New Insights into the Mechanism and Kinetics of Adsorbed CO

Article pubs.acs.org/JPCC

New Insights into the Mechanism and Kinetics of Adsorbed CO Electrooxidation on Platinum: Online Mass Spectrometry and Kinetic Monte Carlo Simulation Studies Hongsen Wang,†,‡ Zenonas Jusys,‡ R. Jürgen Behm,‡ and Héctor D. Abruña*,† †

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States Institute of Surface Chemistry and Catalysis, Ulm University, Albert-Einstein-Allee 47, D-89081 Ulm, Germany

‡

S Supporting Information *

ABSTRACT: The electrooxidation of saturated CO adlayers on Pt/Vulcan and polycrystalline Pt has been studied by potential step techniques combined with differential electrochemical mass spectrometry (DEMS) and kinetic Monte Carlo (KMC) simulations. DEMS was used to selectively monitor the COad electrooxidation, via the CO2 formation rate, without interference from the pseudocapacitive double-layer charging and electrode surface oxidation, while the KMC simulations were employed to understand the mechanism and kinetics of COad electrooxidation at the molecular level. Our DEMS data show that the current transients of COad electrooxidation on polycrystalline Pt and Pt/Vulcan exhibit an initial spike immediately after the potential step, followed by a slow current decay and finally a broad main peak. The temporal evolution of the transients depends strongly on the oxidation potential applied, resulting in the overlap of the initial spike and the main peak for high potentials. A model is proposed to account for the observed phenomena. On the basis of this model, we developed a kinetic Monte Carlo simulation code specific to the electrooxidation of adsorbed CO on Pt. The simulations reproduce the experimental data very well, confirming the robustness of our model. the kinetics of COad electrooxidation.2,10,11,13,14,22−24 On the basis of the observation that the current transients of CO adlayer oxidation on Pt nanoparticles, smaller than 3 nm, become quite asymmetric with a long tail, Maillard et al. proposed a diffusion model (COad diffusion to the active sites and subsequent oxidation at the active sites).16 This concept is similar to Koper’s proposal that COad oxidation only takes place at the step sites on stepped single crystals.25 A kinetic model of COad saturated monolayer oxidation on carbon-supported Pt nanoparticles had been proposed, accounting for a limited surface mobility of COad on small (∼2 nm) nanoparticles.10,13,15 Moreover, in some cases, it appears that the Eley−Rideal (E−R) mechanism can describe the experimental data, in which the reaction rate decreases with a decrease in COad coverage.9,26 From the above discussions, it is clear that determining the diffusion rate of COad on platinum is very important for understanding the kinetics and mechanism of COad electrooxidation and for designing effective fuel cell catalysts. However, there are still very limited reliable methods to directly determine COad’s diffusion coefficient on platinum, although some methods have been suggested.

1. INTRODUCTION The adsorption and electrooxidation behavior of CO on platinum electrodes have been studied for several decades due to their importance in fuel cell related electrocatalysis.1−21 In spite of such efforts, there is still a lack of consensus regarding the most appropriate kinetic model for describing the electrooxidation of adsorbed CO. With respect to the mechanism of COad electrooxidation on Pt, it is generally believed that the reaction follows the Langmuir−Hinshelwood (L−H) mechanism, which can be simply described as Pt + H 2O ⇄ Pt−OH + H+ + e−

(1)

Pt + CO ⇄ Pt−CO

(2)

Pt−CO + Pt−OH → CO2 + H+ + e− + 2Pt

(3)

Regarding COad’s mobility, some authors have found that the reaction kinetics can be treated within the mean-field approximation,2,3 in which COad and oxygen-containing adspecies (likely OHad) diffuse rapidly and thus homogeneously distribute on the Pt surface, so that the reaction rate is proportional to the surface coverage of COad and OHad. In contrast, others have proposed a nucleation and growth model to describe the experimental data, where COad is immobile, and the reaction takes place only at the boundaries between COad and OHad patches.4−6 Using dynamic Monte Carlo simulations, several groups have discussed the effect of COad diffusion on © 2012 American Chemical Society

Received: February 8, 2012 Revised: April 12, 2012 Published: April 13, 2012 11040

dx.doi.org/10.1021/jp301292p | J. Phys. Chem. C 2012, 116, 11040−11053

The Journal of Physical Chemistry C

Article

nanoparticle aggregation on the appearance of the prepeak, and the potential of the main peak in the potentiodynamic oxidation of COad, have also been reported.56,57 Although the origin of the prepeak has been discussed for several decades, its interpretation still remains unresolved. Several explanations about its origin have been proposed. Kunimatsu et al. assigned the prepeak to the preferential oxidation of bridge-bonded CO.59 In contrast, Kita et al. claimed that the pre- and main peaks of the voltammogram do not correspond, separately, to linearly and bridge-bonded CO and that the appearance of the pre- and main peaks does not reflect a difference in the adsorption mode but rather is due to other kinetic factors, which control the ease of CO ad oxidation. 60 Some authors 61 assumed that during the adsorption of CO both weakly and strongly bonded CO are formed and that the prepeak and main peak correspond to the oxidation of weakly and strongly bonded CO, respectively. Markovic et al.62 suggested that for a saturated CO adlayer CO is adsorbed in the weakly bound state due to the repulsion of CO molecules. As a fraction of the adsorbed CO is oxidized in the prepeak, the rest of the CO adlayer relaxes, leading to the formation of strongly bonded CO, which is more difficult to oxidize and can only be oxidized at more positive potentials to form the main peak. Akemann et al. suggested that on Pt(111) the prepeak corresponds to the oxidation of CO adsorbed in the vicinity of steps.63 Cuesta proposed that the prepeak corresponds to the oxidation of CO molecules adsorbed at the steps and a small amount of CO molecules adsorbed on the terraces that diffuse to the steps and that the main peak appears when nucleation of oxygenated species on the terraces also occurs.19 Strmcnik et al. also suggested that the prepeak originates from the oxidation of CO adsorbed on Pt(111) adislands with monatomic height and average size of 1.5 nm.64 Using dynamic Monte Carlo to simulate COad electrooxidation on stepped Rh single-crystal surfaces, Koper et al. assigned the prepeak to COad oxidation on the steps and nearest sites. However, the experimental data did not fully support this.24 Osawa et al. ascribed the prepeak to the partial oxidation of linearly bonded CO triggered by the potential-induced site conversion of bridge-bonded CO to atop sites.8 It was found that the prepeak magnitude decreased as the quality of the Pt(111) surfaces increased63,65,66 and that the oxidation of adsorbed CO on stepped Pt(111) surfaces exhibited a larger prepeak than on Pt(111).7,67 Therefore, the prepeak appears to be indirectly related to steps (or defects) but does not directly correspond to the oxidation of CO adsorbed on step and nearest sites.24 It was also claimed that the adsorption potential strongly affects the amplitude of the prepeak, leading to a distinct signal upon adsorption at cathodic potentials (0 V vs RHE), while it decreases with increasing adsorption potential and is negligible for adsorption in the double-layer region.46,68 In alkaline solutions, three CO oxidation peaks are often observed on polycrystalline Pt, disordered Pt(111), and stepped single-crystal Pt surfaces. The Koper and Wieckowski groups assigned the prepeak, intermediate, and main peaks to the oxidation of CO adsorbed at kinks (step defects), steps, and terraces, respectively.34,35,52 So what is the origin of the prepeak? Is it related to the oxidation of CO on defect sites or weakly bonded CO oxidation or both? In this work, we propose a model that can explain the pre- and the main peaks for the cyclic voltammetric stripping COad on Pt surfaces.

Two different kinds of surface diffusion are often discussed. Chemical and tracer diffusion refer, respectively, to adsorbate migration in the presence and absence of a chemical potential gradient. Tracer diffusion describes the migration of individual ad-particles in an adlayer due to Brownian motion, and can take place at equilibrium. Chemical diffusion describes the migration of ad-particles due to a concentration gradient, resulting in a net mass transport. Therefore, when measuring COad’s surface diffusion coefficient, one needs to clearly define the conditions. Wieckowski and co-workers attempted to determine the surface diffusion coefficient of COad on platinum black using EC-NMR. On the basis of the assumption that surface diffusion was due to COad exchange between different surface sites, driven by the chemical potential gradient, they estimated that COad’s diffusion coefficient on platinum black was 3.6 × 10−13 cm2/s at a fractional coverage of 1.0 monolayers (ML) and 1.5 × 10−12 cm2/s at a fractional coverage of 0.36 ML.27,28 Friedrich et al. estimated a lower limit value for COad surface diffusion on Ru-modified Pt(111) to be ca. 4 × 10−14 cm2/s and suggested a surface diffusion coefficient in the range of 10−13− 10−12 cm2/s.29 Koper et al. used a mean field model to fit the current transients of COad oxidation on stepped Pt(111) surfaces and estimated a surface diffusion coefficient of >10−11 cm2/s for COad diffusion on stepped Pt single crystals.30 However, some groups have reported much lower surface diffusion coefficients. The Baltruschat and Wieckowski groups7,31 found two separate peaks for COad stripping from Ru-modified Pt(111), suggesting that COad diffusion on this surface is slow, and the Baltruschat group estimated the surface diffusion coefficient of COad on stepped Pt surfaces to be below 3 × 10−17 cm2/s.32 Maillard et al. found that COad surface diffusion on platinum nanoparticles depends on the particle size and estimated a COad surface diffusion coefficient of 10−17 cm2/ s on 2 nm Pt particles and 10−13 cm2/s on 3 nm Pt particles, respectively.16 Savinova et al. estimated that the upper limit of the COad surface diffusion coefficient at Pt nanoparticles is 4 × 10−15 cm2/s.33 The Koper group also reported that COad diffusion on platinum in alkaline solutions is much slower than in acid solutions and that COad’s mobility on rhodium in sulfuric acid solution is significantly hindered when compared to perchloric acid solution.24,34−38 FTIR data from Weaver et al. indicated that islands of COad were formed during its oxidation, suggesting a low mobility.39 Considering adsorbate− adsorbate lateral interactions, Zhdanov et al. indicated that CO ad electrooxidation might be accompanied by CO ad segregation, provided that surface COad diffusion is fast.40 Similarly, an anion-induced compression of the CO adlayers upon COad oxidation23,41,42 or the adlayer build-up upon CO adsorption43 has been reported. Another important issue is the COad oxidation prepeak, which may be related to surface defects and/or surface diffusion.19 It is well-known that the stripping voltammograms of adsorbed CO on polycrystalline and single-crystalline platinum surfaces are adsorption potential-dependent21,44−49 and also are different in acid and alkaline solutions.34,35,46,50−52 When CO is adsorbed in the hydrogen region in acidic solutions on polycrystalline Pt, and less perfect Pt single crystals, two oxidation peaks (or regions) appear: a small prepeak in the double-layer region and a large main peak at more positive potentials (sometimes the main peak splits into two or three due to anion effects, different crystallites, or different nanoparticle sizes).18,53−58 When CO is adsorbed in the double-layer region, the prepeak disappears. The effects of 11041



Article

electrochemical reactions and the lower one for mass spectrometric detection, which are connected through four capillaries. In the upper compartment, the working electrode was placed against a ∼100 μm thick Teflon gasket with an inner diameter of 6 mm. This leaves an exposed area of 0.28 cm2 and results in an electrolyte volume of ∼3 μL at the electrode surface. In the lower compartment, a porous Teflon membrane (Scimat) was supported on a stainless steel frit and served as the interface between the electrolyte and vacuum. It was pressed against a ∼100 μm thick Teflon gasket with an inner diameter of 12 mm. The Scimat Teflon membrane had a mean thickness of 60 μm, a mean pore size of 0.02 μm, and a porosity of 50%. Two Pt wires at the inlet and outlet of the thin-layer flow cell, which were connected through an external resistance (1 MΩ), were used as counter electrodes. A saturated calomel electrode (SCE) connected to the outlet of the DEMS cell through a Teflon capillary served as the reference electrode. All potentials, however, are quoted against that of the reversible hydrogen electrode (RHE). The electrolyte flow was driven by the hydrostatic pressure in a supply bottle (flow rate about 10 μL/s), which ensured a fast transport of species formed at the electrode to the mass spectrometric compartment, where the volatile products entered into the vacuum system of the mass spectrometer (time constant, ca. 1 s) through the porous Teflon membrane. The supporting electrolyte was prepared using Millipore Milli Q water (18.2 MΩ·cm) and ultrapure sulfuric acid (Merck, suprapur). CO (N4.7) was obtained from MesserGriesheim. Before measurements, all solutions were deaerated with high-purity Ar (MIT Gase, N6.0). All experiments were carried out at room temperature (23 ± 1 °C). Adsorption of CO was performed by injecting 2 mL of CO saturated 0.5 M H2SO4 solution into the DEMS cell for 2−5 min. The potential of the working electrode was kept at 0.06 V in the course of adsorption. After the formation of a saturated adlayer of CO, as indicated by the drop of the displacement current to zero, the solution with CO was exchanged with a CO-free solution.

From the above literature overview, it follows that cyclic voltametry and potential step chronoamperometry are the most often used techniques to study the kinetics and mechanism of COad oxidation. The rate of CO monolayer oxidation is affected by both potential and time, in potential step chronoamperometry, in which the potential is constant, only one parameter (time) changes, compared to two parameters (potential and time) in cyclic voltammetry, which results in a complex response. Thus, the investigation of CO electrooxidation under potentiostatic conditions is less complicated and offers information on the reaction kinetics and CO’s mobility on the metal surface. In most previous studies, however, relatively high final step potentials for potentiostatic COad oxidation were generally used to get a pronounced Faradaic current response, leading to a small kinetic time window, especially for the initial period/ stages of COad oxidation. Moreover, the double-layer charging current overlaps the Faradaic current of the initial COad electrooxidation, so the information on the initial oxidation of COad is difficult to analyze and particularly difficult to quantify. It is, thus, difficult to derive any reasonable model for describing the initial stages of COad oxidation, which predetermine the residual adlayer coverage and thus the subsequent electrooxidation kinetics. Therefore, obtaining detailed quantitative information regarding the earliest period of CO adlayer electrooxidation and the long time transient is still challenging. In the present work, following the approach used in refs 69 and 70, online differential electrochemical mass spectrometry (DEMS) in combination with a dual thin-layer flow cell71 were used to follow the process of oxidation of a CO monolayer on a Pt/Vulcan (E-TEK) catalyst and on polycrystalline Pt. DEMS can detect and quantify very small amounts of CO2 generated (in the range of 10−10 mol); hence, we can use a relatively low final step potential to obtain a long time window for COad electrooxidation at low rates to study lateral diffusion. More importantly is the fact that the mass spectrometric current for CO2 eliminates the interference of double-layer charging and electrode surface oxidation from the observed response, so that one can study the initial period and the following COad electrooxidation selectively and quantitatively without such interference. On the basis of the experimental observations, a kinetic model for COad electrooxidation is proposed, and kinetic Monte Carlo (KMC) simulations were performed to demonstrate the validity of the model.

3. RESULTS 3.1. Experimental Observations. The mass spectrometric current transients of CO2 generation during the oxidation of a saturated CO adlayer on Pt/Vulcan in 0.5 M H2SO4, after a potential step from 0.05 to 0.56 and 0.61 V, are shown in Figures 1a and 1b, respectively. It should be noted that the mass spectrometric current does not include any double-layer contribution and only corresponds to the oxidation of COad, which is essential for the kinetic analysis of the oxidation transients. Following the initial potential step, a spike is always observed at very short times. Subsequently, the ion current for CO2 formation decays slowly down to its minimum (Period I in Figures 1a and 1b). The time for reaching the minimum depends on the final step potential: the higher the final step potential, the shorter the time (see also Figure S1, Supporting Information). After reaching the minimum, the current starts to increase with time, passes through a maximum, and then decreases (Period II in Figures 1a and 1b). The current transients in Period II exhibit a symmetric shape for low final step potentials (0.56 and 0.61 V). Upon increasing the final step potential, both currents, that for the spike and that for the main peak, increase, while the time corresponding to the main peak maximum decreases. Moreover, for high final step potentials, the main peak of the current transients becomes

2. EXPERIMENTAL SECTION The DEMS setup consisted of two differentially pumped chambers, a Balzers QMS 112 quadrupole mass spectrometer, a Pine Instruments potentiostat, and a computerized data acquisition system. The thin film Pt/Vulcan (20 wt % metal, E-TEK Inc.) electrodes were prepared by pipetting and drying 20 μL of an aqueous catalyst suspension (2 mg/mL) and then 20 μL of an aqueous Nafion solution in the center of a mirrorpolished glassy carbon disk (Sigradur G from Hochtemperatur Werkstoffe GmbH, 9 mm in diameter). The resulting catalyst thin film had a diameter of ca. 6 mm, a geometric surface area of 0.28 cm2, and a Pt loading of 28 μg/cm2. The Pt particles had a mean diameter of 3.7 ± 1.0 nm. A dual thin-layer flow cell made of Kel-F was connected to the main chamber via an angle valve for DEMS experiments. The construction of this cell has been described in detail in ref 71. In this cell, there are two compartments: the upper one for 11042



Article

CV, suggesting that a potential value of around 0.61 V could divide the prepeak and the main peak regions. The spike and the plateau are found to increase by increasing the final step potential, whereas the plateau becomes ill-resolved due to the above-mentioned overlap of the initial spike and the main peak at high oxidation potentials. The logarithm of the spike and plateau currents versus the final step potential are plotted in Figures S3a and S3b (Supporting Information), being linear with slopes of 250 and 66 mV dec−1, respectively, allowing us to assess the Tafel plots for the COad oxidation in the initial stages, for the first time. With an increase in the final step potential, the main peak current increases, and the time corresponding to the main peak decreases. The logarithm of the main peak currents versus the final step potentials is plotted in Figure S3c (Supporting Information). The plot is linear, with a slope of 97 mV dec−1, which is quite similar to that obtained from the main peak in Faradaic current transients in refs 3−5, 30, and 33. To further highlight the potential-dependent changes of the current transients, the normalized mass spectrometric current transients of COad electrooxidation on Pt/Vulcan at different final step potentials are presented in Figure 2. With an increase

Figure 1. Mass spectrometric current transients of CO2 at m/z = 44 for the oxidation of a saturated CO adlayer on a Pt/Vulcan catalyst electrode in 0.5 M H2SO4 solution. The potential was stepped from 0.06 to 0.56 V (a) and 0.61 V (b), respectively. CO was adsorbed at 0.06 V (vs RHE) from a CO saturated 0.5 M H2SO4 solution.

asymmetric and overlaps with the initial spike (see Figure S1, Supporting Information). Following the potential steps, all current transients exhibited a spike at short times, a subsequent slow decay, and subsequently a bell-shaped maximum (Figure S1, Supporting Information), which could not be fully predicted by either the mean field model,2,3 nucleation and growth mechanism,4,5 or diffusion model (CO diffusion to active sites).10,16 About 5−7% of the total CO adlayer was oxidized in Period I for all studied final step potentials, and this value increased slightly with increasing the final step potential. In other words, COad oxidation in Period II can only take place after ca. 6% of a full monolayer of CO is oxidized. It appears that two different oxidation pathways are involved in the two periods, respectively. Other research groups have also found similar current transient curves;3,6,13,15 however, they mainly focused on the mechanism of the second period since they were not able to assess the COad oxidation rate in the initial spike due to a predominant pseudocapacitive response (see previous discussion). The two-oxidation periods are quite similar to and reminiscent of the prepeak and main peak for COad oxidation in potentiodynamic experiments (see Figure S2, Supporting Information). In the mass spectrometric cyclic voltammogram (MSCV) (Figure S2b, Supporting Information) of COad stripping on Pt/Vulcan, the prepeak and main peak can clearly be seen, albeit overlapped. However, only ca. 3% of the CO adlayer is oxidized in the prepeak, which is ca. half of that in the first period of the current transients in Figure 1. This indicates that a fraction of the adsorbed CO (ca. 3%), which is oxidized in the first period of the current transients, is oxidized in the main peak of the CV. In the first period of the ion current transients, the oxidation current can be deconvolved into two contributions: the spike and the plateau (the subsequent slow decay). (While for low oxidation potentials, they can be clearly distinguished, for high oxidation potentials, they merge and cannot be discerned.) The amount of adsorbed CO that is oxidized in the spike for the low final step potential of 0.61 V is close to that of the prepeak in the CV. Hence, the spike in the current transient for a final step potential of 0.61 V could be equivalent to the prepeak in the

Figure 2. Normalized mass spectrometric current transients of CO2 at m/z = 44 for the oxidation of a saturated CO adlayer on a Pt/Vulcan catalyst electrode in 0.5 M H2SO4 solution. The data are from Figures 1 and S1 (Supporting Information). The final potentials are indicated in the figure. The curve at 0.56 V is quite similar to that at 0.61 V (not shown).

in the final step potential, the normalized main peak becomes broader and changes from a symmetric bell shape to an asymmetric one with pronounced tailing on the descending part. Both the progressive nucleation and growth model and the simple L−H mechanism (mean field approximation) predict a symmetric bell shape of the main peak with a maximum at a (relative) coverage of Θ = 0.5. However, the main peak predicted by the mean field approximation is narrower and more slender than that predicted by the progressive nucleation and growth model due to fast COad surface diffusion and the resulting mixture between COad and OHad species.40 According to the instantaneous nucleation and growth model, the maximum rate of COad oxidation at constant potential occurs at a fractional coverage of 0.61 (relative to the COad saturation coverage) with a tail on the descending part.32 In practice, for low final step potentials (0.56−0.66 V) the main peak of COad oxidation occurs symmetrically (if we consider the coverage of 11043



Article

by a few percent relative to the saturation coverage. It seems that a small fraction of the adsorbed CO molecules, which might adsorb on low-coordination defect sites, is very active and can be rapidly oxidized in the spike. The spike, which is related to the oxidation of very active adsorbed CO molecules, could be due to the following processes. After the potential step from the adsorption potential (0.06 V vs RHE) to the higher oxidation potential, oxygen-containing ad-species prefer to form on low-coordination defect sites and force CO molecules adsorbed on those sites onto neighboring terrace sites. This is similar to UHV studies, in which oxygen adsorption displaces adsorbed CO from step sites to terrace sites.73,74 The increase in local COad coverage on terraces causes the adsorption energy to decrease due to COad−COad repulsion, i.e., resulting in weaker bonding of COad molecules. These weakly bonded CO molecules can be oxidized by neighboring adsorbed oxygencontaining species formed on low-coordination defect sites with fast kinetics due to a lowering of the activation energy. In the literature, some research groups assumed that at a saturation COad coverage both the weakly bonded and strongly bonded CO molecules are formed on platinum surfaces.61 Others assumed that at a saturation COad coverage only weakly bonded CO molecules are present due to COad−COad repulsion and that the partial oxidation of CO molecules results in the relaxation of the CO adlayer to form the strongly bonded CO molecules.62 We believe that weakly bonded CO molecules, which can be oxidized in the spike, occur only after the potential step to the higher oxidation potential, due to the displacement of CO molecules adsorbed on the lowcoordination defect sites to the neighboring terraces by oxygen-containing ad-species. The following experiments also support this viewpoint. In Figure 3, a multiple potential-step method was used to study the features of the initial spike after repetitive stepping, for a short time period to the oxidation potential and back. A spike was always observed after each potential step from 0.06 to 0.61 V for different CO coverages

COad that is oxidized in the main peak as 1, the maximum takes place at a coverage of ca. 0.5) and closer to that predicted by the mean field approximation rather than the progressive nucleation and growth model. This suggests that COad surface diffusion could be much faster, relative to COad oxidation on 3−4 nm Pt particles, at least for low oxidation potentials. For higher step potentials (0.71−0.76 V), the maximum is shifted to higher coverage. This is often interpreted by the instantaneous nucleation and growth model rather than the mean field approximation or progressive nucleation and growth models. However, in the Discussion section, we will propose that this asymmetric main peak could be caused by three overlapping processes: weakly bonded CO oxidation around defect sites, COad diffusion toward defect sites, and subsequent oxidation by neighboring oxygen-containing species adsorbed there and COad oxidation by neighboring oxygen-containing species on terraces. For comparison, the oxidation of COad on polycrystalline Pt (pc-Pt) in 0.5 M H2SO4 was also studied by DEMS under the conditions of potential step and potential scan, respectively. The results are shown in Figures S4 and S5 (Supporting Information). The profiles of the mass spectrometric current transients and mass spectrometric cyclic voltammogram for pcPt are qualitatively similar to those for Pt/Vulcan, except for (i) the relative amount of CO2 in the first and second oxidation periods (potential step) or in the prepeak and main peak (cyclic voltammety); (ii) a much shorter time window for the COad oxidation transients on pc-Pt at the same potentials, compared to those on Pt/Vulcan; (iii) the main peak in the potentiodynamic oxidation on pc-Pt which appears at a potential that is ca. 80 mV more negative when compared to COad stripping on Pt/Vulcan (particle size effect); and (iv) the ratio of the charge in the prepeak to the main peak in potentiodynamic COad stripping on pc-Pt which is 3 times larger, compared to that on Pt/Vulcan. After the potential step, all current transients also exhibited a spike at short times, a subsequent slow decay (plateau), and afterward, a main peak. About 8.5−13.5% of the total CO adlayer is oxidized in the first oxidation period (the spike and the plateau region). This value is larger than that for Pt/Vulcan and decreases with increasing oxidation potential from 0.56 to 0.66 V. In contrast, it slightly increases with increasing potential for Pt/Vulcan. The MSCV shows a prepeak (8.5% of the total CO adlayer, compared to 3% for Pt/Vulcan) and a main peak, which is consistent with previous results. After the main peak, a shoulder or small peak, which could arise from different crystalline faces, often occurs, and thus an asymmetric main peak is observed.55 Compared to the main peak, the mechanism of adsorbed CO oxidation at constant potential in the spike and plateau regions has seldom been studied due to the above-discussed overlap of the current response with capacitive contribution. The observation that the COad oxidation current monotonically decreases in the first period of the current transient cannot be appropriately explained within the framework of either the mean field approximation or nucleation and growth models. In terms of the L−H mechanism, the oxidation rate for a saturated CO adlayer is very low. Therefore, it appears that the E−R mechanism or noncompetitive L−H mechanism (i.e., OH and CO preferably adsorb at different kinds of sites) may be operative in the first period, as suggested by some authors.9,52 As found from the experimental data presented above and elsewhere,69,70 the COad oxidation current decreases dramatically after the spike, though the COad coverage decreases only

Figure 3. Mass spectrometric current transient of CO2 at m/z = 44 (b) for the oxidation of adsorbed CO on a Pt/Vulcan catalyst electrode in 0.5 M H2SO4 solution. Potential step procedures are shown in (a). CO was adsorbed at 0.06 V (vs RHE) from a CO saturated 0.5 M H2SO4 solution. 11044



Article

Petukhov discussed the effects of COad diffusion on the current transients of COad electrooxidation in the main peak.22 In the limit of negligible COad diffusion, the main peak is asymmetric and can be described by the Johnson−Mehl−Avrami− Kolmogorov equation (the instantaneous nucleation and growth model). In the limit of fast COad diffusion, the main peak would be symmetric and would approach the prediction of the main-field approximation (simple L−H mechanism). The reaction time is the shortest and independent of COad diffusion rate. In the medium region, where the COad hopping rate is comparable to the COad oxidation rate, molecular migration over the surface will significantly affect the reaction kinetics. 3.2. Kinetic Model of COad Oxidation. For complex reaction mechanisms involving reaction, diffusion of adsorbed species, and their interplay, an analytical solution of the kinetic equations is often difficult to obtain. Therefore, a full numerical “solution” of the kinetic equation is required. In this case, kinetic (or dynamic) Monte Carlo (KMC or DMC) simulations provide a suitable method. To better understand the mechanism and kinetics of COad electrooxidation at the molecular level, we performed KMC simulations using a home written code, specific to our model system. The Pt/Vulcan nanoparticle catalyst was represented by a Pt(111) surface with some random defects. Although the electrocatalytic activity among different sites on the heterogeneous particle surface is likely to be different, we used a simplified approach, in which, except for the activated defect sites, all other sites had the same reaction rate. As a further simplification, anion adsorption/ desorption was neglected. Although we believe that anion adsorption affects both the reaction rate and the COad diffusion rate, its effects can be partly included in the reaction rate and COad diffusion rate and thus should not significantly affect the qualitative features of COad oxidation. The simulated oxidation rate corresponds only to the formation of CO2 and does not include the current corresponding to the formation of oxygencontaining ad-species or the electrode surface oxidation. This is consistent with the mass spectrometric current of CO2 in the potential step chronoamperometric oxidation of COad. In our model, we have made the following assumptions, which are outlined in Scheme 1: (1) For simplicity, the Pt(111) surface is

and was not affected by the time period spent in the low potential limit (0.06 V), although its amplitude decreased with a diminution of the CO coverage. The spike is always present, even for submonolayer coverages of COad,70 which do not show the prepeak during CO stripping in cyclic voltammograms. Therefore, we believe that the spike originates from the oxidation of weakly bonded CO molecules formed by the displacement of CO adsorbed on low-coordination defect sites by adsorbed oxygen-containing species following the step to the higher oxidation potentials. Since a broad distribution of lowcoordination defects is present, the increase of the final step potential would affect the dynamics, as more low-coordination defect sites would be occupied by oxygen-containing ad-species, which would displace more COad molecules to terrace sites so that more weakly bonded CO molecules are formed. As a result, the spike current increases with an increase of the final step potential. After the spike in the current transient profiles, a slow decay (plateau) occurs before the main peak emerges. In the plateau region, the amount of oxygen-containing species does not increase, as indicated by the fact that the current gradually decays. This could be due to a high COad coverage and to COad−OHad repulsion at the high-coordination sites (e.g., terrace sites). The observation that after the spike the current increases immediately for low COad coverages supports this viewpoint. We believe that the plateau could originate from the diffusion of adsorbed CO molecules toward neighboring lowcoordination defect sites and their subsequent oxidation by oxygen-containing species adsorbed there. It should be noted that in the plateau region the oxidation kinetics could also depend on the COad coverage since COad−COad repulsion might still be present due to the high COad coverage, albeit such repulsion would be much smaller than in the spike. In other words, the oxidation rate decreases with a decrease in COad coverage in the plateau region due to an increase in the CO adsorption energy. The plateau current also increases with an increase in the final step potential since more oxygencontaining ad-species (nuclei) can be formed on lowcoordination defect sites, so that the kinetics of COad oxidation increase as well. If the plateau is related to COad diffusion and subsequent COad oxidation around defect sites, another question arises. In the plateau region, is the rate of COad oxidation controlled by reaction kinetics or diffusion of COad? Cuesta assumed that the plateau was due to slow COad diffusion.19 We will address these issues below. As the relative COad coverage decreases to a certain value (e.g., 0.94 for the Pt/Vulcan used here), the oxygen-containing species nucleation can start on the high-coordination sites. As a result, a main peak (the characteristics of a competitive L−H mechanism) appears. The shape of the main peak depends on the mobility of COad. The mobility of COad on metal surfaces has been discussed extensively in the past. Under vacuum conditions, COad molecules diffuse quite fast on Pt surfaces.75,76 However, under electrochemical conditions, COad-free sites are occupied by OH, H2O, or even bisulfate/sulfate anions. COad diffusion thus requires the displacement of OH, H2O, and/or bisulfate/sulfate. Therefore, in aqueous solution, the COad diffusion rate on Pt surfaces is likely to be significantly lowered.4,5,22,32,33 It should be noted that only individual molecules on a bare surface are very mobile, whereas mobility in a close packed adlayer is rather low. This leads, e.g., to poorly ordered adlayers upon adsorption at lower temperatures. The latter resembles the behavior under electrochemical conditions.

Scheme 1. Outline of CO Electrooxidation Model in Our KMC Simulations

fully covered by linearly bonded CO molecules (including weakly and strongly bonded CO) at the initial potential step, except for defect sites covered with oxygen-containing adspecies, (even though COad coverage is, in practice, ca. 0.68, and a small fraction of bridge-bonded and multiply bonded CO also exist).3 The real saturation coverage of COad does not affect the simulation results since a relative coverage, compared 11045



Article

containing species adsorbed on adjacent low-coordination defect sites is determined by the average reaction rate ki. Here, we just use an average reaction rate, regardless of the number of nearest-neighbor oxygen-containing species adsorbed on low-coordination defect sites. The probability of COad oxidation by adjacent oxygen-containing species adsorbed on high-coordination sites (e.g., terraces) is controlled by the reaction rate kj, which depends on the number of oxygencontaining ad-species sitting at nearest-neighbor sites. A simple linear dependence (kj = njk1) is assumed, where k1 is the reaction rate for the reaction between one COad and one nearest-neighbor oxygen-containing ad-species; clearly, nj ≤ 6 for a Pt(111) surface. The probability of COad hopping is determined by the effective hopping rate kd, which depends on the number of non-COad covered nearest-neighbor sites. A simple linear dependence (kd = ndk2) is again assumed, where k2 is the effective rate for COad hopping to one nearestneighbor site occupied with water, other oxygen-containing species, or sulfate/bisulfate through displacement; again, nd ≤ 6 for a Pt(111) surface. The algorithm of our simulations is as follows: (i) a random number q is generated, where q ∈ (0,1]. (ii) If 0 < q < pr, a reaction event is performed. The chosen COad reaction event occurs with a probability proportional to its rate coefficient, i.e., kr/kr,total (r = i or j, kr is the individual COad oxidation reaction rate, and kr,total = Σkr, which is the total sum of the COad oxidation reaction rates). The time step is obtained as: Δt = −log u/(kr,total + kd,total), where u ∈ (0,1] is a uniform random number, kr,total + kd,total = Σkr + Σkd, which is the total rate of all possible processes. (iii) If q ≥ pr, a diffusion event is executed. The chosen COad hopping event occurs with a probability proportional to its rate coefficient, i.e., kd/kd,total (kd is the individual COad hopping rate, and kd,total = Σkd, which is the total sum of the COad hopping rates). The time step is obtained as: Δt = −log u/(kr,total + kd,total), where u ∈ (0,1] is a uniform random number. (iv) Once Δt is selected and the chosen event is performed, the process list is updated for choosing an event in the next VTSMC step. The current is described in terms of the number of oxidized COad molecules per second. (Five VTSMC runs were averaged to obtain the current−time transients or cyclic voltammograms.) The KMC code was written with Python, and the simulation was run on a HP Pavilion with quad-core CPUs. Except for the reaction of weakly bonded CO with nearestneighbor oxygen-containing species adsorbed on low-coordination defect sites, all other reaction rates for potentiodynamic and potential step transient simulations were modeled using the Butler−Volmer equation as follows

to CO saturation coverage, can be used in simulations. Oxygencontaining ad-species (likely OHad) are also adsorbed on atop sites. (2) Weakly bonded CO molecules are formed due to the displacement of COad molecules by oxygen-containing adspecies on defect sites following the step to high potentials. The displaced COad molecules need empty neighboring sites and thus compress the surrounding CO adlayer, leading to the formation of weakly bonded CO molecules. With an increase in the final step potential, more weakly bonded CO molecules are formed since more oxygen-containing ad-species (nuclei) form on more low-coordination defect sites and displace more COad molecules to the neighboring high-coordination sites (e.g., terrace sites), assuming that a wide distribution of defect sites, and thus activities, is present. (3) COad molecules can diffuse on the Pt(111) surface through hopping. The COad hopping rate is independent of COad coverage and electrode potential. (4) In the initial period of COad oxidation, the weakly bonded CO molecules are oxidized by neighboring oxygen-containing species adsorbed on defect sites. (5) The formation of oxygencontaining ad-species on the high-coordination sites (e.g., terraces) starts at a given relative COad coverage (here we used 0.94 in agreement with the experimental results). At high COad coverages, the formation of oxygen-containing ad-species on high-coordination sites (e.g., terraces) can be suppressed due to repulsion between COad and oxygen-containing ad-species and the requirement of sufficiently large ensembles of free Pt sites for H2O dissociation. (6) The formation of oxygen-containing ad-species from the dissociation of adsorbed H2O and their desorption are sufficiently fast to be in equilibrium at all times, and the adsorption and desorption of oxygen-containing species follow the Frumkin isotherm [θOH/(1 − θOH) = k1/ k2·exp(−2θOH), where θOH is the coverage of OHad, k1 and k2 are the rate constants for OH formation and desorption, repectively]. (7) COad molecules can be oxidized only by nearest-neighbor oxygen-containing ad-species to form CO2 and free sites. (8) At relative COad coverages between 1.0 and 0.94, the reaction rate of strongly bonded CO oxidation decreases with decreasing COad coverage due to the increase in the CO adsorption energy (see eq 9).62 3.3. Kinetic Monte Carlo Simulations. The simulations were performed on a 40 × 40 or 60 × 60 Pt(111) lattice with periodic boundary conditions. To keep the coverage of COad constant for different potential steps, the low-coordination defect sites covered with adsorbed oxygen-containing species are not included in the lattice, but neighboring weakly bonded CO molecules are included in the lattice with a random spatial distribution. The oxidation of weakly bonded CO takes place through a noncompetitive L−H mechanism. After the oxidation of weakly bonded CO, the strongly bonded CO diffuses to the sites previously occupied by weakly bonded CO and can thus be oxidized by oxygen-containing species adsorbed on defect sites also through a noncompetitive L−H mechanism. To perform the simulations, we introduce a parameter pr, the relative probability of COad oxidation.

k1 = k10 exp[(1 − α)nF(E − E 0)/RT ]

(4)

(oxygen-containing ad-species formation) k 2 = k 2 0 exp[αnF(E 0 − E)/RT ]

(5)

(oxygen-containg ad-species desorption)

pr = k r,total /(k r,total + kd,total)

k 3 = k 30 exp[(1 − α)nF(E − E 0)/RT ]

where kr,total is the total sum of the COad oxidation rates and kd,total is the total sum of the COad hopping rates. The simulation used a variable time step Monte Carlo (VTSMC) to update the time and configurations at each MC step. In a VTSMC algorithm, a time step, Δt, is chosen so that exactly one event occurs during that time step. The events are taken at random. The probability of COad oxidation by oxygen-

(6)

(oxidation of strongly bonded CO by nearest-neighbor oxygencontaining species adsorbed on high-coordination sites such as terraces) where α = 0.5 is the transfer coefficient; k0 is the reaction rate at the thermodynamic potential (0.1 V); E0 is thermodynamic potential (0.1 V); E is the final step potential for potential step or the dynamic potential for potential scan 11046



Article

experiments, respectively; n = 1; R is the gas constant; and T is the absolute temperature. For simplification, we keep the average rate of weakly bonded CO oxidation by nearest-neighbor oxygen-containing species adsorbed on low-coordination defect sites (k5 = 0.3) constant assuming that the chemical reaction of weakly adsorbed species is less affected by the applied potential. Meanwhile, the amount of weakly bonded CO is exponentially increased with the increasing final step potential since the formation of oxygen-containing ad-species on defect sites, which displace adsorbed CO molecules, is exponentially increased with the increasing final step potential. The amount of weakly bonded CO molecules (N) can be described as follows N = N 0 exp[0.24nF(E − E 0)/RT ]

(7) −1

where 0.24 is calculated from the slope of 250 mV dec in Figure S3a (Supporting Information), and N0 is the amount of weakly bonded CO molecules at the thermodynamic potential of 0.1 V. The oxidation of strongly bonded CO by nearest-neighbor oxygen-containing species adsorbed on low-coordination defect sites can then be described by k4 = k4 0 exp[(1 − α)nF(E − E 0)/RT ]

Figure 4. Simulated mass spectrometric current transients of CO2 at m/z = 44 for the oxidation of a CO adlayer on Pt(111) with defects (0.03% weakly bonded CO at 0.1 V). The final potentials are indicated in the figure. The CO hopping rate: 0.1 s−1.

(8)

currents (I/Imax) are plotted vs normalized times (t/tmax) in Figure 5. These normalized current transients are, likewise, in

(for COad coverages ΘCO ≤ 0.94) or k4 = k4 0 exp[(1 − α)nF(E − E 0)/RT ] exp[50(ΘCO − 0.94)]

(9)

(for COad coverages ΘCO > 0.94, the latter exponential component is used to correct for the COad−COad repulsion, 50 is an empirical value, describing the extent of the repulsion effect). In Table 1 we summarize the parameters in our model. k10 and k20 are chosen to have large values, so that the formation Table 1. Values for the Model Parameters at the Thermodynamic Potential of 0.1 V model parameter k10 k20 k30 k40 k5

parameter value (s−1) 1.0 3.0 1.5 1.5 0.3

× × × ×

10−4 103 10−7 10−7

Figure 5. Normalized simulated mass spectrometric current transients of CO2 at m/z = 44 for the oxidation of adsorbed CO on Pt(111) with defects (0.03% weakly bonded CO at 0.1 V). The data are from Figure 4. The final step potentials are indicated in the figure.

and desorption of oxygen-containing ad-species are always at equilibrium. 3.3.1. Effect of Step Potential. Figure 4 presents some selected potential step transient simulation results. The simulated current transients reproduce the experimental results in Figure 1 and Figure S1 (Supporting Information) reasonably well. The simulated spike, the plateau, and the main peaks increase with the final step potential, while the time corresponding to the main peak decreases. The simulated currents at the spike, the plateau, and the main peak are semilogarithmically plotted vs the oxidation potential in Figure S6 (Supporting Information). The values of the slopes for the spike, the plateau, and the main peak are in excellent agreement with the experimental results, supporting the validity of the model despite the assumptions/simplifications made (Figure S3, Supporting Information). The normalized simulated

very good agreement with those observed experimentally (see Figure 2). With an increase in the final step potential, the main peak becomes broader and changes from a symmetric bellshape to an asymmetric one with pronounced tailing in the descending part. Thus, our proposed COad oxidation model appears to be a faithful representation of the experimental findings. As an example, snapshots of potential step transient simulations at 0.61 V are shown in Figure 6. The red dots represent strongly bonded CO (COad,s) on Pt(111); blue dots indicate oxygen-containing ad-species on Pt(111); yellow dots are free Pt(111) sites; and black dots and green dots denote weakly bonded CO (COad,w) and strongly bonded CO next to oxygen-containing species-covered low-coordination defect sites, respectively. In the spike, the weakly bonded CO is oxidized by nearest-neighbor oxygen-containing species ad11047



Article

Figure 7. Simulated mass spectrometric cyclic voltammograms of CO2 at m/z = 44 for the oxidation of a CO adlayer on Pt(111) with defects (0.03% weakly bonded CO at 0.1 V). Scan rate: 10 mV s−1 (black line) and 50 mV s−1 (read line). The CO hopping rate: 0.1 s−1.

electrooxidation changes from the simple L−H mechanism to the progressive nucleation and growth mechanism for low final step potentials. For low final step potentials, there are two overlapping processes in the main peak: COad diffusion toward active defect sites and subsequent oxidation, which leaves empty sites to accommodate new nuclei of oxygen-containing ad-species, and COad oxidation on terraces through the L−H mechanism. Since the former process has a relatively small contribution, it does not significantly affect the symmetry of the main peak. For the final step potential of 0.76 V, the spike and the main peak processes overlap, leading to an asymmetry of the main peak for all studied COad hopping rates. With a decrease in the diffusion rate of COad, the oxidation of COad at 0.76 V also takes longer; the magnitude of the main peak decreases; and the main peak as well as the normalized main peak broaden. The kinetics of COad electrooxidation also develops from the simple L−H mechanism to the progressive nucleation and growth mechanism for high step potentials with decreasing diffusion rates. An asymmetric main peak has often been assigned to the instantaneous nucleation and growth mechanism. However, our simulations suggest that an asymmetric main peak could be caused by the overlap of the spike and the main peak (which appears usually in commonly used chronoamperometric measurements with steps to high potentials) rather than a dominant instantaneous nucleation and growth mechanism. 3.3.3. Effects of Defect Density. It is generally accepted that the electrooxidation of COad on Pt surfaces is a structuresensitive process and that the presence of low-coordination defect sites can significantly enhance this process. The role of defects is often investigated through the use of stepped or deliberately perturbed single-crystalline surfaces, which serve as models for unveiling the dependence of the electrocatalytic activity toward COad electrooxidation on the step or defect density.30,80 Generally, with an increase in step or defect density, the main peak of COad oxidation on Pt surfaces is shifted toward lower potentials in potentiodynamic experiments or shorter times in potential step chronoamperometric experiments, respectively. Assuming that the density of defects does not affect the kinetics of COad electrooxidation or the COad diffusion rate on nondefect (terrace) Pt sites and only affects the amount of adsorbed oxygen-containing species on these defects, and thus

Figure 6. Snapshots for the oxidation of a CO adlayer on Pt(111) with defects (0.03% weakly bonded CO at 0.1 V) at 0.61 V. COad,s on terraces (red dots), COad,w next to defects (black), oxygen-containing ad-species (blue dots), free Pt(111) sites (yellow dots), and COad,s next to defects (green dots). The CO hopping rate: 0.1 s−1.

sorbed on low-coordination defect sites. In the plateau region, strongly bonded CO molecules diffuse to the sites next to oxygen-containing species-covered, low-coordination defects and then are oxidized there. Once the COad coverage is below 0.94 (this value varies for different Pt surfaces and changes slightly for different final step potentials), oxygen-containing ad-species can start to form on other sites (terraces), and thus the current increases again, reaches a maximum, and then decreases. To simulate the potentiodynamic COad oxidation, the total amount of weakly bonded CO (oxidized and nonoxidized) is supposed to increase with potential, similar to the case of the potential step, assuming a series of small potential steps are applied. The simulated mass spectrometric linear sweep voltammogram of CO2 for COad oxidation is presented in Figure 7. The simulated mass spectrometric voltammogram, which is qualitatively similar to that observed experimentally (Figure S2, Supporting Information), exhibits a preignition oxidation peak or shoulder in the low potential region and a main peak at relative high potentials. With an increase in the scan rate, the main peak is shifted toward higher potentials, and the magnitude of the main peak increases, in agreement with experimental data.77−79 3.3.2. Effect of COad Mobility. As mentioned previously, the mobility of COad plays a significant role in COad electrooxidation kinetics. The effect of the COad diffusion rate on the kinetics of COad electrooxidation is simulated and illustrated in Figure 8. For a final step potential of 0.61 V, with a decrease in the diffusion rate of COad, the oxidation of COad takes longer; the magnitude of the main peak decreases; and the normalized main peak becomes broad, suggesting that the kinetics of COad 11048



Article

Figure 8. Simulated mass spectrometric current transients of CO2 at m/z = 44 for the oxidation of a CO adlayer on Pt(111) with defects (0.03% weakly bonded CO at 0.1 V) at different CO hopping rates (s−1). The step potentials are 0.61 V (a) and 0.76 V (b), respectively. The insets show the normalized simulated main peaks of current transients. The CO hopping rate is indicated in the figures.

Figure 9. Simulated mass spectrometric current transients of CO2 at m/z = 44 for the oxidation of a CO adlayer on Pt(111) with different defect densities at the final step potentials of 0.61 V (a) and 0.76 V (b), respectively. The insets show the normalized simulated current transients. The CO hopping rate: 0.1 s−1. Different amounts of weakly bonded CO at 0.1 V are indicated in the figures.

phenomenon, an active site model has been proposed,13,16 in which adsorbed oxygen-containing species form only on defect sites and adsorbed CO molecules diffuse toward the defect sites and are subsequently oxidized by oxygen-containing species adsorbed on these sites. If one assumes that adsorbed oxygen-containing species are very hard to form on nondefect sites, so that the coverage of oxygen-containing species on nondefect sites is negligible, then our proposed model is equivalent to the active site model. This limiting case can be reached, as low final step potentials are applied. Both experimental and simulation results suggest that as the final step potential decreases the magnitude of the main peak diminishes more rapidly than the spike. For example, Figure 10 shows the KMC simulated mass spectrometric current transients for COad oxidation at 0.51 and 0.46 V. The complete oxidation of COad takes more than 8 h (at 0.51 V) and ca. 5 days (at 0.46 V), and the magnitude of the main peaks is very small due to the extremely low oxidation rate. If the final step potential is further decreased, almost no adsorbed oxygencontaining species are formed at nondefect sites, and thus only a spike and then a slow and very long decay are expected. However, this is experimentally inaccessible since over such a long time period and low signal intensity (low S/N) the

the amount of displaced CO molecules (i.e., the amount of weakly bonded CO), COad electrooxidation on Pt(111) with different amounts of weakly bonded CO (associated with different defect densities) was simulated and compared in Figure 9. With an increase in the defect density, and thus the amount of weakly bonded CO, the spike, the plateau, and the main peak currents increase, while the main peak is shifted toward shorter times. This can be explained by having more weakly bonded CO molecules formed and oxidized in the spike, followed by strongly bonded CO molecules, with a higher probability of being oxidized by oxygen-containing species adsorbed on defect sites, due to the presence of more defect sites and thus shorter diffusion distances. The effects of defects on the potentiodynamic electrooxidation of COad are also simulated and illustrated in Figure S7 (Supporting Information). The preignition peak or shoulder current increases, and the main peak is shifted toward lower potentials with an increase in the defect density. These simulated results are in qualitative agreement with reported experimental results.66,81 3.3.4. Active Site Model. Besides the oxidation potential, defect site density, and COad mobility, the shapes of the current transients are also observed to depend on particle size.13,15,57,82 The current transients for small Pt nanoparticles ( 0.01 s−1) the plateau interval for low final step potentials is not significantly affected by the diffusion coefficient. Thus, even for a COad diffusion coefficient of 10−17 cm2 s−1 (the reported low limit), in the plateau region, the COad oxidation reaction is controlled by the kinetics of the reaction between COad and oxygen-containing ad-species rather than by COad diffusion. As for the main peak, the mechanism of COad electrooxidation can change from a simple L−H mechanism to the progressive nucleation and growth mechanism with decreasing diffusion coefficient. At a diffusion coefficient of 10−17 cm2 s−1, the COad oxidation in the main peak could be close to the prediction of the mean field approximation for low final step potentials, while it holds true for high final step potentials when the diffusion coefficient is higher than 10−17 cm2 s−1. The diffusion coefficient of COad on Pt surfaces estimated by EC-NMR measurements is ca. 10−13−10−12 cm2 s−1,27,28 which would enable COad to be oxidized through a simple L−H mechanism. This is in agreement with the findings by Koper et al.30 From our experimental results, it follows that the relative amplitude ratio of the spike to the main peak for nanoparticles is larger than for the extended surfaces. This could be due to the fact that more edge and vertex atoms, relative to terrace surface atoms, which may act as defect sites, are present on small nanoparticles, and thus a relatively smaller amount of

5. SUMMARY AND CONCLUSIONS We have studied the potential step chronoamperometric and potentiodynamic behavior of COad electrooxidation at Pt/ Vulcan and polycrystalline Pt electrodes using DEMS to quantitatively monitor the CO2 formation rate. In this way, we could avoid the interference from both double-layer charging and electrode surface oxidation in the current response to the applied electrode potential. It was observed that the mass spectrometric current transients of COad electrooxidation on Pt surfaces exhibit an initial spike, a plateau, and then a main peak for low final step potentials. For high final step potentials, the spike and the main peak overlap, and thus the plateau disappears. The spike, plateau, and main peak currents all increase with increasing step potential, while the plateau interval time becomes shorter. The main peak can only be initiated after oxidation of a small fraction (ca. 6% for Pt/ Vulcan or ca. 10% for polycrystalline Pt) of adsorbed CO from the saturation coverage. The spike and plateau appear to represent the induction period for oxidation of the saturated CO adlayer. The shape of the main peak in current transients depends on the final step potential. For low reaction potentials, it exhibits a bell-shaped symmetry, while for higher reaction potentials it becomes increasingly asymmetric with a long tail on the trailing end. We propose a model to account for the observed behavior of COad electrooxidation. Like the prepeak in potentiodynamic COad stripping, the spike is associated with both the saturation coverage of the CO adlayer obtained upon adsorption at low 11051



Article

potentials (

New Insights into the Mechanism and Kinetics of Adsorbed CO

Recommend Documents