Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

Electrochemistry Communications 5 (2003) 317–320 www.elsevier.com/locate/elecom Electrocatalytic oxidation of sugars on silver-UPD single crystal gol...

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Electrochemistry Communications 5 (2003) 317–320 www.elsevier.com/locate/elecom

Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions Sami Ben Aoun a, Gyeong Sook Bang a, Tesshu Koga a, Yasuhiro Nonaka a, Tadashi Sotomura b, Isao Taniguchi a,* a

Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, 2-39-1, Kurokami, Kumamoto 860-8555, Japan b Matsushita Electric Industrial Co. Ltd, 3-1, Moriguchi, Osaka 570-8501, Japan Received 24 February 2003; received in revised form 6 March 2003; accepted 6 March 2003

Abstract A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. )0.44 V (vs. AgjAgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Sugar oxidation; Electrocatalytic activity; Au single crystal electrodes; UPD; Fuel cells; Cyclic voltammetry

1. Introduction Electrocatalytic oxidation of sugars is of a high interest from several points of view ranging from medical applications of the blood sugar sensing to ecological approaches like wastewater treatment in food industry. We are interested in the fuel cells application [1] and a key point for this purpose is the oxidation of sugars at a relatively negative potential compared to oxygen reduction. For this purpose, gold electrodes showed interesting catalytic effect [2–7], however, we are in need of more effective electrodes. Metal electrode surfaces modified with deposited metal ad-atoms are different in physical and chemical properties from both substrate metal and isolated bulk deposited metal. Under potential deposition (UPD) of catalytic-active metals on various noble-metal electrodes *

Corresponding author. Tel./fax: +81-96-342-3655. E-mail address: [email protected] (I. Taniguchi).

has been widely studied through the past [8,9] and recently [10–14]. Ad-atoms modified noble-metal electrodes have been applied to electrochemical reactions of a variety of compounds such as glucose [15], methanol [16,17], carbon monoxide [18–21] and dioxygen [22–24]. Structures of UPD modified gold surfaces with metal ad-atoms have also been studied like Ru [25], Ag [14,26,27], Cu [28] and Pt [29–31] ad-metals. The present work aims to investigate the electrocatalytic activity of metal ad-atoms onto gold single crystal surfaces towards the electrooxidation of sugars. Cyclic voltammetry was used as analytical tool to judge the performance of studied systems. 2. Experimental Au wire (99.99% in purity) was used for preparing the single crystal electrodes. D -glucose, D -xylose, lactose, sucrose and maltose purchased from Wako Pure

1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00055-9

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Chemical Industries, Ltd., together with D -mannose and D -mannolactone (Tokyo Kasei Kogyo Co., Ltd.) were used to prepare samples for cyclic voltammetry measurements. NaOH and H2 SO4 (ultra-pure grade, Kanto Chemical Co., Inc.) were used to prepare electrolytes. Ag2 SO4 (99.5%, Wako Pure Chemical Industries, Ltd.) was also used for UPD experiments. All solutions were prepared using ultra-pure water (Milli-Q 18:2 MX cm, Millipore system). Au(1 1 1) and Au(1 0 0) single crystal electrodes were prepared using the flame-annealing-quenching method with successively polishing mechanically to obtain enough surface areas for electrochemical measurements [32,33]. The freshly annealed single crystal surface was cooled in a hydrogen gas stream and then immediately immersed into hydrogen saturated ultra-pure water. The electrode surface was protected with a water droplet prior to immersion into electrochemical cell. Platinum plate electrode and home-made AgjAgCl (sat. KCl) were used as counter and reference electrodes, respectively, except for UPD experiments where Pt plate was used as a reference electrode. To close the circuit, Au(1 1 1) and Au(1 0 0) single crystal electrodes were put in contact with the solution using the hanging meniscus geometry. After filling the electrochemical cell, the electrolyte was deaerated by purging with high purity nitrogen for 20 min, and during measurements the nitrogen was kept to flow over the solution in the cell. Characterization of the single crystal surfaces were done by measuring the cyclic voltammogram in 0.1 M H2 SO4 solution [34] and the roughness of the surface was determined from the charge of the oxidative desorption of the adsorbed iodine [35]. UPD of Ag ad-atoms was made in a 1 mM Ag2 SO4 þ 1 M H2 SO4 solution. After the UPD modification, the electrode surface was washed thoroughly with ultra-pure water prior to transferring to sugarcontaining solution, and scan was started after 3 min of waiting time in order to stabilize the conditions. Cyclic voltammetry measurements were carried out using either a PS-06 manual polarization unit (Toho Giken, Japan) or CV-50 W voltammetric analyzer (Bioanalytical Systems, Inc.).

Silver-UPD onto Au(1 1 1) single crystal electrode (ca. 0:085 cm2 in surface area) was conducted and a well-defined cyclic voltammogram was obtained (see Fig. 1), which is in good agreement with published works [26,27]. The deposition process was conducted by starting the scan immediately after immersion of the gold electrode at ca. +0.35 V (vs. Pt plate) at a scan rate of 5 mV s1 . The scan was stopped either for a deposition of one-third or one monolayer occurring, respectively, at ca. +0.21 and ca. )0.29 V (vs. Pt plate) [26,27]. The Ag modified gold electrode was then transferred to the sugar-containing cell after thoroughly rinsing with Milli-Q water, and cyclic voltammograms were measured at a scan rate of 50 mV s1 . Fig. 2 shows the cyclic voltammograms obtained with a bare Au(1 1 1) single crystal electrode and Ag adatoms modified electrodes. Oxidation of glucose starts

Fig. 1. Cyclic voltammogram for Ag-UPD on Au(1 1 1) in a 1 M H2 SO4 þ 103 M Ag2 SO4 solution. Scan rate was 5 mV s1 .

3. Results and discussion Oxidation of sugars at Au(1 1 1) single crystal electrodes was studied at various pH solutions and cyclic voltammograms clearly show that in alkaline media, sugars are oxidized at relatively negative potentials, however, when pH decreased towards the acidic media, positive shift in the oxidation potential was observed. Results mentioned in this work were carried on mainly in 1–10 mM sugar dissolved in a 0.1 M NaOH solution.

Fig. 2. Cyclic voltammograms of 10 mM glucose in a 0.1 M NaOH solution at Au(1 1 1) single crystal electrode (solid line), Au(1 1 1)– (1  1) Ag monolayer electrode (dash-dotted line) and Ag–Au(1 1 1)– p p ( 3  3)R30° Ag one-third monolayer electrode (dashed line) at a scan rate of 50 mV s1 .

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around )0.55 V (vs. AgjAgCl) on a bare Au electrode and this potential shiftedp to around )0.65 V (vs. p AgjAgCl) at an Au(1 1 1)–( 3  3)R30° Ag one-third monolayer electrode with oxidation current peaks shifting from ca. )0.26 to ca. )0.40 V (vs. AgjAgCl), respectively. On the other hand, deposition of Ag monolayer leads to a little decrease in the oxidation peak current. By using an Au(1 0 0) electrode, similar catalytic effect of the Ag ad-layer [36] was observed on glucose oxidation. The peak potential for the oxidation of glucose was ca. )0.5 V (vs. AgjAgCl) at Ag ad-layer modified Au(1 0 0) electrodes (see Fig. 3), which is slightly more negative than at Ag ad-layer modified Au(1 1 1) electrodes. This work was extended to a variety of sugars and we obtained different behavior depending on sugar structures: Aldose type monosaccharides such as glucose, mannose and xylose showed one typical oxidation peak around 0:3  0:1 V (vs. AgjAgCl) at a bare Au electrode shifting to ca. 0:4  0:1 V (vs. AgjAgCl) at Ag deposited Au electrodes with a slight decrease in peak current in the case of one monolayer deposition of Ag and an increase in current on one-third monolayer deposition of Ag. However, ketose type monosaccharides such as fructose and sorbose did not give any catalytic oxidation current around 0:3  0:1 V (vs. AgjAgCl). Disaccharides containing aldose type monosaccharides such as maltose and lactose gave the same behavior with difference of ca. )0.1 V in oxidation peak potential in all cases examined. Disaccharides like sucrose did not give any significant current at potential more negative than )0.05 or )0.25 V vs. AgjAgCl at bare or modified gold electrodes, respectively. These results suggest that the aldehyde (or hemiacetal) is oxidized first at C1 carbon atom at such negative potentials and that the oxidation

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of the acetal part is more difficult whether the glycoside bond is a (maltose) or b (lactose) on gold electrodes [3,4,7,37]. Possible explanation of the catalytic activity of Ag ad-layer modified Au electrodes is that the change in charge of the electrode surface after Ag deposition allowed the formation of M–OH (M ¼ Au) at more negative potentials than at a bare Au electrode. On the other hand, two-electron oxidation products such as gluconolactone (or gluconic acid) and mannolactone (or mannonic acid) did not give any oxidation current at potential more negative than )0.15 and )0.30 V vs. AgjAgCl at bare and Ag ad-layer modified gold electrodes, respectively, showing the absence of more than two-electron oxidation at both electrodes. Well-defined cyclic voltammogram of the prepared p p Au(1 1 1)–( 3  3)R30° Ag one-third monolayer electrode was still observed in a pure 0.1 M NaOH solution with oxidation and reduction peaks of the deposited silver after using for cyclic voltammograms in the presence of sugars, showing the stability of this kind of electrodes in the conditions of the present work. We tried to study ad-layer deposition of other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.), but Ag was of the best catalyst. Results ranged from negative catalytic activity (Cu, Co and Cd cases) to quasi-similar activity with Au(1 1 1) itself (Ru and Pt cases) or even to no UPD like Ir case under the conditions used in the present work. We also tried to combine two UPD metals (e.g., Cu and Ag), however, at present, the catalytic activity was not better than Ag modified Au itself. However, an improvement in the number of electrons would be of high interest and a variety of noble metals and their combinations are being studied.

4. Conclusion Ag ad-layer modified Au electrodes acted as good catalysts for sugar oxidation, and the best improvement compared to the bare Au(1 1 1) and Au(1 0 0) single crystal electrodes was obtained with Ag one-third monolayer modified Au electrodes, on which a negative shift of the oxidation potential was obtained (ca. 0.15– 0.25 V) accompanied with a slight increase in peak current (probably due to an increase in the surface area). This positive catalytic activity would be of great interest for the preparation of sugar–air battery and we are undertaking some preliminary battery performance measurements, which will be published separately. Fig. 3. Cyclic voltammograms of 1 mM glucose in a 0.1 M NaOH solution at Au(1 0 0) single crystal electrode (solid line), Au(1 0 0)– (1  1) Ag monolayer electrode (dash-dotted line) and Au(1 0 0)– p p c( 2  5 2)R45° Ag one-third monolayer electrode (dashed line) at a scan rate of 50 mV s1 .

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