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Journal of Electroanalytical Chemistry 532 (2002) 49 /55 www.elsevier.com/locate/jelechem
Oxidation of CO adsorbed from CO saturated solutions on the Pt(111)/Ru electrode G.-Q. Lu, P. Waszczuk, A. Wieckowski * Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 18 January 2002; received in revised form 6 May 2002; accepted 17 May 2002 Dedicated to Professor Sergio Trasatti on the occasion of his 65th birthday and in recognition of his contribution to Electrochemistry.
Abstract Using Pt(111) surfaces covered by ruthenium by a spontaneous deposition method we conducted voltammetric oxidation of chemisorbed CO in clean sulfuric acid electrolyte at 50 mV s 1 and under variable sweep rate conditions. The deposition yields Pt(111) surfaces decorated by two-dimensional nanosized ruthenium islands previously reported. At 50 mV s 1, the CO stripping produces two well-resolved current peaks, at 0.55 and 0.67 V versus RHE. The first peak originates from CO chemisorbed on (and strictly around) ruthenium islands deposited on Pt(111), while another comes from CO chemisorbed on ‘pure’ Pt(111) phases of the Pt(111)/Ru electrode, away from the Ru islands. Using double-potential step chronoamperometry we investigated the CO oxidation on these two surface phases, and found that the low potential oxidation of chemisorbed CO occurred via the Langmuir / Hinshelwood mechanism. This shows that diffusion of CO on ruthenium to the Ru edge is not needed for a complete stripping of CO from such islands, i.e. the oxidation process is activated uniformly across an island. In contrast, CO chemisorbed on Pt sites not occupied by Ru is oxidized at the Pt(111)/Ru edge, and there may be a surface diffusion contribution to a complete removal of CO from the surface. However, a part of the CO adlayer oxidation in the 0.67 V peak area occurs with no participation of surface diffusion, according to a pseudo-first order surface reaction kinetics, and the reaction may involve a ‘reactant pair’ (Pt/O /H O / C /Pt) breakdown on the electrode surface. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Pt(111)/Ru; Spontaneous deposition; CO; Electrooxidation mechanism; Potential step; Cyclic voltammetry
1. Introduction There has been broad interest in Pt(111) electrodes ‘decorated’ by ruthenium by the use of forced and spontaneous electrochemical methods [1 /9]. Electrochemical and UHV deposition procedures [10 /14] produce Pt(111) surfaces covered by ruthenium islands of nanosized dimensions [2,3,7,8,15 /19], as first reported by Stimming, Friedrich and collaborators [2,3]. Carbon monoxide chemisorbed on such surfaces has also been investigated by both experimental [2 /4,10,11] and theoretical methods [17,19]. Under some specific conditions including slow CO surface diffusion [17], Koper et al. demonstrated theoretically that the CO could be
* Corresponding author. Tel.: /1-217-333-7943; fax: /1-217-2448068 E-mail address: [email protected] (A. Wieckowski).
oxidatively removed from the Ru islands (and directly near the islands), and from the Pt sites away from the Pt/ Ru edge as two separate oxidation processes, giving rise to a split cyclic voltammogram (CV) [17]. The presence of such two distinctive CO phases was also inferred from infrared measurements [2,11,15]. However, the link between the distinctive CO populations and the predicted split voltammetry had until quite recently been missing. The confirmation came from the Baltruschat group using the Pt(111) electrode [4] and by our group using CO generated from methanol, voltammetry and electrochemical NMR on nanoparticle Pt/Ru electrodes [20]. The Koper group also presented evidence that the current /potential chronoamperometric response due to CO oxidation on Pt(111) can be better interpreted using the ‘mean-field’ mechanism [21,22], rather than the nucleation and growth mechanism [23]. The model
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 0 9 5 2 - X
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assumes that, overall, CO diffusion on Pt(111) is fast, but argues that if slow diffusion is involved in the CO oxidation reaction near the Pt/Ru edge, it is manifested as a ‘tailing’ of the current /time plot in a chronoamperometric experiment. Such a tailing has not been found either with the clean Pt(111) electrode, or with stepped Pt(hkl ) surfaces. However, the CO stripping data on Pt(111)/Ru in Ref. [4] and in this report give credit to the idea that a slow surface diffusion of CO may be involved [17]. If this is so, the argument would be that due to the low binding energy of CO at the Pt/Ru edge site, the CO surface motion near the edge may experience a retardation due to a high-energy barrier at such a site [22]. Alternatively, Pt sites close to Ru can be less CO populated due to the unfavorable binding energy, and hence these Pt /Ru sites may not be active enough in oxidizing CO to CO2 [22]. Below, we demonstrate that both voltammetric and chronoamperometric CO oxidation data may be interpreted without involving the slow diffusion component in the reaction mechanism. However, our data do not reject the possibility that the slow diffusion can be considered. In the latter case, the second neighbor Pt site, counting from the Pt/Ru edge, is the one that presents the barrier for the CO surface mass transport rather than the first neighbor Pt site. This means that overall we support the previous CO oxidation model [17], but with some meaningful modifications.
2. Experimental All experiments were carried out at ambient temperature (259/2 8C). The potentials were measured against a commercial (BAS) silver j silver chloride electrode, but are reported versus RHE. The chemicals used were sulfuric and perchloric acid (GFS, twice distilled from Vycor), RuCl3 ×/xH2O (Aldrich), CO (Matheson, research purity) and Millipore water. Cyclic voltammetric and chronoamperometric measurements were carried out using a PAR 273 potentiostat and associated auxiliaries. A spontaneous deposition method was used to obtain the Ru covered Pt(111) surfaces. The procedure involves immersing the platinum substrate in a ruthenium containing perchloric acid solution and flushing the cell with either Millipore water or the supporting electrolyte. A negative polarization is then applied to the surface that breaks the chemisorbed admetal precursors immobilized on the surface to desired, catalytically effective ruthenium fragments. The later phase of the deposition takes place in an electrochemical cell where electrocatalytic reactions are tested. The surfaces such prepared are usually ‘stabilized’ by three voltammetric cycles between 0.06 and 0.85 V in 0.1 M H2SO4 electrolyte. For the project described below, a platinum single crystal bead
of 2.5 mm in diameter, cut and polished to the (111) orientation was used. The final surface was prepared by annealing the crystal in the hydrogen/air flame, cooling in an Ar/H2 atmosphere and protected by ultrapure water [24]. The deposition of ruthenium was carried out from solutions of 1 mM of RuCl3 in 0.1 M of HClO4, as described in Refs [5,7,8].
3. Results 3.1. Cyclic voltammetry Cyclic voltammetry of the clean Pt(111) electrode in 0.1 M H2SO4 at 50 mV s 1 is shown in Fig. 1A (dotted line). The CV demonstrates the high quality of the electrode surface, and confirms the cleanliness of our electrochemical systems. By introducing approximately 0.15 ML of spontaneously deposited Ru to the surface [5,7,8], two characteristic changes on the voltammetric profile were observed (Fig. 1A, solid line). First, the negative- and positive-going spikes at 0.49 V (belonging to the anomalous region) are significantly suppressed [16] and second the pseudocapacitive current in the 0.50 /0.85 V region is enlarged, characteristic of a Rutype electrode [16,25]. Carbon monoxide was next admitted to the cell, and CO adsorption was carried out on both Pt(111) and Pt(111)/Ru electrodes for 5 min under active CO bubbling conditions at 0.25 V. This was followed by purging the solution with argon for 20 min (unless stated otherwise) to remove dissolved CO from solution, while holding the electrode at the same potential of 0.25 V. The CO stripping profile from Pt(111) (Fig. 1B, dotted line) indicates that the voltammetric CO oxidation peaks at approximately 0.85 V (at 50 mV s 1), and that the coverage of CO is close to 0.75 ML, confirming previous data [17,21,26 /30]. On Pt(111)/Ru, the main new voltammetric feature shown in Fig. 1B (solid line) with respect to the CO stripping from Pt(111) is the appearance of two, instead of one voltammetric peak due to CO oxidation. This is a clear confirmation of the voltammetric splitting involved in CO oxidation on the Pt(111)/Ru electrode (see Section 1). However, data in Fig. 2 demonstrate that the peak separation depends on the CO oxidation charge, or on the CO coverage. In this study, the CO coverage was regulated by the time of argon bubbling through solution with the Pt(111)/Ru / CO electrode held at 0.25 V. Apparently (Fig. 2), the separation changes from 0.52 / /0.67 to 0.61 / /0.67 V at 20 and 10 min argon exposure, that is from the lower to higher CO coverage, respectively. These effects certainly need further investigations. Also, the peak located at a more positive potential, 0.67 V, is shifted in the negative direction due to Ru present with respect to
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Fig. 1. (A) CVs of clean Pt(111) (dotted line) and Pt(111)/Ru (solid line) in 0.1 M H2SO4. Approximately 0.15 ML of Ru was added onto Pt(111) by spontaneous deposition. (B) Stripping of a CO adlayer from Pt(111) (dotted line) and from Pt(111)/Ru (solid line) in a clean 0.1 M H2SO4. The scan rate was 50 mV s 1. Exposure of the electrodes to CO at 0.25 V for 5 min was followed by Ar bubbling for 20 min.
Fig. 2. Peak separation influenced by the CO coverage on the Ru phase (which was regulated by the length of argon purging time following CO adsorption). Argon purging time is indicated in the figure. Other conditions as in Fig. 1B.
CO oxidation from pure Pt(111), at 0.85 V (see Section 4). As inferred from previous studies [17,20,31], the presence of two voltammetric stripping maxima on Pt(111)/Ru indicates that a part of the CO adlattice is chemisorbed onto the ruthenium islands deposited on Pt(111), the Pt(111)/Ru /CO phase (or the pre-peak), while the more positive peak originates from the CO chemisorbed on ‘pure’ Pt(111), the (Ru)Pt(111)/CO phase. The splitting between the two stripping maxima may be as high as 0.15 V, and is very close to that obtained for CO stripping from the Pt/Ru nanoparticle surfaces [20]. It is interesting to note that the voltammetric morphology and the peak widths at 0.52 and 0.67 V in Fig. 1B are different, being indicative of the different nature of the electrochemical processes exam-
ined (there is also an interesting anion effect between surface sulfate and perchlorate, which will soon be reported). As mentioned in Section 1 Koper et al. believe that split may imply slow CO diffusion to the reaction site [17]. However, the authors did not exclude the possibility that the Pt/Ru was simply inactive to the oxidation of Pt /CO at low potentials in the pre-peak area [22]. We will address this model further below. Voltammetric, sweep rate dependent measurements of CO stripping, such as those shown in Fig. 1B were carried out in the range from v/2/103 to 1 V s 1 (time window /1/v, from 500 to 1 s V 1). We found no dependence of the voltammetric charge distribution between the two peaks on the sweep rate in this range. Therefore, on the time scales indicated above, there is either no effective diffusion from the Pt surface phase to the Ru edge (that would lead to the increase in the prepeak at the expense of the main peak at progressively longer times) or the Pt/Ru edge is not reactive to CO chemisorbed at the Pt atoms adjacent to the Ru edge at lower potentials. However, these conclusions may not be valid for longer experimental times than used in this study, the issue to be addressed in the future. Recent results of NMR investigations show no motional intermixing between the CO chemisorbed molecules on the Pt and Pt/Ru surface phases, albeit at lower potentials than those studied in this report [20]. This clearly indicates unique chemistry of the Pt/Ru edge with respect to surface CO behaviors and dynamics. 3.2. Chronoamperometry To obtain more advanced insights into the nature of the voltammetric splitting presented in Figs. 1B and 2, we employed electric current measurements under potential-step conditions: a single step and double
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potential step chronoamperometry. First, we applied the single potential step into the lower potential voltammetric peak, at 0.52 V, and the data are shown on the left hand side of Fig. 3, where the results from the second potential step experiment are also shown. The first step was applied for 2 s, that is, from 2 to 4 s in Fig. 3. As a consequence of the first step to 0.52 V, a symmetric current /time peak was obtained, characteristic of the Langmuir /Hinshelwood-type [32] electrooxidation of carbon monoxide. After the 2 s delay, the subsequent second step was applied into the potential region of, or near the second voltammetric-stripping peak. In Fig. 3, the potential was stepped to 0.52 V for 2 s (as above), and then from 0.52 to 0.67 V for the remaining time of the experiment (the right-hand side of the figure). From the comparison of the voltammetric (Fig. 1B) and chronoamperometric (Fig. 3) data, we conclude that after the 2 s pulse width, that is, when the current due to the CO oxidation goes to a near zero value, the (Ru)Pt(111) /CO population still remaining on the surface is barely affected. After the second potential step was applied, the remaining CO was completely oxidized after the experimental time 5.5 s (in ca. 1.5 s). Notably, at 0.67 V (and at other potentials in the main peak area), a current decay rather than a peaked-type profile is observed. Clearly, the CO oxidation does not follow the Langmuir /Hinshelwood mechanism, as does the CO oxidation at the lower potentials. The character of the current decay strongly suggests an exponential relationship between current and time. However, fitting all decay current /time data obtained in this series to a simple semi-logarithmic plot
Fig. 3. A representative curve for the double-potential step chronoamperometry for a CO adlayer on Pt(111)/Ru in clean 0.1 M H2SO4: first, the potential is stepped from 0.25 to 0.520 V for 2 s, and then from 0.52 to 0.67 V until the CO was removed from the surface.
was not successful, and a more advanced treatment is required (see below).
4. Discussion It is already well documented that surface CO, either generated from CO solutions or from methanol decomposition oxidizes at less positive potentials on Pt/Ru than on clean Pt electrodes [1 /4,10,11,20,31]. On Pt(111)/Ru, under some specific CO and ruthenium coverage conditions discussed in this paper (also, see below), a split in the voltammetric CO oxidation on the Pt(111)/Ru electrode occurs [4,22]. The split gives rise to two maxima, one at lower potentials, corresponding to the CO oxidation from ruthenium deposited on platinum, Pt(111)/Ru /CO, including the oxidation of CO from the Pt sites adjacent to the Ru edge [17,20], and the second, at more positive potentials, from the (Ru)Pt(111)/CO [20] (Fig. 1B). There appears no doubts that the broadly understood Langmuir /Hinshelwood mechanism applies to the CO oxidation in the prepeak area. If so, this shows that diffusion of CO on ruthenium to the Ru edge is not needed for a complete stripping of CO from such islands, i.e. the oxidation process is activated uniformly across a Ru island. This is interesting behavior given that the events we report occur on small, nanosized island surfaces. It is noteworthy that the oxidation of CO from Pt(111)/Ru /CO and from the Ru perimeter upon the first step occurs without any meaningful change in the local (Ru)Pt(111)/CO coverage (Fig. 3). Or, in the time window up to 500 s V 1, CO molecules away from the Pt/Ru edge are not noticeably oxidized at this edge at potentials of the pre-peak area. As already detailed above, this is either because of the slow surface diffusional CO transfer to the edge, or because the edge is not reactive to CO. There is also a complete removal of CO at potentials from the second CO stripping peak area. Using chronoamperometry (Fig. 3) we report, for the first time, that there is a change in CO oxidation from the Langmuir /Hinshelwood mechanism to a mechanism represented by a current /time decay, which is usually associated with first order oxidation kinetics. A further observation to consider is that the potential shift in the oxidation of (Ru)Pt(111) /CO and Pt(111)/ CO is 0.18 V (from ca. 0.85 V on clean Pt(111) to 0.67 V on Pt(111)/Ru, Figs. 1B and 2, respectively). The shift may be due to an electronic effect, that is due to the modification of local DOSs of platinum by the ruthenium additive [20,33]. Indeed, McBreen and Mukerjee using XAS [34] and Tong et al. using NMR [20] provided evidence as to the reality of such an effect, which in principle would allow the CO from (Ru)Pt(111)/CO to react with adsorbed OH at lower
G.-Q. Lu et al. / Journal of Electroanalytical Chemistry 532 (2002) 49 /55
potentials than at clean Pt(111). However, this effect may not be large enough, and could add only a fraction to the 0.85 /0.67 V voltage gain observed [33]. A rapid OHads spillover from the Pt/Ru edge to the (Ru)Pt(111) /CO phase, and its subsequent involvement in the CO oxidation process, could also be considered on the Ru modified Pt phases [35]. However, this is not very likely since quantum-chemical data indicate that the OHads binding energy does not increase on Pt modified by Ru [17] (which would then be more reactive to CO [21]). The correct explanation of all our data is most likely related to the high density of Pt/Ru edge sites on (Ru)Pt(111) /CO, and the preferential adsorption of OH at the Pt/Ru edge, versus the absence of such Pt/Ru defects on Pt(111)/CO. The edge OH species on (Ru)Pt(111) are active at lower potentials than the OH on Pt(111), causing the removal of CO at lower potentials. The first model of the events within the main voltammetric (more positive) peak area in the chronoamperometric measurements would then postulate that there is an availability of reactive sites at the Pt/Ru edge to which the CO is transferred via rapid surface diffusion on the Pt part of the surface. This would lead to a pseudo first order kinetics and a linear log j versus t plot covering the full current /potential range (within the main peak area). Data presented in Fig. 4A prove this simple hypothesis incorrect. As shown in this figure, the data may be approximated by two consecutive log j versus t straight lines accounting for approximately 90% of the total current decay. Notice that the first straight log j versus t line at short times accounts for approximately 50% current decay (Fig. 4B), and the log j versus t linearity at short times is as significant as the linearity at higher times. The results of the Tafel analysis of the data in the second voltammetric peak area are shown in Fig. 5, where j is the current density obtained from data in Fig. 4A via the extrapolation to 0 time (instantaneous current densities [36]). We found two different Tafel slopes, a high slope approximately 0.12 V per decade for the first phase of the CO oxidation (at short times) and a low slope, approximately 0.06 V per decade for the nearly complete CO oxidation process. Therefore, we believe that the preliminary interpretation of our data may be as follows. (1) CO diffusion to the Ru edge is fast . Before the second potential step, CO molecules adsorbed on Pt sites are stable and the Pt sites adjacent to the Pt/Ru sites of the ruthenium islands are covered either by OH [21] or by chemisorbed water [36,37]. Assuming the former, the stability of CO in the pre-peak potential range allows ‘reactant pairs’ [37]to be created between the CO and surface OH stabilized e.g. by hydrogen bonding: Pt/O /H O /C /Pt. The first exponential decay for CO oxidation would then be due to breakdown of such OH OC pairs according to Eq. (1):
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Fig. 4. (A) log j (current density) vs. time straight lines for CO oxidation accounting for approximately 90% of the total current decay. Data are shown after CO adsorption (Fig. 1), a pre-step to 0.520 V, followed by the second potential step to 0.695 V (Fig. 3). (B) A j vs. time plot showing the areas for high and low log j vs. time slopes (see text). Other details as in (A).
Fig. 5. Tafel, log j (current density) vs. electrode potential lines for the first order kinetics of CO oxidation in time domains depicted in Fig. 4. The Tafel slopes are 0.12 V per decade for the first phase of the CO oxidation (short times, solid symbols) and 0.06 V per decade (longer times, open symbols).
rds
Pt(111)=Ru=PtOH OCPt 0 Pt(111)=Ru=PtPt CO2 H e
(1)
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The OH is associated with Pt(111)/Ru/Pt sites (Pt sites near the Pt/Ru edge) rather than with the Pt(111)/Ru sites because: (i) CO is removed from the Pt(111)/Ru sites in the pre-peak area [17,20,22], (ii) Pt/Ru/Pt sites must be already populated by OH (or H2O) since the Tafel slope for the first phase of the CO oxidation is approximately 0.12 V per decade (Fig. 5, solid symbols). After the breakdown of the O /H OC surface pair, corresponding to the log j versus t straight line at short times (Fig. 4A) and to reaction 1, the OHs need to be replenished in a reversible electrochemical reaction: Pt(111)=Ru=PtH2 O UPt(111)=Ru=PtOHH e
(2)
If the CO surface diffusion is fast, step 2 is followed by a chemical rate-determining step, yielding: rds
Pt(111)=Ru=PtOHPtCO 0 Pt=Ru=PtCOOH
(3)
see the second straight line in Fig. 4, and: fast
Pt(111)=Ru=PtCOOH 0 Pt=Ru=PtPtCO2 H e (4) The Tafel slope associated with the second exponent is around 0.06 V per decade (Fig. 5, open symbols) as expected for the consecutive reactions 2 /4 [21,30]. (2) CO diffusion to the Ru edge after reaction 1 is slow . Following reaction 1, the slow diffusion model by Koper et al. [17,22] may also account for the data shown in Fig. 4A. In this model, the diffusional barrier is established at the vacant (H2O occupied) Pt site. If this is so, the transfer of CO via some water occupied Pt sites, electronically modified by the Ru presence [20] is rate limiting, and the current /time decay approximated by the second exponent visualizes the ‘diffusional tailing’ from the low CO binding energy model [22] (see Section 1). The difference of our model (Fig. 6) with respect to the previous model [22] is that the Pt(111)/Ru/Pt/OH site, not the Pt(111)/Ru /OH site is involved. This is because the data discussed above indicate that the Pt(111)/Ru/Pt/OH (Pt near the Ru edge) is highly active (reaction 1) and regenerates an active OH form fast (reaction 2). Therefore, a diffusional barrier cannot be formed at the first Pt ‘ring’ around the Ru island.
Fig. 6. CO oxidation reactivity diagram, see text.
Notice that the ligand effect on platinum extends to at least two neighboring sites (or Pt rings) as indicated by recent electrochemical NMR results [different paper] [20]. This is in accordance to the Friedel /Heine theorem [20,38] and puts our modeling considerations on a rational basis. Evidence as to the validity of the schemes discussed above becomes apparent if we consider what will happen when the density of the Ru islands on Pt(111) increases. This will be tantamount to a situation in which the ratio of the CO sites away from the Pt(111)/ Ru edge to the reactive Ru edge sites decreases. Or, in other words, at a certain critical Ru island density, the second voltammetric peak in Fig. 1B should disappear as the increasing number of CO molecules bound to Pt is near the reactive Pt/OH sites. This is what is observed experimentally, with the critical island density of 0.55 ML (Fig. 7). Certainly however, more details on these observations and mechanisms in the electrochemistry and surface science perspective have to be elaborated.
5. Conclusions We have proposed a mechanistic bridge to reconcile concepts already developed, or being developed in several laboratories interested in the CO oxidation reaction on the Pt(111) electrode modified by ruthenium. Specific new discoveries vs. data available in the subject literature are the following: . There is no dependence of the voltammetric charge distribution between the two peaks on the sweep rate down to 2 mV s 1.
Fig. 7. The dependence of the CO stripping peaks on the ruthenium coverage from 0.10 to 0.55 ML obtained by the method of repetitive spontaneous deposition [39]: ( */) one short Ru deposition, ( */ */) one regular deposition (0.15 ML), ( */ ×/ */) two depositions, (. . .), four depositions. Data obtained for 15 min argon bubbling are processed, see Fig. 2.
G.-Q. Lu et al. / Journal of Electroanalytical Chemistry 532 (2002) 49 /55
. The oxidation mechanism in the second voltammetric peak area differs from that in the pre-peak area, and is not of Langmuir /Hinshelwood character. . The voltammetric peak splitting disappears with an increase in Ru coverage. The detailed focus was on the reactivity in the potential range of the second voltammetric peak (Fig. 1B). Double potential step chronoamperometric data focused on this electrode potential range reveal that there are two components encoded into the current / time decay (Fig. 4). The first exponential decay is due to the oxidation of CO chemisorbed at the Pt sites neighboring the Pt/Ru sites, involving the reactant pair formation. The second decay can be rationalized by assuming (i) a surface reaction between rapidly diffusing CO with the Pt /OH site at the Pt/Ru edge, or (ii) a diffusional tailing proposed previously. In such a case, the second neighbor Pt site, counting from the Pt/Ru edge is the one that presents the barrier for the CO transfer process in its edge-type oxidation of CO to CO2, in the realm of the model formulated by Koper et al. [22].
Acknowledgements Numerous discussions with Dr. M.T.M. Koper from the Eindhoven University of Technology are acknowledged, including him sending us preprints of his papers. Mr. Tomas M. Barnard’s assistance in the data treatment is appreciated. This work is supported by the National Science Foundation Grant CHE 9985837. G.Q.L. appreciates support from the Department of Energy grant DEFG02-96ER45439, administered by the Frederick Seitz Materials Research Laboratory at the University of Illinois.
References [1] E. Herrero, K. Franaszczuk, A. Wieckowski, J. Electroanal. Chem. 361 (1993) 269. [2] K.A. Friedrich, K.P. Geyzers, U. Linke, U. Stimming, J. Stumper, J. Electroanal. Chem. 402 (1996) 123. [3] K.A. Friedrich, K.P. Geyzers, A. Marmann, U. Stimming, R. Vogel, Z. Phys. Chem. 208 (1999) 137. [4] H. Massong, H.S. Wang, G. Samjeske, H. Baltruschat, Electrochim. Acta 46 (2000) 701. [5] W. Chrzanowski, A. Wieckowski, Langmuir 13 (1997) 5974. [6] W. Chrzanowski, A. Wieckowski, in: A. Wieckowski (Ed.), Interfacial Electrochemistry: Theory, Experiment, and Applications, Marcel Dekker, New York, 1999, p. 937. [7] A. Crown, I.R. Moraes, A. Wieckowski, J. Electroanal. Chem. 500 (2001) 333.
55
[8] A. Crown, A. Wieckowski, Phys. Chem. Chem. Phys. 3 (2001) 3290. [9] W. Chrzanowski, H. Kim, A. Wieckowski, Catal. Lett. 50 (1998) 69. [10] W.F. Lin, M.S. Zei, M. Eiswirth, G. Ertl, T. Iwasita, W. Vielstich, J. Phys. Chem. B 103 (1999) 6968. [11] T. Iwasita, H. Hoster, A. John-Anacker, W.F. Lin, W. Vielstich, Langmuir 16 (2000) 522. [12] H. Hoster, T. Iwasita, H. Baumgartner, W. Vielstich, Phys. Chem. Chem. Phys. 3 (2001) 337. [13] T.D. Jarvi, T.H. Madden, E.M. Stuve, Electrochem. Solid State Lett. 2 (1999) 224. [14] A. Lamouri, Y. Gofer, Y. Luo, G.S. Chottiner, D.A. Scherson, J. Phys. Chem. B 105 (2001) 6172. [15] K.A. Friedrich, K.P. Geyzers, F. Henglein, A. Marmann, U. Stimming, W. Unkauf, R. Vogel, Proc. Electrochem. Soc. 96 (1996) 119. [16] E. Herrero, J.M. Feliu, A. Wieckowski, Langmuir 15 (1999) 4944. [17] M.T.M. Koper, J.J. Lukkien, A.P.J. Jansen, R.A. van Santen, J. Phys. Chem. B 103 (1999) 5522. [18] P. Waszczuk, A. Crown, S. Mitrovski, A. Wieckowski, in: W. Vielstich (Ed.), Fuel Cell Handbook: Fuel Cell Electrocatalysis, Wiley-VCH, in press. [19] M.T.M. Koper, T.E. Shubina, R.A. van Santen, J. Phys. Chem. B 106 (2002) 686. [20] Y. Tong, H.S. Kim, P.K. Babu, P. Waszczuk, A. Wieckowski, E. Oldfield, J. Am. Chem. Soc. 124 (2002) 468. [21] N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van Santen, J. Phys. Chem., submitted. [22] M.T.M. Koper, N.P. Lebedeva, C.G.M. Hermse, Faraday Discuss., in press. [23] B. Love, J. Lipkowski, in: M.P. Soriaga (Ed.), Electrochemical Surface Science, Molecular Phenomena at Electrode Surfaces, American Chemical Society, Washington, DC, 1988, p. 484. [24] J. Clavilier, in: A. Wieckowski (Ed.), Interfacial Electrochemistry: Theory, Experiment, and Applications, Marcel Dekker, 1999. [25] B.E. Conway, Prog. Surf. Sci. 49 (1995) 331. [26] R. Gomez, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf. Sci. 410 (1998) 48. [27] J.M. Feliu, J.M. Orts, A. Fernandez Vega, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 296 (1990) 191. [28] J.M. Feliu, J.M. Orts, R. Gomez, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 372 (1994) 265. [29] E. Santos, E.P.M. Leiva, W. Vielstich, Electrochim. Acta 36 (1991) 555. [30] N.P. Lebedeva, M.T.M. Koper, E. Herrero, J.M. Feliu, R.A. van Santen, J. Electroanal. Chem. 487 (2000) 37. [31] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, Electrochim. Acta 39 (1994) 1825. [32] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. [33] C. Lu, R.I. Masel, J. Phys. Chem. B 105 (2001) 9793. [34] J. McBreen, S. Mukerjee, J. Electrochem. Soc. 142 (1995) 3399. [35] J.C. Davies, B.E. Hayden, D.J. Pegg, M.E. Rendall, Surf. Sci. 496 (2002) 110. [36] E. Herrero, K. Franaszczuk, A. Wieckowski, J. Phys. Chem. 98 (1994) 5074. [37] S. Gilman, J. Phys. Chem. 68 (1964) 70. [38] Y.Y. Tong, C. Rice, A. Wieckowski, E. Oldfield, J. Am. Chem. Soc. 122 (2000) 11921. [39] A. Crown, C. Johnston, A. Wieckowski, Surf. Sci. Lett. 506 (2002) L268.
Oxidation of CO adsorbed from CO saturated solutions on the Pt(111)/Ru electrode G.-Q. Lu, P. Waszczuk, A. Wieckowski * Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 18 January 2002; received in revised form 6 May 2002; accepted 17 May 2002 Dedicated to Professor Sergio Trasatti on the occasion of his 65th birthday and in recognition of his contribution to Electrochemistry.
Abstract Using Pt(111) surfaces covered by ruthenium by a spontaneous deposition method we conducted voltammetric oxidation of chemisorbed CO in clean sulfuric acid electrolyte at 50 mV s 1 and under variable sweep rate conditions. The deposition yields Pt(111) surfaces decorated by two-dimensional nanosized ruthenium islands previously reported. At 50 mV s 1, the CO stripping produces two well-resolved current peaks, at 0.55 and 0.67 V versus RHE. The first peak originates from CO chemisorbed on (and strictly around) ruthenium islands deposited on Pt(111), while another comes from CO chemisorbed on ‘pure’ Pt(111) phases of the Pt(111)/Ru electrode, away from the Ru islands. Using double-potential step chronoamperometry we investigated the CO oxidation on these two surface phases, and found that the low potential oxidation of chemisorbed CO occurred via the Langmuir / Hinshelwood mechanism. This shows that diffusion of CO on ruthenium to the Ru edge is not needed for a complete stripping of CO from such islands, i.e. the oxidation process is activated uniformly across an island. In contrast, CO chemisorbed on Pt sites not occupied by Ru is oxidized at the Pt(111)/Ru edge, and there may be a surface diffusion contribution to a complete removal of CO from the surface. However, a part of the CO adlayer oxidation in the 0.67 V peak area occurs with no participation of surface diffusion, according to a pseudo-first order surface reaction kinetics, and the reaction may involve a ‘reactant pair’ (Pt/O /H O / C /Pt) breakdown on the electrode surface. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Pt(111)/Ru; Spontaneous deposition; CO; Electrooxidation mechanism; Potential step; Cyclic voltammetry
1. Introduction There has been broad interest in Pt(111) electrodes ‘decorated’ by ruthenium by the use of forced and spontaneous electrochemical methods [1 /9]. Electrochemical and UHV deposition procedures [10 /14] produce Pt(111) surfaces covered by ruthenium islands of nanosized dimensions [2,3,7,8,15 /19], as first reported by Stimming, Friedrich and collaborators [2,3]. Carbon monoxide chemisorbed on such surfaces has also been investigated by both experimental [2 /4,10,11] and theoretical methods [17,19]. Under some specific conditions including slow CO surface diffusion [17], Koper et al. demonstrated theoretically that the CO could be
* Corresponding author. Tel.: /1-217-333-7943; fax: /1-217-2448068 E-mail address: [email protected] (A. Wieckowski).
oxidatively removed from the Ru islands (and directly near the islands), and from the Pt sites away from the Pt/ Ru edge as two separate oxidation processes, giving rise to a split cyclic voltammogram (CV) [17]. The presence of such two distinctive CO phases was also inferred from infrared measurements [2,11,15]. However, the link between the distinctive CO populations and the predicted split voltammetry had until quite recently been missing. The confirmation came from the Baltruschat group using the Pt(111) electrode [4] and by our group using CO generated from methanol, voltammetry and electrochemical NMR on nanoparticle Pt/Ru electrodes [20]. The Koper group also presented evidence that the current /potential chronoamperometric response due to CO oxidation on Pt(111) can be better interpreted using the ‘mean-field’ mechanism [21,22], rather than the nucleation and growth mechanism [23]. The model
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 0 9 5 2 - X
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assumes that, overall, CO diffusion on Pt(111) is fast, but argues that if slow diffusion is involved in the CO oxidation reaction near the Pt/Ru edge, it is manifested as a ‘tailing’ of the current /time plot in a chronoamperometric experiment. Such a tailing has not been found either with the clean Pt(111) electrode, or with stepped Pt(hkl ) surfaces. However, the CO stripping data on Pt(111)/Ru in Ref. [4] and in this report give credit to the idea that a slow surface diffusion of CO may be involved [17]. If this is so, the argument would be that due to the low binding energy of CO at the Pt/Ru edge site, the CO surface motion near the edge may experience a retardation due to a high-energy barrier at such a site [22]. Alternatively, Pt sites close to Ru can be less CO populated due to the unfavorable binding energy, and hence these Pt /Ru sites may not be active enough in oxidizing CO to CO2 [22]. Below, we demonstrate that both voltammetric and chronoamperometric CO oxidation data may be interpreted without involving the slow diffusion component in the reaction mechanism. However, our data do not reject the possibility that the slow diffusion can be considered. In the latter case, the second neighbor Pt site, counting from the Pt/Ru edge, is the one that presents the barrier for the CO surface mass transport rather than the first neighbor Pt site. This means that overall we support the previous CO oxidation model [17], but with some meaningful modifications.
2. Experimental All experiments were carried out at ambient temperature (259/2 8C). The potentials were measured against a commercial (BAS) silver j silver chloride electrode, but are reported versus RHE. The chemicals used were sulfuric and perchloric acid (GFS, twice distilled from Vycor), RuCl3 ×/xH2O (Aldrich), CO (Matheson, research purity) and Millipore water. Cyclic voltammetric and chronoamperometric measurements were carried out using a PAR 273 potentiostat and associated auxiliaries. A spontaneous deposition method was used to obtain the Ru covered Pt(111) surfaces. The procedure involves immersing the platinum substrate in a ruthenium containing perchloric acid solution and flushing the cell with either Millipore water or the supporting electrolyte. A negative polarization is then applied to the surface that breaks the chemisorbed admetal precursors immobilized on the surface to desired, catalytically effective ruthenium fragments. The later phase of the deposition takes place in an electrochemical cell where electrocatalytic reactions are tested. The surfaces such prepared are usually ‘stabilized’ by three voltammetric cycles between 0.06 and 0.85 V in 0.1 M H2SO4 electrolyte. For the project described below, a platinum single crystal bead
of 2.5 mm in diameter, cut and polished to the (111) orientation was used. The final surface was prepared by annealing the crystal in the hydrogen/air flame, cooling in an Ar/H2 atmosphere and protected by ultrapure water [24]. The deposition of ruthenium was carried out from solutions of 1 mM of RuCl3 in 0.1 M of HClO4, as described in Refs [5,7,8].
3. Results 3.1. Cyclic voltammetry Cyclic voltammetry of the clean Pt(111) electrode in 0.1 M H2SO4 at 50 mV s 1 is shown in Fig. 1A (dotted line). The CV demonstrates the high quality of the electrode surface, and confirms the cleanliness of our electrochemical systems. By introducing approximately 0.15 ML of spontaneously deposited Ru to the surface [5,7,8], two characteristic changes on the voltammetric profile were observed (Fig. 1A, solid line). First, the negative- and positive-going spikes at 0.49 V (belonging to the anomalous region) are significantly suppressed [16] and second the pseudocapacitive current in the 0.50 /0.85 V region is enlarged, characteristic of a Rutype electrode [16,25]. Carbon monoxide was next admitted to the cell, and CO adsorption was carried out on both Pt(111) and Pt(111)/Ru electrodes for 5 min under active CO bubbling conditions at 0.25 V. This was followed by purging the solution with argon for 20 min (unless stated otherwise) to remove dissolved CO from solution, while holding the electrode at the same potential of 0.25 V. The CO stripping profile from Pt(111) (Fig. 1B, dotted line) indicates that the voltammetric CO oxidation peaks at approximately 0.85 V (at 50 mV s 1), and that the coverage of CO is close to 0.75 ML, confirming previous data [17,21,26 /30]. On Pt(111)/Ru, the main new voltammetric feature shown in Fig. 1B (solid line) with respect to the CO stripping from Pt(111) is the appearance of two, instead of one voltammetric peak due to CO oxidation. This is a clear confirmation of the voltammetric splitting involved in CO oxidation on the Pt(111)/Ru electrode (see Section 1). However, data in Fig. 2 demonstrate that the peak separation depends on the CO oxidation charge, or on the CO coverage. In this study, the CO coverage was regulated by the time of argon bubbling through solution with the Pt(111)/Ru / CO electrode held at 0.25 V. Apparently (Fig. 2), the separation changes from 0.52 / /0.67 to 0.61 / /0.67 V at 20 and 10 min argon exposure, that is from the lower to higher CO coverage, respectively. These effects certainly need further investigations. Also, the peak located at a more positive potential, 0.67 V, is shifted in the negative direction due to Ru present with respect to
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Fig. 1. (A) CVs of clean Pt(111) (dotted line) and Pt(111)/Ru (solid line) in 0.1 M H2SO4. Approximately 0.15 ML of Ru was added onto Pt(111) by spontaneous deposition. (B) Stripping of a CO adlayer from Pt(111) (dotted line) and from Pt(111)/Ru (solid line) in a clean 0.1 M H2SO4. The scan rate was 50 mV s 1. Exposure of the electrodes to CO at 0.25 V for 5 min was followed by Ar bubbling for 20 min.
Fig. 2. Peak separation influenced by the CO coverage on the Ru phase (which was regulated by the length of argon purging time following CO adsorption). Argon purging time is indicated in the figure. Other conditions as in Fig. 1B.
CO oxidation from pure Pt(111), at 0.85 V (see Section 4). As inferred from previous studies [17,20,31], the presence of two voltammetric stripping maxima on Pt(111)/Ru indicates that a part of the CO adlattice is chemisorbed onto the ruthenium islands deposited on Pt(111), the Pt(111)/Ru /CO phase (or the pre-peak), while the more positive peak originates from the CO chemisorbed on ‘pure’ Pt(111), the (Ru)Pt(111)/CO phase. The splitting between the two stripping maxima may be as high as 0.15 V, and is very close to that obtained for CO stripping from the Pt/Ru nanoparticle surfaces [20]. It is interesting to note that the voltammetric morphology and the peak widths at 0.52 and 0.67 V in Fig. 1B are different, being indicative of the different nature of the electrochemical processes exam-
ined (there is also an interesting anion effect between surface sulfate and perchlorate, which will soon be reported). As mentioned in Section 1 Koper et al. believe that split may imply slow CO diffusion to the reaction site [17]. However, the authors did not exclude the possibility that the Pt/Ru was simply inactive to the oxidation of Pt /CO at low potentials in the pre-peak area [22]. We will address this model further below. Voltammetric, sweep rate dependent measurements of CO stripping, such as those shown in Fig. 1B were carried out in the range from v/2/103 to 1 V s 1 (time window /1/v, from 500 to 1 s V 1). We found no dependence of the voltammetric charge distribution between the two peaks on the sweep rate in this range. Therefore, on the time scales indicated above, there is either no effective diffusion from the Pt surface phase to the Ru edge (that would lead to the increase in the prepeak at the expense of the main peak at progressively longer times) or the Pt/Ru edge is not reactive to CO chemisorbed at the Pt atoms adjacent to the Ru edge at lower potentials. However, these conclusions may not be valid for longer experimental times than used in this study, the issue to be addressed in the future. Recent results of NMR investigations show no motional intermixing between the CO chemisorbed molecules on the Pt and Pt/Ru surface phases, albeit at lower potentials than those studied in this report [20]. This clearly indicates unique chemistry of the Pt/Ru edge with respect to surface CO behaviors and dynamics. 3.2. Chronoamperometry To obtain more advanced insights into the nature of the voltammetric splitting presented in Figs. 1B and 2, we employed electric current measurements under potential-step conditions: a single step and double
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potential step chronoamperometry. First, we applied the single potential step into the lower potential voltammetric peak, at 0.52 V, and the data are shown on the left hand side of Fig. 3, where the results from the second potential step experiment are also shown. The first step was applied for 2 s, that is, from 2 to 4 s in Fig. 3. As a consequence of the first step to 0.52 V, a symmetric current /time peak was obtained, characteristic of the Langmuir /Hinshelwood-type [32] electrooxidation of carbon monoxide. After the 2 s delay, the subsequent second step was applied into the potential region of, or near the second voltammetric-stripping peak. In Fig. 3, the potential was stepped to 0.52 V for 2 s (as above), and then from 0.52 to 0.67 V for the remaining time of the experiment (the right-hand side of the figure). From the comparison of the voltammetric (Fig. 1B) and chronoamperometric (Fig. 3) data, we conclude that after the 2 s pulse width, that is, when the current due to the CO oxidation goes to a near zero value, the (Ru)Pt(111) /CO population still remaining on the surface is barely affected. After the second potential step was applied, the remaining CO was completely oxidized after the experimental time 5.5 s (in ca. 1.5 s). Notably, at 0.67 V (and at other potentials in the main peak area), a current decay rather than a peaked-type profile is observed. Clearly, the CO oxidation does not follow the Langmuir /Hinshelwood mechanism, as does the CO oxidation at the lower potentials. The character of the current decay strongly suggests an exponential relationship between current and time. However, fitting all decay current /time data obtained in this series to a simple semi-logarithmic plot
Fig. 3. A representative curve for the double-potential step chronoamperometry for a CO adlayer on Pt(111)/Ru in clean 0.1 M H2SO4: first, the potential is stepped from 0.25 to 0.520 V for 2 s, and then from 0.52 to 0.67 V until the CO was removed from the surface.
was not successful, and a more advanced treatment is required (see below).
4. Discussion It is already well documented that surface CO, either generated from CO solutions or from methanol decomposition oxidizes at less positive potentials on Pt/Ru than on clean Pt electrodes [1 /4,10,11,20,31]. On Pt(111)/Ru, under some specific CO and ruthenium coverage conditions discussed in this paper (also, see below), a split in the voltammetric CO oxidation on the Pt(111)/Ru electrode occurs [4,22]. The split gives rise to two maxima, one at lower potentials, corresponding to the CO oxidation from ruthenium deposited on platinum, Pt(111)/Ru /CO, including the oxidation of CO from the Pt sites adjacent to the Ru edge [17,20], and the second, at more positive potentials, from the (Ru)Pt(111)/CO [20] (Fig. 1B). There appears no doubts that the broadly understood Langmuir /Hinshelwood mechanism applies to the CO oxidation in the prepeak area. If so, this shows that diffusion of CO on ruthenium to the Ru edge is not needed for a complete stripping of CO from such islands, i.e. the oxidation process is activated uniformly across a Ru island. This is interesting behavior given that the events we report occur on small, nanosized island surfaces. It is noteworthy that the oxidation of CO from Pt(111)/Ru /CO and from the Ru perimeter upon the first step occurs without any meaningful change in the local (Ru)Pt(111)/CO coverage (Fig. 3). Or, in the time window up to 500 s V 1, CO molecules away from the Pt/Ru edge are not noticeably oxidized at this edge at potentials of the pre-peak area. As already detailed above, this is either because of the slow surface diffusional CO transfer to the edge, or because the edge is not reactive to CO. There is also a complete removal of CO at potentials from the second CO stripping peak area. Using chronoamperometry (Fig. 3) we report, for the first time, that there is a change in CO oxidation from the Langmuir /Hinshelwood mechanism to a mechanism represented by a current /time decay, which is usually associated with first order oxidation kinetics. A further observation to consider is that the potential shift in the oxidation of (Ru)Pt(111) /CO and Pt(111)/ CO is 0.18 V (from ca. 0.85 V on clean Pt(111) to 0.67 V on Pt(111)/Ru, Figs. 1B and 2, respectively). The shift may be due to an electronic effect, that is due to the modification of local DOSs of platinum by the ruthenium additive [20,33]. Indeed, McBreen and Mukerjee using XAS [34] and Tong et al. using NMR [20] provided evidence as to the reality of such an effect, which in principle would allow the CO from (Ru)Pt(111)/CO to react with adsorbed OH at lower
G.-Q. Lu et al. / Journal of Electroanalytical Chemistry 532 (2002) 49 /55
potentials than at clean Pt(111). However, this effect may not be large enough, and could add only a fraction to the 0.85 /0.67 V voltage gain observed [33]. A rapid OHads spillover from the Pt/Ru edge to the (Ru)Pt(111) /CO phase, and its subsequent involvement in the CO oxidation process, could also be considered on the Ru modified Pt phases [35]. However, this is not very likely since quantum-chemical data indicate that the OHads binding energy does not increase on Pt modified by Ru [17] (which would then be more reactive to CO [21]). The correct explanation of all our data is most likely related to the high density of Pt/Ru edge sites on (Ru)Pt(111) /CO, and the preferential adsorption of OH at the Pt/Ru edge, versus the absence of such Pt/Ru defects on Pt(111)/CO. The edge OH species on (Ru)Pt(111) are active at lower potentials than the OH on Pt(111), causing the removal of CO at lower potentials. The first model of the events within the main voltammetric (more positive) peak area in the chronoamperometric measurements would then postulate that there is an availability of reactive sites at the Pt/Ru edge to which the CO is transferred via rapid surface diffusion on the Pt part of the surface. This would lead to a pseudo first order kinetics and a linear log j versus t plot covering the full current /potential range (within the main peak area). Data presented in Fig. 4A prove this simple hypothesis incorrect. As shown in this figure, the data may be approximated by two consecutive log j versus t straight lines accounting for approximately 90% of the total current decay. Notice that the first straight log j versus t line at short times accounts for approximately 50% current decay (Fig. 4B), and the log j versus t linearity at short times is as significant as the linearity at higher times. The results of the Tafel analysis of the data in the second voltammetric peak area are shown in Fig. 5, where j is the current density obtained from data in Fig. 4A via the extrapolation to 0 time (instantaneous current densities [36]). We found two different Tafel slopes, a high slope approximately 0.12 V per decade for the first phase of the CO oxidation (at short times) and a low slope, approximately 0.06 V per decade for the nearly complete CO oxidation process. Therefore, we believe that the preliminary interpretation of our data may be as follows. (1) CO diffusion to the Ru edge is fast . Before the second potential step, CO molecules adsorbed on Pt sites are stable and the Pt sites adjacent to the Pt/Ru sites of the ruthenium islands are covered either by OH [21] or by chemisorbed water [36,37]. Assuming the former, the stability of CO in the pre-peak potential range allows ‘reactant pairs’ [37]to be created between the CO and surface OH stabilized e.g. by hydrogen bonding: Pt/O /H O /C /Pt. The first exponential decay for CO oxidation would then be due to breakdown of such OH OC pairs according to Eq. (1):
53
Fig. 4. (A) log j (current density) vs. time straight lines for CO oxidation accounting for approximately 90% of the total current decay. Data are shown after CO adsorption (Fig. 1), a pre-step to 0.520 V, followed by the second potential step to 0.695 V (Fig. 3). (B) A j vs. time plot showing the areas for high and low log j vs. time slopes (see text). Other details as in (A).
Fig. 5. Tafel, log j (current density) vs. electrode potential lines for the first order kinetics of CO oxidation in time domains depicted in Fig. 4. The Tafel slopes are 0.12 V per decade for the first phase of the CO oxidation (short times, solid symbols) and 0.06 V per decade (longer times, open symbols).
rds
Pt(111)=Ru=PtOH OCPt 0 Pt(111)=Ru=PtPt CO2 H e
(1)
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The OH is associated with Pt(111)/Ru/Pt sites (Pt sites near the Pt/Ru edge) rather than with the Pt(111)/Ru sites because: (i) CO is removed from the Pt(111)/Ru sites in the pre-peak area [17,20,22], (ii) Pt/Ru/Pt sites must be already populated by OH (or H2O) since the Tafel slope for the first phase of the CO oxidation is approximately 0.12 V per decade (Fig. 5, solid symbols). After the breakdown of the O /H OC surface pair, corresponding to the log j versus t straight line at short times (Fig. 4A) and to reaction 1, the OHs need to be replenished in a reversible electrochemical reaction: Pt(111)=Ru=PtH2 O UPt(111)=Ru=PtOHH e
(2)
If the CO surface diffusion is fast, step 2 is followed by a chemical rate-determining step, yielding: rds
Pt(111)=Ru=PtOHPtCO 0 Pt=Ru=PtCOOH
(3)
see the second straight line in Fig. 4, and: fast
Pt(111)=Ru=PtCOOH 0 Pt=Ru=PtPtCO2 H e (4) The Tafel slope associated with the second exponent is around 0.06 V per decade (Fig. 5, open symbols) as expected for the consecutive reactions 2 /4 [21,30]. (2) CO diffusion to the Ru edge after reaction 1 is slow . Following reaction 1, the slow diffusion model by Koper et al. [17,22] may also account for the data shown in Fig. 4A. In this model, the diffusional barrier is established at the vacant (H2O occupied) Pt site. If this is so, the transfer of CO via some water occupied Pt sites, electronically modified by the Ru presence [20] is rate limiting, and the current /time decay approximated by the second exponent visualizes the ‘diffusional tailing’ from the low CO binding energy model [22] (see Section 1). The difference of our model (Fig. 6) with respect to the previous model [22] is that the Pt(111)/Ru/Pt/OH site, not the Pt(111)/Ru /OH site is involved. This is because the data discussed above indicate that the Pt(111)/Ru/Pt/OH (Pt near the Ru edge) is highly active (reaction 1) and regenerates an active OH form fast (reaction 2). Therefore, a diffusional barrier cannot be formed at the first Pt ‘ring’ around the Ru island.
Fig. 6. CO oxidation reactivity diagram, see text.
Notice that the ligand effect on platinum extends to at least two neighboring sites (or Pt rings) as indicated by recent electrochemical NMR results [different paper] [20]. This is in accordance to the Friedel /Heine theorem [20,38] and puts our modeling considerations on a rational basis. Evidence as to the validity of the schemes discussed above becomes apparent if we consider what will happen when the density of the Ru islands on Pt(111) increases. This will be tantamount to a situation in which the ratio of the CO sites away from the Pt(111)/ Ru edge to the reactive Ru edge sites decreases. Or, in other words, at a certain critical Ru island density, the second voltammetric peak in Fig. 1B should disappear as the increasing number of CO molecules bound to Pt is near the reactive Pt/OH sites. This is what is observed experimentally, with the critical island density of 0.55 ML (Fig. 7). Certainly however, more details on these observations and mechanisms in the electrochemistry and surface science perspective have to be elaborated.
5. Conclusions We have proposed a mechanistic bridge to reconcile concepts already developed, or being developed in several laboratories interested in the CO oxidation reaction on the Pt(111) electrode modified by ruthenium. Specific new discoveries vs. data available in the subject literature are the following: . There is no dependence of the voltammetric charge distribution between the two peaks on the sweep rate down to 2 mV s 1.
Fig. 7. The dependence of the CO stripping peaks on the ruthenium coverage from 0.10 to 0.55 ML obtained by the method of repetitive spontaneous deposition [39]: ( */) one short Ru deposition, ( */ */) one regular deposition (0.15 ML), ( */ ×/ */) two depositions, (. . .), four depositions. Data obtained for 15 min argon bubbling are processed, see Fig. 2.
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. The oxidation mechanism in the second voltammetric peak area differs from that in the pre-peak area, and is not of Langmuir /Hinshelwood character. . The voltammetric peak splitting disappears with an increase in Ru coverage. The detailed focus was on the reactivity in the potential range of the second voltammetric peak (Fig. 1B). Double potential step chronoamperometric data focused on this electrode potential range reveal that there are two components encoded into the current / time decay (Fig. 4). The first exponential decay is due to the oxidation of CO chemisorbed at the Pt sites neighboring the Pt/Ru sites, involving the reactant pair formation. The second decay can be rationalized by assuming (i) a surface reaction between rapidly diffusing CO with the Pt /OH site at the Pt/Ru edge, or (ii) a diffusional tailing proposed previously. In such a case, the second neighbor Pt site, counting from the Pt/Ru edge is the one that presents the barrier for the CO transfer process in its edge-type oxidation of CO to CO2, in the realm of the model formulated by Koper et al. [22].
Acknowledgements Numerous discussions with Dr. M.T.M. Koper from the Eindhoven University of Technology are acknowledged, including him sending us preprints of his papers. Mr. Tomas M. Barnard’s assistance in the data treatment is appreciated. This work is supported by the National Science Foundation Grant CHE 9985837. G.Q.L. appreciates support from the Department of Energy grant DEFG02-96ER45439, administered by the Frederick Seitz Materials Research Laboratory at the University of Illinois.
References [1] E. Herrero, K. Franaszczuk, A. Wieckowski, J. Electroanal. Chem. 361 (1993) 269. [2] K.A. Friedrich, K.P. Geyzers, U. Linke, U. Stimming, J. Stumper, J. Electroanal. Chem. 402 (1996) 123. [3] K.A. Friedrich, K.P. Geyzers, A. Marmann, U. Stimming, R. Vogel, Z. Phys. Chem. 208 (1999) 137. [4] H. Massong, H.S. Wang, G. Samjeske, H. Baltruschat, Electrochim. Acta 46 (2000) 701. [5] W. Chrzanowski, A. Wieckowski, Langmuir 13 (1997) 5974. [6] W. Chrzanowski, A. Wieckowski, in: A. Wieckowski (Ed.), Interfacial Electrochemistry: Theory, Experiment, and Applications, Marcel Dekker, New York, 1999, p. 937. [7] A. Crown, I.R. Moraes, A. Wieckowski, J. Electroanal. Chem. 500 (2001) 333.
55
[8] A. Crown, A. Wieckowski, Phys. Chem. Chem. Phys. 3 (2001) 3290. [9] W. Chrzanowski, H. Kim, A. Wieckowski, Catal. Lett. 50 (1998) 69. [10] W.F. Lin, M.S. Zei, M. Eiswirth, G. Ertl, T. Iwasita, W. Vielstich, J. Phys. Chem. B 103 (1999) 6968. [11] T. Iwasita, H. Hoster, A. John-Anacker, W.F. Lin, W. Vielstich, Langmuir 16 (2000) 522. [12] H. Hoster, T. Iwasita, H. Baumgartner, W. Vielstich, Phys. Chem. Chem. Phys. 3 (2001) 337. [13] T.D. Jarvi, T.H. Madden, E.M. Stuve, Electrochem. Solid State Lett. 2 (1999) 224. [14] A. Lamouri, Y. Gofer, Y. Luo, G.S. Chottiner, D.A. Scherson, J. Phys. Chem. B 105 (2001) 6172. [15] K.A. Friedrich, K.P. Geyzers, F. Henglein, A. Marmann, U. Stimming, W. Unkauf, R. Vogel, Proc. Electrochem. Soc. 96 (1996) 119. [16] E. Herrero, J.M. Feliu, A. Wieckowski, Langmuir 15 (1999) 4944. [17] M.T.M. Koper, J.J. Lukkien, A.P.J. Jansen, R.A. van Santen, J. Phys. Chem. B 103 (1999) 5522. [18] P. Waszczuk, A. Crown, S. Mitrovski, A. Wieckowski, in: W. Vielstich (Ed.), Fuel Cell Handbook: Fuel Cell Electrocatalysis, Wiley-VCH, in press. [19] M.T.M. Koper, T.E. Shubina, R.A. van Santen, J. Phys. Chem. B 106 (2002) 686. [20] Y. Tong, H.S. Kim, P.K. Babu, P. Waszczuk, A. Wieckowski, E. Oldfield, J. Am. Chem. Soc. 124 (2002) 468. [21] N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van Santen, J. Phys. Chem., submitted. [22] M.T.M. Koper, N.P. Lebedeva, C.G.M. Hermse, Faraday Discuss., in press. [23] B. Love, J. Lipkowski, in: M.P. Soriaga (Ed.), Electrochemical Surface Science, Molecular Phenomena at Electrode Surfaces, American Chemical Society, Washington, DC, 1988, p. 484. [24] J. Clavilier, in: A. Wieckowski (Ed.), Interfacial Electrochemistry: Theory, Experiment, and Applications, Marcel Dekker, 1999. [25] B.E. Conway, Prog. Surf. Sci. 49 (1995) 331. [26] R. Gomez, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf. Sci. 410 (1998) 48. [27] J.M. Feliu, J.M. Orts, A. Fernandez Vega, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 296 (1990) 191. [28] J.M. Feliu, J.M. Orts, R. Gomez, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 372 (1994) 265. [29] E. Santos, E.P.M. Leiva, W. Vielstich, Electrochim. Acta 36 (1991) 555. [30] N.P. Lebedeva, M.T.M. Koper, E. Herrero, J.M. Feliu, R.A. van Santen, J. Electroanal. Chem. 487 (2000) 37. [31] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, Electrochim. Acta 39 (1994) 1825. [32] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. [33] C. Lu, R.I. Masel, J. Phys. Chem. B 105 (2001) 9793. [34] J. McBreen, S. Mukerjee, J. Electrochem. Soc. 142 (1995) 3399. [35] J.C. Davies, B.E. Hayden, D.J. Pegg, M.E. Rendall, Surf. Sci. 496 (2002) 110. [36] E. Herrero, K. Franaszczuk, A. Wieckowski, J. Phys. Chem. 98 (1994) 5074. [37] S. Gilman, J. Phys. Chem. 68 (1964) 70. [38] Y.Y. Tong, C. Rice, A. Wieckowski, E. Oldfield, J. Am. Chem. Soc. 122 (2000) 11921. [39] A. Crown, C. Johnston, A. Wieckowski, Surf. Sci. Lett. 506 (2002) L268.