Nanostructured Porous Gold for Methanol Electro-Oxidation - The

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Nanostructured Porous Gold for Methanol Electro-Oxidation Jintao Zhang, Pengpeng Liu, Houyi Ma,* and Yi Ding* Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China ReceiVed: March 24, 2007; In Final Form: May 2, 2007

Dealloying single phase alloys is known to generate a type of nanostructured porous metals with intriguing properties. In this study, nanoporous gold (NPG) made by dealloying Au-Ag is investigated as a novel electrode material for methanol electro-oxidation. Compared to bulk Au electrode, oxidation and subsequent reduction of NPG occur at significantly negative potentials in both acid and alkaline solutions. NPG shows great catalytic activity for methanol electro-oxidation, but the structure quickly coarsens upon long time potential cycling. Interestingly, after surface modification with only a tiny amount of platinum, NPG exhibits greatly enhanced electrocatalytic activity toward methanol oxidation in the alkaline solutions, which is exemplified by a broad and high anodic peak during the positive scan and two secondary oxidation peaks in the subsequent reverse scan. At the same time, SEM observation and long-time potential cycling both prove that Pt-NPG has much enhanced structure stability as compared with bare NPG.

Introduction Porous metallic materials, especially nanoporous gold,1-4 platinum,5,6 and nickel,7 have received increasing attention in view of their potential applications in catalysis, chemical sensors, selective filtration, and electrochemistry.8-11 Porous nanostructured metal can be prepared by selectively etching a singlephase binary alloy that consists of two kinds of metals with different chemical activity in appropriate corrosion conditions.1,2,4 Among various alloy systems that have been studied, Au-Ag alloy is the most interesting one because both metals have an identical FCC lattice structure with similar atomic radii, while their electrochemical properties are significantly different from each other.12,13 Due to its unique properties substantially deviating from those of bulk Au or individual Au nanoparticles, the nanoporous gold (NPG) made by this type of simple dealloying method has become the focus of intensive research in recent years and is expected to find enormous potential applications.14-20 Au is generally considered as a rather inert metal, but recent studies discovered that Au could exhibit a series of unusual properties when its feature size decreases down to the nanoscale.21-24 For example, Au nanoparticles 2-3 nm in diameter were well-known for their high catalytic activity for CO oxidation at or below ambient temperatures.25,26 Recently, supported Au nanoparticles were found to display high activity for methanol oxidation.27-29 At the present time, platinum group metals (PGMs) are used most extensively as the catalysts for various industrial applications. However, they often suffer from poisoning by strongly chemisorbed carbonaceous intermediates such as CO.30 Being highly effective for CO oxidation, Au and Au-based functional catalysts may potentially act as promising alternatives for electro-oxidation of methanol in fuel cells.31,32 We are particularly interested in the electrocatalytic properties of NPG for oxidation of small organic molecules with an eye * Corresponding authors. H.M.: e-mail, [email protected]; phone, +86531-88364959; fax, +86-531-88564464. Y.D.: e-mail, [email protected]; phone, +86-531-88366513; fax, +86-531-88366280.

on its potential application as high efficiency catalyst in methanol fuel cells. In this work, we systematically studied for the first time the catalytic activity of NPG electrodes with welldefined porous structures using methanol electro-oxidation as a test reaction. Compared to polycrystalline Au electrode, NPG exhibited much higher electrocatalytic activity toward methanol oxidation in alkaline media, but the porous structure quickly coarsened upon long-term potential cycling. Interestingly, modified with only a tiny amount of Pt, the catalytic activity of NPG could be significantly enhanced possibly due to the synergistic effect between Au and Pt. These new findings are of fundamental importance to give a better understanding of the catalytic ability of Au and also advantageous to the development of more effective catalysts suitable for direct methanol fuel cells (DMFCs). Experimental Section All chemicals were of analytical grade and were used as received from the suppliers. NPG electrode samples were prepared by chemically etching Au42Ag58 alloys (2.5 mm × 2.5 mm in size and 25 µm in thickness) in concentrated HNO3 (65%) for 40 min. An NPG electrode was carefully transferred into an electrochemical cell after being washed repeatedly with triply distilled water. A reversible hydrogen electrode (RHE) or a saturated calomel electrode (SCE) was selected as the reference electrode depending on the experimental requirements, and a bright Pt plate with a surface area of 4 cm2 worked as the counter electrode. The reference electrode was led to the surface of the working electrode through a Luggin capillary. A mixed solution of 0.5 mol dm-3 KOH and 1.0 mol dm-3 CH3OH was used as the electrolyte solution to characterize the electrocatalytic activity of NPG samples for methanol oxidation reactions. The solutions were deoxygenated by bubbling with high-purity N2 for at least 10 min prior to each measurement. All electrochemical measurements were performed with a Zahner IM6 electrochemical workstation at room temperature (∼22 °C).

10.1021/jp072333p CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007

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Figure 1. Cyclic voltammograms (CVs) of NPG and polycrystalline Au (poly-Au) electrodes in N2-saturated 0.1 mol dm-3 HClO4 solutions at 20 mV s-1. For the NPG electrode, the CV enlarged 50 times on the current scale (red curve) was performed first in the potential range between 0 and 1.0 V, then the whole CV from -0.05 to 1.8 V was recorded.

The Pt modified NPG electrodes (abbreviated to NPG-Pt) were prepared by means of the following “immersion-electrodeposition” (abbreviated to IE) procedure: an NPG slice was first immersed in 96.5 mmol dm-3 H2PtCl6 solution for 10 min to allow soak; then the sample was transferred into 0.1 mol dm-3 HClO4 solution. Pt was electrodeposited onto the NPG substrate by continuously sweeping the potential at 50 mV s-1 between 0 and 1.1 V (vs RHE) for 3 cycles in this solution. Assuming 100% faradaic efficiency for the electrochemical reduction process, the amount of Pt deposited onto the NPG electrodes, which gradually increased with the number of IE treatments, was directly related to the total charge involved in Pt (IV) ion reduction. The real surface area of NPG working electrodes was estimated from the charge needed to form gold surface oxide monolayers according to the oxygen adsorption measurement method.33,34 All current densities have been normalized by the real surface area of the NPG electrodes. Surface morphologies of NPG and NPG-Pt electrodes before and after the methanol oxidation reactions were observed by a JSM-6700F field emission scanning electron microscope (FESEM) operating at 10 kV. Results and Discussion Voltammetric Characterization of NPG Electrodes in Acidic and Alkaline Media. The freshly prepared NPG electrodes and a clean poly crystalline Au (abbreviated to polyAu) electrode were characterized first in acidic and alkaline solutions by means of cyclic voltammetry to compare the difference in electrochemical properties between NPG and bulk Au. Figure 1 shows the cyclic voltammogram (CV) of an NPG electrode in 0.1 mol dm-3 HClO4 solution at a sweeping rate of 20 mV s-1. This CV profile contains an extended doublelayer charging region between 0 and 1.0 V (see enlarged CV), a broad oxidation peak, and a characteristic yet negatively shifted reduction peak in comparison with that of poly-Au electrode. The oxidation peak is ascribed to formation of gold surface oxides and the reduction peak to subsequent removal of the oxides. As compared with the voltammetric behavior of the poly-Au electrode, both peaks occur at more negative potentials, indicating that the NPG was oxidized more readily than bulk Au under otherwise identical conditions. In particular, it is of interest to note that in the double-layer charging region NPG shows quasi-symmetrical anodic and cathodic behavior, which

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Figure 2. Cyclic voltammograms (CVs) of NPG and polycrystalline gold (poly-Au) electrodes at 20 mV s-1 in 0.5 mol dm-3 KOH solutions deoxygenated by N2. The CV (red dotted curve) was measured in a small potential range between -0.8 and 0.03 V and then was enlarged 10 times on the current scale to observe more clearly the preoxidation wave.

can be correlated to the charging of double-layer capacitance17 and the potential-induced surface reconstruction.35 The doublelayer capacitance can be calculated approximately from the enlarged CV by using:36

(dEdt ) ) C υ

j ) Cdl

dl

(1)

where j is the charging current density, υ the potential scan rate, and Cdl the double-layer capacitance. The value of Cdl for the NPG in 0.1 mol dm-3 HClO4 was then estimated to be was about 10 µF/cm2. For both NPG and poly-Au electrodes, the surface oxidation and subsequent reduction take place at much more negative potentials in alkaline solutions than in acidic solutions, as indicated by Figure 2. Again, the oxidation and reduction peaks of NPG shift to negative potentials as compared with those of the poly-Au. The CV of NPG exhibits a pair of anodic and cathodic curves in the potential range between -0.8 and 0.03 V (see enlarged CV). A broad “pre-oxidation wave” was observed to appear at -0.52 V in the anodic curve, which corresponds to a partial charge-transfer due to the chemisorption of OH- anions.37 The strong chemisorption of OH- anions on the NPG surface results in an increased double-layer capacitance (53 µF/cm2), which is 5 times larger than that in acidic solutions. The formation of gold surface oxides commenced at more positive potential (0.1 V), whereas during the cathodic sweep, gold oxides were electrochemically reduced, leading to a large reduction peak, followed by the desorption of OH- at more negative potentials (about -0.2 V). Similar behavior was also observed for the poly-Au electrode, but chemisorption of OHanions occurred at -0.3 V, 220 mV more positive than that of NPG. The generation of an oxide precursor in the potential region prior to the gold oxide formation is an important property of Au electrodes,38,39 which plays a key role in enhancing the catalytic activity of Au in alkaline solutions.39b The influence of the pore size on voltammetric behavior of NPG electrodes was investigated. By comparing the CVs of three NPG electrodes with different pore sizes (Figure 3), one will find that, in both acidic and alkaline solutions, decreasing the pore size of NPG will cause a slight negative shift of the reduction peak potential, indicating a more active surface state of NPG electrodes with small feature sizes. Electrocatalysis of NPG in Alkaline Media. Electrooxidation of methanol is commonly used to evaluate the catalytic

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CH3OH + 5OH- ) HCOO- + 4 H2O + 4e-

(2)

whereas at higher potentials, the methanol molecules were oxidized to carbonates with the exchange of six electrons, according to the reaction given below:40a,42

CH3OH + 8OH- ) CO32- + 6 H2O + 6e-

Figure 3. Cyclic voltammograms (CVs) of NPG electrodes with different pore sizes in 0.1 mol dm-3 HClO4 (a) and in 0.5 mol dm-3 KOH (b) at 20 mV s-1. The average pore size of NPG samples is indicated in the plots.

Figure 4. Cyclic voltammograms (CVs) of NPG electrodes in 0.5 mol dm-3 KOH solutions with and without 1.0 mol dm-3 CH3OH. The potential scan rate was 10 mV s-1.

activity of NPG. Because the adsorption of oxygen species (such as OH- anion) on the Au surface has a promoting role to the electro-oxidation of methanol or other alcohol molecules,40 Au exhibits much higher electrocatalytic activity in alkaline media than in acidic media. Figure 4 shows the CVs of the NPG electrodes in 0.5 mol dm-3 KOH solution with and without 1.0 mol dm-3 methanol. The comparison of two CVs clearly shows that methanol oxidation occurs in two different potential regions.40a The first region is from -0.5 to 0.40 V (vs SCE), which partially overlaps with the NPG surface oxide formation, and the second one starts at ∼0.44 V, on oxidized NPG surfaces. It has been reported previously that methanol oxidation on Au electrode proceeds independently in two potential regions, with different forms of mechanisms.40,41 At lower potentials, methanol was mainly oxidized to formates via an overall four-electrontransfer reaction as shown by eq 2:40a,41

(3)

We focus on the electro-oxidation of methanol in the low potential region at first. From Figure 4, the methanol oxidation at the oxide-free NPG surface commences at about -0.5 V and proceeds in a slow reduction rate until about -0.25 V. The onset of this reaction depends on the OH- anions chemisorbed on the NPG surface; the slow increase in the anodic current between -0.5 and -0.25 V is due to the chemisorption of a small amount of OHads. As the potential sweeps positively, more OH anions adsorb onto the NPG surface and react with gold surface atoms, resulting in the formation of “pre-oxidation precursors”. On partially oxidized NPG surface, the anodic current rises significantly with the increase of potential and reaches a maximum at 0.25 V, which is about 40 mV more positive than the oxidation peak potential of NPG in methanol-free solution. It is believed that electro-oxidation of methanol is not only associated with the chemisorbed OH- anions but also dependent strongly on the pre-oxidation species,40a,42 such as AuOH(1-λ), where “ads” denotes the chemisorbed species on ads NPG and λ the charge-transfer coefficient that varies between 0 and 1.42 Here both methanol electro-oxidation and Au surface oxidation take place in the first potential region between -0.5 and 0.4 V. Because it needs to consume enough Au-OH(1-λ)ads species to form relatively dense and ordered gold oxides, evidently, the gradual exhaustion of Au-OH(1-λ)species ads slows down and eventually stops the methanol oxidation reaction. All these features can be seen from Figure 4, where an inhibition to the methanol oxidation takes place at the beginning of gold oxide monolayer formation (0.25 V), and the current density decreases rapidly from a maximum of 88.4 µA cm-2 at 0.25 V to 8.3 µA cm-2 at 0.4 V. At more positive potentials (>0.5 V), one observes a sharp increase of the anodic current for methanol oxidation; while in methanol-free solution there is only a slow current increase at potentials higher than 0.6 V. This difference is clearly associated with the reactivation of the anodic oxidation of methanol by gold oxides.40b As discussed earlier, the four-electron-transfer reaction described by eq 1 cannot proceed on oxidized NPG surfaces. This oxide-induced reaction involves a six-electron charge-transfer process and the reaction products are carbonates instead of formates (eq 2). This is an important characteristic of Au as the electrocatalyst, which provides more reaction channels for methanol oxidation, and allows reaction intermediates produced at low potentials to be further oxidized, thereby eliminating possible catalyst poisoning. Au is usually considered as a poor catalyst toward methanol oxidation, but a great advantage of Au as the catalyst over PGM catalysts is that poisoning intermediates are not formed.40a,42 Borkowska et al.31 studied electro-oxidation of methanol on rough Au electrodes and found that the presence of surface defects allowed the oxidation to occur at less positive potentials. This is exactly what we have observed here for NPG, which has well-defined, highly active nanoscale ligaments and pores with very high surface area. During the negative-going potential sweep, the electrochemical reduction of the gold surface oxides leads to a reduction peak centered on 0.04 V, which coincides with the reduction

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Figure 5. Cyclic voltammograms (CVs) of the electro-oxidation of methanol at an NPG electrode in a solution of 0.5 mol dm-3 KOH and 1.0 mol dm-3 CH3OH with different potential scan rates. Scan rates were 50, 30, 10, 5, and 2 mV s-1, respectively.

Figure 6. Cyclic voltammograms (CVs) for the electro-oxidation of methanol at an NPG electrode in 0.5 mol dm-3 KOH +1.0 mol dm-3 CH3OH mixed solution with different positive potential limits. Scan rate, 10 mV s-1.

peak of the NPG electrode in the methanol-free KOH solution. With the removal of the gold surface oxides, Au-OH(1-λ)ads species regenerate on the NPG surface, inducing methanol oxidation to reinitiate according to eq 1. As a result, a corresponding oxidation current regains immediately after the potential reaches ∼0 V.40a,42 Figure 5 shows the effect of sweep rates on the electrooxidation of methanol at the NPG electrode in alkaline solution. Based on the results and discussion described above, the methanol electro-oxidation in the low potential region (first region) is coupled with the gold surface oxidation. Because OHanions adsorption is a relatively slow process in comparison with the gold oxidation which inhibits the low potential methanol oxidation, the slow scan rates are advantageous to the methanol oxidation reaction.40,42 We indeed found that the slower the scan rate, the higher the oxidation current in negatively going potential sweep. On the other hand, the slower the scan rate, the smaller the peak corresponding to the gold oxide reduction in the negative-going potential sweep. When the scan rate is reduced to 2 mV s-1, the reduction current peak almost disappears. Figure 6 shows the potential-range dependence of an NPG electrode at a sweep rate of 10 mV s-1 in 0.5 mol dm-3 KOH + 1.0 mol dm-3 CH3OH mixed solution. In the positive-going potential sweep, the large current peak simultaneously involves the methanol oxidation and gold oxide formation. As the positive potential limit gradually decreases, the cathodic peak corre-

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10385 sponding to gold oxide reduction becomes smaller and finally disappears at the potential limit of 0.35 V. Interestingly, when the potential limit reaches 0.25 V, the forward and reverse curves superimpose with each other, indicating that no poisoning or inhibiting effect takes place. It also proves that the gold surface oxide formation occurs at potentials higher than 0.25 V. Although NPG shows greatly enhanced activity for methanol electro-oxidation as compared to bulk Au, we observed that the current intensity would gradually decrease upon long-time potential cycling. At the same time, the color of the NPG electrode apparently changed from brownish to bright yellow (see insets to Figure 7). We have shown above that no catalyst poisoning occurs during this reaction. The current decrease should be associated with the change of surface structures of the NPG electrode during continuous potential cycling. This is confirmed by SEM observation shown in Figure 7. The freshly prepared NPG electrode has an average pore/ligament size around 12 nm; however, after reaction, serious coarsening occurs and the pore size increases to larger than 40 nm. The coarsening and clogging of pores result in the loss of active surface area, which in turn decrease the electro-activity of the NPG electrode. We compared the electrocatalytic activity of three types of NPG electrodes with average pore size of 12, 18 and 24 nm, respectively, and found smaller porosity generally exhibited higher catalytic efficiency, but its stability decreased accordingly, possibly due to its metastable surface state at nanoscale. Preparation of NPG-Pt Electrodes. In view of further improving the catalytic activity of NPG, we modified the NPG surface by depositing a tiny amount of Pt by using a simple IE procedure. The advantage of IE method over the common electrodeposition methods is that only very tiny amount of Pt atoms could be deposited into the nanoporous structure, enabling submonolayer surface functionalization. In addition, the amount of Pt deposited on the NPG substrate can be controlled by numbers of IE cycles. By analyzing the charge involved in the electrodeposition process, it is estimated that ∼2.2 µg of Pt can be deposited onto the NPG surface in each cycle. The success of IE functionalization was verified by cyclic voltammetry which showed very characteristic electrochemical behavior of Pt. As shown in Figure 8, the CVs of the NPG-Pt electrodes display a pair of reversible waves caused by hydrogen adsorption/desorption in the potential region from 0.05 to 0.33 V (vs RHE) and a characteristic reduction peak at 0.7 V (vs RHE) corresponding to the reduction of platinum oxides, as compared with the CVs of pure NPG. It is also observed that the CV profiles vary with increasing IE times. The increase of IE cycles is accompanied by more pronounced Pt oxide reduction peak. Accordingly, the reduction peak for NPG surface oxides becomes smaller, consistent with more Pt coverage on NPG surfaces. Figure 9a,b shows the microstructures of an NPG sample before and after Pt modification. Because very little amount of Pt was deposited, SEM did not reveal obvious structural difference between NPG and NPG-Pt. And after three IE cycles, EDS analysis showed that the composition of Pt was ∼4.3 atom % (Figure 9e). Catalytic Activity of NPG-Pt Electrodes. Figure 10 shows a set of CVs of an NPG-Pt electrode at 20 mV s-1 in a solution of 0.5 mol dm-3 KOH + 1.0 mol dm-3 CH3OH. The CV of an NPG electrode measured under the identical condition was also included in this figure for comparison. The deposited Pt influenced the electrocatalytic activity of NPG toward the methanol oxidation in the following two aspects: (i) onset of the methanol electro-oxidation occurred at ∼ -0.7 V (vs SCE),

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Figure 7. SEM images of an NPG electrode measured before (a) and after (b) electro-oxidation of methanol. Scale bars in two SEM images are 100 nm. Insets in two images show the change of the color of NPG before and after the electrocatalytic reaction.

Figure 8. Cyclic voltammograms (CVs) of NPG and NPG-Pt electrodes in 0.1 mol dm-3 HClO4 solution at a scan rate of 20 mV s-1. The NPG-Pt electrodes were prepared by means of “immersion-electrodeposition” (IE) procedure, and IE times are marked in the figure.

200 mV more negative than that for NPG electrode; (ii) the current density due to the methanol oxidation was significantly enhanced. The electrochemical performance of NPG-Pt was tested with repeated potential cycling. In the first potential scan, the forward curve displays a current plateau between -0.15 and 0.21 V, and the backward curve gives a reduction peak and two anodic peaks. The first anodic peak at -0.12 V during forward scan is attributed to electrocatalysis on Pt and the second one at 0.16 V to electrocatalysis on Au; while during the cathodic sweep, the reduction peak at 0.05 V is related to the reduction of gold surface oxides, and two oxidation peaks (-0.05 and -0.30 V) arise from the electro-oxidation of fresh methanol molecules and adsorbed carbonaceous intermediates at Au and Pt sites respectively.43 Different from the NPG electrode, the anodic current does not increase sharply at more positive potentials (over 0.5 V). This implies that the existence of Pt on NPG suppresses the Au oxidation which in turn hinders the methanol oxidation on gold surface oxides. From the second potential scan to the fifth one, in the positive-going potential sweep the former anodic current plateau gradually evolves into a large peak at ∼0 V with a small shoulder at 0.24 V, while in the negative-going direction, two anodic peaks gradually

decrease. This is probably due to surface alloying between Au and Pt upon continuous potential cycling. It is well-known that Au/Pt surface alloys exhibit enhanced methanol electrocatalytic properties due to the charge-transfer induced synergistic effect, which is exemplified by a decrease in current intensity during cathodic sweep, indicating improved tolerance to carbonaceous intermediates accumulation.44,45 SEM observation (Figure 9b,c) of NPG-Pt demonstrated that the ligament size increased from an original 12 nm to ∼18 nm after long-time potential cycling. Compared to bare NPG, surface modification of Pt significantly improved the structure stability and catalytic performance. And more importantly, the three-dimensional porous structure was well reserved, which is also indicative of poisoning suppression. Conclusion Nanoporous metals represent a new class of electrode materials that exhibit interesting electrocatalytic activity toward the methanol oxidation. The bi-continuous open porosity allows unlimited transport of molecules through the medium,46 while holding promise for further functionalization with other useful materials. NPG discussed here is just the first example. It is not unexpected that NPG shows greatly enhanced performance

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Figure 9. SEM images of a freshly prepared NPG sample (a), and of an NPG-Pt electrode sample before (b) and after (c) using as the catalyst for methanol electro-oxidation in 0.5 mol dm-3 KOH + 1.0 mol dm-3 CH3OH mixed solution. The NPG-Pt electrode was prepared by electrodepositing Pt at an NPG sample once according to the IE procedure. Scale bars in three SEM images are 100 nm. EDS analysis of the NPG sample (d) and the NPG-Pt sample (e) obtained after three times of IE treatments.

Acknowledgment. This work was supported by the National Science Foundation of China (20573068, 20673067, 50601015), the National 863 Program Project (2006AA03Z222) and the 973 Program Project of China (2005CB623601). H.M. acknowledges the Visiting Scholar Foundation of Key Laboratory in Shandong University, and Y.D. is a Tai-Shan Scholar supported by Shandong Province. References and Notes

Figure 10. Cyclic voltammograms (CVs) of electro-oxidation of methanol at NPG and NPG-Pt electrodes in 0.5 mol dm-3 KOH + 1.0 mol dm-3 CH3OH mixed solution at 10 mV s-1. The number of scan cycles is indicated in the figure.

as compared to bulk Au electrodes because a porous morphology has the advantage of trapping OH- anions, which is crucial in methanol oxidation reaction. Methanol molecules can be oxidized on both bare and oxidized NPG surfaces through two different mechanisms. NPG has a highly active, metastable structure and continuous potential cycling causes serious coarsening of ligaments, resulting in a decrease in catalytic activity. However, with submonolayer modification of Pt onto the NPG substrate, the structure stability is significantly improved. More interestingly, the new porous nanocomposite shows new catalytic properties toward the methanol oxidation, such as high anodic activity and improved tolerance to carbonaceous intermediates accumulation.

(1) Li, R.; Sieradzki, K. Phys. ReV. Lett. 1992, 68, 1168-1171. (2) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450-453. (3) Ding, Y.; Erlebacher, J. J. Am. Chem. Soc. 2003, 125, 7772-7773. (4) Hakamada, M.; Mabuchi, M. Nano Lett. 2006, 6, 882-885. (5) Thorp, J. C.; Sieradzki, K.; Tang, L.; Crozier, P. A.; Misra, A.; Nastasi, M.; Mitlin, D. Appl. Phys. Lett. 2006, 88, 033110-3. (6) Liu, H. T.; He, P.; Li, Z. Y.; Li, J. H. Nanotechnology 2006, 17, 2167-2173. (7) Sun, L.; Chien, C.-L.; Searson, P. C. Chem. Mater. 2004, 16, 31253129. (8) Peng, X. S.; Koczkur, K.; Nigro, S.; Chen, A. C. Chem. Commun. 2004, 2872-2873. (9) Weissmu¨ller, J.; Viswanath, R. N.; Kramer, D.; Zimmer, P.; Wu¨rschum, R.; Gleiter, H. Science 2003, 300, 312-315. (10) Ro¨sler, J.; Mukherji, D. AdV. Eng. Mater. 2003, 5, 916-918. (11) Cortie, M. B.; Maaroof, M. B.; Smith, G. B. Gold Bull. 2005, 38, 14-22. (12) Shi, H.; Zhang, L.; Cai, W. J. Appl. Phys. 2000, 87, 1572-1574. (13) Erlebacher, J.; Sieradzki, K. Scr. Mater. 2003, 49, 991-996. (14) Ding, Y.; Kim, Y.-J.; Erlebacher, J. AdV. Mater. 2004, 16, 18971900. (15) Ding, Y.; Chen, M. W.; Erlebacher, J. J. Am. Chem. Soc. 2004, 126, 6876-6877. (16) Basheer, C.; Swaminathan, S.; Lee, H. K.; Valiyaveettil, S. Chem. Commun. 2005, 409-410. (17) Kramer, D.; Viswanath, R. N.; Weissmuller, J. Nano Lett. 2004, 4, 793-796.

10388 J. Phys. Chem. C, Vol. 111, No. 28, 2007 (18) Bonroy, K.; Friedt, J.-M.; Frederix, F.; Laureyn, W.; Langerock, S.; Campitelli, A.; Sa´ra, M.; Borghs, G.; Goddeeris, B.; Declerck, P. Anal. Chem. 2004, 76, 4299-4306. (19) Liu, Z.; Searson, P. C. J. Phys. Chem. B 2006, 110, 4318-4322. (20) Ding, Y.; Mathur, A.; Erlebacher, J. Angew. Chem., Int. Ed. 2005, 44, 4002-4006. (21) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 16471650. (22) Haruta, M. Chem. Rec. 2003, 3, 75-87. (23) Haruta, M. Nature 2005, 437, 1098-1099. (24) Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374-9375. (25) Murphy, C. J. Science 2002, 298, 2139-2141. (26) Jaramillo, T. F.; Baeck, S.-H.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 2018-2019. (27) Zhong, C.-J.; Maye, M. M. AdV. Mater. 2001, 13, 1507-1511. (28) Luo, J.; Jones, V. W.; Maye, M. M.; Han, L.; Kariuki, N. N.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 13988-13989. (29) Maye. M. M.; Luo. J.; Lin. Y. H.; Engelhard. M. H.; Hepel. M.; Zhong, C.-J. Langmuir 2003, 19, 125-131. (30) Kua, J.; Goddard, W. A., III J. Am. Chem. Soc. 1999, 121, 1092810941. (31) Borkowska, Z.; Tymosiak-Zielinska, A.; Nowakowski, R. Electrochim. Acta 2004, 49, 2613-2621. (32) Luo, J.; Njoki, P. N.; Lin, Y.; Mott, D.; Wang, L. Y.; Zhong, C.-J. Langmuir 2006, 22, 2892-2898. (33) Woods, R. Chemisorption at electrodes. Hydrogen and oxygen on noble metals and their alloys. In Electroanalytic Chemistry: A Series of

Zhang et al. AdVances; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. 9, pp 119-125. (34) Trasatti, S.; Petrii, O. A. Pure Appl. Chem. 1991, 63, 711734. (35) Silva, F.; Martins, A. Electrochim. Acta 1998, 44, 919-929. (36) (a) Yong, F.; Ma, H.; Wang, X.; Feng, X.; Huang, S.; Jiang, J.; Chen, S. Electrochim. Acta 2006, 51, 3743-3751. (b) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1-11. (c) Ozoemena, K.; Nyokong, T. Electrochim. Acta 2002, 47, 4035-4043. (37) (a) Sˇ trbac, S.; Adzˇic´, R. R. J. Electroanal. Chem. 1996, 403, 169181. (b) Angerstein-Kozlowska, H.; Conway, B. E.; Barnett, B.; Mozota, J. J. Electroanal. Chem. 1979, 100, 417-446. (38) Chen, A. C.; Lipkowski, J. J. Phys. Chem. B 1999, 103, 682691. (39) (a) Burke, L. D.; Nugent, P. F. Gold Bull. 1997, 30, 43-53. (b) Burke, L. D.; Nugent, P. F. Gold Bull. 1998, 31, 39-50. (c) Burke, L. D. Gold Bull. 2004, 37, 125-135. (40) (a) Borkowska, Z.; Tymosiak-Zielinska, A.; Shul, G. Electrochim. Acta 2004, 49, 1209-1220. (b) Avramov-Ivic´, M.; Jovanovic´, V.; Vlajnic´, G.; Popic´, J. J. Electroanal. Chem. 1997, 423, 119-124. (41) Tremiliosi-Filho, G.; Gonzalez, E. R.; Motheo, A. J.; Belgsir, E. M.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 31-39. (42) Assiongbon, K. A.; Roy, D. Surf. Sci. 2005, 594, 99-119. (43) Prabhuram, J.; Manoharan, R. J. Power Sources 1998, 74, 54-61. (44) Du, B. C.; Tong, Y. T. J. Phys. Chem. B 2005, 109, 17775-17780. (45) Pedersen, M. Ø.; Helveg, S.; Ruban, A.; Stensgaard, I.; Lægsgaard, E.; Nørskov, J. K.; Besenbacher, F. Surf. Sci. 1999, 426, 395-409. (46) Rolison, D. R. Science 2003, 299, 1698-1701.