Temperature-Dependent Kinetic Study of CO Desorption from Pt PEM

Feb 12, 2008 - J. C. Davies* andG. Tsotridis. Institute for Energy, Joint ... Jordan Anderson , Ajay Karakoti , Diego J. Díaz and Sudipta Seal. The J...
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J. Phys. Chem. C 2008, 112, 3392-3397

Temperature-Dependent Kinetic Study of CO Desorption from Pt PEM Fuel Cell Anodes J. C. Davies* and G. Tsotridis Institute for Energy, Joint Research Centre, European Commission, Postbus 2, 1755 ZG Petten, The Netherlands ReceiVed: October 18, 2007; In Final Form: December 14, 2007

The temperature dependence of the CO adsorption/desorption process on a commercial platinum proton exchange membrane fuel cell (PEMFC) anode has been investigated using isotopic exchange experiments. Kinetic data for a range of temperatures from 25 to 150 °C have been obtained. The results are discussed with relation to the poisoning effect of CO on the PEM fuel cell anode.

1. Introduction Proton exchange membrane fuel cells (PEMFC) are expected to achieve commercial application in a wide variety of areas including the automotive industry, because of having the inherent properties of having high current density, being lightweight, operating at low temperatures, and containing no corrosive materials.1 One of the major obstructions to market penetration is the cost of the materials, including the use of precious metals as the catalysts for both the anode and the cathode processes. Platinum is the catalyst of choice for both processes because of the high current densities obtained.2 At the anode, hydrogen oxidation occurs producing protons that then pass through a proton conducting membrane to the cathode. Here, the protons react with oxygen to form water. This process is spontaneous producing electricity and some heat.

H2 f 2H+ + 2e-

anode reaction

2H+ + 2e- + 1/2 O2 f H2O H2 + 1/2 O2 f H2O

(1)

cathode reaction (2) overall reaction

(3)

A possible source of the hydrogen for the anode reaction is from the reformation of hydrocarbons, by partial oxidation or steam reforming. This leads to hydrogen containing, among other impurities, significant levels of CO. CO levels can be reduced down to parts per million levels (by pressure swing adsorption or via the water gas shift reaction) but as little as 20 ppm CO is known to poison the platinum catalyst by blocking sites for hydrogen adsorption.3 The following processes occur at the fuel cell anode (where * represents an adsorption site):

H2 + 2 * T 2Had

(4)

CO + * T COad H2O + * T H2Oad

(5) (6)

* Corresponding author. Phone: +31 224 565462. Fax: +31 224 565625. E-mail: [email protected].

H2Oad + * f OHad + H+ + e-

(7)

OHad + COad f 2 * + CO2 + H+ + e-

(8)

Had f * + H+ + e-

(9)

The only processes occurring involving CO are the adsorption/ desorption process and the CO oxidation reaction. Therefore, it can be seen that the kinetically predominant of these two processes will govern the surface coverage of the species for a given steady-state condition. It has been demonstrated that combining platinum with ruthenium improves the CO tolerance of the catalyst.4,5 There are three possible promotion effects that have been identified: (i) Bifunctional mechanism: The presence of the ruthenium is known to reduce the overpotential for CO oxidation to CO2 by mediating oxygen adsorption through the dissociation of water. Extensive studies have been performed by Gasteiger et al. on PtRu bulk alloys6-10 and an ideal surface ratio of Pt/Ru of 50:50 has been determined. Voltammetric studies showed a maximum oxidation peak shift of -0.25V versus Pt and -0.15V versus Ru.7 (ii) Ligand effect mechanism: It has been demonstrated that adding Ru to Pt leads to a weakening of the Pt-C bond. It has been suggested that this weakening may lead to more facile oxidation of CO from the surface and a lower equilibrium coverage. However, studies have demonstrated this effect to be minor in comparison to the mediation of the oxygen adsorption with regards to the CO oxidation mechanism.11,12 (iii) “Detoxification” mechanism: As stated in ii, the presence of the ruthenium is known to change the electronic properties of platinum leading to changes in the bond strength and subsequently the coverage of the CO.13 It is suggested that this may play a significant role in increasing the CO tolerance of the catalyst, by modifying the equilibrium coverage simply via the adsorption/desorption process. While a considerable amount of research has been performed into the first two mechanisms, considerably fewer studies have been performed relating to the third. To understand more fully the true behavior of carbon monoxide on the carbon supported platinum or platinum/ ruthenium nanoparticles present at the PEM fuel cell anode, it is necessary to initially understand the fundamental behavior of CO adsorption. A significant amount of research has been

10.1021/jp710121w CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008

CO Desorption from Pt PEM Fuel Cell Anodes performed into CO adsorption on platinum single crystals under ultrahigh vacuum conditions; however, the relevance of such data toward the real catalytic conditions has been questioned, because of the so-called pressure and structure gaps. However, advances in techniques in the last 10 years have allowed for a variety of surface techniques to be performed, bridging the pressure gap between ultra-high vacuum (UHV) and realistic system pressures. High-pressure studies of CO on Pt(111) include high-pressure scanning tunneling microscopy (HP-STM) studies14 and studies of the vibrational spectra obtained at these high pressures.15 At 10-10 mbar pressure and -28 °C the c(4 × 2)-2CO overlayer corresponding to a coverage θ ) 0.5 is observed by STM. This structure has also been observed using low-energy electron diffraction (LEED) at room temperature.16 Increasing the partial pressure of CO in the gas phase over the range 10-6 - 760 Torr (1.33 × 10-6 mbar - 1 bar) causes a continuous variation of CO coverage over this range from 0.5 to 0.68 ML. A hexagonal compression occurs forming an incommensurate overlayer as the adsorbed CO atoms lose registry with the Pt surface atoms.14 This variation is however fully reversible. At 1 bar pressure, this corresponds to a structure with the (x19 × x19)R23.4°-13CO unit cell. Density functional theory (DFT) studies of CO adsorption on Pt(111) demonstrate that up to 0.5 coverage there is very little effect on the adsorption energy by lateral interactions and that the adsorption remains unactivated. A high coverage limit of 0.67 is proposed, above which the adsorption energies are strongly reduced and a substantial activation barrier toward adsorption occurs.17 Pressure-dependent vibrational spectroscopy studies using sum frequency generation demonstrate that CO bonds at atop and bridge sites from 10-10 to 10-1 Torr (1.33 × 10-10 to 1.33 × 10-1 mbar)15 and an on-top occupancy increase in raising the pressure of CO from 10-7-500 mbar compatible with that of the structure given.18 Hence, Vestergaard et al. draw the conclusion with regards to the pressure gap that, as far as this system is concerned, raising the pressure is equivalent to decreasing the temperature just so long as the thermodynamical equilibrium structure remains kinetically accessible.19 Recent studies on Pt and PtRu C-supported nanoparticulate catalysts have demonstrated that, under dry conditions at room temperature, a measurable rate of CO exchange occurs even at a concentration of 100 ppm CO in the gas stream. It has been postulated that this rate of exchange is high compared with the equivalent rate of CO oxidation measured under the operating conditions of the fuel cell.14,20 By comparison, Andersen et al. have measured the isotopic exchange rate for CO on Pt(111) at different partial pressures using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and have observed rates 5 orders of magnitude higher than for the supported nanoparticles.21 This difference is attributed to the fact that on the small facets of the nanoparticles there will be a quantization of the possible coverage values; hence, such large unit cells experienced on the single crystal will not be possible. A further study into CO desorption and adsorption rates has been performed by Lakshmanan et al. on polycrystalline platinum.22 They have used CO oxidation by cyclic voltammetry to take a “snapshot” of the catalyst surface after switching between CO/N2 and pure N2. However, they have assumed that the kinetic rate constants are entirely independent of the surface coverage (in sharp contrast to the studies mentioned above),

J. Phys. Chem. C, Vol. 112, No. 9, 2008 3393 and by the nature of the experiment, the rate constants cannot be considered to have been measured under steady-state conditions. The aim of this work is to investigate the effect of increasing temperature over a range that is of relevance to the operating PEM fuel cell on the CO exchange rate on commercial Pt/C catalysts. A temperature range of 25-150 °C has been used for these measurements, due to the drive by the research community to develop new membranes for the PEM fuel cell capable of operating at higher temperatures than the current industry standard, the Nafion membrane, which is limited by its need for humidification by water to less than 100 °C. The aim was to investigate the change in the CO exchange rate with temperature on a real industrial catalyst and to determine fundamental data for this system which has not to date been obtained. In particular, the work was carried out in relation to a European Commission co-funded project, FCAnode (Nonnoble Catalysts for Proton Exchange Membrane Fuel Cell Anodes). This project aims to design and develop novel catalyst materials and to assess them for a variety of potential parameters relevant to their use in PEM fuel cells. It was considered from the onset, that the issue of CO tolerance should be considered both from the point of view of the electrochemical CO oxidation and with regard to the equilibrium attained through the adsorption/desorption process. It has been postulated that for different catalysts (or even for the same catalyst under different conditions) both of these processes may have a significant influence. It was, with relation to this project, considered necessary to supplement the literature data regarding the more traditional Pt catalysts, especially with regard to the effect of temperature. It is recognized that, to understand the working of the fully operational cell, the adsorption/desorption process is one parameter of many. Therefore, the results will be discussed in terms of their relevance relating to the other processes (notably in terms of the competitive adsorption of CO, H2, and H2O, the influence of the structure of the nanoparticulate catalysts, and the influence of the cell potential). 2. Experimental Methods Experiments have been performed using a simple flow cell system based on a conflat design. The experimental setup and a cross-sectional view of the cell are provided in Figure 1. The samples used were commercial Pt catalysts (Electrochem Inc.) supported on Vulcan carbon and bound to Toray carbon paper gas diffusion layers with a Teflon binder (in order to ensure the ability to perform adsorption experiments to higher temperatures than would be possible with a Nafion binder). The catalyst loading was 1 mg cm-2 Pt (20 wt %). The average catalyst particle size is approximately 5-6 nm (as obtained from the Debye-Scherrer broadening of X-ray diffraction data obtained from the samples). Catalyst samples were circular with a radius of 1.8 cm. A sample of the exhaust gas from the cell was extracted via a quartz capillary, and the composition was measured in “real time” using a quadrupole mass spectrometer. A pressure of 1 bar in the exhaust tube leads to a pressure of 7 × 10-6 mbar in the mass spectrometer. Gases used were always of the highest commercially available purities. Data was obtained for a 1000 ppm CO in argon mixture in order to obtain an adequate signal-to-noise ratio for the change in the mass 29 peak. 13CO was “dosed” onto the surface using a 1% 13CO in Ar mixture where the CO has been enriched to 99% 13CO/1% 12CO. This technique was used, as opposed to more traditional steady state isotopic transient kinetic analysis

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Figure 1. (a) Schematic of the experimental setup used for the flow experiments. (b) Exploded cross section of the cell design.

(SSITKA), where it is more typical to swap between two identical concentrations, as with a concentration of 1% 13CO the time required to return to the “initial state” of total 13CO coverage is accelerated, reducing the required time for each data point. This technique has been used successfully before14 as the steady-state equilibrium is approached very quickly on raising the background pressure from 0 to 1000 ppm CO. The nature of the performed experiments was as follows. Initially, isotopically labelled CO was “dosed” on the platinum catalysts surface at a given temperature. This was done using 1% 13CO in Ar (at this CO concentration any exchange with pre-existing 12CO on the surface occurs at a high rate, even at room temperature14 to ensure that 13CO covers the surface). After 30 min, the labeled CO was replaced with a stream of argon. After 20 min, the Ar flow was replaced with natural CO at 1000 ppm in argon, and the desorption profile for 13CO was obtained. This procedure was repeated for a range of temperatures (25, 50, 75, 100, 125, and 150 °C) and flow rates (30, 60, 90, and 120 mL/min). 3. Results and Discussion Exchange experiments were first performed with the empty cell to determine the natural delay of the system when switching gases. It is necessary to demonstrate that there is not a significant increase in the pressure due to the resistance of the cell/sample, as the exchange rate has been previously shown to be dependent on the pressure of CO.14,20 The effect of flow on the onset of the 12CO peak was measured for the utilized concentration of 1000 ppm CO in argon. This was performed for a range of different flow rates from 10 to 120 mL/min for the empty cell and for the cell containing the gas diffusion layer (Figure 2a) without any catalyst. Furthermore, the change in the pressure in a gas manifold just prior to the cell (back pressure) with the change in flow rate was investigated in both cases. Figure 2b displays a plot of the pressure measured in the gas manifold just prior to the reaction cell versus the flow using the 1000 ppm CO in Ar gas. This demonstrates an increase of about 0.13 mbar for each 1 mL/min increase in the flow for the empty cell and a marginally larger increase when the gas diffusion layer has been added to the setup. This effect was deemed to be negligible on the experiment. The exchange of adsorbed isotopically labeled CO with the unlabeled CO in the gas phase was then investigated at a range

Figure 2. (a) Onset of the mass 28 step (12CO) when the gas was switched from argon to 1000 ppm of CO in argon at different flow rates, blank response. (b) Pressure increase over atmospheric for the system versus the flow rate.

of different temperatures from 25 to 150 °C and for different flow rates (30-120 mL/min). An example of the raw data obtained at 25 °C with a flow rate of 90 mL/min is given in Figure 3. This shows the level of isotopically labeled 13CO (mass 29) and the unlabeled 12CO (mass 28) on switching to the unlabeled gas. The desorption profile of the mass 29 provides the kinetics of the desorption process as follows: As outlined in section 1, of the reactions occurring at the fuel cell anode, only reaction

CO Desorption from Pt PEM Fuel Cell Anodes

J. Phys. Chem. C, Vol. 112, No. 9, 2008 3395

Figure 3. Raw exchange data at 25 °C and 90 mL/min for 1000 ppm CO in argon.

5, which determines the steady-state coverage of CO, and reaction 8, the oxidative removal of CO, involve carbon monoxide. Therefore, at steady state in an operating fuel cell the following holds:

Figure 4. Mass spectrometer trace for mass 29 (13CO) on switching to 1000 ppm CO in argon, for different temperatures at 90 mL/min (raw data). Some sets of data have been omitted for visual clarity.

dθCO ) k+ pCO(1 - θCO) - k-θCO - krθCO ) 0 (10) dt where θCO is the total CO coverage, pCO is the gas-phase partial pressure of CO, k+ is the adsorption rate coefficient, k- is the desorption rate coefficient, and kr is the coefficient for the rate of oxidation. In our experiment, however, we are only concerned with the adsorption/desorption equilibrium for CO; however, we must include now the two isotopic species. For our system, we have the following kinetic regime:

CO + * T 12COad

(11)

CO + * T COad

(12)

12

13

13

Figure 5. Normalized coverage value obtained from the 13CO peak areas for exchange at 1000 ppm CO in Ar at a flow rate of 90 mL/ min.

From this, the following kinetic expressions can be derived for the two CO isotopes:

dθ12CO ) k + p12COθ* - k- θ12CO dt

(13)

dθ13CO ) k + p13COθ* - k- θ13CO dt

(14)

θ* ) 1 - θ13CO- θ12CO

(15)

We can assume that, under steady-state conditions, θ*, the number of free sites is constant for a given temperature and partial pressure of CO. It is also presumed that changing the isotope has no effect on either rate constant (i.e., ignoring any slight differences due to the isotope effect). Figure 4 shows the desorption data (mass 29) for a range of temperatures up to 150 °C performed at 90 mL/min. Some data has been omitted for visual clarity. There is a visible increase in the kinetics of the desorption profile across this temperature range, and there is also a decrease in the area under the exchange curve (which gives the relative coverage of CO exchanging, assuming that at an infinite time, all CO will have exchanged). The relative coverage against temperature for the data obtained at a flow of 90 mL/min is given in Figure 5. This is obtained from the area under the 13CO desorption curves. All data is

Figure 6. Relative peak area (normalized to the maximum peak coverage) versus the inverse flow-rate for all sets of data.

normalized against a value of 1 for the data obtained at 25 °C, thus giving a relative, rather than an absolute, coverage. The decrease with increasing temperature is naturally to be expected, and the order of magnitude is not unreasonable when compared with typical temperature programmed desorption data for CO from Pt (e.g., compare to CO TPD on Pt(111) in ref 16). Figure 6 shows a plot of the peak area obtained for each experiment, against 1/q (where q is the flow rate) normalized to the maximum peak area obtained (at 30 mL/min flow and 25 °C). For a given temperature, each set of experiments should

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Davies and Tsotridis

Figure 7. ln (coverage) of 13CO versus time for the exchange experiment performed at 50 °C, demonstrating flow-rate dependence.

provide a straight line passing through zero. The best fit lines have been obtained from the given sets of data. It can be seen that while in general the correct trend is followed there is a slight deviation from linear behavior particularly at the lower temperatures. In addition, it can be shown that the lines will intercept the y axis at slightly negative values. Discrepancies most likely arise from the background approximation and the fact that the areas are calculated numerically assuming zero after a specified time of 10000 s. In refs 14 and 20, it was observed that under the conditions used (room temperature; small sample size) there was insignificant readsorption; hence, eq 14 could be simplified to:

Figure 8. Plot of 1/kapp versus the inverse flow-rate. Data at room temperature has been omitted as it is of a different order of magnitude.

Therefore, the intial gradient of the semilogarithmic plot given can now be defined as an apparent desorption rate constant kapp where:

kapp )

(16)

and therefore: -t

θ13CO ) θ130 CO e-k

(17)

Therefore, the desorption rate constant (k-) could be measured directly from the gradient of a plot of ln(θ) against t. In the current experiments, little dependence on flow was observed at room temperature above 30 mL/min (not shown). However, it is clear that on increasing the temperature even to 50 °C then there is a strong flow dependence of the measured desorption rate (Figure 7). From eq 14, it can be seen that this will occur if there is a significant readsorption of isotopically labeled CO, that is, if the desorbing species reaches significantly high concentrations above the surface of the sample. This will lead to an apparent rate of desorption which is lower than the actual rate. By combining eqs 13 and 14, assuming pseudo-steady-state conditions and then using an analogous method to that used by Xu et al.,23 we find that the decay curve for the 13CO concentration, C, can be given by:

C)

θCOk-

θCOk q+ C0

[

exp -

kt θCOk1+ qC0

]

(18)

where q represents the flow rate and C0 denotes the concentration of 13CO at time zero (note: in this experiment, we begin from zero and only achieve 1000 ppm after the first or second data point; this can clearly be seen from the data given in Figure 7, as the linear region only begins after the first or second data point).

(19)

Rearranging gives:

1 app

k dθ13CO ) - k- θ13CO dt

kθCOk1+ qC0

)

θCO 1 1 + C0 q k

(20)

meaning that a plot of 1/kapp versus 1/q should provide a linear response, with an intercept of 1/k-, the unidirectional desorption rate constant (Figure 8). Realistically, the unidirectional rate constant is already too fast to measure accurately for a concentration of 1000 ppm by the temperature of 100 °C, although values of 4.0 × 10-4 s-1 at 25 °C, 2.5 × 10-2 s-1 at 50 °C, and 1.1 × 10-1 s-1 at 75 °C can be extrapolated from the current data. In general, the obtaining of kinetic data regarding the CO adsorption/desorption process has not been widely considered a priority, as it has previously been considered that the free sites available for hydrogen oxidation were solely limited by the electrooxidation rate. Therefore, in order to determine CO tolerance traditional techniques have involved polarization curves, voltammetry, rotating disk experiments and spectroscopies such as IR and NMR. However, recent studies have demonstrated that there may be a stronger dependence on the desorption rate than previously considered.14,20 It was therefore considered necessary to supplement the existing kinetic data in the literature for the adsorption/desorption of CO from existing standard industry catalysts, in order to investigate the temperature dependence of the CO exchange rate. Previous work has shown a linear dependence of ln(k) with ln(pCO) at room temperature on both Pt and PtRu with a diluent gas of argon.14,20 No significant change from Pt to PtRu was observed in this case as the CO states will be filled to the same desorption energy for both catalysts. However, with a diluent gas of hydrogen a deviation from this linear behavior was observed at low concentrations of CO, indicating a lower binding energy for CO and increased competition for sites.20 This deviation was greater for PtRu than for Pt. Some predictions have also been made for rate constants at higher temperatures, extrapolating from room-temperature data, via the Arrhenius expression, and using a pre-factor value in the range 1013-

CO Desorption from Pt PEM Fuel Cell Anodes 1017 s-1; however, to our knowledge, this is the first time this data has been measured directly. As stated previously, one additional motivation of this work has been to set a benchmark for upcoming studies on novel catalysts. From the data above, it can be seen that, as an upper limit within this benchmarking, a combination of 1000 ppm CO in argon and 75 °C can be utilized, within the limitations of our setup. At lower concentrations, it should naturally be possible to obtain future data to higher temperatures, within the project concentrations of 100-1000 ppm, which shall be used with both argon and hydrogen as diluent gases for comparison of desorption rates to CO oxidation rate for different catalysts. In addition, attempts will be made to simulate more closely the real environment within the fuel cell by including humidification and potential. Lakshmanan et al. have recently attempted to measure adsorption/desorption rates within a PEM half cell.22 Their experiment consisted of “pre-dosing” of the electrode with a flow of 476 ppm CO in N2, then replacing the flow with pure nitrogen, and after a given time, performing a voltammetric stripping experiment to determine the coverage. Then the data was fitted on the basis of a prepared model for adsorption/ desorption. While this data provides some interesting supplementary information, there are a few drawbacks in the original model. First, they suggest that the kinetic rate constants are independent of the surface coverage; second, they suggest that the rate of CO adsorption/desorption is not influenced by the presence of hydrogen. These statements seem in strong contradiction to the SSITKA measurements performed under dry conditions. Finally, by the nature of the experiment, it cannot be considered to be undertaken at steady state, as the CO stream is stopped prior to measurement. It can be shown that the measured rate of desorption in the current experiment is high compared with the rates of oxidation measured from polarization curves obtained with high concentrations of CO in Ar (for example, in ref 9). The rate constant obtained above at 50 °C and 1000 ppm CO of 2.5 × 10-2 s-1 can be converted to an equivalent CO oxidation current density of 120 µA cm-2 (assuming the number of Pt sites per cm2 is 1.50 × 1015 which is the case for close-packed Pt(111) and that 2 electrons are oxidized per CO molecule). This is far higher than the current densities observed for carbon monoxide electro-oxidation on Pt and PtRu in ref 9 for 2% CO in argon at 62 °C even at high overpotentials (>0.6 V for Pt and >0.4 V for PtRu) which would imply that in the fuel cell environment it is the adsorption/desorption equilibrium that has the greater influence on the equilibrium CO coverage. However, care must be taken when drawing conclusions from this data to compare “like-with-like”, and it must be stressed that the current experiments do not contain humidification or a potential field and therefore provide a simplified adsorption environment. Therefore, future studies will attempt to progress from the current simplified adsorption system to more accurately represent the real fuel cell environment. 4. Conclusions Experiments have been performed to determine for the first time the exchange rate of CO on commercial Pt fuel cell catalysts for 1000 ppm CO in Ar for a range of temperatures from 25 to 150 °C. Unidirectional rate constants have been obtained for the lower temperature data, while it has been determined that the rate constants for the desorption process are too large to be measured at these concentrations by the current method above approximately 75 °C, where the measured

J. Phys. Chem. C, Vol. 112, No. 9, 2008 3397 rate will depend on a combination of the temperature, concentration and flow. The rates appear to be significantly higher than previously published CO oxidation rates obtained at similar concentrations and temperatures. These studies provide a foundation for benchmarking novel catalyst materials in future studies, which will be performed at concentrations of (100-1000 ppm) in argon and in hydrogen over the extended temperature range utilized in these measurements. Acknowledgment. This work has been carried out within the multi-annual programme of the European Commission’s Joint Research Centre, under the auspices of the FCTEST/ FCPOINT activities and with relation to the Framework Program 6 (FP6), Specific Targeted Research Project (STREP) “FCAnode” (Non-noble Catalysts for Proton Exchange Membrane Fuel Cell Anodes). The authors would like to thank Roberto Bove and Marc Steen for critical reading of the manuscript. This document does not represent the point of view of the European Commission. Any interpretations or opinions contained in this document are solely those of the authors. References and Notes (1) Hoogers, G.; Thompsett, D. CATTECH 1999, 3, 106. (2) Vogel, W.; Lundquist, J.; Ross, P.; Stonehart, P. Electrochim. Acta 1975, 20, 79. (3) Igarashi, H.; Fujino, T.; Watanabe, M. J. Electroanal. Chem. 1995, 391, 119. (4) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (5) Motoo, S.; Watanabe, M. J. Electroanal. Chem. 1979, 98, 203. (6) Gasteiger, H. A.; Ross, P. N.; Cairns, E. J. Surf. Sci. 1993, 293, 67. (7) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (8) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N.; Cairns, E. J. Electrochim. Acta 1994, 39, 1825. (9) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 8290. (10) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 16757. (11) Davies, J. C.; Hayden, B. E.; Pegg, D. J. Surf. Sci. 2000, 467, 118. (12) Lu, C.; Rice, C.; Masel, R. I.; Babu, P. K.; Waszczuk, P.; Kim, H. S.; Oldfield, E.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 9581. (13) Igarashi, H.; Fujino, T.; Zhu, Y. M.; Uchida, H.; Watanabe, M. Phys. Chem. Chem. Phys. 2001, 3, 306. (14) Davies, J. C.; Nielsen, R. M.; Thomsen, L. B.; Chorkendorff, I.; Logadottir, A.; Lodziana, Z.; Norskov, J. K.; Li, W. X.; Hammer, B.; Longwitz, S. R.; Schnadt, J.; Vestergaard, E. K.; Vang, R. T.; Besenbacher, F. Fuel Cells 2004, 4, 1. (15) Su, X.; Cremer, P. S.; Shen, Y. R.; Somorjai, G. A. Phys. ReV. Lett. 1996, 77, 3858. (16) Ertl, G.; Neumann, M.; Streit, K. M. Surf. Sci. 1977, 64, 393. (17) Steckel, J. A.; Eichler, A.; Hafner, J. Phys. ReV. B. Condens. Matter 2003, 68, 854161. (18) Rupprechter, G.; Delwig, T.; Unterhalt, H.; Freund, H.-J. J. Phys. Chem. B 2001, 105, 3797. (19) Vestergaard, E. K.; Thostrup, P.; An, T.; Laegsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Phys. ReV. Lett. 2002, 88, 259601. (20) Davies, J. C.; Bonde, J.; Logadottir, A.; Norskov, J. K.; Chorkendorff, I. Fuel Cells 2005, 5, 429. (21) Andersen, M.; Johansson, M.; Chorkendorff, I. J. Phys. Chem. B 2005, 109, 10285. (22) Lakshmanan, B.; Weidner, J. W. Proc. Electrochem. Soc. 2002, 31, 149. (23) Xu, M.; Inglesia, E. J. Phys. Chem. B 1998, 102, 961.