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Au bimetallic nanoparticles by UPD-redox replacement: Application in the electrochemical reduction of benzyl chloride
Electrochemistry Communications 12 (2010) 1233–1236
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Fabrication of Ag/Au bimetallic nanoparticles by UPD-redox replacement: Application in the electrochemical reduction of benzyl chloride Guoping Zhang a, Yafei Kuang a,⁎, Jinping Liu a, Yanqing Cui a, Jinhua Chen a,b, Haihui Zhou a,b,⁎ a b
College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China
a r t i c l e
i n f o
Article history: Received 21 May 2010 Received in revised form 8 June 2010 Accepted 18 June 2010 Available online 25 June 2010 Keywords: Bimetallic Nanoparticles Underpotential deposition Electroreduction Benzyl chloride
a b s t r a c t The bimetallic Ag/Au nanoparticles were prepared by underpotential deposition-redox replacement technique on the basis of Au nanoparticles modified glassy carbon (GC) electrode. The as-prepared Ag/Au nanoparticles were characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The Ag/Au bimetallic nanoparticles modified GC electrode with low-Ag loading exhibits much better catalytic activity for the reduction of benzyl chloride than Ag nanoparticles modified GC electrode. The result is attributed to the synergic effect between Ag and Au in the reduction process. The chronoamperometry test shows that the Ag/Au nanoparticles possess long-term performance in the electrolysis. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Electrosynthesis, being an attractive method since the use of electron as a reagent that does not involve the formation of any byproduct, has received significant research interest from both academia and industry [1]. Among electrosynthesis reactions, the electrochemical reduction of organic halides (RX) has been a central topic both from mechanistic and synthetic points of view for the last few decades [2,3]. And it is also investigated as a promising method of preparing fine compounds, treating halogenated wastes, fixing carbon and immobilizing radicals [4–7]. Generally, the materials, which are capable of remarkable positive shifting the reduction potential of RX, are used as cathode. Among various cathode materials (including Ag, Pd, Hg), silver has been found to possess extraordinary electrocatalytic property for the reduction of RX [8]. Because of the interesting optical, microelectronics and catalytic properties of nanomaterial, some nano-Ag materials were employed for the electroreduction of organic halides. For example, silver nanoparticles and nanorods modified glassy carbon (GC) electrodes have been prepared, which possess remarkable electrocatalytic activity for the reduction of benzyl chloride compared to the bulk silver [9,10]. Bimetallic materials have attracted much attention because of their potential application in various technological fields due to their ⁎ Corresponding authors. Tel.: +86 731 88821874; fax: +86 731 88713642. Zhou is to be contacted at State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China. E-mail addresses: [email protected] (Y. Kuang), [email protected] (H. Zhou). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.06.027
unique catalytic activities, which differ from their monometallic counterparts [11]. Underpotential deposition (UPD) can be considered as an important technology to fabricate bimetallic nanomaterials and it has attracted special consideration [12]. During the process of UPD, electrodeposition occurs at potentials more positive than that for bulk deposition of the metal and forms the monolayer or submonolayer of a foreign metal on the surface of noble metal. Epitaxial metallic layers feature reduced ohmic resistance and electromigration can be used for the synthesis of low-metal loading bimetallic catalysts [13–15]. Recently, Li et al. prepared low-metal loading bimetallic Au– Pt and trimetallic Au–Pd–Pt catalysts by UPD-redox replacement technique for the methanol oxidation [16]. In this paper, we employed the UPD-redox replacement technique to fabricate bimetallic Ag/Au nanoparticles on GC electrode. The asprepared modified electrode with Ag/Au nanoparticles was used as cathode for the electrochemical reduction of benzyl chloride for the first time.
2. Experimental 2.1. Chemicals Acetonitrile (CH3CN, HPLC grade, Sinopharm Chemical Reagent Co., Ltd) and Tetraethylammonium tetrafluoroborate ((C2H5)4NBF4, 99%, Alfa Aesar) were used directly without further purification. Poly (Nvinyl-2-pyrrolidone) (PVP, MW-10000), KCl (AR), NaNO3 (AR) and Pb (NO3)2 (AR) were supplied by Tianjin Damao Reagent Factory. AgNO3 and HAuCl4·4H2O were of analytical grade and used as received from
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G. Zhang et al. / Electrochemistry Communications 12 (2010) 1233–1236
Shanghai Chemical Reagent Factory (China). Water used in the experiments was deionized (DI), double distilled prior to use.
2.2. Apparatus and procedure Au nanoparticles were electrodeposited on GC electrode by potentiostatic technique. The electrolyte was a homogenous mixture containing 2.5 mL aqueous PVP (5 g L− 1), 0.5 mL aqueous HAuCl4·4H2O (0.005 M) and 0.5 mL aqueous KCl (0.1 M), in which KCl served as supporting electrolyte, and PVP worked as size- and morphologycontroller to induce preferential growth of Au nanoparticles [17]. The electrodeposition was carried out at −0.4 V (vs. Ag/AgCl) for 600 s. The resultant Au nanoparticles modified GC electrode was denoted as Au/GC. Furthermore, the same method was used to fabricate Ag nanoparticles on the surface of GC except for that the electrolyte was a mixture of 2.5 mL aqueous PVP (5 g L− 1), 0.5 mL aqueous AgNO3 (0.005 M) and 0.5 mL aqueous NaNO3 (0.1 M). The resultant modified electrode was denoted as Ag/GC. Bimetallic nanoparticles were fabricated on the basis of the Au/GC using the UPD-redox replacement technique. A UPD monolayer of Pb was first deposited onto the Au/GC by means of potentiostatic polarization at a designated UPD potential (−0.25 V vs.Ag/AgCl) in a solution of 50 mM Pb(NO3)2 + 0.1 M HNO3. The as-prepared modified electrode was denoted as Pb/Au/GC. The Ag/Au nanoparticles were prepared by replacement of the Pb UPD monolayer in a solution of 0.05 M AgNO3 + 0.5 M H2 SO4 for 1 h. The as-prepared Ag/Au nanoparticles modified GC electrode was denoted as Ag/Au/GC. The morphologies of various modified electrodes were investigated by scanning electron microscopy HITACHI S-4800 (SEM) at an accelerating voltage of 10.0 kV. The element analysis was conducted by energy-dispersive X-ray spectroscopy (EDS) microanalyzer mounted on the HITACHI S-4800. All electrochemical experiments were carried out with a CHI 660C electrochemistry workstation (Shanghai Chenhua Instrument Factory, China) at room temperature. The electrochemical activities of various modified GC electrodes (S = 0.076 cm2) were tested in electrochemical cell with a three-electrode configuration. A Pt wire and an Ag wire were used as counter and reference electrode respectively, and the GC covered with Au, Ag or Ag/Au bimetallic nanoparticles was used as the working electrode. The electrolyte used was a CH3CN solution with 10 mM PhCH2Cl + 0.1 M (C2H5)4NBF4, which was deaerated by N2 (99.99%) in advance.
Fig. 1. Cyclic voltammograms of the Au/GC in 0.1 M HNO3 (a) and 50 mM Pb(NO3)2 + 0.1 M HNO3 (b) solution. Scan rate: 50 mV s− 1.
3. Results and discussion Fig. 1 shows the cyclic voltammograms (CVs) of the Au/GC in 0.1 M HNO3 and 50 mM Pb(NO3)2 + 0.1 M HNO3 solution. Compared with the CV of the Au/GC in 0.1 M HNO3 (Fig. 1a), the underpotential peak (−0.28 V vs. Ag/AgCl) of Pb is more positive than the bulk deposition potential (−0.46 V vs. Ag/AgCl) as shown in the CV of the Au/GC in 50 mM Pb(NO3)2 + 0.1 M HNO3 solution (Fig. 1b) [12]. By controlling the potential at −0.25 V before the commencement of bulk Pb deposition, a monolayer of Pb atoms was deposited underpotentially on the Au nanoparticles surface with ease. Then, a clean Pbmonolayer-modified Au/GC was immersed into an aqueous solution of 0.05 M AgNO3 + 0.5 M H2SO4, the redox replacement between the UPD Pb and Ag+, driven by the large standard reduction potentials gap between Pb2+/Pb (− 0.126 V) and Ag+/Ag (0.799 V) [18], yielded a thin layer of Ag on the surface of Au nanoparticles. The SEM images of Au/GC, Ag/Au/GC and Ag/GC are shown in Fig. 2. It is clear that Au and Ag/Au nanoparticles cover uniformly on the surface of GC electrode, and the mean size of nanoparticles is about 40 nm (Fig. 2a and 2b). In virtue of UPD method, a tiny amount of Ag, the equivalent of a monolayer, is deposited onto the Au nanoparticles. Therefore, there is no much difference in the size and morphology between Ag/Au nanoparticles and Au nanoparticles. The dispersive Ag nanoparticles also cover uniformly on the surface of GC electrode with a mean size of 30 nm (Fig. 2c). The EDS analysis of Pb/ Au/GC is shown in Fig. 2d. It can be seen that there is only 0.02 at.% of Pb in the composition of the Pb/Au/GC. After redox replacement, the EDS analysis of Ag/Au/GC is shown in Fig. 2e. There is not any Pb in the composition and the surface composition of Ag is about 0.03 at.%. Therefore, the UPD-redox replacement technique can be considered as a convenient method for fabricating Ag/Au bimetallic nanoparticles to modify GC electrode with low-Ag loading. The electrocatalytic activities of various modified electrodes were tested for the reduction of benzyl chloride. Fig. 3 shows CVs of PhCH2Cl at Au/GC, Ag/GC and Ag/Au/GC electrodes in CH3CN solution with 10 mM PhCH2Cl + 0.1 M (C2H5)4NBF4. A single irreversible peak (−1.84 V) is observed for the Ag/GC electrode compared with Au/GC electrode, and the current density is 4.84 mA cm− 2. This indicates that Ag nanoparticles possess more prominent activity than Au nanoparticles for the electroreduction of PhCH2Cl. Furthermore, the voltammetric behaviour of benzyl chloride at the Ag/Au/GC is quite similar to that observed at Ag/GC except for the remarkable positive shift of the peak potential (− 1.62 V) and much larger current density (5.39 mA cm− 2). The Ag/Au/GC electrode with low-Ag loading presents the best catalytic activity. Generally, the peak potential usually strongly depends on the properties of electrode materials [10]. Accordingly, the specificity of Ag/Au bimetallic nanoparticle obeys probably a synergic effect between Ag and Au during the reduction process [19,20]. Therefore, the as-prepared Ag/Au nanoparticle can be used as an effective material for the reduction of PhCH2Cl. Chronoamperometry test was performed to investigate the longterm performance of the modified electrode for the reduction of PhCH2Cl in abundant electrolyte. Fig. 4 shows current–time curves for electroreduction of benzyl chloride measured at a fixed potential for 3600 s. The polarization current shows a rapid decay during the initial period because of the adsorption between electrode materials and PhCH2Cl [19]. With the proceeding of electrolysis, the adsorption and desorption of intermediate on the surface of Ag/Au nanoparticles reached a dynamic equilibrium and the current kept at a more comparative steady state than that of Ag/GC. At the end of the 3600 s test, the reduction current on the Ag/Au/GC is 0.18 mA, dropping to ca. 47% of its initial value (0.38 mA) as shown in Fig. 4b. However, the reduction current on the Ag/GC is 0.13 mA, dropping to ca. 35% of its initial value (0.37 mA) after 3600 s as shown in Fig. 4a. These results further confirm better catalytic activity and long-term performance of Ag/Au/GC electrode for the reduction of benzyl chloride.
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Fig. 2. SEM images of the Au/GC (a), Ag/Au/GC (b) and Ag/GC (c). EDS analysis of the Pb/Au/GC (d) and Ag/Au/GC electrode (e). The signals of C come from the GC electrode and the table shows the EDS atomic percentages.
Fig. 3. Cyclic voltammograms of PhCH2Cl recorded at 100 mV s− 1 in CH3CN solution with 10 mM PhCH2Cl + 0.1 M (C2H5)4NBF4 at various modified electrodes Au/GC(a), Ag/GC (b) and Ag/Au/GC (c).
Fig. 4. Chronoamperometric curves for reduction of benzyl chloride at −1.80 V (vs. Ag) on Ag/GC electrode (a) and −1.60 V (vs. Ag) on Ag/Au/GC electrode (b) in CH3CN solution with 10 mM PhCH2Cl + 0.1 M (C2H5)4NBF4.
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4. Conclusions In summary, Ag/Au bimetallic nanoparticles are fabricated by the underpotential deposition (UPD)-redox replacement technique. The as-prepared Ag/Au nanoparticles modified GC electrode with low-Ag loading can be used as cathode for the electroreduction of benzyl chloride. Simultaneously, Ag/Au nanoparticles exhibit excellent catalytic property and long-term performance in the electrolysis. These new findings are not only a better understanding of the synergic effect between different metal compositions on the combined catalytic ability but also helpful for the development of more effective catalysts suitable for the reduction of organic halides. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant no. 20673036, J0830415) and Natural Science Foundation of Hunan Province (No. 09JJ3025). References [1] J.I. Yoshida, K. Kataoka, R. Horcajada, A. Nagaki, Chem. Rev. 108 (2008) 2265. [2] A.A. Isse, L. Falciola, P.R. Mussini, A. Gennaro, Chem. Commun. (2006) 344.
[3] D.F. Niu, A.J. Zhang, T. Xue, J.B. Zhang, S.F. Zhao, J.X. Lu, Electrochem. Commun. 10 (2008) 1498. [4] D.G. Peters, in: H. Lund, O. Hammerich (Eds.), Organic Electrochemistry, fourth ed., Marcel Dekker, New York, 2001, p. 341. [5] Z. Liu, E.A. Betterton, R.G. Arnold, Environ. Sci. Technol. 34 (2000) 804. [6] D.F. Niu, L.P. Xiao, A.J. Zhang, G.R. Zhang, Q.Y. Tan, J.X. Lu, Tetrahedron 64 (2008) 10517. [7] V. Jouikov, J. Simonet, Electrochem. Commun. 12 (2010) 331. [8] S.B. Rondinini, P.R. Mussini, F. Crippa, G. Sello, Electrochem. Commun. 2 (2000) 491. [9] A.A. Isse, S. Gottardello, C. Maccato, A. Gennaro, Electrochem. Commun. 8 (2006) 1707. [10] T. Maiyalagan, Appl. Catal. A 340 (2008) 191. [11] P.H.C. Camargo, Y.J. Xiong, L. Ji, J.M. Zuo, Y.N. Xia, J. Am. Chem. Soc. 129 (2007) 15452. [12] R.R. Adzic, in: H. Gerischer (Ed.), Advances in Electrochemistry and Electrochemical Engineering, vol. 13, Wiley, New York, 1984, p. 159. [13] R. Vasilic, L.T. Viyannalage, N. Dimitrov, J. Electrochem. Soc. 153 (2006) C648. [14] F.W. Campbell, R.G. Compton, Int. J. Electrochem. Sci. 5 (2010) 407. [15] F.W. Campbell, Y.G. Zhou, R.G. Compton, New J. Chem. 34 (2010) 187. [16] W.J. Li, H.Y. Ma, J.T. Zhang, X.Y. Liu, X.L. Feng, J. Phys. Chem. C 113 (2009) 1738. [17] S.X. Huang, H.Y. Ma, X.K. Zhang, F.F. Yong, X.L. Feng, W. Pan, X.N. Wang, Y. Wang, S. H. Chen, J. Phys. Chem. B 109 (2005) 19823. [18] Y.L. Wang, L. Cai, Y.N. Xia, Adv. Mater. 17 (2005) 473. [19] P. Poizot, L. Laffont-Dantras, J. Simonet, J. Electroanal. Chem. 622 (2008) 204. [20] A.Q. Wang, J.H. Liu, S.D. Lin, T.S. Lin, C.Y. Mou, J. Catal. 233 (2005) 186.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Fabrication of Ag/Au bimetallic nanoparticles by UPD-redox replacement: Application in the electrochemical reduction of benzyl chloride Guoping Zhang a, Yafei Kuang a,⁎, Jinping Liu a, Yanqing Cui a, Jinhua Chen a,b, Haihui Zhou a,b,⁎ a b
College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China
a r t i c l e
i n f o
Article history: Received 21 May 2010 Received in revised form 8 June 2010 Accepted 18 June 2010 Available online 25 June 2010 Keywords: Bimetallic Nanoparticles Underpotential deposition Electroreduction Benzyl chloride
a b s t r a c t The bimetallic Ag/Au nanoparticles were prepared by underpotential deposition-redox replacement technique on the basis of Au nanoparticles modified glassy carbon (GC) electrode. The as-prepared Ag/Au nanoparticles were characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The Ag/Au bimetallic nanoparticles modified GC electrode with low-Ag loading exhibits much better catalytic activity for the reduction of benzyl chloride than Ag nanoparticles modified GC electrode. The result is attributed to the synergic effect between Ag and Au in the reduction process. The chronoamperometry test shows that the Ag/Au nanoparticles possess long-term performance in the electrolysis. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Electrosynthesis, being an attractive method since the use of electron as a reagent that does not involve the formation of any byproduct, has received significant research interest from both academia and industry [1]. Among electrosynthesis reactions, the electrochemical reduction of organic halides (RX) has been a central topic both from mechanistic and synthetic points of view for the last few decades [2,3]. And it is also investigated as a promising method of preparing fine compounds, treating halogenated wastes, fixing carbon and immobilizing radicals [4–7]. Generally, the materials, which are capable of remarkable positive shifting the reduction potential of RX, are used as cathode. Among various cathode materials (including Ag, Pd, Hg), silver has been found to possess extraordinary electrocatalytic property for the reduction of RX [8]. Because of the interesting optical, microelectronics and catalytic properties of nanomaterial, some nano-Ag materials were employed for the electroreduction of organic halides. For example, silver nanoparticles and nanorods modified glassy carbon (GC) electrodes have been prepared, which possess remarkable electrocatalytic activity for the reduction of benzyl chloride compared to the bulk silver [9,10]. Bimetallic materials have attracted much attention because of their potential application in various technological fields due to their ⁎ Corresponding authors. Tel.: +86 731 88821874; fax: +86 731 88713642. Zhou is to be contacted at State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China. E-mail addresses: [email protected] (Y. Kuang), [email protected] (H. Zhou). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.06.027
unique catalytic activities, which differ from their monometallic counterparts [11]. Underpotential deposition (UPD) can be considered as an important technology to fabricate bimetallic nanomaterials and it has attracted special consideration [12]. During the process of UPD, electrodeposition occurs at potentials more positive than that for bulk deposition of the metal and forms the monolayer or submonolayer of a foreign metal on the surface of noble metal. Epitaxial metallic layers feature reduced ohmic resistance and electromigration can be used for the synthesis of low-metal loading bimetallic catalysts [13–15]. Recently, Li et al. prepared low-metal loading bimetallic Au– Pt and trimetallic Au–Pd–Pt catalysts by UPD-redox replacement technique for the methanol oxidation [16]. In this paper, we employed the UPD-redox replacement technique to fabricate bimetallic Ag/Au nanoparticles on GC electrode. The asprepared modified electrode with Ag/Au nanoparticles was used as cathode for the electrochemical reduction of benzyl chloride for the first time.
2. Experimental 2.1. Chemicals Acetonitrile (CH3CN, HPLC grade, Sinopharm Chemical Reagent Co., Ltd) and Tetraethylammonium tetrafluoroborate ((C2H5)4NBF4, 99%, Alfa Aesar) were used directly without further purification. Poly (Nvinyl-2-pyrrolidone) (PVP, MW-10000), KCl (AR), NaNO3 (AR) and Pb (NO3)2 (AR) were supplied by Tianjin Damao Reagent Factory. AgNO3 and HAuCl4·4H2O were of analytical grade and used as received from
1234
G. Zhang et al. / Electrochemistry Communications 12 (2010) 1233–1236
Shanghai Chemical Reagent Factory (China). Water used in the experiments was deionized (DI), double distilled prior to use.
2.2. Apparatus and procedure Au nanoparticles were electrodeposited on GC electrode by potentiostatic technique. The electrolyte was a homogenous mixture containing 2.5 mL aqueous PVP (5 g L− 1), 0.5 mL aqueous HAuCl4·4H2O (0.005 M) and 0.5 mL aqueous KCl (0.1 M), in which KCl served as supporting electrolyte, and PVP worked as size- and morphologycontroller to induce preferential growth of Au nanoparticles [17]. The electrodeposition was carried out at −0.4 V (vs. Ag/AgCl) for 600 s. The resultant Au nanoparticles modified GC electrode was denoted as Au/GC. Furthermore, the same method was used to fabricate Ag nanoparticles on the surface of GC except for that the electrolyte was a mixture of 2.5 mL aqueous PVP (5 g L− 1), 0.5 mL aqueous AgNO3 (0.005 M) and 0.5 mL aqueous NaNO3 (0.1 M). The resultant modified electrode was denoted as Ag/GC. Bimetallic nanoparticles were fabricated on the basis of the Au/GC using the UPD-redox replacement technique. A UPD monolayer of Pb was first deposited onto the Au/GC by means of potentiostatic polarization at a designated UPD potential (−0.25 V vs.Ag/AgCl) in a solution of 50 mM Pb(NO3)2 + 0.1 M HNO3. The as-prepared modified electrode was denoted as Pb/Au/GC. The Ag/Au nanoparticles were prepared by replacement of the Pb UPD monolayer in a solution of 0.05 M AgNO3 + 0.5 M H2 SO4 for 1 h. The as-prepared Ag/Au nanoparticles modified GC electrode was denoted as Ag/Au/GC. The morphologies of various modified electrodes were investigated by scanning electron microscopy HITACHI S-4800 (SEM) at an accelerating voltage of 10.0 kV. The element analysis was conducted by energy-dispersive X-ray spectroscopy (EDS) microanalyzer mounted on the HITACHI S-4800. All electrochemical experiments were carried out with a CHI 660C electrochemistry workstation (Shanghai Chenhua Instrument Factory, China) at room temperature. The electrochemical activities of various modified GC electrodes (S = 0.076 cm2) were tested in electrochemical cell with a three-electrode configuration. A Pt wire and an Ag wire were used as counter and reference electrode respectively, and the GC covered with Au, Ag or Ag/Au bimetallic nanoparticles was used as the working electrode. The electrolyte used was a CH3CN solution with 10 mM PhCH2Cl + 0.1 M (C2H5)4NBF4, which was deaerated by N2 (99.99%) in advance.
Fig. 1. Cyclic voltammograms of the Au/GC in 0.1 M HNO3 (a) and 50 mM Pb(NO3)2 + 0.1 M HNO3 (b) solution. Scan rate: 50 mV s− 1.
3. Results and discussion Fig. 1 shows the cyclic voltammograms (CVs) of the Au/GC in 0.1 M HNO3 and 50 mM Pb(NO3)2 + 0.1 M HNO3 solution. Compared with the CV of the Au/GC in 0.1 M HNO3 (Fig. 1a), the underpotential peak (−0.28 V vs. Ag/AgCl) of Pb is more positive than the bulk deposition potential (−0.46 V vs. Ag/AgCl) as shown in the CV of the Au/GC in 50 mM Pb(NO3)2 + 0.1 M HNO3 solution (Fig. 1b) [12]. By controlling the potential at −0.25 V before the commencement of bulk Pb deposition, a monolayer of Pb atoms was deposited underpotentially on the Au nanoparticles surface with ease. Then, a clean Pbmonolayer-modified Au/GC was immersed into an aqueous solution of 0.05 M AgNO3 + 0.5 M H2SO4, the redox replacement between the UPD Pb and Ag+, driven by the large standard reduction potentials gap between Pb2+/Pb (− 0.126 V) and Ag+/Ag (0.799 V) [18], yielded a thin layer of Ag on the surface of Au nanoparticles. The SEM images of Au/GC, Ag/Au/GC and Ag/GC are shown in Fig. 2. It is clear that Au and Ag/Au nanoparticles cover uniformly on the surface of GC electrode, and the mean size of nanoparticles is about 40 nm (Fig. 2a and 2b). In virtue of UPD method, a tiny amount of Ag, the equivalent of a monolayer, is deposited onto the Au nanoparticles. Therefore, there is no much difference in the size and morphology between Ag/Au nanoparticles and Au nanoparticles. The dispersive Ag nanoparticles also cover uniformly on the surface of GC electrode with a mean size of 30 nm (Fig. 2c). The EDS analysis of Pb/ Au/GC is shown in Fig. 2d. It can be seen that there is only 0.02 at.% of Pb in the composition of the Pb/Au/GC. After redox replacement, the EDS analysis of Ag/Au/GC is shown in Fig. 2e. There is not any Pb in the composition and the surface composition of Ag is about 0.03 at.%. Therefore, the UPD-redox replacement technique can be considered as a convenient method for fabricating Ag/Au bimetallic nanoparticles to modify GC electrode with low-Ag loading. The electrocatalytic activities of various modified electrodes were tested for the reduction of benzyl chloride. Fig. 3 shows CVs of PhCH2Cl at Au/GC, Ag/GC and Ag/Au/GC electrodes in CH3CN solution with 10 mM PhCH2Cl + 0.1 M (C2H5)4NBF4. A single irreversible peak (−1.84 V) is observed for the Ag/GC electrode compared with Au/GC electrode, and the current density is 4.84 mA cm− 2. This indicates that Ag nanoparticles possess more prominent activity than Au nanoparticles for the electroreduction of PhCH2Cl. Furthermore, the voltammetric behaviour of benzyl chloride at the Ag/Au/GC is quite similar to that observed at Ag/GC except for the remarkable positive shift of the peak potential (− 1.62 V) and much larger current density (5.39 mA cm− 2). The Ag/Au/GC electrode with low-Ag loading presents the best catalytic activity. Generally, the peak potential usually strongly depends on the properties of electrode materials [10]. Accordingly, the specificity of Ag/Au bimetallic nanoparticle obeys probably a synergic effect between Ag and Au during the reduction process [19,20]. Therefore, the as-prepared Ag/Au nanoparticle can be used as an effective material for the reduction of PhCH2Cl. Chronoamperometry test was performed to investigate the longterm performance of the modified electrode for the reduction of PhCH2Cl in abundant electrolyte. Fig. 4 shows current–time curves for electroreduction of benzyl chloride measured at a fixed potential for 3600 s. The polarization current shows a rapid decay during the initial period because of the adsorption between electrode materials and PhCH2Cl [19]. With the proceeding of electrolysis, the adsorption and desorption of intermediate on the surface of Ag/Au nanoparticles reached a dynamic equilibrium and the current kept at a more comparative steady state than that of Ag/GC. At the end of the 3600 s test, the reduction current on the Ag/Au/GC is 0.18 mA, dropping to ca. 47% of its initial value (0.38 mA) as shown in Fig. 4b. However, the reduction current on the Ag/GC is 0.13 mA, dropping to ca. 35% of its initial value (0.37 mA) after 3600 s as shown in Fig. 4a. These results further confirm better catalytic activity and long-term performance of Ag/Au/GC electrode for the reduction of benzyl chloride.
G. Zhang et al. / Electrochemistry Communications 12 (2010) 1233–1236
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Fig. 2. SEM images of the Au/GC (a), Ag/Au/GC (b) and Ag/GC (c). EDS analysis of the Pb/Au/GC (d) and Ag/Au/GC electrode (e). The signals of C come from the GC electrode and the table shows the EDS atomic percentages.
Fig. 3. Cyclic voltammograms of PhCH2Cl recorded at 100 mV s− 1 in CH3CN solution with 10 mM PhCH2Cl + 0.1 M (C2H5)4NBF4 at various modified electrodes Au/GC(a), Ag/GC (b) and Ag/Au/GC (c).
Fig. 4. Chronoamperometric curves for reduction of benzyl chloride at −1.80 V (vs. Ag) on Ag/GC electrode (a) and −1.60 V (vs. Ag) on Ag/Au/GC electrode (b) in CH3CN solution with 10 mM PhCH2Cl + 0.1 M (C2H5)4NBF4.
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4. Conclusions In summary, Ag/Au bimetallic nanoparticles are fabricated by the underpotential deposition (UPD)-redox replacement technique. The as-prepared Ag/Au nanoparticles modified GC electrode with low-Ag loading can be used as cathode for the electroreduction of benzyl chloride. Simultaneously, Ag/Au nanoparticles exhibit excellent catalytic property and long-term performance in the electrolysis. These new findings are not only a better understanding of the synergic effect between different metal compositions on the combined catalytic ability but also helpful for the development of more effective catalysts suitable for the reduction of organic halides. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant no. 20673036, J0830415) and Natural Science Foundation of Hunan Province (No. 09JJ3025). References [1] J.I. Yoshida, K. Kataoka, R. Horcajada, A. Nagaki, Chem. Rev. 108 (2008) 2265. [2] A.A. Isse, L. Falciola, P.R. Mussini, A. Gennaro, Chem. Commun. (2006) 344.
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