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oxidation at model platinum surfaces
Electrochemistry Communications 33 (2013) 39–42
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Electrocatalysis of H2O2 reduction/oxidation at model platinum surfaces E. Sitta, A.M. Gómez-Marín, A. Aldaz, J.M. Feliu ⁎ Instituto de Electroquímica, Universidad de Alicante, Apt. 99, 03080 Alicante, Spain
a r t i c l e
i n f o
Article history: Received 20 March 2013 Received in revised form 11 April 2013 Accepted 11 April 2013 Available online 23 April 2013 Keywords: Hydrogen peroxide oxidation Hydrogen peroxide reduction Platinum single crystals Hanging meniscus rotating electrode
a b s t r a c t Oxidation (HPOR) and reduction (HPRR) reactions of hydrogen peroxide were studied on platinum single crystalline surfaces. Experimental curves were fitted to a simple model in which the whole process is described by the sum of two independent, mass controlled reactions. It is shown that stepped surfaces are more catalytic than basal planes for both HPOR and HPRR reactions. On the contrary, on oxide covered surfaces, those having large {111} terraces are better electrocatalysts for HPRR, but worse for HPOR. These results shed light concerning H2O2 reactivity on platinum surfaces and may help to unveil oxygen reduction reaction mechanism. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The electro-catalysis of oxygen reduction reaction (ORR) is an active research field aiming (a) to find low cost catalysts with appreciable activity for fuel cell devices, (for a short review see Ref. [1]), and (b) unraveling the ORR mechanism [2–10]. ORR to water is a 4 electrons mechanism that can proceed dissociatively or associatively. In the latter case a serial pathway via the formation of hydrogen peroxide as stable intermediate has been commonly proposed [11]. H2O2 has been identified during ORR over platinum, using rotating ring-disk electrodes, when the potential reaches the hydrogen UPD region or in the presence of strong adsorbing anions (e.g. [12–14] and references therein) thus indicating incomplete electron transfer. Generally speaking, H2O2 can be reduced to water (hydrogen peroxide reduction reaction, HPRR), E0 = 1.76 V, or oxidized to O2 (hydrogen peroxide oxidation reaction, HPOR), E0 = 0.695 V, depending on the electrode potential [3,15–21]. This dual character makes this molecule an interesting workhorse to build electrochemical models. In addition, near to the ORR standard potential, E0 = 1.23 V, the theoretical overpotential for HPOR is high (more than 0.5 V in standard conditions). This means that any H2O2 eventually formed would be immediately oxidized, resulting in a zero net current for the oxygen reduction if the dominant pathway would follow the associative mechanism. The present work aims to investigate the structure sensitivity of hydrogen peroxide oxidation and reduction reactions (HPORR) at single crystal platinum electrode surfaces.
Experiments were made following the general protocol described in [22]. Electrodes with basal orientations and stepped surfaces belonging to the series (Pt(s) [n(111) × (100)]) and (Pt(s) [(n − 1)(111) × (110)]) were used in the hanging meniscus rotating disk configuration [23] (Radiometer, EDI-101). Working solutions were prepared with HClO4 (Merck suprapur) and H2O2 (Panreac) in ultrapure water (PureLab 18.2 MΩ·cm−1).
⁎ Corresponding author. Tel.: +34 965 909 301; fax: +34 965 903 537. E-mail address: [email protected] (J.M. Feliu). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.04.014
3. Results Fig. 1 shows cyclic voltammograms of low index Pt surfaces in 0.1 M HClO4 containing 2 mM of H2O2. At low potentials (E b 0.8 V) H2O2 is reduced to water under mass diffusion control, giving − 5.2 mA cm −2 at 2500 rpm, independently of the surface structure, until E = 0.3 V. For lower potentials the activity is strongly reduced at Pt(111) while the other electrodes are slightly affected. At high potentials, H2O2 is oxidized to O2 [21], reaching the same limiting current, but with opposite sign. A kinetically limited transition region connects these two contributions [21]. The activity in this region depends on the sweep direction; i.e. there is a hysteresis, which depends on the sweep rate and on the upper potential limit (Eup), between the curves obtained at the positive and negative going scans. This fact highlights the important role of surface oxides on HPORR electrode activity, similar to what has been reported for ORR on platinum surfaces [6,10]. In the positive-going scan, limiting reduction currents start to decrease at E > 0.8 V crossing the y-axis (zero net current) at around 0.95 V for all basal planes before reaching the anodic limiting current.
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E. Sitta et al. / Electrochemistry Communications 33 (2013) 39–42
Fig. 1. Cyclic voltammograms of low index platinum single crystals in 0.1 M HClO4 + 2 mM H2O2. 2500 rpm, 0.05 V·s − 1.
There is a clear discontinuity on the oxidative branch of Pt(111) around 1.0 V. In the negative-going scan, the onset of HPRR is different and follows the order Pt(111) > Pt(100) ≈ Pt(110), provided that Eup does not exceed 1.2 V. This tendency is in agreement with theoretical predictions [8] and early experimental results [3]. It is important, however, to emphasize that only the well oriented Pt(111) maintains its structure after the excursion up to 1.2 V, the other planes show modifications of the fingerprint hydrogen adsorption/desorption region (HUPD) in the immediate negative-going sweep, although the changes are quite small for Pt(110). On the transition region, platinum surfaces are covered by surface (hydro)oxides. Interestingly, PtOH can be considered a common intermediate to both the HPRR and HPOR reactions and then the stability of PtOH would drive the HPORR through its oxidative or reductive pathway [19]. To highlight surface contribution effects, experiments with low H2O2 concentration were performed. The black line in Fig. 2 corresponds to the experimental profile of Pt(111) at 2500 rpm in 0.2 mM of
Fig. 2. Cyclic voltammograms of Pt(111) in 0.2 mM H2O2. The Pt(111) blank is shown in dotted line. Other experimental conditions as in Fig. 1.
H2O2 solution. The red line shows the same curve after subtraction of the blank (dotted line). Clearly, the reduction process is inhibited at low potentials, in the HUPD region. On the other hand, the reaction is not affected by OH adsorption between 0.6 V and 0.8 V, slight variations on the butterfly peak region are probably due to the shift of this process in the presence of H2O2. The overall process seems to be kinetically controlled in the region between the butterfly peak and the beginning of the second oxidation process, usually associated with PtO formation. This second oxidation process involves adsorption of additional OH followed by its conversation to Oads [24,25]. By comparison with the blank, HPOR dominates at the PtOH surface, after the butterfly peak. It appears that HPOR seems to compete with PtO formation, as observed by the depression on HPOR current at the peak potential of PtO formation. However, once the PtO formation has been completed (E ≳ 1.06 V) the HPOR current reaches values similar to those in the HPRR, i.e. the process becomes diffusion controlled. During the negative-going scan, which is shifted towards higher potential values, the shape modifications of the j/E curve are almost negligible, in agreement with the progressive reduction of PtO that does not occur in a single peak [24,25]. It should be noted that both the HPOR and HPRR reactions reach the diffusion limiting current at the PtO and PtOH covered surfaces, respectively. In addition the HPRR maintains this diffusional limit at the Pt(111) electrode covered mainly with water and it is only inhibited by HUPD in the lower potential range. Interestingly, Pt(110) and Pt(100) do not show the inhibition of the HPOR caused by oxide formation in the positive-going scan, as described for Pt(111). CV's in absence of H2O2 of these planes certificate nonsymmetrical profiles when the surface is oxidized until 1.2 V, indicating the formation of a more irreversible PtO(H)x (x = 2, 1 or 0) structure and even surface disorder. Pt(100) is the most affected by potential excursions above 0.8 V, resulting in surface disorder as proved by changes on the HUPD region. Small changes were observed at Pt(110) in the Hupd region, but the HPORR profile is maintained in successive cycles, indicating non aggressive changes on surface structure. The largest separation related to PtO formation happens only at Pt(111). The presence of {110} or {100} steps leads to a series of small peaks after the butterfly region and the process at 1.06 V becomes smaller [26], consequently differences between positive- and negative-going sweeps also become smaller. Considering that HPORR reflects two independent contributions that become diffusion controlled at both sides of the wave, it would be possible to tentatively describe the overall process as the sum of two S-shaped j/E curves that follow the linearized equation [27]: j −j E ¼ E1=2 þ m log lim j
ð1Þ
being E as the applied potential; j as the current density and jlim as the limiting current density. The haft wave potential, E1/2, characteristic of each S-shaped curve gives an idea of the overpotential for both reactions. At this stage no attempts are made to characterize each individual mechanism, m is just the slope of the plot. Fig. 3(a1) depicts the predicted curves to the oxidation (ox) and reduction (red) of hydrogen peroxide over Pt(110) during the positive going scan, considering m = 60 mV. It is clear that the sum of these contributions (dotted line) fits well with the experimental curve (black line). The same procedure was applied for the negative going scan (panel (a2)). The good fit allows the attempt to analyze in the future the individual contributions, HPOR and HPRR, as a function of several experimental parameters. The method was extended to Pt(100), Pt(111) and for several stepped surfaces on the {111} pole. For sake of comparison, E1/2 values are depicted on Fig. 3(b) as a function of step density. On the adopted nomenclature, right and left increasing numbers indicate increases in {110} and {100} step densities respectively.
E. Sitta et al. / Electrochemistry Communications 33 (2013) 39–42
41
Fig. 3. (a) Adjusted curves (dashed lines) for HPOR and HPRR during positive (a1) and negative (a2) going scans. Dotted lines correspond to the sum of fitted branches and black line the experimental curves. (b) E1/2 values for the basal planes and stepped surfaces Pt[n(111) × (100)] and Pt[(n − 1)(111) × (110)]. Arrows indicate the sweep direction. Experimental conditions are the same as in Fig. 1.
Curves in Fig. 3(b) are quite symmetric with respect to the Pt(111), indicating similar effect of defects, irrespective of its symmetry. On the positive scan, (b1), both the HPOR and HPRR are catalyzed by the presence of defects, proved by the negative and positive shifts on E1/2 for the oxidation and reduction processes, respectively. When the step density increases towards the other basal planes, the reactions are again inhibited leading to similar E1/2 values Fig. 1(a). On the negative going scan, (b2), the branches reach the maximum E1/2 value at Pt(111), indicating that surfaces having large {111} terraces are the best catalysts for HPRR and the worst for the HPOR when covered by PtO. This would represent the best scenario for ORR. However, these values depend strongly on Eup value, e.g., on Pt(111) changing it from 1.2 to 1.4 V shifts E1/2 ox and E1/2 red by −70 and −60 mV, respectively (data not shown). At 1.4 V surface defects are created and the E1/2 value will lay on those found to the stepped surfaces. The presence of {110} or {100} defects on vicinal Pt(111) electrodes leads to a series of small peaks after the butterfly region and the process at 1.06 V become smaller [26]. Consequently, the differences between positive- and negative-going sweeps become smaller after potential excursions up to 1.2 V. Herein, it is important to remark that the inhibition observed around 1.0 V on HPOR at Pt(111) is not described by Eq. (1). Future works with more robust equations, incorporating the kinetics of platinum surface oxidation, are in progress. 4. Conclusions Experimental j/E curves for the HPORR at different single crystal electrode surfaces were decomposed into two independent contributions. Despite the very simple and approximate method applied to fit experimental data, it was possible to follow the dependence of individual processes with surface structure. Stepped Pt(111) surfaces, with both {110} and {100} steps, are more catalytic to the oxidation and reduction of H2O2. However, when the surface is covered by oxygen, large {111} terraces are better catalyst for HPRR but worse for HPOR. This would represent the best conditions for ORR taking place via the
associative mechanism. The H2O2 reactivity dependence with surface steps points out the importance of the employment of well-oriented surfaces to study HPORR and/or include the surface inhomogeneity on theoretical models. Another point is to identify if the OH intermediate, likely involved in HPORR, is the same that plays a role in surface oxidation. A similar problem was discussed for hydrogen evolution [28]. Acknowledgments Support from the Spanish MICYNN through the project CTQ201016271 and GV through PROMETEO/2009/045 (FEDER) is greatly acknowledged. ES would like to thank CNPq (Brazil) for the scholarship (Grant No. 200939/2012-2). References [1] H.A. Gasteiger, N.M. Markovic, Science 324 (2009) 48. [2] N.M. Markovic, R.R. Adzid, B.D. Cahan, E.B. Yeager, Journal of Electroanalytical Chemistry 377 (1994) 249. [3] N.M. Markovic, H.A. Gasteiger, P.N. Ross Jr., 99 (1995) 3411. [4] N.M. Markovic, T.J. Schmidt, V. Stamenkovic, P.N. Ross, Fuel Cells 1 (2001) 105. [5] P.B. Balbuena, S.R. Calvo, E.J. Lamas, P.F. Salazar, J.M. Seminario, The Journal of Physical Chemistry. B 110 (2006) 17452. [6] A. Kuzume, E. Herrero, J.M. Feliu, Journal of Electroanalytical Chemistry 599 (2007) 333. [7] V. Viswanathan, H.A. Hansen, J. Rossmeisl, J.K. Norskov, Journal of Physical Chemistry Letters 3 (2012) 2948. [8] V. Viswanathan, H.A. Hansen, J. Rossmeisl, J.K. Norskov, ACS Catalysis 2 (2012) 1654. [9] J.A. Keith, T. Jacob, Angewandte Chemie International Edition 49 (2010) 9521. [10] A.M. Gómez-Marín, J.M. Feliu, ChemSusChem (2013), http://dx.doi.org/10.1002/ cssc.201200847. [11] Y. Xu, M. Shao, M. Mavrikakis, R.R. Adzic, in: M.T.M. Koper (Ed.), Fuel Cells Catalysis: A Surface Science Approach, Wiley, 2009, pp. 271–315. [12] T.J. Schmidt, U.A. Paulus, H.A. Gasteiger, R.J. Behm, Journal of Electroanalytical Chemistry 508 (2001) 41. [13] V. Stamenkovic, N.M. Markovic, P.N. Ross, Journal of Electroanalytical Chemistry 500 (2001) 44. [14] N.M. Markovic, P.N. Ross, Surface Science Reports 45 (2002) 117. [15] N.M. Markovic, H.A. Gasteiger, P.N. Ross, The Journal of Physical Chemistry 100 (1996) 6715.
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[16] B.N. Grgur, N.M. Markovic, P.N. Ross, Canadian Journal of Chemistry 75 (1997) 1465. [17] R.J. Bowen, H.B. Urbach, The Journal of Chemical Physics 49 (1968) 1206. [18] H.B. Urbach, R.J. Bowen, Electrochimica Acta 14 (1969) 927. [19] I. Katsouraros, W.B. Schneider, J.C. Meier, U. Benedikt, P.U. Biedermann, A.A. Auer, K.J.J. Mayrhofer, Physical Chemistry Chemical Physics 14 (2012) 7384. [20] I. Katsouraros, W.B. Schneider, J.C. Meier, U. Benedikt, P.U. Biedermann, A.A. Auer, A. Cuesta, K.J.J. Mayrhofer, Physical Chemistry Chemical Physics (2013), http://dx.doi.org/10.1039/C3CP50649E, (in press). [21] A.M. Gómez-Márin, K.J.P. Schouten, M.T.M. Koper, J.M. Feliu, Electrochemistry Communications 22 (2012) 153.
[22] C. Korzeniewsky, V. Climent, J.M. Feliu, in: A.J. Bard, C.G. Zoski (Eds.), Electroanalytical Chemistry. A series of Advances, vol. 24, CRC Press, 2012, pp. 75–170. [23] H.M. Villullas, M. López Teijero, Journal of Electroanalytical Chemistry 384 (1995) 24. [24] A.M. Gómez-Marín, J. Clavilier, J.M. Feliu, Journal of Electroanalytical Chemistry 688 (2013) 360. [25] A.M. Gómez-Marín, J.M. Feliu, Electrochimica Acta 82 (2012) 558. [26] A. Bjorling, E. Herrero, J.M. Feliu, Journal of Physical Chemistry C 115 (2011) 15509. [27] A.J. Bard, L.R. Faulkner, Electrochemical Methods — Fundamentals and Applications, 2nd ed. John Wiley and Sons, 2000. [28] B.E. Conway, B.V. Tilak, Electrochimica Acta 47 (2002) 3571.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Electrocatalysis of H2O2 reduction/oxidation at model platinum surfaces E. Sitta, A.M. Gómez-Marín, A. Aldaz, J.M. Feliu ⁎ Instituto de Electroquímica, Universidad de Alicante, Apt. 99, 03080 Alicante, Spain
a r t i c l e
i n f o
Article history: Received 20 March 2013 Received in revised form 11 April 2013 Accepted 11 April 2013 Available online 23 April 2013 Keywords: Hydrogen peroxide oxidation Hydrogen peroxide reduction Platinum single crystals Hanging meniscus rotating electrode
a b s t r a c t Oxidation (HPOR) and reduction (HPRR) reactions of hydrogen peroxide were studied on platinum single crystalline surfaces. Experimental curves were fitted to a simple model in which the whole process is described by the sum of two independent, mass controlled reactions. It is shown that stepped surfaces are more catalytic than basal planes for both HPOR and HPRR reactions. On the contrary, on oxide covered surfaces, those having large {111} terraces are better electrocatalysts for HPRR, but worse for HPOR. These results shed light concerning H2O2 reactivity on platinum surfaces and may help to unveil oxygen reduction reaction mechanism. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The electro-catalysis of oxygen reduction reaction (ORR) is an active research field aiming (a) to find low cost catalysts with appreciable activity for fuel cell devices, (for a short review see Ref. [1]), and (b) unraveling the ORR mechanism [2–10]. ORR to water is a 4 electrons mechanism that can proceed dissociatively or associatively. In the latter case a serial pathway via the formation of hydrogen peroxide as stable intermediate has been commonly proposed [11]. H2O2 has been identified during ORR over platinum, using rotating ring-disk electrodes, when the potential reaches the hydrogen UPD region or in the presence of strong adsorbing anions (e.g. [12–14] and references therein) thus indicating incomplete electron transfer. Generally speaking, H2O2 can be reduced to water (hydrogen peroxide reduction reaction, HPRR), E0 = 1.76 V, or oxidized to O2 (hydrogen peroxide oxidation reaction, HPOR), E0 = 0.695 V, depending on the electrode potential [3,15–21]. This dual character makes this molecule an interesting workhorse to build electrochemical models. In addition, near to the ORR standard potential, E0 = 1.23 V, the theoretical overpotential for HPOR is high (more than 0.5 V in standard conditions). This means that any H2O2 eventually formed would be immediately oxidized, resulting in a zero net current for the oxygen reduction if the dominant pathway would follow the associative mechanism. The present work aims to investigate the structure sensitivity of hydrogen peroxide oxidation and reduction reactions (HPORR) at single crystal platinum electrode surfaces.
Experiments were made following the general protocol described in [22]. Electrodes with basal orientations and stepped surfaces belonging to the series (Pt(s) [n(111) × (100)]) and (Pt(s) [(n − 1)(111) × (110)]) were used in the hanging meniscus rotating disk configuration [23] (Radiometer, EDI-101). Working solutions were prepared with HClO4 (Merck suprapur) and H2O2 (Panreac) in ultrapure water (PureLab 18.2 MΩ·cm−1).
⁎ Corresponding author. Tel.: +34 965 909 301; fax: +34 965 903 537. E-mail address: [email protected] (J.M. Feliu). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.04.014
3. Results Fig. 1 shows cyclic voltammograms of low index Pt surfaces in 0.1 M HClO4 containing 2 mM of H2O2. At low potentials (E b 0.8 V) H2O2 is reduced to water under mass diffusion control, giving − 5.2 mA cm −2 at 2500 rpm, independently of the surface structure, until E = 0.3 V. For lower potentials the activity is strongly reduced at Pt(111) while the other electrodes are slightly affected. At high potentials, H2O2 is oxidized to O2 [21], reaching the same limiting current, but with opposite sign. A kinetically limited transition region connects these two contributions [21]. The activity in this region depends on the sweep direction; i.e. there is a hysteresis, which depends on the sweep rate and on the upper potential limit (Eup), between the curves obtained at the positive and negative going scans. This fact highlights the important role of surface oxides on HPORR electrode activity, similar to what has been reported for ORR on platinum surfaces [6,10]. In the positive-going scan, limiting reduction currents start to decrease at E > 0.8 V crossing the y-axis (zero net current) at around 0.95 V for all basal planes before reaching the anodic limiting current.
40
E. Sitta et al. / Electrochemistry Communications 33 (2013) 39–42
Fig. 1. Cyclic voltammograms of low index platinum single crystals in 0.1 M HClO4 + 2 mM H2O2. 2500 rpm, 0.05 V·s − 1.
There is a clear discontinuity on the oxidative branch of Pt(111) around 1.0 V. In the negative-going scan, the onset of HPRR is different and follows the order Pt(111) > Pt(100) ≈ Pt(110), provided that Eup does not exceed 1.2 V. This tendency is in agreement with theoretical predictions [8] and early experimental results [3]. It is important, however, to emphasize that only the well oriented Pt(111) maintains its structure after the excursion up to 1.2 V, the other planes show modifications of the fingerprint hydrogen adsorption/desorption region (HUPD) in the immediate negative-going sweep, although the changes are quite small for Pt(110). On the transition region, platinum surfaces are covered by surface (hydro)oxides. Interestingly, PtOH can be considered a common intermediate to both the HPRR and HPOR reactions and then the stability of PtOH would drive the HPORR through its oxidative or reductive pathway [19]. To highlight surface contribution effects, experiments with low H2O2 concentration were performed. The black line in Fig. 2 corresponds to the experimental profile of Pt(111) at 2500 rpm in 0.2 mM of
Fig. 2. Cyclic voltammograms of Pt(111) in 0.2 mM H2O2. The Pt(111) blank is shown in dotted line. Other experimental conditions as in Fig. 1.
H2O2 solution. The red line shows the same curve after subtraction of the blank (dotted line). Clearly, the reduction process is inhibited at low potentials, in the HUPD region. On the other hand, the reaction is not affected by OH adsorption between 0.6 V and 0.8 V, slight variations on the butterfly peak region are probably due to the shift of this process in the presence of H2O2. The overall process seems to be kinetically controlled in the region between the butterfly peak and the beginning of the second oxidation process, usually associated with PtO formation. This second oxidation process involves adsorption of additional OH followed by its conversation to Oads [24,25]. By comparison with the blank, HPOR dominates at the PtOH surface, after the butterfly peak. It appears that HPOR seems to compete with PtO formation, as observed by the depression on HPOR current at the peak potential of PtO formation. However, once the PtO formation has been completed (E ≳ 1.06 V) the HPOR current reaches values similar to those in the HPRR, i.e. the process becomes diffusion controlled. During the negative-going scan, which is shifted towards higher potential values, the shape modifications of the j/E curve are almost negligible, in agreement with the progressive reduction of PtO that does not occur in a single peak [24,25]. It should be noted that both the HPOR and HPRR reactions reach the diffusion limiting current at the PtO and PtOH covered surfaces, respectively. In addition the HPRR maintains this diffusional limit at the Pt(111) electrode covered mainly with water and it is only inhibited by HUPD in the lower potential range. Interestingly, Pt(110) and Pt(100) do not show the inhibition of the HPOR caused by oxide formation in the positive-going scan, as described for Pt(111). CV's in absence of H2O2 of these planes certificate nonsymmetrical profiles when the surface is oxidized until 1.2 V, indicating the formation of a more irreversible PtO(H)x (x = 2, 1 or 0) structure and even surface disorder. Pt(100) is the most affected by potential excursions above 0.8 V, resulting in surface disorder as proved by changes on the HUPD region. Small changes were observed at Pt(110) in the Hupd region, but the HPORR profile is maintained in successive cycles, indicating non aggressive changes on surface structure. The largest separation related to PtO formation happens only at Pt(111). The presence of {110} or {100} steps leads to a series of small peaks after the butterfly region and the process at 1.06 V becomes smaller [26], consequently differences between positive- and negative-going sweeps also become smaller. Considering that HPORR reflects two independent contributions that become diffusion controlled at both sides of the wave, it would be possible to tentatively describe the overall process as the sum of two S-shaped j/E curves that follow the linearized equation [27]: j −j E ¼ E1=2 þ m log lim j
ð1Þ
being E as the applied potential; j as the current density and jlim as the limiting current density. The haft wave potential, E1/2, characteristic of each S-shaped curve gives an idea of the overpotential for both reactions. At this stage no attempts are made to characterize each individual mechanism, m is just the slope of the plot. Fig. 3(a1) depicts the predicted curves to the oxidation (ox) and reduction (red) of hydrogen peroxide over Pt(110) during the positive going scan, considering m = 60 mV. It is clear that the sum of these contributions (dotted line) fits well with the experimental curve (black line). The same procedure was applied for the negative going scan (panel (a2)). The good fit allows the attempt to analyze in the future the individual contributions, HPOR and HPRR, as a function of several experimental parameters. The method was extended to Pt(100), Pt(111) and for several stepped surfaces on the {111} pole. For sake of comparison, E1/2 values are depicted on Fig. 3(b) as a function of step density. On the adopted nomenclature, right and left increasing numbers indicate increases in {110} and {100} step densities respectively.
E. Sitta et al. / Electrochemistry Communications 33 (2013) 39–42
41
Fig. 3. (a) Adjusted curves (dashed lines) for HPOR and HPRR during positive (a1) and negative (a2) going scans. Dotted lines correspond to the sum of fitted branches and black line the experimental curves. (b) E1/2 values for the basal planes and stepped surfaces Pt[n(111) × (100)] and Pt[(n − 1)(111) × (110)]. Arrows indicate the sweep direction. Experimental conditions are the same as in Fig. 1.
Curves in Fig. 3(b) are quite symmetric with respect to the Pt(111), indicating similar effect of defects, irrespective of its symmetry. On the positive scan, (b1), both the HPOR and HPRR are catalyzed by the presence of defects, proved by the negative and positive shifts on E1/2 for the oxidation and reduction processes, respectively. When the step density increases towards the other basal planes, the reactions are again inhibited leading to similar E1/2 values Fig. 1(a). On the negative going scan, (b2), the branches reach the maximum E1/2 value at Pt(111), indicating that surfaces having large {111} terraces are the best catalysts for HPRR and the worst for the HPOR when covered by PtO. This would represent the best scenario for ORR. However, these values depend strongly on Eup value, e.g., on Pt(111) changing it from 1.2 to 1.4 V shifts E1/2 ox and E1/2 red by −70 and −60 mV, respectively (data not shown). At 1.4 V surface defects are created and the E1/2 value will lay on those found to the stepped surfaces. The presence of {110} or {100} defects on vicinal Pt(111) electrodes leads to a series of small peaks after the butterfly region and the process at 1.06 V become smaller [26]. Consequently, the differences between positive- and negative-going sweeps become smaller after potential excursions up to 1.2 V. Herein, it is important to remark that the inhibition observed around 1.0 V on HPOR at Pt(111) is not described by Eq. (1). Future works with more robust equations, incorporating the kinetics of platinum surface oxidation, are in progress. 4. Conclusions Experimental j/E curves for the HPORR at different single crystal electrode surfaces were decomposed into two independent contributions. Despite the very simple and approximate method applied to fit experimental data, it was possible to follow the dependence of individual processes with surface structure. Stepped Pt(111) surfaces, with both {110} and {100} steps, are more catalytic to the oxidation and reduction of H2O2. However, when the surface is covered by oxygen, large {111} terraces are better catalyst for HPRR but worse for HPOR. This would represent the best conditions for ORR taking place via the
associative mechanism. The H2O2 reactivity dependence with surface steps points out the importance of the employment of well-oriented surfaces to study HPORR and/or include the surface inhomogeneity on theoretical models. Another point is to identify if the OH intermediate, likely involved in HPORR, is the same that plays a role in surface oxidation. A similar problem was discussed for hydrogen evolution [28]. Acknowledgments Support from the Spanish MICYNN through the project CTQ201016271 and GV through PROMETEO/2009/045 (FEDER) is greatly acknowledged. ES would like to thank CNPq (Brazil) for the scholarship (Grant No. 200939/2012-2). References [1] H.A. Gasteiger, N.M. Markovic, Science 324 (2009) 48. [2] N.M. Markovic, R.R. Adzid, B.D. Cahan, E.B. Yeager, Journal of Electroanalytical Chemistry 377 (1994) 249. [3] N.M. Markovic, H.A. Gasteiger, P.N. Ross Jr., 99 (1995) 3411. [4] N.M. Markovic, T.J. Schmidt, V. Stamenkovic, P.N. Ross, Fuel Cells 1 (2001) 105. [5] P.B. Balbuena, S.R. Calvo, E.J. Lamas, P.F. Salazar, J.M. Seminario, The Journal of Physical Chemistry. B 110 (2006) 17452. [6] A. Kuzume, E. Herrero, J.M. Feliu, Journal of Electroanalytical Chemistry 599 (2007) 333. [7] V. Viswanathan, H.A. Hansen, J. Rossmeisl, J.K. Norskov, Journal of Physical Chemistry Letters 3 (2012) 2948. [8] V. Viswanathan, H.A. Hansen, J. Rossmeisl, J.K. Norskov, ACS Catalysis 2 (2012) 1654. [9] J.A. Keith, T. Jacob, Angewandte Chemie International Edition 49 (2010) 9521. [10] A.M. Gómez-Marín, J.M. Feliu, ChemSusChem (2013), http://dx.doi.org/10.1002/ cssc.201200847. [11] Y. Xu, M. Shao, M. Mavrikakis, R.R. Adzic, in: M.T.M. Koper (Ed.), Fuel Cells Catalysis: A Surface Science Approach, Wiley, 2009, pp. 271–315. [12] T.J. Schmidt, U.A. Paulus, H.A. Gasteiger, R.J. Behm, Journal of Electroanalytical Chemistry 508 (2001) 41. [13] V. Stamenkovic, N.M. Markovic, P.N. Ross, Journal of Electroanalytical Chemistry 500 (2001) 44. [14] N.M. Markovic, P.N. Ross, Surface Science Reports 45 (2002) 117. [15] N.M. Markovic, H.A. Gasteiger, P.N. Ross, The Journal of Physical Chemistry 100 (1996) 6715.
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