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On the apparent lack of preferential site occupancy and electrooxidation of CO adsorbed at low coverage onto stepped platinum surfaces
Electrochemistry Communications 13 (2011) 338–341
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
On the apparent lack of preferential site occupancy and electrooxidation of CO adsorbed at low coverage onto stepped platinum surfaces Manuel J.S. Farias a, Auro A. Tanaka b, Germano Tremiliosi-Filho a,⁎, Juan M. Feliu c,⁎⁎ a b c
Instituto de Química de São Carlos, Universidade de São Paulo, Caixa Postal 780, 13560-970 São Carlos, SP, Brazil Departamento de Química, Universidade Federal do Maranhão, 65.085-580 São Luís, MA, Brazil Instituto de Electroquímica, Universidad de Alicante, Ap. 99, E-03080, Alicante, Spain
a r t i c l e
i n f o
Article history: Received 25 November 2010 Received in revised form 18 January 2011 Accepted 20 January 2011 Available online 31 January 2011 Keywords: Platinum CO adsorption CO occupancy Stepped surface
a b s t r a c t We report time evolution studies of low coverage CO adsorption (surface hydrogen site blocking b 40%) and oxidative stripping on stepped Pt(776) and Pt(554) surfaces. It was observed that there is no preferential site occupancy for CO adsorption on step or terrace. It is proposed that CO adsorption onto these surfaces is a random process, and after CO adsorption there is no appreciable shift from CO-(111) to CO-(110) sites. This implies that after adsorption, CO molecules either have a very long residence time, or that the diffusion coefficient is much lower than previously thought. After CO electrooxidation the sites released included both terrace (111) and step (110) orientations. For surface hydrogen site blocking N40%, the lateral interactions might play a role in the preferential CO site occupancy. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In an ultra-high vacuum (UHV) environment, the pattern of CO adsorption is well known to be strongly influenced by the microfeatures of single crystals [1–4]. Due to the higher heat of CO adsorption on step sites compared to terrace sites [2], it is recognized that CO filling on stepped surfaces starts at step sites and then proceeds to terrace sites. This preferential site occupancy occurs due to fast rearrangement of the CO adlayer and fast CO diffusion onto the surface. At low CO coverage, after CO molecules are adsorbed onto the surface there is a high probability of their diffusion from terrace sites (such as (111) domains), which are predominant on the surface, towards step sites (with symmetry (110) or (100)), since the adsorbed molecules are more stable at the latter. Under electrochemical environments, for a short terrace (111) stepped surface it has been suggested that the filling of CO at low coverage follows a mechanism similar to that described for the UHV environment [5–9]. Wang et al. [5] and Kim et al. [6,7] based this conclusion on an observed weak dipole coupling intensity at low coverage by FTIR, and increased dipole coupling intensity due to the occupation of the terrace and step sites. Lebedeva et al. [8,9] measured the voltammetric profile of CO adsorbed onto a surface at low coverage, and found that once adsorbed, the CO molecules mainly blocked the hydrogen
⁎ Corresponding author. Tel.: + 55 16 3373 9933. ⁎⁎ Corresponding author: Tel.: + 34 965 909 301. E-mail addresses: [email protected] (G. Tremiliosi-Filho), [email protected] (J.M. Feliu). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.01.019
adsorption at step sites ((100) and/or (110)), while only a small number of (111) terrace sites were occupied. Since the sites on platinum single crystal electrodes are very sensitive to local surface reactions [10], the evolution of the voltammogram can be used to analyze processes on the surface, because the interactions of molecules with the surface produce different modifications depending on whether the interaction occurs with (111), (110) or (100) sites [11]. In this communication, we present experimental evidence that the initial stages of CO adsorption at stepped Pt(hkl) | 0.10 mol L− 1 HClO4 interfaces do not show any preferential site occupation. Once CO
Fig. 1. Cyclic voltammograms of platinum single crystals in 0.10 mol L− 1 HClO4 at 0.05 V s− 1.
M.J.S. Farias et al. / Electrochemistry Communications 13 (2011) 338–341
339
Fig. 2. Cyclic voltammograms between 0.05 V and 0.35 V for Pt(776) and Pt(554) in 0.10 mol L− 1 HClO4. (a): recorded during the growth of the CO adlayer, under potential scanning conditions at 0.10 V s− 1; (b) recorded at 0.20 V s− 1 after the potentiostatic growth of the CO sub-monolayer at 0.10 V; (c): recorded at 0.20 V s− 1 after the potentiostatic growth of the CO sub-monolayer at 0.35 V. The different potential exposure corresponds to different degrees of CO coverage.
molecules are adsorbed, there is no molecule relocation from (111) terrace sites to unoccupied (110) step sites.
electrochemical cell atmosphere. Other conditions have been reported elsewhere [11].
2. Experimental 3. Results Bead platinum (111), (776) and (554) single crystals were prepared and cooled down according to procedure in reference [12]. The stepped Pt(776) and Pt(554) surfaces contain 13 and 9 atom wide (111) terraces, respectively, separated by (110) monoatomic steps. A platinized platinum wire was used as a counter electrode, and a RHE was employed as the reference electrode. The CO (Alpha Gas, 99.997%) sub-monolayer was deposited under either potentiostatic or potentiodynamic conditions by using a small gas flow in the
3.1. Cyclic voltammograms of the platinum single crystal electrodes Cyclic voltammograms of Pt(111), Pt(776) and Pt(554) in the supporting electrolyte are shown in Fig. 1 just to show that the electrolyte solution was free of impurities, and that the surfaces were very well ordered. The pair of peaks at around 0.12 V is characteristic of hydrogen adsorption/desorption at step (110) sites [10].
340
M.J.S. Farias et al. / Electrochemistry Communications 13 (2011) 338–341
3.2. CO adsorption and absence of preferential site occupancy on stepped surfaces
Fig. 3. (a) Charge under the voltammetric peaks (σstep and σterrace) vs. time. (b) σstep/ σterrace vs. time. Data calculated from Fig. 2. Potentiodynamic CO adsorption: Black-Pt (776) and Red-Pt(554), CO adsorption at 0.10 V: Blue-Pt(776) and Dark Cyan-Pt(554), CO adsorption at 0.35 V: Pink-Pt(776), Olive Green-Pt(554).
Fig. 2(a) shows the cyclic voltammograms obtained during formation and growth of the CO sub-monolayer on Pt(776) and Pt (554). The experiment was performed by applying a scan rate of 0.10 V s− 1 between 0.05 and 0.35 V, then introducing a weak flow of CO, which was allowed to reach the meniscus and promote slow formation of the CO sub-monolayer. Progressive hydrogen site blocking was observed as the formation of the CO sub-monolayer occurred. For acquisition of the data shown in Fig. 2(b and c), the electrode potential was maintained at 0.10 V and 0.35 V, respectively, with a weak CO flux provided at the meniscus to promote partial filling of CO on the surface. The extent of partial CO filling was controlled using different times of exposure to the CO flux, and the control cyclic voltammograms were recorded at a scan rate of 0.20 V s− 1. This scan rate reduces any interference caused by CO coming from the solution side, because the CO flux was very slow, and the voltammetric cycle occurred in ~ 3 s. Fig. 2(b) shows that the general pattern of hydrogen site blocking is similar to that observed in Fig. 2(a). Results were the same on Pt (776) and Pt(554) electrodes, for different CO flux exposure times. Experiments performed at 0.35 V suggested that the pattern of hydrogen site blocking during CO sub-monolayer formation was not dependent on potential (Fig. 2(b and c)). Thus, similar patterns of hydrogen site blocking were obtained for partial CO filling in all of the experiments, either at two different dosing potentials or under potentiodynamic conditions. Fig. 3 shows the quantitative analysis of the charge under the voltammetric peaks (data calculated from Fig. 2). Fig. 3(a) illustrates the charges corresponding to step (σstep) and terrace (σterrace) peaks as a function of time for Pt(776) and Pt(554) after CO adsorption at 0.10 V. From the corresponding slopes for terraces −1.1 μC cm− 2 s− 1|
Fig. 4. Cyclic voltammograms between 0.05 V and 0.76 V of (a) Pt(776) and (b) Pt(554), in 0.10 mol L− 1 HClO4, at 0.20 V s− 1 after the potentiostatic growth of the CO sub-monolayer at 0.10 V.
M.J.S. Farias et al. / Electrochemistry Communications 13 (2011) 338–341
and steps −0.19 μC cm− 2 s− 1|, the time for hydrogen site blocking can be calculated as 136 s (terraces) and 117 s (steps), with a difference of only 15%, by considering the maximum hydrogen charge for terrace as ~ 150 μC cm− 2 and steps as ~ 22 μC cm− 2. Similar behavior was observed for the other CO adsorption conditions. Fig. 3 (b) shows for σstep/σterrace vs. time an almost invariant behavior indicating a similar CO occupancy on terraces and steps up to 60 s (corresponding to a surface hydrogen site blocking of ca. 40%). Over 60 s the decrease of σstep/σterrace infers a faster CO occupancy on steps than terraces. 3.3. Release of sites due to electrooxidation of the CO sub-monolayer on a stepped surface Experiments were performed using a sub-monolayer formed as described in Fig. 2(b) but the electrode potential was programmed to execute two consecutive cyclic voltammograms, both between 0.05 V and 0.76 V, at a scan rate of 0.20 V s− 1 (Fig. 4(a and b)). Cycle C1 was recorded to characterize the hydrogen desorption region, below the potential required to oxidize the CO sub-monolayer. For both, Pt(776) and the Pt(554) surfaces, the amount of oxidized CO was controlled by the charge transferred at the upper potential limit (0.76 V). To qualitatively compare the surface blockage before or after partial oxidation of the CO sub-monolayer, Fig. 4(a and b) also include a voltammogram in the hydrogen region without CO (blank). It can be seen that after partial oxidation of the CO sub-monolayer, the sites released are those corresponding to (110) step as well as (111) terrace sites. The quantitative analysis of the charge under the voltammetric peaks (not shown) exhibited a similar behavior observed in Fig. 3(a), with the step and terrace sites released almost equally. 4. Discussion Under potentiodynamic and potentiostatic conditions, growth of the CO sub-monolayer always produced a symmetric pattern of hydrogen site blocking. This surprising behavior indicates that the step site does not play any special role during the initial filling and growth of the CO coverage on a stepped surface. The simultaneous blockage evidences that CO site occupancy occurs randomly, and that after the CO molecules are adsorbed there are no dominant site exchanges between terrace and step sites. This means either that the adsorbed CO molecules have long residence times, or that they have very low mobility at the electrochemical interface. In low CO coverage, there was no evidence of preferential selective CO site occupancy, in contrast to the findings of Kim et al. [6,7] and Lebedeva et al. [8,9], according to whom CO filling on a stepped surface occurred preferentially at the step sites. On the other hand, it was observed for us that up to ~40% site surface block, step sites present most high rate of blocking than terrace sites. This can indicate that under high CO coverage lateral interaction could present influence on site CO occupancy on stepped surface.
341
In the work of Lebedeva et al. [8,9], a CO sub-monolayer was formed, the CO in the solution side was removed by Ar bubbling and the cyclic voltammogram was recorded after ~ 20 min of the CO adsorption step. We have observed (not shown), that after formation of the CO sub-monolayer in a clean electrolyte solution and degassing the solution with Ar for 40–60 min, step sites were mainly blocked, with only a smaller quantity of terrace sites blocked. Hence, for low CO coverage on a stepped surface, after a long degassing time both experiments coincide. A surprising aspect of oxidation of the CO sub-monolayer is that the profiles generated after partial CO oxidation indicate that the electrooxidation occurs on surfaces everywhere. The finding that the step and terrace sites are released during the short oxidative removal of the CO adlayer suggests that a reappraisal is needed of the role of step sites in initiation of electrooxidation of CO from the adlayer. This behavior seems to be independent of the extent of CO coverage. The results suggest that CO sub-monolayer electrooxidation takes place everywhere on the surface.
5. Concluding remarks In this communication we have shown for the first time that CO adsorption occurs randomly during the growth of a CO submonolayer, and that CO filling on a stepped surface exhibits no preferential site occupancy. This means that under electrochemical environments the CO molecule adsorbed on the surface does not experience site changes, such as from CO-(111) to CO-(110). On a stepped surface, at low CO coverage, electrooxidation is initiated everywhere on the surface.
Acknowledgements This study was supported by FAPESP, CNPq and CAPES, Brazil. JMF acknowledges support of MICINN (Spain) through project CTQ201016271 (Feder).
References [1] B.E. Hayden, K. Kretzschmar, A.M. Bradshaw, Surf. Sci. 149 (1985) 394. [2] J.S. Lou, R.G. Tobin, D.K. Lambert, G.B. Fisher, C.L. DiMaggio, Surf. Sci. 274 (1992) 53. [3] J. Xu, J.T. Yates Jr., J. Chem. Phys. 99 (1993) 725. [4] J.T. Yates Jr., J. Vac. Sci. Technol. 13 (1995) 1359. [5] H. Wang, D.K. Tobin, J. Chem. Phys. 101 (1994) 4277. [6] C.S. Kim, C. Korzeniewski, W. Tornquist, J. Chem. Phys. 100 (1994) 628. [7] C.S. Kim, C. Korzeniewski, Anal. Chem. 69 (1997) 2349. [8] N.P. Lebedeva, A. Rodes, J.M. Feliu, M.T.M. Koper, R.A. van Santen, J. Phys. Chem. B 106 (2002) 9863. [9] N.P. Lebedeva, M.T.M. Koper, E. Herrero, J.M. Feliu, R.A. van Santen, J. Electroanal. Chem. 487 (2000) 37. [10] V. Climent, N. García-Araez, E. Herrero, M.J. Feliu, Russ. J. Electrochem. 42 (2006) 1145. [11] E. Herrero, V. Climent, J.M. Feliu, Electrochem. Commun. 2 (2000) 636. [12] J. Clavilier, D. Armand, S.G.M. Sun, M. Petit, J. Electroanal. Chem. 205 (1986) 267.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
On the apparent lack of preferential site occupancy and electrooxidation of CO adsorbed at low coverage onto stepped platinum surfaces Manuel J.S. Farias a, Auro A. Tanaka b, Germano Tremiliosi-Filho a,⁎, Juan M. Feliu c,⁎⁎ a b c
Instituto de Química de São Carlos, Universidade de São Paulo, Caixa Postal 780, 13560-970 São Carlos, SP, Brazil Departamento de Química, Universidade Federal do Maranhão, 65.085-580 São Luís, MA, Brazil Instituto de Electroquímica, Universidad de Alicante, Ap. 99, E-03080, Alicante, Spain
a r t i c l e
i n f o
Article history: Received 25 November 2010 Received in revised form 18 January 2011 Accepted 20 January 2011 Available online 31 January 2011 Keywords: Platinum CO adsorption CO occupancy Stepped surface
a b s t r a c t We report time evolution studies of low coverage CO adsorption (surface hydrogen site blocking b 40%) and oxidative stripping on stepped Pt(776) and Pt(554) surfaces. It was observed that there is no preferential site occupancy for CO adsorption on step or terrace. It is proposed that CO adsorption onto these surfaces is a random process, and after CO adsorption there is no appreciable shift from CO-(111) to CO-(110) sites. This implies that after adsorption, CO molecules either have a very long residence time, or that the diffusion coefficient is much lower than previously thought. After CO electrooxidation the sites released included both terrace (111) and step (110) orientations. For surface hydrogen site blocking N40%, the lateral interactions might play a role in the preferential CO site occupancy. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In an ultra-high vacuum (UHV) environment, the pattern of CO adsorption is well known to be strongly influenced by the microfeatures of single crystals [1–4]. Due to the higher heat of CO adsorption on step sites compared to terrace sites [2], it is recognized that CO filling on stepped surfaces starts at step sites and then proceeds to terrace sites. This preferential site occupancy occurs due to fast rearrangement of the CO adlayer and fast CO diffusion onto the surface. At low CO coverage, after CO molecules are adsorbed onto the surface there is a high probability of their diffusion from terrace sites (such as (111) domains), which are predominant on the surface, towards step sites (with symmetry (110) or (100)), since the adsorbed molecules are more stable at the latter. Under electrochemical environments, for a short terrace (111) stepped surface it has been suggested that the filling of CO at low coverage follows a mechanism similar to that described for the UHV environment [5–9]. Wang et al. [5] and Kim et al. [6,7] based this conclusion on an observed weak dipole coupling intensity at low coverage by FTIR, and increased dipole coupling intensity due to the occupation of the terrace and step sites. Lebedeva et al. [8,9] measured the voltammetric profile of CO adsorbed onto a surface at low coverage, and found that once adsorbed, the CO molecules mainly blocked the hydrogen
⁎ Corresponding author. Tel.: + 55 16 3373 9933. ⁎⁎ Corresponding author: Tel.: + 34 965 909 301. E-mail addresses: [email protected] (G. Tremiliosi-Filho), [email protected] (J.M. Feliu). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.01.019
adsorption at step sites ((100) and/or (110)), while only a small number of (111) terrace sites were occupied. Since the sites on platinum single crystal electrodes are very sensitive to local surface reactions [10], the evolution of the voltammogram can be used to analyze processes on the surface, because the interactions of molecules with the surface produce different modifications depending on whether the interaction occurs with (111), (110) or (100) sites [11]. In this communication, we present experimental evidence that the initial stages of CO adsorption at stepped Pt(hkl) | 0.10 mol L− 1 HClO4 interfaces do not show any preferential site occupation. Once CO
Fig. 1. Cyclic voltammograms of platinum single crystals in 0.10 mol L− 1 HClO4 at 0.05 V s− 1.
M.J.S. Farias et al. / Electrochemistry Communications 13 (2011) 338–341
339
Fig. 2. Cyclic voltammograms between 0.05 V and 0.35 V for Pt(776) and Pt(554) in 0.10 mol L− 1 HClO4. (a): recorded during the growth of the CO adlayer, under potential scanning conditions at 0.10 V s− 1; (b) recorded at 0.20 V s− 1 after the potentiostatic growth of the CO sub-monolayer at 0.10 V; (c): recorded at 0.20 V s− 1 after the potentiostatic growth of the CO sub-monolayer at 0.35 V. The different potential exposure corresponds to different degrees of CO coverage.
molecules are adsorbed, there is no molecule relocation from (111) terrace sites to unoccupied (110) step sites.
electrochemical cell atmosphere. Other conditions have been reported elsewhere [11].
2. Experimental 3. Results Bead platinum (111), (776) and (554) single crystals were prepared and cooled down according to procedure in reference [12]. The stepped Pt(776) and Pt(554) surfaces contain 13 and 9 atom wide (111) terraces, respectively, separated by (110) monoatomic steps. A platinized platinum wire was used as a counter electrode, and a RHE was employed as the reference electrode. The CO (Alpha Gas, 99.997%) sub-monolayer was deposited under either potentiostatic or potentiodynamic conditions by using a small gas flow in the
3.1. Cyclic voltammograms of the platinum single crystal electrodes Cyclic voltammograms of Pt(111), Pt(776) and Pt(554) in the supporting electrolyte are shown in Fig. 1 just to show that the electrolyte solution was free of impurities, and that the surfaces were very well ordered. The pair of peaks at around 0.12 V is characteristic of hydrogen adsorption/desorption at step (110) sites [10].
340
M.J.S. Farias et al. / Electrochemistry Communications 13 (2011) 338–341
3.2. CO adsorption and absence of preferential site occupancy on stepped surfaces
Fig. 3. (a) Charge under the voltammetric peaks (σstep and σterrace) vs. time. (b) σstep/ σterrace vs. time. Data calculated from Fig. 2. Potentiodynamic CO adsorption: Black-Pt (776) and Red-Pt(554), CO adsorption at 0.10 V: Blue-Pt(776) and Dark Cyan-Pt(554), CO adsorption at 0.35 V: Pink-Pt(776), Olive Green-Pt(554).
Fig. 2(a) shows the cyclic voltammograms obtained during formation and growth of the CO sub-monolayer on Pt(776) and Pt (554). The experiment was performed by applying a scan rate of 0.10 V s− 1 between 0.05 and 0.35 V, then introducing a weak flow of CO, which was allowed to reach the meniscus and promote slow formation of the CO sub-monolayer. Progressive hydrogen site blocking was observed as the formation of the CO sub-monolayer occurred. For acquisition of the data shown in Fig. 2(b and c), the electrode potential was maintained at 0.10 V and 0.35 V, respectively, with a weak CO flux provided at the meniscus to promote partial filling of CO on the surface. The extent of partial CO filling was controlled using different times of exposure to the CO flux, and the control cyclic voltammograms were recorded at a scan rate of 0.20 V s− 1. This scan rate reduces any interference caused by CO coming from the solution side, because the CO flux was very slow, and the voltammetric cycle occurred in ~ 3 s. Fig. 2(b) shows that the general pattern of hydrogen site blocking is similar to that observed in Fig. 2(a). Results were the same on Pt (776) and Pt(554) electrodes, for different CO flux exposure times. Experiments performed at 0.35 V suggested that the pattern of hydrogen site blocking during CO sub-monolayer formation was not dependent on potential (Fig. 2(b and c)). Thus, similar patterns of hydrogen site blocking were obtained for partial CO filling in all of the experiments, either at two different dosing potentials or under potentiodynamic conditions. Fig. 3 shows the quantitative analysis of the charge under the voltammetric peaks (data calculated from Fig. 2). Fig. 3(a) illustrates the charges corresponding to step (σstep) and terrace (σterrace) peaks as a function of time for Pt(776) and Pt(554) after CO adsorption at 0.10 V. From the corresponding slopes for terraces −1.1 μC cm− 2 s− 1|
Fig. 4. Cyclic voltammograms between 0.05 V and 0.76 V of (a) Pt(776) and (b) Pt(554), in 0.10 mol L− 1 HClO4, at 0.20 V s− 1 after the potentiostatic growth of the CO sub-monolayer at 0.10 V.
M.J.S. Farias et al. / Electrochemistry Communications 13 (2011) 338–341
and steps −0.19 μC cm− 2 s− 1|, the time for hydrogen site blocking can be calculated as 136 s (terraces) and 117 s (steps), with a difference of only 15%, by considering the maximum hydrogen charge for terrace as ~ 150 μC cm− 2 and steps as ~ 22 μC cm− 2. Similar behavior was observed for the other CO adsorption conditions. Fig. 3 (b) shows for σstep/σterrace vs. time an almost invariant behavior indicating a similar CO occupancy on terraces and steps up to 60 s (corresponding to a surface hydrogen site blocking of ca. 40%). Over 60 s the decrease of σstep/σterrace infers a faster CO occupancy on steps than terraces. 3.3. Release of sites due to electrooxidation of the CO sub-monolayer on a stepped surface Experiments were performed using a sub-monolayer formed as described in Fig. 2(b) but the electrode potential was programmed to execute two consecutive cyclic voltammograms, both between 0.05 V and 0.76 V, at a scan rate of 0.20 V s− 1 (Fig. 4(a and b)). Cycle C1 was recorded to characterize the hydrogen desorption region, below the potential required to oxidize the CO sub-monolayer. For both, Pt(776) and the Pt(554) surfaces, the amount of oxidized CO was controlled by the charge transferred at the upper potential limit (0.76 V). To qualitatively compare the surface blockage before or after partial oxidation of the CO sub-monolayer, Fig. 4(a and b) also include a voltammogram in the hydrogen region without CO (blank). It can be seen that after partial oxidation of the CO sub-monolayer, the sites released are those corresponding to (110) step as well as (111) terrace sites. The quantitative analysis of the charge under the voltammetric peaks (not shown) exhibited a similar behavior observed in Fig. 3(a), with the step and terrace sites released almost equally. 4. Discussion Under potentiodynamic and potentiostatic conditions, growth of the CO sub-monolayer always produced a symmetric pattern of hydrogen site blocking. This surprising behavior indicates that the step site does not play any special role during the initial filling and growth of the CO coverage on a stepped surface. The simultaneous blockage evidences that CO site occupancy occurs randomly, and that after the CO molecules are adsorbed there are no dominant site exchanges between terrace and step sites. This means either that the adsorbed CO molecules have long residence times, or that they have very low mobility at the electrochemical interface. In low CO coverage, there was no evidence of preferential selective CO site occupancy, in contrast to the findings of Kim et al. [6,7] and Lebedeva et al. [8,9], according to whom CO filling on a stepped surface occurred preferentially at the step sites. On the other hand, it was observed for us that up to ~40% site surface block, step sites present most high rate of blocking than terrace sites. This can indicate that under high CO coverage lateral interaction could present influence on site CO occupancy on stepped surface.
341
In the work of Lebedeva et al. [8,9], a CO sub-monolayer was formed, the CO in the solution side was removed by Ar bubbling and the cyclic voltammogram was recorded after ~ 20 min of the CO adsorption step. We have observed (not shown), that after formation of the CO sub-monolayer in a clean electrolyte solution and degassing the solution with Ar for 40–60 min, step sites were mainly blocked, with only a smaller quantity of terrace sites blocked. Hence, for low CO coverage on a stepped surface, after a long degassing time both experiments coincide. A surprising aspect of oxidation of the CO sub-monolayer is that the profiles generated after partial CO oxidation indicate that the electrooxidation occurs on surfaces everywhere. The finding that the step and terrace sites are released during the short oxidative removal of the CO adlayer suggests that a reappraisal is needed of the role of step sites in initiation of electrooxidation of CO from the adlayer. This behavior seems to be independent of the extent of CO coverage. The results suggest that CO sub-monolayer electrooxidation takes place everywhere on the surface.
5. Concluding remarks In this communication we have shown for the first time that CO adsorption occurs randomly during the growth of a CO submonolayer, and that CO filling on a stepped surface exhibits no preferential site occupancy. This means that under electrochemical environments the CO molecule adsorbed on the surface does not experience site changes, such as from CO-(111) to CO-(110). On a stepped surface, at low CO coverage, electrooxidation is initiated everywhere on the surface.
Acknowledgements This study was supported by FAPESP, CNPq and CAPES, Brazil. JMF acknowledges support of MICINN (Spain) through project CTQ201016271 (Feder).
References [1] B.E. Hayden, K. Kretzschmar, A.M. Bradshaw, Surf. Sci. 149 (1985) 394. [2] J.S. Lou, R.G. Tobin, D.K. Lambert, G.B. Fisher, C.L. DiMaggio, Surf. Sci. 274 (1992) 53. [3] J. Xu, J.T. Yates Jr., J. Chem. Phys. 99 (1993) 725. [4] J.T. Yates Jr., J. Vac. Sci. Technol. 13 (1995) 1359. [5] H. Wang, D.K. Tobin, J. Chem. Phys. 101 (1994) 4277. [6] C.S. Kim, C. Korzeniewski, W. Tornquist, J. Chem. Phys. 100 (1994) 628. [7] C.S. Kim, C. Korzeniewski, Anal. Chem. 69 (1997) 2349. [8] N.P. Lebedeva, A. Rodes, J.M. Feliu, M.T.M. Koper, R.A. van Santen, J. Phys. Chem. B 106 (2002) 9863. [9] N.P. Lebedeva, M.T.M. Koper, E. Herrero, J.M. Feliu, R.A. van Santen, J. Electroanal. Chem. 487 (2000) 37. [10] V. Climent, N. García-Araez, E. Herrero, M.J. Feliu, Russ. J. Electrochem. 42 (2006) 1145. [11] E. Herrero, V. Climent, J.M. Feliu, Electrochem. Commun. 2 (2000) 636. [12] J. Clavilier, D. Armand, S.G.M. Sun, M. Petit, J. Electroanal. Chem. 205 (1986) 267.