Effect of Ag modification on catalytic activity of Pd electrode for allyl alcohol oxidation in alkaline solution

Electrochimica Acta 87 (2013) 860–864

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Effect of Ag modification on catalytic activity of Pd electrode for allyl alcohol oxidation in alkaline solution Changchun Jin ∗ , Zhuo Zhang, Zhidong Chen, Qun Chen ∗ Department of Applied Chemistry, Changzhou University, Jiangsu 213164, China

a r t i c l e

i n f o

Article history: Received 27 June 2012 Received in revised form 7 September 2012 Accepted 4 October 2012 Available online 13 October 2012 Keywords: Surface modification Pd electrode Silver Electrocatalytic activity Allyl alcohol

a b s t r a c t The surface modification of Pd polycrystalline electrodes with silver and the electrocatalytic oxidation of allyl alcohol on Ag-modified Pd electrodes in alkaline solution have been investigated. Ag-modified Pd electrodes with different silver loadings were prepared by means of potentiostatic deposition of silver. Scanning electron microscope images demonstrate that Ag particles and cloud-like clusters of different sizes and shapes are formed on the Pd substrate, indicating three-dimensional deposition of silver, and that a large part of the Pd substrate is not covered by silver. The results of cyclic voltammetric measurement display that allyl alcohol oxidation on the Ag-modified Pd electrodes shows a slight negative shift in peak potential and a significant variation in peak current compared to those on Pd electrode. The peak current on the Ag-modified Pd electrodes is closely related to the amount of the deposited silver, much higher peak current than that on Pd electrode can be obtained. The results of chronoamperometric measurement show enhanced anti-poisoning ability of the Ag-modified Pd electrodes. Silver modification is found to be an effective method to improve electrocatalytic activity and stability of Pd electrode for allyl alcohol oxidation. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Surface modification is a very useful method in electrochemistry, and surface modified electrode can show improved activity in electrocatalytic reactions. Underpotential deposition (UPD) and overpotential deposition (OPD) are widely used electrochemical methods for surface modification [1–3]. Silver is one of important metals and can be used not only as modification metal but also as substrate metal in surface modification. For example, Ag UPD on Au(1 1 1), Au(1 0 0), Pt(1 1 1) and Pt(1 0 0), Pb UPD on Ag(1 1 1), Ag(1 0 0) and Ag(1 1 0), Tl UPD on Ag(1 0 0) and Ag(1 1 0) [1], and Ag OPD on Au(1 1 1) [4], have been reported. Silver modification can also be performed by other methods such as chemical reduction and UPD-redox displacement, for example, silver modification of copper surface through reduction of diamminesilver(I) cation with formaldehyde [5] and silver modification of gold surface through displacement of a Pb UPD monolayer formed on Au nanoparticles by silver in a sulfuric acid solution containing silver nitrate [6]. Palladium is one of widely used electrode materials and is emerging as an attractive replacement for platinum in direct alcohol fuel cells [7], since Pd-based electrodes can be highly active

∗ Corresponding authors. Tel.: +86 519 86330263; fax: +86 519 86330927. E-mail addresses: [email protected], [email protected] (C. Jin), [email protected] (Q. Chen). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.10.014

for the oxidation of a large variety of substances in alkaline media. The preparation, structure analysis and electrochemical property of Pd-based electrodes for the oxidation of alcohols and hydrogen and the reduction of oxygen have been investigated intensively, and the progresses achieved have been summarized in recent reviews [7–9]. Much effort has also been made in the surface modification of different kinds of metal substrates with palladium, for example, the palladium modification of monocrystalline and polycrystalline Au and Pt [10–15]. Palladium can as well be used as substrate metal in surface modification, for example, platinum modification of Pd substrates through displacement by platinum of a Cu UPD monolayer formed on the Pd substrates [16]. However, such reports on the surface modification of Pd substrate with other metal are very limited. One of the reasons is the difficulty in the preparation of Pd single crystal surface [10]. On the other hand, Ag electrode is attractive for its high catalytic activity toward the reduction of organic halides [17–20], for example, Ag electrode for reductive dehalogenation of polyhalogenated phenols [18] and Ag nanoparticles and nanorods deposited on glassy carbon substrates for the reduction of benzyl chloride [19,20]. But Ag electrode shows low catalytic activity for the oxidation of alcohols such as methanol, ethylene glycol and glycerol [21,22]. Silver is also used as second metal in the preparation of bimetallic electrodes, and enhanced catalytic activities of AgM nanoparticle-based electrodes and Ag-modified M electrodes (M: metal) in various electrochemical reactions, for example, AgPd

C. Jin et al. / Electrochimica Acta 87 (2013) 860–864

4

j / mA cm –2

3 2 1 0 -1 -2 -1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

E / V vs. SCE Fig. 1. Cyclic voltammograms of Ag electrode in 0.5 mol L−1 NaOH + 0.1 mol L−1 allyl alcohol (solid line) and 0.5 mol L−1 NaOH (dotted line). Scan rate: 50 mV s−1 .

861

potentiostatic deposition of silver at −0.20 V vs. Pt (ca. +0.70 V vs. RHE), using two Pt coils as the counter electrode and the reference electrode, respectively. The amount of the deposited silver was controlled by the deposition time, and the Ag-modified Pd electrodes with different silver loadings were prepared. The electrocatalytic oxidation of allyl alcohol on Ag, Pd and Agmodified Pd electrodes was conducted by cyclic voltammetric (CV) and chronoamperometric methods in a 0.5 mol L−1 NaOH solution containing 0.1 mol L−1 allyl alcohol, using a Pt coil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode, respectively. CV measurement was performed between − 0.8 and + 0.6 V in the first cycle, then upper limit of the sweep was set at +0.2 V, and stable CV curves of the electrodes in this potential region were used in the article. Currents are normalized to geometric area of the electrodes, and all potentials are reported with respect to SCE, except as specifically indicated. 3. Results and discussion 3.1. Electrocatalytic oxidation of allyl alcohol on Ag electrode

electrodes for the oxidation of methanol [23] and ethanol [24–26] and the reduction of primary alkyl bromides and iodides [27,28], AgPt electrodes for the oxidation of methanol [29], AgAu electrodes for the oxidation of glucose [30] and CO [31], Ag-modified Au(1 1 1) electrodes for the oxidation of glucose [32], Ag-modified Au nanoparticles supported on glassy carbon substrates for the reduction of benzyl chloride [6], and Ag-modified Pt(1 1 1) electrodes for the adsorption of CO [33], have been reported. But in general, not many articles on electrochemical property of AgM bimetallic electrodes, especially Ag-modified metal electrodes, have been reported. As the simplest unsaturated alcohol, allyl alcohol is interesting for the investigation of the characteristics of unsaturated alcohols in electrochemical reactions and how the adsorption of C C bond in the molecular chain onto electrode surface affects the activity of the electrode. We have recently reported enhanced catalytic activity of Pd-modified Au electrodes and no obvious catalytic activity of Pt electrode for allyl alcohol oxidation in alkaline solution [34]. It is necessary to further investigate the preparation and property of Pd-based bimetallic electrodes to increase their catalytic activities toward allyl alcohol oxidation and examine the effect of the second metal on the property of palladium. This article describes the silver modification of polycrystalline Pd electrodes and the electrocatalytic oxidation of allyl alcohol on Ag-modified Pd electrodes in alkaline solution. Ag-modified Pd electrodes with different silver loadings were prepared potentiostatically, and the surface structure of the electrodes, the effect of the silver modification on catalytic activity of Pd electrode, and the characteristics of allyl alcohol oxidation on Ag-modified Pd electrodes compared to those on Ag and Pd electrodes, have been investigated. To the best of our knowledge, no article on the silver modification of Pd electrode and the electrocatalytic property of Ag-modified Pd electrode has been reported.

Fig. 1 shows cyclic voltammograms of Ag electrode in alkaline solution in the presence and absence of allyl alcohol. A comparison of the two curves shows that for the curve of allyl alcohol oxidation, the peak at −0.31 V with peak current of 2.5 mA cm−2 in the positive sweep comes from allyl alcohol oxidation, and the second peak is mainly ascribed to the oxidation of the Ag electrode surface, since the cathodic peak in the negative sweep represents the reduction of silver oxide formed in the positive sweep. The anodic peak at −0.41 V with peak current of 1.1 mA cm−2 in the negative sweep is due to further oxidation of the intermediates of allyl alcohol oxidation, adsorbed on the electrode surface, by regenerated Ag active sites. One of the characteristics of allyl alcohol oxidation on Ag electrode is that the oxidation peak is not sharp. No oxidation peak was observed in propan-1-ol oxidation on Ag electrode under the same reaction condition as that in Fig. 1. Since both allyl alcohol and propan-1-ol are C3 primary alcohols, the above results reveal high activity of allyl alcohol, caused by the C C bond in the molecular chain. The CV curve of allyl alcohol oxidation in Fig. 1 shows a great difference from that of methanol oxidation. In methanol oxidation on Ag electrode in alkaline solution, the oxidation takes place at the positive potential region where Ag(II) oxide is formed, and AgO acts as the main active sites [21,22]. In glucose oxidation on Ag nanoparticle-modified composite electrodes prepared by incorporating silver nanoparticle/carbon black mixture into a polystyrene matrix, glucose is also oxidized by AgO and then the intermediates of glucose oxidation are further oxidized by Ag2 O [35]. On the other hand, in electrocatalytic oxidation of methanol on Ag/MWCNT catalysts (silver nanoparticles electrocrystallized on 4-aminobenzene monolayer-grafted multi-walled carbon nanotubes) in alkaline solution, the oxidation takes place in a practical window prior to the formation of silver oxide [36], which is similar to allyl alcohol oxidation on Ag electrode shown in Fig. 1.

2. Experimental 3.2. Silver modification of Pd electrode Electrochemical experiments were performed with a LK98BII (Lanlike Ltd, China) electrochemical workstation at room temperature (about 18 ◦ C). Polycrystalline Pd and Ag electrodes of 4 mm in diameter embedded in PTFE were used in the experiments. The preparation of Ag-modified Pd electrode is described as follows: prior to the deposition of silver, Pd electrode was polished with alumina-water slurry and cleaned ultrasonically in twice-distilled water. The silver modification of the Pd electrode was carried out in a 0.05 mol L−1 H2 SO4 solution containing 1 mmol L−1 AgNO3 by

Fig. 2 shows the deposition of silver on a polycrystalline Pd electrode by the reduction of silver cation under potentiostatic condition. No formation of hydrogen bubbles on the Pd surface was observed. The amount of the deposited silver is dependent on the deposition time; the longer the deposition time, the more the silver loading. The silver deposition time of 20, 30, 60 and 100 s, respectively, was chosen in the experiments, and charge values obtained from these deposition processes were ca. 60, 130,

862

C. Jin et al. / Electrochimica Acta 87 (2013) 860–864

6

4

5

2 0

j / mA cm –2

j / mA cm–2

4 3 2

-2 -4 -6

1

-8

0

-10

-1

-12 0

20

40

60

80

100

-1

120

-0.8

-0.6 -0.4

390 and 610 ␮C, respectively, corresponding to 0.07, 0.15, 0.44, and 0.68 ␮g of the silver loading on the Pt substrates (4 mm in diameter). The Ag-modified Pd electrodes prepared are denoted as Ag(x) /Pd, in which x represents the silver deposition time. For example, Ag(100 s) /Pd indicates the Ag/Pd electrode prepared by the silver deposition time of 100 s. Both substrate surface structure and electrode potential affect the growth mode and morphology of second metal overlayer. Fig. 3 shows scanning electron microscope (SEM) images of the Ag(100 s) /Pd electrode. Ag particles and cloud-like clusters of different sizes and shapes are observed, indicating three-dimensional deposition of silver on the Pd substrate. At the same time, it is seen that a large part of the Pd substrate is not covered by silver, which will be further confirmed below by electrochemical behaviors of the Ag/Pd electrodes. In Ag UPD modification of Au and Pt single crystal electrodes, by contrast, the deposition of silver proceeds two-dimensionally and one or two monolayers of silver are generally formed [1,33,37]. A uniform Ag monolayer formed on Au(1 1 1) electrode surface can be converted to Ag islands with a globular or particle shape then to a complex nano-pattern, upon the control of the potential and the silver cation concentration [38]. Fig. 4 shows cyclic voltammograms of Pd and Ag/Pd electrodes in a 0.5 mol L−1 NaOH solution, in which positive sweep was performed until +0.5 V to form first monolayer of PdO [39]. For the curve of Pd electrode, the low cathodic peak at −0.42 V in the negative sweep is ascribed to the reduction of palladium oxide, and the

0

0.2

0.4

0.6

E / V vs. SCE

Time / sec Fig. 2. Potentiostatic deposition of silver on polycrystalline Pd electrode in 0.05 mol L−1 H2 SO4 + 1 mmol L−1 AgNO3 . Potential: −0.20 V vs. Pt.

-0.2

Fig. 4. Cyclic voltammograms of (a) Ag(20 s) /Pd, (b) Ag(30 s) /Pd, (c) Ag(60 s) /Pd, (d) Ag(100 s) /Pd and (e) Pd electrodes in 0.5 mol L−1 NaOH. Scan rate: 50 mV s−1 . Inset shows an enlarged part of the CVs.

peak of palladium oxidation in the positive sweep can not be seen clearly due to low current. For the curve of each Ag/Pd electrode, the peak at +0.28 V in the positive sweep is mainly ascribed to the oxidation of the deposited silver, and the high peak at −0.01 V and the low peak at −0.42 V in the negative sweep are ascribed to the reduction of silver oxide and palladium oxide, respectively. As can be seen from the enlarged peaks of palladium oxide reduction shown in the inset of Fig. 4, the onset potentials of these peaks are close to one another while peak potentials show a slight positive shift with the increase of the silver loading, and the peaks of the Ag/Pd electrodes are smaller than that of Pd electrode, indicating a decrease in electrochemically active surface area of palladium with the increase of the deposited silver on the Pd substrates. Table 1 shows a variation in electrochemically active surface area of palladium for Pd and Ag/Pd electrodes. The charge values under the peaks of palladium oxide reduction shown in Fig. 4 and the charge density of 424 ␮C cm−2 for the reduction of 1 monolayer of PdO [39] are used in the calculation. In the case of the Ag(100 s) /Pd electrode, about 72% of the Pd surface is found not being covered by silver, which is in consistent with the large uncovered Pd substrate observed from the SEM images in Fig. 3. At the same time, as can be expected, the decrease in electrochemically active surface area of palladium with increasing silver loading is relatively small, due to the threedimensional deposition of silver. For example, the uncovered Pd surface of the Ag(100 s) /Pd electrode is about 22% smaller than that of the Ag(20 s) /Pd electrode, though the difference in the silver loading of these two electrodes is about ten times. These results also indicate that the Pd surface of the Ag/Pd electrodes is not completely covered by the deposited silver. On the other hand, the peaks representing the oxidation of silver and the reduction of silver oxide increase significantly with the increase of the silver loading, and very high peaks are observed for the Ag/Pd electrodes with higher silver loading. This is due to that the oxidation of silver takes place not only at the surface but also inside of the Ag particles as the upper limit of the sweep becomes more positive in the positive sweep. The Table 1 Electrochemically active surface areas (EASA) of palladium.

Fig. 3. SEM images of Ag/Pd electrode prepared by the silver deposition time of 100 s.

Electrodes

EASA (cm2 )

Uncovered Pd substrate (%)

Pd Ag(20 s) /Pd Ag(30 s) /Pd Ag(60 s) /Pd Ag(100 s) /Pd

0.36 0.34 0.32 0.29 0.26

100 94 89 81 72

C. Jin et al. / Electrochimica Acta 87 (2013) 860–864

12

863

2.5

1.2

2

0.8

1

j / mA cm –2

6

j / mA cm –2

d c b

8

e

4

a

2

j / jo

10

d c

0.6 0.4

1.5

b a c

e

0.2

d

c

1

0

b

-3 0 3 6 9 12 15 18 21 24 27 30 33

e

a

Time / min

0.5

0

0

-2 -1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

E / V vs. SCE Fig. 5. Cyclic voltammograms of (a) Ag(20 s) /Pd, (b) Ag(30 s) /Pd, (c) Ag(60 s) /Pd, (d) Ag(100 s) /Pd and (e) Pd electrodes in 0.5 mol L−1 NaOH + 0.1 mol L−1 allyl alcohol. Scan rate: 50 mV s−1 .

formation of multilayer silver oxide on Ag electrode surface in this potential region has been reported [21,40]. 3.3. Electrocatalytic oxidation of allyl alcohol on Pd and Ag/Pd electrodes Fig. 5 shows cyclic voltammograms of allyl alcohol oxidation on Pd and Ag/Pd electrodes. An oxidation peak at −0.06 V with peak current of 5.5 mA cm−2 is observed for Pd electrode, showing a 0.25 V more positive peak potential but a more than two times higher peak current than those on Ag electrode. This indicates the advantage of Ag electrode in oxidation potential and of Pd electrode in oxidation current. For Ag/Pd electrodes, oxidation peaks at ca. −0.08 V with different peak currents are observed, and these peaks show a slight negative shift in peak potential and an increase in peak current with the increase of the silver loading. For example, peak currents on Ag(20 s) /Pd and Ag(100 s) /Pd electrodes are 3.9 and 9.1 mA cm−2 , respectively. The improvement in catalytic activity of Pd electrode for allyl alcohol oxidation by the silver modification can be found from the comparison of the CV curves of Ag/Pd and Pd electrodes, and the following two differences are remarkable in Fig. 5: one is that the peak potentials on the Ag/Pd electrodes are a little more negative than that on the Pd electrode, and the other is that the peak current on the Ag/Pd electrodes is higher or lower than that on the Pd electrode, depending on the amount of the deposited silver. For example, the peak current on the Pd electrode is higher than that on the Ag(20 s) /Pd electrode but is lower than that on the Ag(30 s) /Pd electrode. This suggests that the Ag/Pd electrode with a certain amount (a very small amount) of the deposited silver exhibits higher catalytic activity than the Pd electrode in both peak potential and current. In Fig. 5, the advantage of the Ag/Pd electrodes, especially in peak current, is very obvious. On the other hand, a comparison of the CV curves of Ag and Ag/Pd electrodes shows about 0.23 V more positive peak potentials but significantly higher peak currents on the Ag/Pd electrodes than those on the Ag electrode. Furthermore, unlike the Ag electrode, no peak of silver surface oxidation at the potential region close to +0.20 V in the positive sweep is observed for the Ag/Pd electrodes. The different electrochemical behavior of the deposited silver on the Pd substrate from that of Ag electrode is evidently due to the influence of the Pd substrate. Two important points can also be found from Figs. 1 and 5: (i) the shape and position of the peak of allyl alcohol oxidation on the Ag/Pd electrodes are similar to those on the Pd electrode rather than those on the Ag electrode, and (ii) the peak current on the Ag/Pd

0

3

6

9 12 15 18 21 24 27 30 33

Time / min Fig. 6. Chronoamperometric curves (a) Ag(20 s) /Pd, (b) Ag(30 s) /Pd, (c) Ag(60 s) /Pd, (d) Ag(100 s) /Pd and (e) Pd electrodes in 0.5 mol L−1 NaOH + 0.1 mol L−1 allyl alcohol. Potential: −0.07 V vs. SCE. Inset shows j/jo curves.

electrodes increases with the increase of the deposited silver, while the peak current on the Ag electrode is the lowest. These characteristics suggest important roles of both uncovered Pd substrate and Ag loading in the function of the Ag/Pd electrodes and also indicate different roles of silver in the function of Ag and Ag/Pd electrodes. Taking into account of similar oxidation potentials (onset potential and peak potential) of allyl alcohol on Ag/Pd and Pd electrodes, it is reasonable to conclude that in the function of the Ag/Pd electrodes, palladium act as the main active metal to oxidize allyl alcohol and silver acts as the promoting metal to provide oxygencontaining species, such as Ag OHads , and Ag2 O OHads , for allyl alcohol oxidation on neighboring palladium active sites. In our previous study on allyl alcohol oxidation on Pd-modified Au electrodes, oxidation potential is close to that on Pd electrode rather than that on Au electrode and peak current becomes higher with the increase of the deposited palladium on Au substrate, indicating that palladium acts as the main active sites [34]. Since palladium is used as second metal to modify Au substrate, electrochemically active surface area of palladium is increased with the increase of the palladium loading and as a result, the peak current of allyl alcohol oxidation becomes higher. In the present study, however, palladium is used as substrate metal and modified by silver, so electrochemically active surface area of palladium is decreased by the deposited silver, as seen in Fig. 4. The possible reason why peak current on the Ag/Pd electrodes increases with the increase of the silver loading is that more oxygen-containing species are provided with the formation of new silver particles and silver particles growing larger. Thus, the rate of allyl alcohol oxidation on neighboring palladium active sites is accelerated, and as a result oxidation current becomes larger. In the oxidation of methanol and ethanol on Pdx Agy electrodes [23–26], slightly more negative or positive peak potentials than that on Pd electrode were observed, and the highest catalytic activity, mainly in peak current, of Pdx Agy /C with x:y atomic ratio of 1:1 [23,24], of Pdx Agy /C with Ag content of 25–33% [25], and of Pdx Agy alloy electrode with Ag content of 21% [26], were reported. And it was proposed that palladium act as the main active metal and silver acts as the promoting metal, on the basis of bifunctional mechanism and d-band theory [23–25]. In glucose oxidation on about one third monolayer of Ag-modified Au(1 1 1) electrode in alkaline solution, a ca. 0.10 V more negative oxidation potential and a little higher peak current than those on Au(1 1 1) electrode were observed [32]. Fig. 6 shows chronoamperometric curves of Ag/Pd and Pd electrodes for the oxidation of allyl alcohol at −0.07 V. Currents decrease rapidly at the initial stage of the reaction, caused by the

864

C. Jin et al. / Electrochimica Acta 87 (2013) 860–864

poisoning of the active sites, and then gradually become stable. The curve of the Pd electrode is located over the curve of the Ag(20 s) /Pd electrode. The order of these curves is in consistent with the order of peak currents of allyl alcohol oxidation shown in Fig. 5. After 30 min of the reaction, the currents on Ag(100 s) /Pd and Pd electrodes are 0.37 and 0.10 mA cm−2 , respectively. For each curve shown above, all current values constituting the curve are divided by its initial current (j/jo ), and the resulting curves are displayed in the inset of Fig. 6, which indicates activity decay of these electrodes during the reaction. In this case, the curve of Pd electrode is the lowest, and the curves of the Ag/Pd electrodes with higher silver loading decline more slowly, with survival ratios of 22% and 5% on Ag(100 s) /Pd and Pd being observed, respectively. Enhanced resistance of the Ag/Pd electrodes to poisoning is very obvious. The improvement in catalytic activity and stability of Pd electrode toward allyl alcohol oxidation by silver modification can be clearly seen from Figs. 5 and 6. 4. Conclusions The Ag-modified Pd electrodes with different silver loadings were prepared by means of potentiostatic deposition of silver on Pd polycrystalline substrates. The deposition of silver proceeds threedimensionally, and Ag particles and cloud-like clusters of different sizes and shapes were formed on the Pd substrates. Allyl alcohol oxidation on the Ag/Pd electrodes shows a slight negative shift in peak potential and a significant variation in peak current compared to those on Pd electrode. The peak current on the Ag/Pd electrodes increases with the increase of the deposited silver. The Ag/Pd electrodes also show higher resistance to poisoning. The results of this study reveal that silver modification leads to a significant improvement in electrocatalytic activity and stability of Pd electrode toward allyl alcohol oxidation and that catalytic activity of the Ag/Pd electrodes is related to both the Pd substrates and the mount of the deposited silver. Acknowledgment This work is supported by the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD2012–11). References ˜ Underpotential deposition at single crystal [1] E. Herrero, L.J. Buller, H.D. Abruna, surfaces of Au, Pt, Ag and other materials, Chemical Reviews 101 (2001) 1897. [2] P. Allongue, F. Maroun, Metal electrodeposition on single crystal metal surfaces mechanisms, structure and applications, Current Opinion in Solid State and Materials Science 10 (2006) 173. [3] L.P. Bicelli, B. Bozzini, C. Mele, L. D’Urzo, A review of nanostructural aspects of metal electrodeposition, International Journal of Electrochemical Science 3 (2008) 356. [4] M.J. Esplandiu, M.A. Schneeweiss, D.M. Kolb, An in situ scanning tunneling microscopy study of Ag electrodeposition on Au(1 1 1), Physical Chemistry Chemical Physics 1 (1999) 4847. [5] X. Yan, G. Xu, Effect of surface modification of Cu with Ag by ball-milling on the corrosion resistance of low infrared emissivity coating, Materials Science and Engineering B 166 (2010) 152. [6] G. Zhang, Y. Kuang, J. Liu, Y. Cui, J. Chen, H. Zhou, Fabrication of Ag/Au bimetallic nanoparticles by UPD-redox replacement: Application in the electrochemical reduction of benzyl chloride, Electrochemistry Communications 12 (2010) 1233. [7] C. Bianchini, P.K. Shen, Palladium-based electrocatalysts for alcohol oxidation in half cells and in direct alcohol fuel cells, Chemical Reviews 109 (2009) 4183. [8] E. Antolini, Palladium in fuel cell catalysis, Energy & Environmental Science 2 (2009) 915. [9] M. Shao, Palladium-based electrocatalysts for hydrogen oxidation and oxygen reduction reactions, Journal of Power Sources 196 (2011) 2433. [10] M. Baldauf, D.M. Kolb, Formic acid oxidation on ultrathin Pd films on Au(hkl) and Pt(hkl) electrodes, Journal of Physical Chemistry 100 (1996) 11375. [11] T.J. Schmidt, V. Stamenkovic, M. Arenz, N.M. Markovic, P.N. Ross Jr., Oxygen electrocatalysis in alkaline electrolyte: Pt(hkl), Au(hkl) and the effect of Pdmodification, Electrochimica Acta 47 (2002) 3765.

[12] M.J. Ball, C.A. Lucas, N.M. Markovic, V. Stamenkovic, P.N. Ross, From submonolayer to multilayer—an in situ X-ray diffraction study of the growth of Pd films on Pt(1 1 1), Surface Science 518 (2002) 201. [13] J. Tang, M. Petri, L.A. Kibler, D.M. Kolb, Pd deposition onto Au(1 1 1) electrodes from sulphuric acid solution, Electrochimica Acta 51 (2005) 125. [14] J. Zhang, M. Huang, H. Ma, F. Tian, W. Pan, S. Chen, High catalytic activity of nanostructured Pd thin films electrochemically deposited on polycrystalline Pt and Au substrates towards electro-oxidation of methanol, Electrochemistry Communications 9 (2007) 1298. [15] L. Luo, M. Futamata, Competitive adsorption of water and CO on Pd modified Pt electrode from CH3 OH solution, Electrochemistry Communications 8 (2006) 231. [16] J. Zhang, Y. Mo, M.B. Vukmirovic, R. Klie, K. Sasaki, R.R. Adzic, Platinum monolayer electrocatalysts for O2 reduction: Pt monolayer on Pd(1 1 1) and on carbon-supported Pd nanoparticles, Journal of Physical Chemistry B 108 (2004) 10955. [17] S. Rondinini, P.R. Mussini, P. Muttini, G. Sello, Silver as a powerful electrocatalyst for organic halide reduction: the critical role of molecular structure, Electrochimica Acta 46 (2001) 3245. [18] Y. Xu, Y. Zhu, F. Zhao, C. Ma, Electrocatalytic reductive dehalogenation of polyhalogenated phenols in aqueous solution on Ag electrodes, Applied Catalysis A: General 324 (2007) 83. [19] A.A. Isse, S. Gottardello, C. Maccato, A. Gennaro, Silver nanoparticles deposited on glassy carbon: electrocatalytic activity for reduction of benzyl chloride, Electrochemistry Communications 8 (2006) 1707. [20] T. Maiyalagan, Synthesis, characterization and electrocatalytic activity of silver nanorods towards the reduction of benzyl chloride, Applied Catalysis A: General 340 (2008) 191. ´ V. Jovanovic, ´ G. Vlajnic, ´ J. Popic, ´ The electrocatalytic prop[21] M. Avramov-Ivic, erties of the oxides of noble metals in the electro-oxidation of some organic molecules, Journal of Electroanalytical Chemistry 423 (1997) 119. [22] G. Orozco, M.C. Pérez, A. Rincón, C. Gutiérrez, Electrooxidation of methanol on silver in alkaline medium, Journal of Electroanalytical Chemistry 495 (2000) 71. [23] Y. Wang, Z.M. Sheng, H. Yang, S.P. Jiang, C.M. Li, Electrocatalysis of carbon blackor activated carbon nanotubes-supported Pd–Ag towards methanol oxidation in alkaline media, International Journal of Hydrogen Energy 35 (2010) 10087. [24] S.T. Nguyen, H.M. Law, H.T. Nguyen, N. Kristian, S. Wang, S.H. Chan, X. Wang, Enhancement effect of Ag for Pd/C towards the ethanol electro-oxidation in alkaline media, Applied Catalysis B: Environmental 91 (2009) 507. [25] M.C. Oliveira, R. Rego, L.S. Fernandes, P.B. Tavares, Evaluation of the catalytic activity of Pd–Ag alloys on ethanol oxidation and oxygen reduction reactions in alkaline medium, Journal of Power Sources 196 (2011) 6092. [26] G. Li, L. Jiang, Q. Jiang, S. Wang, G. Sun, Preparation and characterization of PdxAgy/C electrocatalysts for ethanol electrooxidation reaction in alkaline media, Electrochimica Acta 56 (2011) 7703. [27] P. Poizot, L. Laffont-Dantras, J. Simonet, Palladized silver as new cathode material: evidence of a one-electron scission for primary alkyl iodides, Journal of Electroanalytical Chemistry 622 (2008) 204. [28] P. Poizot, O. Durand-Drouhin, M. Lejeune, J. Simonet, Changes in a glassy carbon surface by the cathodic generation of free alkyl radicals mediated by a silverpalladium catalyst, Carbon 50 (2012) 73. [29] Y.Y. Feng, L.X. Bi, Z.H. Liu, D.S. Kong, Z.Y. Yu, Significantly enhanced electrocatalytic activity for methanol electro-oxidation on Ag oxide-promoted PtAg/C catalysts in alkaline electrolyte, Journal of Catalysis 290 (2012) 18. [30] Z. Liu, L. Huang, L. Zhang, H. Ma, Y. Ding, Electrocatalytic oxidation of D-glucose at nanoporous Au and Au–Ag alloy electrodes in alkaline aqueous solutions, Electrochimica Acta 54 (2009) 7286. [31] A.Q. Wang, J.H. Liu, S.D. Lin, T.S. Lin, C.Y. Mou, A novel efficient Au–Ag alloy catalyst system: preparation, activity, and characterization, Journal of Catalysis 233 (2005) 186. [32] S.B. Aoun, G.S. Bang, T. Koga, Y. Nonaka, T. Sotomura, I. Taniguchi, Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions, Electrochemistry Communications 5 (2003) 317. [33] N.S. Marinkovic, J.X. Wang, J.S. Marinkovic, R.R. Adzic, Unusual adsorption properties of silver adlayers on the Pt(1 1 1) electrode surface, Journal of Physical Chemistry B 103 (1999) 139. [34] C. Jin, X. Sun, R. Dong, Z. Chen, Electrocatalytic oxidation of allyl alcohol on Pd and Pd-modified Au electrodes in alkaline solution, Applied Catalysis A: General 431–432 (2012) 57. [35] H. Quan, S.U. Park, J. Park, Electrochemical oxidation of glucose on silver nanoparticle-modified composite electrodes, Electrochimica Acta 55 (2010) 2232. [36] D.J. Guo, H.L. Li, Highly dispersed Ag nanoparticles on functional MWNT surfaces for methanol oxidation in alkaline solution, Carbon 43 (2005) 1259. [37] T. Kondo, J. Morita, M. Okamura, T. Saito, K. Uosaki, In situ structural study on underpotential deposition of Ag on Au(1 1 1) electrode using surface X-ray scattering technique, Journal of Electroanalytical Chemistry 532 (2002) 201. [38] S. Takakusagi, K. Kitamura, K. Uosaki, Electrodeposition of Ag and Pd on a reconstructed Au(1 1 1) electrode surface studied by in situ scanning tunneling microscopy, Electrochimica Acta 54 (2009) 5137. ´ M. Łukaszewski, G. Jerkiewicz, A. Czerwinski, ´ [39] M. Grden, Electrochemical behavior of palladium electrode: oxidation, electrodissolution and ionic adsorption, Electrochimica Acta 53 (2008) 7583. [40] I. Boskovic, S.V. Mentus, M. Pjescic, Electrochemical behavior of an Ag/TiO2 composite surfaces, Electrochimica Acta 51 (2006) 2793.