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Kinetics of surface modification induced by submonolayer electrochemical oxygen adsorption on Pt(1 1 1)
Electrochemistry Communications 12 (2010) 359–361
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Kinetics of surface modification induced by submonolayer electrochemical oxygen adsorption on Pt(1 1 1) Alexander Björling a, Elisabet Ahlberg b, Juan M. Feliu a,* a b
Instituto de Electroquímica, Universidad de Alicante, Apt 99, E-03080 Alicante, Spain Department of Chemistry, University of Gothenburg, Sweden
a r t i c l e
i n f o
Article history: Received 14 December 2009 Accepted 21 December 2009 Available online 24 December 2009 Keywords: Pt(1 1 1) electrodes Surface modification kinetics Electrochemical oxygen adsorption Electrocatalysis Place-exchange disordering
a b s t r a c t Electrochemical oxygen adsorption/desorption below monolayer level leads to a disordering of platinum single-crystal surfaces vicinal to the (1 1 1) plane. The kinetics can be described by means of a consecutive reaction from (1 1 1)-terrace sites to (1 1 0)-defect sites, in which (1 0 0)-defects act as intermediates. The first oxidation of the electrode reflects independent contributions from terrace and step sites, the latter being structure sensitive. Oxygen adsorption charges amount to a mean value of one electron per step site. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Studies on single-crystal platinum surfaces are usually limited to low potentials because of the disordering caused by electrochemical surface oxidation and reduction. This disordering destroys the initial surface structure and leads to a perturbed surface, the structure of which is undefined at the atomic level [1–3]. Thus the utility of using well-defined surfaces, in which the atomic neighbourhood is known, will no longer be valid. Surface oxide formation on polycrystalline platinum samples was extensively studied and concepts such as place-exchange kinetics were described by Conway and co-workers in several contributions [4–6]. In these studies the electrodes were extensively cycled as a part of the pretreatment, and as a result, only the properties of locally disordered surfaces were investigated. In this respect there are only a few reports on surface oxide formation and its effect on the surface structure. Pt(1 1 1) electrodes in sulphuric acid deserve special attention, because their surface oxidation is largely inhibited owing to the stability of the adsorbed anion layer [7]. It is known that the oxidation and reduction leads to surface disorder to an extent that depends on the potential perturbation and the initial surface order of the electrode [2,3]. In situ structural information can be gained in the potential region in which hydrogen and anion adsorption takes place by using the well established fingerprint signals present in the positive-going sweep of the voltammetric profile [8]. * 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 Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.12.034
In this paper attempts are made to understand the kinetics of surface modification after electrochemical oxygen adsorption on well-defined single-crystal surfaces. Special attention is paid to the first electrooxidation cycle. 2. Experimental Single-crystal electrodes were prepared from small Pt beads (2 mm diameter) [9]. Pt(1 1 1) and its vicinal surfaces with relatively large terraces were chosen. The electrodes were flame annealed, cooled down in a reductive atmosphere and protected with ultrapure water (Elga-Vivendi) [8]. Suprapur sulphuric acid (Merck) was used to prepare 0.5 M solutions and the voltammetric experiments were conducted in a three electrode cell [8]. All potentials were measured against the Reversible Hydrogen Electrode (RHE). 3. Results and discussion Fig. 1A describes the evolution of the voltammetric profile of a Pt(1 1 1) electrode upon repeated cycling up to 1.4 V. The upper switching potential was chosen well below the completion of the oxide monolayer and the start of oxygen evolution [1,3,7] to study better the progressive evolution of the surface. The stable blank voltammogram after flame annealing, as it is recorded between 0.85 V and the beginning of hydrogen evolution, is given as a reference of the initial state of the electrode. Fig. 1A shows that the potential excursions lead to a marked shift of the surface oxidation
360
A. Björling et al. / Electrochemistry Communications 12 (2010) 359–361
A
100
A4
0 (111)−terraces
70 Δq (μC cm−2)
A1−3
−2
j (μA cm )
50
80
Pt(111)
0 −50
−10
60
−20 (110)−defects
50 40
−40
30
−50
20 −100
−60
(100)−defects
10 0
0.2
0.4
0.6
0.8
1
1.2
1.4
B j (μA cm−2)
−30
Pt(111)
60
A4
A3
40
A1
Pt(20 20 19)
20 0 0.05
A2
0.1
0.15 0.2 0.25 E (V vs RHE)
0.3
Fig. 1. (A) Evolution of the voltammetric profile of Pt(1 1 1) in 0.5 M H2SO4 as the electrode is cycled at 50 mV/s between 0.06 and 1.4 V vs RHE. Black shows initial stable profile up to 0.85 V, blue the profile attained after 12 cycles. Arrows indicate gradual development of the peaks. (B) Enlargement of Fig. 1A for Pt(1 1 1) together with the corresponding data for Pt(20,20,19).
towards lower potentials. Also, (i) the adsorption states at around 0.12 V, labelled A4, increasingly grow and broaden and (ii) the anion adsorption states decrease in charge and shift toward positive potentials. The adsorption states around 0.28 V (A1–3) show some interesting dynamics as the electrode is cycled. These states first increase and then decrease and shift towards lower potentials. The evolution of the A1–3 peaks is better observed in the enlargement of the lower potential region, Fig. 1B, for Pt(1 1 1) and Pt(20,20,19). It should be noted that three adsorption energies are clearly observed, the A1 and A2 being progressively transformed into the A3 state at 0.27 V, i.e. at the same potential as observed with Pt(S)[n(1 1 1) (1 0 0)] electrodes [10]. The A3 state finally disappears in the case of the Pt(1 1 1) electrode, but remains on stepped surfaces with a limiting charge that increases on electrodes with higher initial step density. Also, different upper potential limits in the range 1.3–1.4 V lead to similar changes, although these become faster as the upper limit is increased. After several cycles, the voltammograms attain a stationary profile, markedly different from the initial one. It is evident that oxygen evolution is still absent and therefore the anodic currents during the early cycles are only related to partial surface oxidation of the electrode. The dominant feature is the peak at 0.12 V, which can be assigned to the presence of (1 1 0)-defect sites on the surface [11]. In the same way we assume that the A1–3 states correspond to (1 0 0)-defect sites [10]. The anion adsorption charge density can be considered as representative of the amount of (1 1 1)-terrace sites that continuously decreases in the cycling process. The overall charge between 0.06 and 0.7 V remains almost constant, only a small increase of 20 lC cm2 is measured in the overall cycling process owing to the increase of surface roughness
−70
0
−80 0
2
4
6 n (cycles)
8
10
Fig. 2. Evolution of charge densities from Fig. 1A associated with (1 1 0)-defects (red), (1 0 0)-defects (black) and the change in coverage of (1 1 1) terraces (green, right-hand axis). The latter was approximated by subtracting the defect charges from that of the total double-layer corrected charge from the lower limit to 0.77 V. Solid lines show fitting according to the reaction mechanism in the text, with the parameters k1 = 0.19, k2 = 0.9, k3 = 0 cycle1 and k4 = 0.01 lC1 cm2 cycle1. Dashed lines show fitting to the unmodified consecutive mechanism (k3 = k4 = 0). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[3]. The terrace modification can also be estimated from the overall voltammetric profile after subtraction of the (1 1 0)- and (1 0 0)defect contributions. All changes can be referred to the corresponding initial blank voltammogram and plotted as a function of the number of cycles (Fig. 2). The evolution of the different charges is reminiscent of the concentration–time profiles for first-order consecutive reactions, and the two defect types can be seen as chemical species formed from (1 1 1) terrace sites in such a simple mechanism. In a further refinement the direct formation of (1 1 0)-defects is not discarded a priori and an autocatalytic path is added to account for the more facile oxidation of previously cycled surfaces: k3X
X
k1X
k 2Y
Y
X = (111)-terraces
Z
Y = (100)-defects
k 4X (Y+Z)
If we accept the number of cycles as a continuous time unit, a cycle being defined by excursion from 0.7 to 1.4 V followed by the reverse scan to 0.06 V and the subsequent positive scan up to 0.7 V, then the time unit is 53.6 s. This should be considered as a mean dynamic time unit because potential and surface coverage and thus surface mobilities of various species are changing in a very complex way. The consecutive reaction path (k3 = k4 = 0) agrees qualitatively with experimental data (dotted curves in Fig. 2), but the refined mechanism gives a much better fit (solid curves in the same figure). The best agreement is apparently obtained with k3 = 0. This model has also been used for stepped surfaces (not shown) and the most interesting result is that the maximum in the amount of (1 0 0)-defects is reached after longer times as the initial step density is increased. This unexpected result suggests that welldefined steps may have a lower activity than random defects resulting from place-exchange processes. Ongoing systematic investigations on stepped surfaces with large (1 1 1) terraces separated by (1 1 0) or (1 0 0) monoatomic steps will shed light on the role of defects in surface modification upon oxygen adsorption and its dependence on potential and competitive anion
A. Björling et al. / Electrochemistry Communications 12 (2010) 359–361
(533)
−2
j (μA cm )
30
20
(544) (554) (776) (11,10,10) (20,20,19) 0 0.1 0.2 d⋅N
80 60
(332)
40 20 0.3
the oxidation of the steps is complete, increases as the terraces become narrower. Further, the charges under the step oxidation peaks (shaded areas in Fig. 3) fit linearly with the step density for terraces up to 10 atoms wide (inset of Fig. 3). The slope of the linear plot agrees with the theoretical value for a monoelectronic process at each step site. At present it is difficult to define the surface stoichiometry of the step oxidation and more work is needed to discriminate between the species that could be involved, including sulphate anions that will require spectroscopic information.
100
0 0.4
−2
(221)
q (μC cm )
40
n= 13 21 39
∞
10
0.9
1
1.1 1.2 E (V vs RHE)
361
1.3
Fig. 3. Positive-going voltammetric sweeps at well-ordered surfaces vicinal to Pt(1 1 1) in 0.5 M H2SO4 at 50 mV/s. The (1 0 0)-stepped surfaces are represented by Pt(11,10,10) (blue) while Pt(20,20,19) and Pt(7 7 6) show the behaviour of (1 1 0) steps (green and red). The inset shows peak charge versus step density for various surfaces of both symmetries together with a line for that expected from an ideal surface. Shading demonstrates how this integration was carried out and n is the terrace length. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
adsorption. Constant-potential oxidation measurements need to be performed and included in the analysis. Preliminary experiments suggest that the conversion between defect sites is not allowed when the potential is held constant, showing the role of potential cycling in surface modification. These experiments will supply data for modelling the first steps of place-exchange kinetics on platinum single-crystal electrodes. Another point, particularly important in electrocatalysis, is the adsorption of oxygen on well-defined surfaces without previous modification, i.e. the first oxygen adsorption cycle of each electrode. Relevant data are shown in Fig. 3, comparing the behaviour of electrodes with different (1 1 1) terrace lengths (n = 1, 39, 21, 13 atoms), two of them separated by (1 1 0) steps (n = 39, 13) and one by (1 0 0) steps (n = 21). At first sight it appears that it is possible to separate the oxidation at the terraces from that on the steps. The oxidation current at the terraces appears to obey an exponential-type law, faster as the terrace width decreases. The oxidation on the steps is superimposed on this terrace oxidation and involves a limited charge density in a defined potential range. It is noteworthy that the (1 0 0) steps oxidize at lower potentials in a relatively sharp single peak, whereas the (1 1 0) step oxidation takes place in two broader contributions. The different shapes reflect differences in the interactions within the surface oxide adlayer, which develops one-dimensionally along the steps. This adlayer would seem to influence the oxidation of the terraces, since changes in the exponential current on the terraces can be observed. It is clear that the current at 1.2 V, i.e. a potential at which
4. Conclusions The kinetics of surface disordering after electrochemical oxygen adsorption/desorption cycles can be followed by monitoring the changes in the population of (1 1 1)-terrace, (1 0 0)- and (1 1 0)-defect sites. The process is described by a consecutive first order mechanism with the addition of an autocatalytic contribution. The reaction becomes slower as the terraces become narrower. The first oxidation of the electrodes shows two contributions. One can be described as an exponential increase of current density with potential and is faster as the terrace length decreases. The other is related to oxide formation on the steps and depends on the step symmetry. The charge involved in this second contribution increases linearly with the step density, with a slope of one electron per step atom. Acknowledgements This study has been carried out in the framework of the European Commission FP7 Initial Training Network ‘‘ELCAT”, Grant Agreement No. 214936-2. Partial support of MICINN through Project CTQ2006-04071/BQU and Generalitat Valenciana through project PROMETEO/2009/045 is also acknowledged. References [1] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205– 209. [2] D. Aberdam, R. Durand, R. Faure, F. El-Omar, Surf. Sci. 171 (1986) 303–330. [3] K. Itaya, S. Sugawara, K. Sashikata, N. Furuya, J. Vac. Sci. Technol. A 8 (1990) 515–519. [4] B.E. Conway, Prog. Surf. Sci. 49 (1995) 331–452. [5] D.A. Harrington, J. Electroanal. Chem. 420 (1997) 101–109. [6] G. Jerkiewicz, G. Vatankhah, J. Lessard, M.P. Soriaga, Y.-S. Park, Electrochim. Acta 49 (2004) 1451–1459. [7] J. Clavilier, in: M.P. Soriaga (Ed.), ACS Symposium Series 378, ACS, Washington, 1988, Ch.14. [8] J. Solla-Gullón, P. Rodríguez, E. Herrero, A. Aldaz, J.M. Feliu, PCCP 10 (2008) 1359–1373. [9] J. Clavilier, D. Armand, S. Sun, M. Petit, J. Electroanal. Chem. 205 (1986) 267– 277. [10] A. Rodes, K. El Achi, M. Zamakhchari, J. Clavilier, J. Electroanal. Chem. 284 (1990) 245–253. [11] J. Clavilier, K. El Achi, A. Rodes, Chem. Phys. 141 (1990) 1–14.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Kinetics of surface modification induced by submonolayer electrochemical oxygen adsorption on Pt(1 1 1) Alexander Björling a, Elisabet Ahlberg b, Juan M. Feliu a,* a b
Instituto de Electroquímica, Universidad de Alicante, Apt 99, E-03080 Alicante, Spain Department of Chemistry, University of Gothenburg, Sweden
a r t i c l e
i n f o
Article history: Received 14 December 2009 Accepted 21 December 2009 Available online 24 December 2009 Keywords: Pt(1 1 1) electrodes Surface modification kinetics Electrochemical oxygen adsorption Electrocatalysis Place-exchange disordering
a b s t r a c t Electrochemical oxygen adsorption/desorption below monolayer level leads to a disordering of platinum single-crystal surfaces vicinal to the (1 1 1) plane. The kinetics can be described by means of a consecutive reaction from (1 1 1)-terrace sites to (1 1 0)-defect sites, in which (1 0 0)-defects act as intermediates. The first oxidation of the electrode reflects independent contributions from terrace and step sites, the latter being structure sensitive. Oxygen adsorption charges amount to a mean value of one electron per step site. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Studies on single-crystal platinum surfaces are usually limited to low potentials because of the disordering caused by electrochemical surface oxidation and reduction. This disordering destroys the initial surface structure and leads to a perturbed surface, the structure of which is undefined at the atomic level [1–3]. Thus the utility of using well-defined surfaces, in which the atomic neighbourhood is known, will no longer be valid. Surface oxide formation on polycrystalline platinum samples was extensively studied and concepts such as place-exchange kinetics were described by Conway and co-workers in several contributions [4–6]. In these studies the electrodes were extensively cycled as a part of the pretreatment, and as a result, only the properties of locally disordered surfaces were investigated. In this respect there are only a few reports on surface oxide formation and its effect on the surface structure. Pt(1 1 1) electrodes in sulphuric acid deserve special attention, because their surface oxidation is largely inhibited owing to the stability of the adsorbed anion layer [7]. It is known that the oxidation and reduction leads to surface disorder to an extent that depends on the potential perturbation and the initial surface order of the electrode [2,3]. In situ structural information can be gained in the potential region in which hydrogen and anion adsorption takes place by using the well established fingerprint signals present in the positive-going sweep of the voltammetric profile [8]. * 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 Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.12.034
In this paper attempts are made to understand the kinetics of surface modification after electrochemical oxygen adsorption on well-defined single-crystal surfaces. Special attention is paid to the first electrooxidation cycle. 2. Experimental Single-crystal electrodes were prepared from small Pt beads (2 mm diameter) [9]. Pt(1 1 1) and its vicinal surfaces with relatively large terraces were chosen. The electrodes were flame annealed, cooled down in a reductive atmosphere and protected with ultrapure water (Elga-Vivendi) [8]. Suprapur sulphuric acid (Merck) was used to prepare 0.5 M solutions and the voltammetric experiments were conducted in a three electrode cell [8]. All potentials were measured against the Reversible Hydrogen Electrode (RHE). 3. Results and discussion Fig. 1A describes the evolution of the voltammetric profile of a Pt(1 1 1) electrode upon repeated cycling up to 1.4 V. The upper switching potential was chosen well below the completion of the oxide monolayer and the start of oxygen evolution [1,3,7] to study better the progressive evolution of the surface. The stable blank voltammogram after flame annealing, as it is recorded between 0.85 V and the beginning of hydrogen evolution, is given as a reference of the initial state of the electrode. Fig. 1A shows that the potential excursions lead to a marked shift of the surface oxidation
360
A. Björling et al. / Electrochemistry Communications 12 (2010) 359–361
A
100
A4
0 (111)−terraces
70 Δq (μC cm−2)
A1−3
−2
j (μA cm )
50
80
Pt(111)
0 −50
−10
60
−20 (110)−defects
50 40
−40
30
−50
20 −100
−60
(100)−defects
10 0
0.2
0.4
0.6
0.8
1
1.2
1.4
B j (μA cm−2)
−30
Pt(111)
60
A4
A3
40
A1
Pt(20 20 19)
20 0 0.05
A2
0.1
0.15 0.2 0.25 E (V vs RHE)
0.3
Fig. 1. (A) Evolution of the voltammetric profile of Pt(1 1 1) in 0.5 M H2SO4 as the electrode is cycled at 50 mV/s between 0.06 and 1.4 V vs RHE. Black shows initial stable profile up to 0.85 V, blue the profile attained after 12 cycles. Arrows indicate gradual development of the peaks. (B) Enlargement of Fig. 1A for Pt(1 1 1) together with the corresponding data for Pt(20,20,19).
towards lower potentials. Also, (i) the adsorption states at around 0.12 V, labelled A4, increasingly grow and broaden and (ii) the anion adsorption states decrease in charge and shift toward positive potentials. The adsorption states around 0.28 V (A1–3) show some interesting dynamics as the electrode is cycled. These states first increase and then decrease and shift towards lower potentials. The evolution of the A1–3 peaks is better observed in the enlargement of the lower potential region, Fig. 1B, for Pt(1 1 1) and Pt(20,20,19). It should be noted that three adsorption energies are clearly observed, the A1 and A2 being progressively transformed into the A3 state at 0.27 V, i.e. at the same potential as observed with Pt(S)[n(1 1 1) (1 0 0)] electrodes [10]. The A3 state finally disappears in the case of the Pt(1 1 1) electrode, but remains on stepped surfaces with a limiting charge that increases on electrodes with higher initial step density. Also, different upper potential limits in the range 1.3–1.4 V lead to similar changes, although these become faster as the upper limit is increased. After several cycles, the voltammograms attain a stationary profile, markedly different from the initial one. It is evident that oxygen evolution is still absent and therefore the anodic currents during the early cycles are only related to partial surface oxidation of the electrode. The dominant feature is the peak at 0.12 V, which can be assigned to the presence of (1 1 0)-defect sites on the surface [11]. In the same way we assume that the A1–3 states correspond to (1 0 0)-defect sites [10]. The anion adsorption charge density can be considered as representative of the amount of (1 1 1)-terrace sites that continuously decreases in the cycling process. The overall charge between 0.06 and 0.7 V remains almost constant, only a small increase of 20 lC cm2 is measured in the overall cycling process owing to the increase of surface roughness
−70
0
−80 0
2
4
6 n (cycles)
8
10
Fig. 2. Evolution of charge densities from Fig. 1A associated with (1 1 0)-defects (red), (1 0 0)-defects (black) and the change in coverage of (1 1 1) terraces (green, right-hand axis). The latter was approximated by subtracting the defect charges from that of the total double-layer corrected charge from the lower limit to 0.77 V. Solid lines show fitting according to the reaction mechanism in the text, with the parameters k1 = 0.19, k2 = 0.9, k3 = 0 cycle1 and k4 = 0.01 lC1 cm2 cycle1. Dashed lines show fitting to the unmodified consecutive mechanism (k3 = k4 = 0). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[3]. The terrace modification can also be estimated from the overall voltammetric profile after subtraction of the (1 1 0)- and (1 0 0)defect contributions. All changes can be referred to the corresponding initial blank voltammogram and plotted as a function of the number of cycles (Fig. 2). The evolution of the different charges is reminiscent of the concentration–time profiles for first-order consecutive reactions, and the two defect types can be seen as chemical species formed from (1 1 1) terrace sites in such a simple mechanism. In a further refinement the direct formation of (1 1 0)-defects is not discarded a priori and an autocatalytic path is added to account for the more facile oxidation of previously cycled surfaces: k3X
X
k1X
k 2Y
Y
X = (111)-terraces
Z
Y = (100)-defects
k 4X (Y+Z)
If we accept the number of cycles as a continuous time unit, a cycle being defined by excursion from 0.7 to 1.4 V followed by the reverse scan to 0.06 V and the subsequent positive scan up to 0.7 V, then the time unit is 53.6 s. This should be considered as a mean dynamic time unit because potential and surface coverage and thus surface mobilities of various species are changing in a very complex way. The consecutive reaction path (k3 = k4 = 0) agrees qualitatively with experimental data (dotted curves in Fig. 2), but the refined mechanism gives a much better fit (solid curves in the same figure). The best agreement is apparently obtained with k3 = 0. This model has also been used for stepped surfaces (not shown) and the most interesting result is that the maximum in the amount of (1 0 0)-defects is reached after longer times as the initial step density is increased. This unexpected result suggests that welldefined steps may have a lower activity than random defects resulting from place-exchange processes. Ongoing systematic investigations on stepped surfaces with large (1 1 1) terraces separated by (1 1 0) or (1 0 0) monoatomic steps will shed light on the role of defects in surface modification upon oxygen adsorption and its dependence on potential and competitive anion
A. Björling et al. / Electrochemistry Communications 12 (2010) 359–361
(533)
−2
j (μA cm )
30
20
(544) (554) (776) (11,10,10) (20,20,19) 0 0.1 0.2 d⋅N
80 60
(332)
40 20 0.3
the oxidation of the steps is complete, increases as the terraces become narrower. Further, the charges under the step oxidation peaks (shaded areas in Fig. 3) fit linearly with the step density for terraces up to 10 atoms wide (inset of Fig. 3). The slope of the linear plot agrees with the theoretical value for a monoelectronic process at each step site. At present it is difficult to define the surface stoichiometry of the step oxidation and more work is needed to discriminate between the species that could be involved, including sulphate anions that will require spectroscopic information.
100
0 0.4
−2
(221)
q (μC cm )
40
n= 13 21 39
∞
10
0.9
1
1.1 1.2 E (V vs RHE)
361
1.3
Fig. 3. Positive-going voltammetric sweeps at well-ordered surfaces vicinal to Pt(1 1 1) in 0.5 M H2SO4 at 50 mV/s. The (1 0 0)-stepped surfaces are represented by Pt(11,10,10) (blue) while Pt(20,20,19) and Pt(7 7 6) show the behaviour of (1 1 0) steps (green and red). The inset shows peak charge versus step density for various surfaces of both symmetries together with a line for that expected from an ideal surface. Shading demonstrates how this integration was carried out and n is the terrace length. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
adsorption. Constant-potential oxidation measurements need to be performed and included in the analysis. Preliminary experiments suggest that the conversion between defect sites is not allowed when the potential is held constant, showing the role of potential cycling in surface modification. These experiments will supply data for modelling the first steps of place-exchange kinetics on platinum single-crystal electrodes. Another point, particularly important in electrocatalysis, is the adsorption of oxygen on well-defined surfaces without previous modification, i.e. the first oxygen adsorption cycle of each electrode. Relevant data are shown in Fig. 3, comparing the behaviour of electrodes with different (1 1 1) terrace lengths (n = 1, 39, 21, 13 atoms), two of them separated by (1 1 0) steps (n = 39, 13) and one by (1 0 0) steps (n = 21). At first sight it appears that it is possible to separate the oxidation at the terraces from that on the steps. The oxidation current at the terraces appears to obey an exponential-type law, faster as the terrace width decreases. The oxidation on the steps is superimposed on this terrace oxidation and involves a limited charge density in a defined potential range. It is noteworthy that the (1 0 0) steps oxidize at lower potentials in a relatively sharp single peak, whereas the (1 1 0) step oxidation takes place in two broader contributions. The different shapes reflect differences in the interactions within the surface oxide adlayer, which develops one-dimensionally along the steps. This adlayer would seem to influence the oxidation of the terraces, since changes in the exponential current on the terraces can be observed. It is clear that the current at 1.2 V, i.e. a potential at which
4. Conclusions The kinetics of surface disordering after electrochemical oxygen adsorption/desorption cycles can be followed by monitoring the changes in the population of (1 1 1)-terrace, (1 0 0)- and (1 1 0)-defect sites. The process is described by a consecutive first order mechanism with the addition of an autocatalytic contribution. The reaction becomes slower as the terraces become narrower. The first oxidation of the electrodes shows two contributions. One can be described as an exponential increase of current density with potential and is faster as the terrace length decreases. The other is related to oxide formation on the steps and depends on the step symmetry. The charge involved in this second contribution increases linearly with the step density, with a slope of one electron per step atom. Acknowledgements This study has been carried out in the framework of the European Commission FP7 Initial Training Network ‘‘ELCAT”, Grant Agreement No. 214936-2. Partial support of MICINN through Project CTQ2006-04071/BQU and Generalitat Valenciana through project PROMETEO/2009/045 is also acknowledged. References [1] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205– 209. [2] D. Aberdam, R. Durand, R. Faure, F. El-Omar, Surf. Sci. 171 (1986) 303–330. [3] K. Itaya, S. Sugawara, K. Sashikata, N. Furuya, J. Vac. Sci. Technol. A 8 (1990) 515–519. [4] B.E. Conway, Prog. Surf. Sci. 49 (1995) 331–452. [5] D.A. Harrington, J. Electroanal. Chem. 420 (1997) 101–109. [6] G. Jerkiewicz, G. Vatankhah, J. Lessard, M.P. Soriaga, Y.-S. Park, Electrochim. Acta 49 (2004) 1451–1459. [7] J. Clavilier, in: M.P. Soriaga (Ed.), ACS Symposium Series 378, ACS, Washington, 1988, Ch.14. [8] J. Solla-Gullón, P. Rodríguez, E. Herrero, A. Aldaz, J.M. Feliu, PCCP 10 (2008) 1359–1373. [9] J. Clavilier, D. Armand, S. Sun, M. Petit, J. Electroanal. Chem. 205 (1986) 267– 277. [10] A. Rodes, K. El Achi, M. Zamakhchari, J. Clavilier, J. Electroanal. Chem. 284 (1990) 245–253. [11] J. Clavilier, K. El Achi, A. Rodes, Chem. Phys. 141 (1990) 1–14.