effect being larger in the case of the hydrogen Page 4 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 adsorption region. Hydrogen evolution, on the contrary, shows the expected Nernstian behavior, and occurs at the same potential on the RHE scale at all pHs studied. We have recently identified the region appearing at E < 0.6 V at pH 1 as a PCET to adsorbed CN (CN ad ) to yield adsorbed CNH (CNH ad ), 20 an idea already proposed by Schardt et al. 21 The exact nature of the adsorption process on cyanide-modified Pt(111) electrodes at E > 0.8 V is still unknown, so we will concentrate our analysis on the hydrogen adsorption region. Figure 1. (A) Cyclic voltammograms at 50 mV s -1 of a cyanide-modified Pt(111) electrode in phosphate-buffer solutions of pH 2 (black line), 4 (red line), 8 (green line) and 11 (blue line). Adequate amounts of NaClO 4 were added to the solutions in order to keep the ionic strength and the concentration of Na + constant. (B) Potential (in the SHE scale) vs. pH at constant hydrogenadsorption charge densities of 20 µC cm -2 (black), 30 µC cm -2 (red), 45 µC cm -2 (green), and 60 µC cm -2 (blue). For the sake of comparison a line with a slope of -0.059 V, corresponding to the Nernstian behaviour expected for a PCET has also been included (dashed line). Page 5 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 Metal cations in the electrolyte can interact with the nitrogen atoms of CN ad , thus blocking the sites where the PCET occurs, 6,20 and increasing the concentration of alkali-metal cations above a cation-dependent threshold concentration provokes a negative shift of the onset of hydrogen adsorption on cyanide-modified Pt(111) electrodes. 6 However, in the experiments reported in Fig. 1 we paid particular attention to keeping the ionic strength and the Na + concentration constant (see Supporting Information), so we can exclude any variable other than pH as the cause of the shift. The total charge density in the hydrogen adsorption region remains constant, within experimental error, at pH ≤ 8 (i.e., within this pH range, the maximum CNH ad coverage ( θ CNH ) achievable is pH-independent). At the highest pHs studied (10 and 11) it decreases slowly, because hydrogen evolution starts before the maximum θ CNH has been attained. Figure 1(B) shows plots of the potential (vs. the standard hydrogen electrode, SHE) at constant charge density (i.e., at constant θ CNH ) vs. pH. A line with a slope of -0.059 V has been included as a guide to the eye. At all coverages, the potential shift is larger than the -0.059 V per pH unit expected for a PCET. The super-Nernstian contribution decreases slightly as θ CNH approaches the maximum coverage possible (see Table S2), but at all θ CNH the shift of the equilibrium potential with pH remains far from the expected Nernstian value. Hydrogenation of CN ad on cyanide-modified Pt(111) is a very reversible process, as reflected by the symmetric shape of the CV in the corresponding potential region, even at high scanning rates. Accordingly, every potential within the hydrogen adsorption region corresponds to a welldefined equilibrium, i.e., when measured vs. the RHE, the potential at which a given θ CNH is reached corresponds to -∆G at 0 V vs. RHE for the hydrogenation of CN ad up to that θ CNH (at the potential at which a given equilibrium θ CNH is reached, ∆G for the hydrogenation of CN ad up to Page 6 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 that θ CNH is obviously zero). ∆G only depends on the initial (CN ad + H + + e) and the final (CNH ad ) states of the reaction. The Nernstian shift accounts for the decrease in the chemical potential of H + as the pH increases. A super-Nernstian shift of the equilibrium potential for a given θ CNH suggests, hence, an additional stabilization of the initial state and/or an additional destabilization of the final state with increasing pH, by an amount identical to the super- Nernstian contribution. Figure 2. (A) Ball model of the structure of cyanide-modified Pt(111), with cations (red balls) interacting non-covalently with CN ad (blue balls) forming the experimentally observed honeycomb structure. 6 Grey balls correspond to the substrate Pt atoms. (B) Cross section along the solid line in (A), and schematic representation of the electrical double layer, indicating the location of the plane of proton-electron transfer, a, and of the plane of maximum approach of cations and protons, b (which, for the sake of simplicity, we have made coincide with the OHP, see text). Black balls correspond to the carbon atoms, and blue balls to the nitrogen atoms, respectively, of the CN groups. Yellow balls correspond to hydrated protons. The inset illustrates the polar nature of the chemisorption bond. (C) Electronic density difference with respect to the Page 7 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 separated fragments, CN and Pt slab, represented on a plane that cuts through the Pt-CN bond. The separation between contiguous isocurves is 7.5 x 10 -4 electrons / Bohr 3 , ranging from - 0.0075 in blue to +0.0075 in red. N atoms are blue, C atoms are black, and Pt atoms are light yellow. Fig. 2(A) shows a ball model of the structure of the cyanide-modified Pt(111) surface with cations adsorbed at sites surrounded by 3 CNs, as in the experimentally observed honeycomb structure. 6 Fig. 2(B) shows a cross section along the line in Fig. 2(A), and a schematic representation of the electrode-electrolyte interface. The PCET will occur at plane a, defined by the position of the specifically adsorbed hydrogen acceptor/proton donor, namely, the N atom of CN ad /CNH ad . The presence of the cations interacting non-covalently with CN ad defines a plane of maximum approach b, where the proton donor (H 3 O + in acidic solutions, H 2 O in alkaline solutions) or acceptor (H 2 O in acidic solutions, OH - in alkaline solutions) has to be transferred from the limit of the diffuse double layer (which, in concentrated solutions as the ones used here will coincide with the outer Helmholtz plane, OHP), before reaching a for the PCET to occur. In other words, protons have to displace the cations from this plane, and this explains the recently reported effect of increasing the cation concentration at constant pH on the hydrogen adsorption region of cyanide-modified Pt(111) electrodes. 6 Table 1. Hirshfeld and Voronoi atomic charge analysis CN ad on Pt(111) in the experimentally observed (2√3 x 2√3)R30 o structure (see Fig. 3A). Atoms Atomic charges / e Hirshfeld Voronoi C -0.20 -0.278 Page 8 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 Equations (6) and (7) predict that, once the threshold concentration for a given M + is reached, the slope of these two experiments must be identical. For M + = Na + , and for a hydrogen adsorption charge of 20 µC cm -2 , the experiments reported here yield _=1−(1−% 4 )=0.61, in perfect agreement with the value obtained in reference [4]. The relevance of the discussion above is that it can be generalised for any PCET to any anion specifically adsorbed on an electrode surface, including OH ad (see inset in Fig. 3(B)) or O ad . These species play an essential role in many fuel-cell reactions, like, e.g., the electrooxidation of CO, the methanol oxidation reaction (MOR), or the oxygen reduction reaction (ORR), where they provide the oxygen atom necessary for the formation of CO 2 (CO electrooxidation, MOR), or can act as a catalytic poison (ORR). In alkaline solutions, the formation of surface oxygenated species often occurs at potentials more negative than expected from a merely Nernstian shift, 1,2 and this has been suggested to be the reason for the higher electrocatalytic activity of Pt and other metals in alkaline media towards the oxidation of some organic molecules, as compared to acidic environments. 24-27 Our work provides an explanation for these phenomena. Furthermore, the presence of different alkaline and alkaline-earth metal cations has been shown to affect the activity of Pt towards several fuel-cell reactions, both in alkaline 5,28-30 and in acidic solutions. 31 Strmcnik et al. 5 qualitatively attributed this effect to the stabilisation of OH ad via non-covalent interactions between the adsorbate and the cations, and the differences between cations to the cation-dependent strength of these interactions. Our model suggests that this apparent stabilisation of OH ad is due to the energy required to remove the cations from plane b. Moreover, our model additionally predicts that the cation effect will only be observed above a threshold concentration, and that the magnitude of the shift of OH adsorption with increasing pH or cation concentration will be different for different cations. Page 15 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 16 In order to confirm that our model can be generalised to the formation of OH ad and O ad , and, consequently, to the fuel-cell reactions in which they are involved, we studied the effect of the concentration of alkaline-metal cations on the electrooxidation of adsorbed CO (an archetypal electrocatalytic reaction) in 0.1 M H 2 SO 4 . The results for Li + , Na + , K + and Cs + are shown in Fig. 3(B) (the corresponding CO-stripping voltammograms are shown in Fig. S2). In all the cases, addition of an alkaline-metal cation at so low a concentration as 10 -4 M provokes a positive shift of the CO-stripping peak potential, although this shift does not show any clear trend. Interestingly, the reproducibility of the CO-stripping voltammogram is clearly higher at c M ≥ 10 -4 M than at lower, or nil, concentrations of alkaline-metal cations. It has been shown that a slight increase in the amount of defect sites on a Pt(111) electrode, too small to be detected in its CV in sulphuric acid solutions, can provoke a significant decrease in the CO-stripping peak potential. 32 Accordingly, we attribute the small positive shift, and the higher reproducibility, of the CO- stripping peak potential at c M ≥ 10 -4 M to the preferential adsorption of MSO 4- pairs 14 at defect sites, which are thus blocked as ignition sites for the CO-stripping reaction. In all cases, increasing the cation concentration above a cation-dependent threshold provokes a negative shift of the CO-stripping peak potential. The minimum concentration at which the alkaline-metal cations start provoking a shift follows the trend: Li + < Na + ≈ K + < Cs + , in good agreement with the effect of the alkaline-metal cations on the CO-stripping voltammogram of Pt(100) in alkaline solutions, 33 and with the effect of the alkaline-metal cations on OH adsorption on Pt(111) in alkaline media. 5 This trend can be related to the corresponding trend in the association constant, K as , of our model (Li + > Na + ≈ K + > Cs + ), i.e., with the strength of the interaction between the cation and OH ad . The analysis of the slope in the region where the peak potential shows a dependence on the cation concentration is not straightforward, because in this Page 16 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 17 case the peak potential does not correspond to an equilibrium situation. Furthermore, sulfate and OH will compete for the adsorption sites left free during CO stripping and, since the cations also affect the adsorption of sulfate, 14 θ OH at the peak will not necessarily be always the same, and independent of the cation concentration. In spite of this, it is obvious that, as predicted by our model, the shift of the voltammetric peak with increasing cation concentration is different for different cations. Within the frame of our model, this is due to the creation, by the cations interacting non-covalently with OH ad , of a plane of maximum approach that separates the plane where the proton-electron transfer occurs from the OHP, the location of this plane being cationdependent. 4. CONCLUSIONS In summary, we have shown that PCETs to specifically adsorbed anions will show a super- Nernstian shift with pH if the chemisorption bond is polar, and if the plane where the protonelectron transfer occurs does not coincide with the electrode surface. These PCETs include OH adsorption/desorption (which can be considered as the dehydrogenation/hydrogenation of (H 2 O) ad /OH ad ), and has therefore profound implications in fuel-cells electrocatalysis. Among them, the frequently easier electrooxidation of organic fuels in alkaline as compared to acidic media, for which our model provides an explanation. Above a cation-dependent threshold concentration, the presence in the electrolyte of cations (other than H + ) which can interact noncovalently with specifically adsorbed anions, including OH ad , has a double effect: (i) They will provoke an additional negative shift of the PCET, due to the energy required to remove the cation layer before the proton can access the plane where the hydrogenation occurs. This can be viewed as a purely thermodynamic effect, which Page 17 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18 increases the apparent adsorption energy of the dehydrogenated form of the specifically adsorbed species (CN ad , OH ad …). This effect was the only one taken into account in previous interpretations of the effect of non-covalent interactions on electrocatalytic reactions. 5 (ii) They will affect the magnitude of the super-Nernstian shift, by separating the plane of hydrogenation and the OHP. This can be viewed as a double-layer effect that counteracts the one above, and whose relevance had not be considered in previous work. 5 As has been shown repeatedly, 5,28-31 these two effects can be used to tune the catalytic activity of electrode-electrolyte interfaces. ASSOCIATED CONTENT Experimental details and computational methodology. Full deduction of Equation (4). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected]. Tel: +44 (0)1224 274538 ACKNOWLEDGMENT The support of the University of Aberdeen is gratefully acknowledged. CW acknowledges a summer studentship from the Carnegie Trust for the Universities of Scotland. EPML acknowledges SeCYT (Universidad Nacional de Córdoba), CONICETPIP 11220110100992, Program BID (PICT 2012-2324) and PME 2006-01581 for financial support. REFERENCES Page 18 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 19 (1) Burke, L. D.; Lyons, M. E.; Whelan, D. P. Influence of ph on the Reduction of Thick Anodic Oxide Films on Gold. J. Electroanal. Chem. 1982, 139, 131-142. (2) Burke, L. D.; Roche, M. B. C. The Possible Importance of Hydrolysis Effects in the Early Stages of Metal Surface Electrooxidation Reactions — With Particular Reference to Platinum. J. Electroanal. Chem. 1983, 159, 89-99. (3) Liu, C.; Bocchicchio, D.; Overmyer, P.; Neuman, M. 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Page 20 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 21 TOC Graphic Page 21 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60