Electro-oxidation kinetics of adsorbed CO on platinum electrocatalysts

Chemical Engineering Science 64 (2009) 4765 -- 4771

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Electro-oxidation kinetics of adsorbed CO on platinum electrocatalysts Patrick McGrath a,b , Aurora Marie Fojas a,b , Jeffrey A. Reimer a,b , Elton J. Cairns a,b, ∗ a b

Department of Chemical Engineering, University of California Berkeley, Berkeley, CA 94720, USA Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

A R T I C L E

I N F O

Article history: Received 12 August 2008 Received in revised form 18 May 2009 Accepted 22 May 2009 Available online 21 June 2009 Keywords: Electrocatalysis Carbon monoxide Platinum Fuel cells Adsorption

A B S T R A C T

We describe the voltammetric measurement of the full oxidation of adsorbed CO on unsupported platinum electrocatalysts, with concomitant cyclic voltammetry of the hydrogen adsorption and desorption. The hydrogen region of platinum is used to parse the platinum surface into sites associated with weakly bound (WB) hydrogen and strongly bound (SB) hydrogen. By monitoring changes in the hydrogen region while following the two observed CO oxidation peaks, we are able to identify the WB sites as being the most active sites for COads electro-oxidation. The full oxidation peak is fitted to a model based on a modified Butler–Volmer equation that includes the two families of sites. Excellent agreement with experimental results is obtained, and the resulting fits yield the kinetic parameters for the two families of sites. When combined with coulometry, these kinetic analyses also show the importance of linearand bridged-COads species in the electro-oxidation process. Limitations of the model and the role of COads dynamics amongst the various surface sites are discussed. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction The electro-oxidation of carbon monoxide from platinum surfaces has long been studied as a benchmark for activity of these catalysts toward carbonaceous adsorbates. For fuel cells, this is important from both the standpoint of CO tolerant catalysts that are resistant to poisoning by impurities in the fuel stream, and for fuel cells that operate on liquid fuels, where CO is generally recognized as the primary intermediate in the electro-oxidation of carbonaceous fuels (Steele and Heinzel, 2001; Liu et al., 2006; Lamy et al., 2001; Arico et al., 2001; Beden and Lamy, 1998). Methanol electro-oxidation (Ross, 1991; Lu et al., 2000; Tripkovic et al., 2002; Seland et al., 2006) in particular has been studied extensively in an attempt to improve electrocatalysts for this application. However, a complete fundamental understanding of how COads behaves on platinum surfaces remains elusive. Often, voltammetric techniques are applied to study the characteristic CO oxidation from the surface in electrolyte with and without the presence of a bulk-CO source. This source can be from dissolved CO (g), an alcohol, or other carbonaceous fuel in the electrolyte. The characteristic size and shape of cyclic voltammetry (CV) peaks for platinum can be used as a fingerprint for surface sites and electrooxidative activity.

∗ Corresponding authorat: Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. Tel.: +1 510 486 5028. E-mail address: [email protected] (Elton J. Cairns). 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.05.053

The hydrogen-region characteristic current peaks, which result from H-adsorption (cathodic) and desorption (anodic) during low potential sweeps on platinum, are known from single crystal studies to be associated with the location or site of the hydrogen adsorbate (Markovic and Ross, 2002a; Love et al., 1986; Kita et al., 1990; Markovic et al., 1991, 1997a; Lopez-Cudero et al., 2003; Teliska et al., 2004). These peaks do not translate in a straightforward manner to nanoparticle platinum surfaces because the disordered nanoparticle electrodes are unlikely to be composed of extended planar faces. The general position and shape of the peaks, however, is attributed to a family of energetically similar sites on the platinum surface. For polycrystalline platinum, two major peaks are typically observed in the hydrogen desorption region, with a lower potential peak comprising hydrogen from disordered (1 1 0)-types of sites, and a higher potential peak associated with hydrogen from disordered (1 0 0)-like types of sites, with both peaks containing mixed contributions from (1 1 1)-types of sites. These peaks are referred to as weakly and strongly adsorbed hydrogen on platinum (Markovic et al., 1991, 1997a; Lopez-Cudero et al., 2003; Teliska et al., 2004). In our previous work (McGrath et al., 2007), we also grouped the sites on Pt nanoparticles into two categories, weakly bound (WB) and strongly bound (SB), associated with the low- and highpotential hydrogen desorption peaks, respectively, from the voltammogram. The WB and SB designation simply refers to the family of sites associated with Hads . In that work, we observed fundamental differences in the hydrogen region of the platinum voltammogram resulting from different preparations of COads on Pt/C. These surface layers were prepared from partial adsorptions or partial oxidations

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of adsorbates created from CO (g) bubbled through electrolyte, or electrochemically adsorbed CH3 OH. We found that the evolution of the hydrogen region resulting from various surface preparations is a result of the different activities of WB and SB sites toward CO (g) or CH3 OH adsorption and COads oxidation. This was in agreement with single crystal studies in the past that have reported trends in the ability of different surface sites to adsorb species from the bulk (Bittins-Cattaneo et al., 1988), and the activity of different surface sites toward COads oxidation (Markovic et al., 1997b, 1999a,b). In this work, we extend the voltammetric analysis to the behavior of the COads oxidation peak resulting from various surface preparations on Pt black. Because the complete oxidation of the CO intermediate is recognized as the slowest step in the mechanism, it is possible to prepare a surface layer from the accumulation of COads . Surfaces are prepared through partial adsorption or partial oxidation of COads from CO (g) or CH3 OH in 0.5 M H2 SO4 . Careful replacement of the CO or CH3 OH-containing electrolyte with clean electrolyte allows one to study the size and shape of the peak associated with the oxidation of this prepared surface layer, without obfuscation from the adsorption and oxidation of dissolved bulk reactants. We observe differences in the total COads oxidation current (which appears to be composed of two separate COads oxidation peaks) related to the preparation of the COads on the surface. A modified Butler–Volmer model is developed to probe the separate behavior of these two peaks resulting from different surface preparations. This analysis affords insight into the kinetic parameters governing the COads oxidation reactions. We parse the corresponding hydrogen region into WB and SB families of sites, as in our previous work (McGrath et al., 2007). Changes in WB- and SB-site occupation are correlated to the separate CO oxidation peaks. Limitations of this modeling scheme are discussed. 2. Experimental apparatus and procedures Cyclic voltammetry (CV) experiments were performed in a modified 50 mL 3-neck flask that allowed for liquid flow from a 1 L reservoir of clean, 0.5 M H2 SO4 (Certified ACS Plus Grade, Fisher Scientific with DI-water filtered in a Millipore Organ-X system, 18 M). The cell compartment and the reservoir were degassed with UHP Argon gas (Praxair). The working electrode consisted of a Pt-black layer bonded with Nafion to a 0.5 cm2 glass slide coated with a 1 m gold film. The catalysts used in this work are unsupported, fuel cell grade platinum black catalysts (Sigma-Aldrich). Inks were prepared using a mixture of Nafion-117 solution (Scientific Polymer Products, Inc., hydrogen ion form, 5% solids, 10% water, balance lower aliphatic alcohols) and the platinum black. Cleaning of the platinum surface was performed by potential cycling between 50 and 980 mV vs. RHE using a PAR Model 263A potentiostat. Our flow cell uses a platinum mesh counter-electrode, and a Hg/HgSO4 reference electrode (Koslow Scientific). Adlayers of CO were deposited on the electrode surface by bubbling CO gas (Praxair, 99%) through the electrolyte or by the injection of CH3 OH (Optima, Fisher Scientific) into the 0.5 M H2 SO4 electrolyte in the cell. In this study, we define our saturated or full COads layer as that resulting from a 1 h exposure to CO (g) or a 12 h exposure to 300 mM CH3 OH in 0.5 M H2 SO4 at a fixed potential. Both of these preparations resulted in a fully suppressed hydrogen region. After exposure to the COads source, either Ar was bubbled through the electrolyte for 1 h to remove bulk CO, or the electrolyte in the cell was flushed out with methanol-free 0.5 M H2 SO4 . Unless otherwise noted, all the data presented in this study derive from CO adlayers created under fixed potential adsorptions at 250 mV vs. RHE. The CV was always performed in clean electrolyte (i.e., all CO oxidation currents represent COads , and not CObulk or CH3 OHbulk ). For both the CO (g) and the CH3 OH preparations,

sub-saturation coverages were created in two ways: by partial adsorptions (short exposure times) to electrolyte containing the carbonaceous species of interest or by partial oxidations of the saturated layer (defined above) at 450 mV vs. RHE for varying lengths of time. The results from the CV traces for a given surface preparation were reproducible within about 2%. The electrode was subjected to voltammetry after the various pretreatments. The anodic traces of the voltammograms presented are corrected for double layer and sulfate anion adsorption effects by subtracting a coverage weighted anodic baseline current of the clean electrode from the covered trace (Rush et al., 2001; Rush, 1998). As in the previous studies, the effect of trace impurities on the voltammogram of the electrode in the absence of adsorbate was determined to be negligible. The surface coverage of the CO adlayer was determined by integrating the corrected anodic currents in the hydrogen region of the voltammogram. The distribution of WB- and SB-site occupation is estimated by dividing the hydrogen region of the first anodic sweep of the voltammogram into two regions: all current 50–200 mV is ascribed to WB sites, and all current from 200 to 400 mV to SB sites (McGrath et al., 2007). All experiments were carried out at ∼25 ◦ C. 3. Results Fig. 1 shows voltammograms (obtained at a sweep speed of 25 mV/s) for a clean Pt surface and for the same electrode after exposure to CO (g). The missing area in the hydrogen region (below 400 mV) for the CO-treated electrode is used to calculate the number of sites covered by COads . The total charge of COads oxidation is calculated from the area under the dashed curve (CO-covered) after correction for the double-layer charging current and background currents shown by the solid curve (clean surface). The WB- and SBassociated sites are represented by the lighter diagonal and darker grey shading, respectively. Fig. 2 shows the COads oxidation current for an adlayer prepared by open circuit adsorption of CO (g) (solid curve) and by fixed potential adsorption at 250 mV vs. RHE (dashed curve). Though not shown, both voltammograms exhibit completely suppressed behavior in the low potential hydrogen region, like that shown by the dashed curve in Fig. 1, indicating that both preparations of COads yield full coverage of the Pt surface. The integration of the COads current gives an estimate of the amount of COads through the number of electrons passed with the oxidation of the surface adsorbate. The oxidation of CO to CO2 is a two electron process. Dividing this by the number of available (freed-up) surface sites, calculated by the “missing” current from the corrected hydrogen region, gives the number of electrons

Fig. 1. Voltammograms for clean (solid) and COads covered (dashed) Pt surface. The shading in the low potential region of the clean voltammogram (less than 0.4 V) denotes the hydrogen region. The region shaded by diagonal lines represents the WB sites and the region shaded by grey represents the SB sites. The adlayer for the covered surface was created by a 45 min exposure to CO (g) at 250 mV. The sweep rate was 25 mV/s.

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Table 1 Calculated coverages and Neps values from CO oxidation peaks in Figs. 3–5. Surface preparation

Coverage

Neps

Total Ox. charge (mC/cm2 )

CO 5-min P. Ox. CO 20-min P. Ox. CO 60-min P. Ox. CO 120-min P. Ox. MeOH 50 mM P. Ad. MeOH 300 mM P. Ad. MeOH 300 mM Sat'd MeOH 20-min P. Ox. MeOH 60-min P. Ox.

1 0.99 0.74 0.46 0.62 0.77 0.94 0.81 0.57

1.89 1.69 1.46 1.14 1.34 1.42 1.50 1.37 1.30

0.411 0.361 0.239 0.124 0.180 0.233 0.319 0.224 0.167

Fig. 2. COads oxidation peak from a fully covered surface prepared from an open circuit (solid) and 250 mV adsorption (dashed). The inset shows the COads oxidation current for a 30 s open circuit adsorption.

Fig. 4. COads oxidation curve from a partially oxidized, CH3 OH derived adlayer. × is saturated with 300 mM CH3 OH,  is 20 min., and –is 60 min at 450 mV. Sweep rate = 25 mV/s.

Fig. 3. COads oxidation current after partial oxidation at 450 mV. × is 5 min,  is 20 min., −is 60 min., and 䊉 is 120 min. Sweep rate = 25 mV/s.

required for CO oxidation per platinum site (Neps ). The inset depicts the COads oxidation current after only 30 s of CO adsorption at open circuit. Fig. 3 shows COads oxidation curves from a series of partial oxidations of an adlayer of CO adsorbed at 250 mV. The top curve (exes) is the oxidation curve of COads after 5 min of oxidation at 450 mV, the second curve (circles) represents an adlayer after 20 min at 450 mV, the third curve (dashes) 60 min, and the bottom curve (diamonds) is the oxidation curve of COads after 120 min at 450 mV. Note that the partial oxidation results in no pre-ignition currents for all curves in Fig. 3. The coverage and Neps values for all curves are summarized in Table 1. Fig. 4 shows the oxidation curves produced by partially oxidized adlayers of CH3 OH-derived COads . Fig. 5 shows the COads oxidation curves of adlayers from time-limited adsorptions of CH3 OH from 300 and 50 mM CH3 OH solutions. The coverage and Neps data for these figures are also summarized in Table 1. 4. Discussion Trends in the surface coverage and Neps values can lead to valuable insight about surface species resulting from different preparations on platinum black. We recognize that a platinum surface saturated with COads does not correspond to a true monolayer

Fig. 5. COads oxidation curve from partial adsorptions of CH3 OH at 250 mV. × is saturated with 300 mM CH3 OH,  is 60 s adsorption with 300 mM CH3 OH, and +is 60 s adsorption with 50 mM CH3 OH. Sweep rate = 25 mV/s.

(Rush et al., 2001; Rush, 1998) equivalent to the amount of adsorbate that one can obtain from Hads in acidic media. With corrections for double layer effects and concurrent adsorption of anions from sulfuric acid, however, we can use our calculated values to study trends from the resulting surface layers. In this work, we refer to our highest surface coverage (resulting from the preparations described in the experimental section), as our saturated or full coverage.

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4.1. Qualitative assessment of oxidation peak structure from CO (g) Fig. 2 shows that qualitative differences arise in the oxidation curves resulting from different preparation methods using CO (g), even when these preparation methods yield similar Neps values and correspond to complete suppression of the hydrogen region. In Fig. 2, the solid curve (long open circuit adsorption of CO (g)) shows a single CO oxidation peak centered at 780 mV, but the dashed curve (250 mV vs. RHE adsorption of CO (g)) shows three distinct regions: a small “pre-ignition” peak (Breiter, 1968; Grambow and Bruckenstein, 1977; Markovic and Ross, 2002b,c; Maillard et al., 2004a; Lebedeva et al., 2002a; Jiang and Kucernak, 2002) beginning at approximately 400 mV, and two apparent COads oxidation peaks centered at 700 and 770 mV, respectively. (The deconvolution of the two peaks will be discussed later in this paper.) The two-part CO oxidation current has been observed previously, including studies of high surface area Pt/C electrodes (Rush et al., 2001; Maillard et al., 2004b). The absence of a pre-ignition peak in the solid curve may indicate that the open circuit adsorption of CO produces a crowded surface layer with limited access to surface OHads , preventing the relaxation of the surface layer through the partial oxidation of some of the surface species. The contrast of a single oxidation peak at higher potential (solid) compared to the two main lower potential oxidation peaks (dashed) in Fig. 2 also points to the higher resistance to oxidation for COads in the case of the full open circuit adsorption. Decreased access to OHads could explain the single peak at higher potentials for this case, as it is well known that access to OHads species is necessary for the complete oxidation of CO to CO2 (Lebedeva et al., 2002a,b; Gilman, 1964; Giorgi et al., 2001). Adsorption at 250 mV vs. RHE does not prevent this access to OHads , and enables the COads to be oxidized at lower potentials. The CV of a short (30 s) open circuit adsorption of CO (g), which produces a calculated surface coverage of COads below 40%, exhibits two-peak structure for COads oxidation, as seen in the inset of Fig. 2. We surmise that at low surface coverage there are sites available for OHads . This allows for a lower potential of COads oxidation, even for COads adsorbed at open circuit. Thus, the lack of evidence of a two-peak structure after the long open circuit adsorption in Fig. 2 is not a consequence of open circuit adsorption, but could instead reflect the accessibility of OHads for that preparation. Regardless of the method of adsorption, it appears that lowering the potential of COads oxidation requires the presence of OHads . 4.2. Quantitative assessment of COads oxidation peaks We have applied a modified form of the Butler–Volmer equation (Bard and Faulkner, 2001) to the COads oxidation curves in order to get a more complete and quantitative understanding of the factors that contribute to improved electro-oxidative activity of Pt towards COads . This approach is similar to a previous treatment of CO stripping (Koper et al., 2001). In the present approach, the differential equations described in Koper et al. (2001) were used as a starting point, with modifications for the lack of bulk CO (and thus, a negligible reverse reaction). They are i = 2FkOH NCO Exp[2f (E − Erev )]

(1)

dNCO i =− = −kOH NCO Exp[2f (E − Erev )] 2F dt

(2)

where i is the current density (A/cm2 ), F is Faradays constant (C/equivalent), k is the rate constant of the oxidation reaction (1/s.), OH is the fractional surface coverage of OHads , NCO is the surface concentration of COads (mol/cm2 ),  is the transfer coefficient, f is equal to F/RT, and E is the electrode potential (volts). In order to

Fig. 6. Representative fit of equation 3–60 min partial oxidation of a CO (g)-derived adlayer. Peak 1 is dashes, Peak 2 is circles, total is thin solid line, and data are the hollow diamonds. The inset shows the crossing point of the current associated with Peak 1 and Peak 2 at lower potentials. Sweep rate = 25 mV/s.

model the voltammetric currents shown above, a two-component system obeying these equations was solved for current as a function of potential. Each component has its own values of  and k, and x is the fraction of the current associated with k1 and 1 :    k1 Exp[21 fE] + 21 fE i = A xk1 Exp − 21 f   k2 +(1 − x)k2 Exp − Exp[22 fE] + 22 fE (3) 22 f Eq. (3) uses the following definitions:

=

dE dt

Erev = 0 A = 2FNCO, t=0 k = kOH

(4)

The final simplification shown in Eq. (4) is vital to the analytical solution of Eqs. (1) and (2). The combination of the rate constant k with OH implies that OH itself is a constant. This is not an ideal assumption, as the surface concentration of OHads is known to be a function of potential. However, full numerical solutions (of the form in Koper et al., 2001) that allow for OH (E) show that assuming OH to be constant over a small range of potentials is reasonable. Without this simplification, an analytical solution to Eq. (3) is not available. Voltammetric data were fit to Eq. (3) with x, i , and ki used as fitting parameters, and A is evaluated from the data (2F NCO,t=0 , the total charge available from surface COads , is given by the time integration of the CO oxidation current). Fig. 6 shows a representative fit for a 60-min partial oxidation of the CO-derived full COads layer. For the following discussion, “Peak 1” will refer to the peak whose maximum appears at a lower potential (dashes), and “Peak 2” will refer to the other peak whose maximum appears at higher potential (circles). The fitting parameters found for all surface preparations are summarized in Table 2. Using this approach, the individual behavior of the independent peaks resulting from various preparations can be monitored. Fig. 7a shows ln(k1 ) and ln(k2 ) plotted against Neps for the CVs presented above, and Fig. 7b shows 1 and 2 vs. Neps . In both plots of Fig. 7, squares represent the values from a CH3 OH-derived surface population, and circles represent CO-derived population. Hollow points are from Peak 1, and filled points are from Peak 2. The points for Neps = 2 in both figures come from a 5-min adsorption of

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Table 2 Fitting parameters for COads oxidation peaks. Surface preparation

A (mC/cm2 )

x

k1 (s−1 )

1

k2 (s−1 )

2

CO 5-min P. Ox. CO 20-min P. Ox. CO 60-min P. Ox. CO 120-min P. Ox. MeOH 50 mM P. Ad. MeOH 300 mM P. Ad. MeOH 300 mM Sat'd MeOH 20-min P. Ox. MeOH 60-min P. Ox.

0.41 0.36 0.24 0.12 0.18 0.23 0.32 0.22 0.17

0.37 0.42 0.32 0.08 0.45 0.49 0.53 0.43 N/A

6.2×10−14 3.7×10−11 4.4×10−10 1.6×10−9 3.9×10−10 6.9×10−10 1.7×10−10 3.8×10−10 N/A

0.56 0.44 0.41 0.40 0.42 0.40 0.41 0.40 N/A

1.2×10−10 2.0×10−9 2.7×10−8 1.4×10−9 3.5×10−7 1.6×10−7 9.1×10−9 2.6×10−7 N/A

0.39 0.34 0.29 0.34 0.25 0.27 0.31 0.25 N/A

monolayer encompassing all the sites available for Hads is not possible with COads (Gomez et al., 1998), but with correction for double layer effects and anion adsorption, the Neps value can give an indication of the distribution of linear- and bridged species on the platinum surface. For Neps values approaching 2e− /site , the surface should primarily be composed of linear-bound species. As Neps approach 1e− /site , bridge-bound species dominate the surface. Fig. 7 shows significant increases in both k1 and k2 as Neps decreases from close to 2e− /site towards 1e− /site . Each set of data spans 4 orders of magnitude, indicating an increase in reactivity for surface populations with greater concentrations of bridged COads . Such an effect could result from increased surface mobility with increasing percentages of bridged COads (lower activation barrier for motion) (Maillard et al., 2004b; Kobayashi et al., 2007), or from a lower activation energy for reaction of bridged COads as compared to linear COads (Alavi et al., 1998; Eichler, 2002). (This effect is not solely due to the distribution of linear- and bridged-species since we have assumed a constant OH , which is not exact.) 4.3. Correlation of oxidation peaks with hydrogen region voltammetry

Fig. 7. (a) ln(ki ) vs. Neps for various preparations.  is for CO-derived adlayers and  is for CH3 OH-derived adlayers. (b) i vs. Neps for various preparations.  is for CO-derived adlayers and  are for CH3 OH-derived adlayers. Hollow points correspond to Peak 1 and filled correspond to Peak 2.

CO gas that was not presented above (CV not shown). The data for the 120-min partial oxidation of a CO (gas)-derived CO (ads) and the 60-min partial oxidation of CH3 OH-derived COads are not included in the following analysis because the model failed to provide a satisfactory fit for the data. The omission of these data will be discussed later. The presence of bridged COads on Pt is well documented, but its exact role in COads oxidation (Maillard et al., 2004a; Lebedeva et al., 2002a; Jiang and Kucernak, 2002; Markovic and Ross, 2002c; Vidal et al., 2004; Heinen et al., 2007a; Kunimatsu et al., 2008) is unclear. By considering the trends in Neps values for the COads left after surface preparation, insight into the presence of linear- and bridged-species can be obtained. It was shown previously that a true

There have been a number of studies of COads oxidation on Pt electrocatalysts suggesting the presence of active sites on the surface of a Pt nanoparticle. According to the model proposed in Maillard et al. (2004b), COads oxidation occurs primarily at special sites, and that rates of oxidation are controlled by diffusion of COads to those sites. This model has been successful in capturing the behavior of COads oxidation in potential step experiments. This model, however, does not determine the nature of the active site. It was shown previously (McGrath et al., 2007) that WB sites appear to be more active towards the oxidation of COads , consistent with results from single crystal studies. We surmise that a quantitative analysis of the WB/SB sites in the hydrogen region should be linked to the 2component model of COads oxidation. There is clear evidence that the WB sites are (or at least contain) the active centers for COads oxidation, but, as will be seen below, a complete picture is not available from potential sweep experiments alone. The data presented herein provide a compelling case that Peak 1 and Peak 2 in the COads oxidation are correlated with SB and WB hydrogen sites, respectively, but without a better understanding of COads dynamics, this case is not conclusive. Initial examination of the fits of Eq. (3) to the measured oxidation currents may suggest that the lower potential Peak 1 should represent the more active WB sites that preferentially oxidize COads . However, a closer inspection of the fitted oxidation curves reveals that the current crosses over, as shown in the inset of Fig. 6. The maximum for Peak 1 occurs at a lower potential than the maximum for Peak 2, but the current from Peak 2 at E = 450 mV (where the partial oxidations in this study were performed) is larger than the current from Peak 1. For the 60-min partial oxidation of the CO-derived adlayer, the Peak 2 current at 450 mV is twice as large as the Peak 1 current, and the crossover occurs at approximately 550 mV. A similar

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Fig. 8. Charge from Peak 2 vs. missing hydrogen charge from WB sites. Circles are from CO-derived and squares are from CH3 OH-derived adlayer.

Fig. 9. Charge from Peak 1 vs. missing hydrogen charge from SB sites. Circles are from CO-derived and squares are from CH3 OH-derived adlayer.

crossover occurs for all fits, in both CO-derived and CH3 OH-derived adlayers. The crossing of Peak 1 and Peak 2 currents in this model is caused by the difference between the symmetry factors 1 and 2 . In Eq. (3),  has a dramatic effect on the shape of the oxidation curve: an increase in  lowers the potential of an oxidation peak, as well as narrows the peak. Because 2 is always smaller than 1 across all coverages (see Fig. 7), Peak 2 is broader and exhibits a higher current at low potentials, despite the occurrence of its maximum at potentials higher than that of Peak 1. For this reason, Peak 2 correlates with the more active WB sites, and Peak 1, exhibiting smaller currents at E = 450 mV, is then correlated with the less active SB sites. Applying this scheme, the trends in the separate behavior of COads on each type of site can be monitored. In Figs. 8 and 9, the ranges of “missing” hydrogen charge for WB and SB sites are plotted on the x-axis, with the total oxidation charge of COads for Peak 2 and Peak 1, respectively, are plotted. Since the “missing” hydrogen charge is due to CO blockage on that type of site, larger Q values correspond to higher CO coverage on that type of site. The black lines in the figures represent the ideal values for the ratio of QCO to Qmissing H corresponding to simple bridged- (Neps = 1) and linear- (Neps = 2) COads . The points represent the integrated areas of Peak 2 and Peak

1 for each data set, with error bars for the 95% confidence intervals of the fits, and 5% in Neps based upon reproducibility. In all cases, the trends indicate that a decrease in coverage on each type of site corresponds to the formation of more bridge-bound species. The amounts of the linear and the bridged vary separately for the two families of sites. Recent work by Kunimatsu et al. (2008) supports this , where they observed the evolution of linear- and bridged-species during oxidation of CO on platinum thin films using an in situ ATR-FTIR method. They explain shifts in absorbance data as a result of the amounts of species on the different types of sites. Our model has limitations. As noted above, the COads oxidation peaks for adlayers resulting from the long partial oxidations of COads do not follow the trends described above. For the 60-min oxidation of the CH3 OH-derived adlayer, Eq. (3) does not fit the COads oxidation peak well. A fit is achieved for the 120-min partial oxidation of the CO-derived adlayer, but the values of 2 and k2 fall far outside the trends shown in Fig. 7. Some explanation for these outlying data is required. The surface of an electrocatalyst is presumed to be a highly dynamic system: water and anions are continually adsorbing and desorbing, and COads adsorbates move rapidly over the surface, quite possibly through the interconversion of linear- and bridged COads (Markovic and Ross, 2002a; Vidal et al., 2004; Kunimatsu et al., 2008; Kobayashi et al., 2007; Desai and Neurock, 2003). The model described by Eq. (3) does not explicitly include any of these effects – the timescales of these phenomena may be too short to access by cyclic voltammetry – but they may nonetheless affect the outcome of the model indirectly. For example, during the partial oxidations at 450 mV vs. RHE, the relative concentrations of bridged- and linearCOads change. This is clear from the 5- and 20-min partial oxidations of the CO-derived adlayer: though there is minimal change in COads surface coverage (Pt sites blocked), the distribution of the surface population changed from 10% bridged to 30% bridged. As has been reported for COads on Pt/C catalysts, the linear/bridged interconversion clearly occurs during oxidation of COads (Rush et al., 2001; Rush, 1998). This interconversion can occur during the voltammetric sweep as well (Kunimatsu et al., 2008; Heinen et al., 2007b). In addition, movement of COads between WB and SB sites may occur. It was shown previously (McGrath et al., 2007) that disparities in the activity between WB and SB sites can create long-lived non-equilibrium distributions. We varied the sweep rate of the CV experiments to explore explicitly how dynamic processes affect the 2-component model. Fig. 10 shows the integrated Peak 1 and Peak 2 currents for the CV after the 20-min partial oxidation of a CO-derived adlayer for sweep rates of 10, 25, and 40 mV/s. The COads oxidation charges associated with Peak 1 and Peak 2 are affected by the sweep rate of the CV; with increasing sweep rate, more COads is oxidized in Peak 2 and less in Peak 1 (the total amount of COads oxidized remains unchanged). Fig. 10 shows that i and ln(ki ) vary with sweep rate, where both  and k exhibit a noticeable effect for Peak 2 with changing sweep rate, but show little effect for Peak 1. The values of 1 remain close to 0.40 for all sweep rates, but 2 changes significantly. Similarly, k1 is relatively stable with changing sweep rate, but the values of k2 vary over an order of magnitude. These experiments are not sufficient to draw conclusions about the mechanism of COads oxidation, but they suggest that the dynamics on the surface alter the oxidation behavior associated with Peak 2 significantly, but that the nature (though not the scale) of the oxidation from Peak 1 remains relatively unaffected. The dependence on sweep rate of the COads oxidation behavior presents a difficulty if the two components are to be quantitatively correlated with the WB and SB hydrogen sites. While there may be some reconstruction of the catalyst surface during electro-oxidation, the number of WB and SB sites on the Pt surface is presumed constant. If the individual contributions in Eq. (3) are each associated

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peak present a compelling case for a fundamental model of COads oxidation on Pt nanoparticles. Such a model, however, requires an understanding of the surface motion COads ; this is not forthcoming from the electrochemical data in this study, nor are other models of surface motion based on electrochemical data sufficient. An independent understanding of COads surface motion is required. Acknowledgements This material is based upon work supported by the US Army Research Laboratory and the US Army Research Office under contract/Grant no. 48713CH. References

Fig. 10. QCO (circles),  (squares) and ln k (diamonds) as a function of voltammetric sweep rate. These parameters are from fits for COads oxidation after a 20-min partial oxidation of a CO (g)-derived adlayer. Unfilled data points correspond to Peak 1 parameters, and filled data points correspond to Peak 2 parameters. Note: QCO and  values are on the left axis and ln k values are on the right axis.

with a specific kind of site, we might expect that the total charge for each contribution should also be a constant. Clearly the relative contributions of Peak 1 and Peak 2 do change as the timescale of the oxidation sweep changes, showing that the dynamics of the system are important. If the modeling of COads oxidation currents is to be used as a tool for assessing adsorption site(s), these dynamic effects must be accounted for. Such effects may be accessed via variable temperature studies, as well as concomitant spectroscopic analyses where dynamics can presumably be monitored directly. 5. Conclusions Electroanalytical modeling the COads oxidation peak has yielded some insight into the behavior of adsorbates on the platinum black surface. Previous work prompted us to parse the total oxidation current into contributions from two different families of Pt surface sites, with each family of sites resulting in an oxidation peak. These peaks correspond well with the behavior on the individual families of sites (WB and SB) that are labeled based on the potential at which hydrogen desorbs from them. By monitoring hydrogen coverage on these sites, and the total COads oxidation current, we probed the distribution of linear- and bridged-species on the surface. Using our model, we conclude that the behavior of COads depends strongly on the type of site it is on. The conversion from a linear- to a bridge-bound species may be a more important step in the oxidation mechanism than previously recognized. We are currently working to understand the dynamics of these different species on the surface where motion could play an important role in the interpretation of our results. This remaining question of dynamics is vital to the understanding of the system. Given the results presented in McGrath et al. (2007), any motion of COads on the Pt surface must allow for long-lived disparities in the occupation of COads between WB and SB hydrogen sites. This condition is not met by the idea of free diffusion of COads over the entire particle, which would lead to a random distribution of COads across all sites. Perhaps with an improved understanding of the motion of COads , the present observations could be reconciled with the active site model from Maillard et al. (2004b). The observed activity of WB sites towards COads oxidation and the 2-component structure to the COads voltammetric oxidation

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