Electrocatalytic Oxidation of Glucose on Natural ...

Electrocatalytic oxidation of by A Chtaini et al., Bulletin of the Catalysis Society of India, 8 (2009) 11-15

Electrocatalytic Oxidation of Glucose on Natural Phosphate Modified Iron Electrode A. Chtaini *, A. El Mhammedi, R. Najih and M. Bakasse Equipe d’Electrochimie Moléculaire et Matériaux Inorganiques, Faculté des Sciences et Techniques de Beni Mellal, BP: 523, Beni Mellal, Morocco [email protected], Tel : 0021265129257, Fax : 0021223485201

This work aims to investigate the electrolytic oxidation of glucose on natural phosphate modified electrode was investigated the cyclic voltammetry. The electrodes were obtained by depositing the natural phosphate onto substrates. The natural phosphate modified iron electrode display high electrocatalytic activity for the glucose oxidation. The effect of the under-potential deposition of lead in the the reaction is studied. Key Words: Natural phosphate; Glucose oxidation; Modified electrodes; Nyquist plots; Lead modified electrode; cyclic voltammetry. 1. Introduction The electrocatalytic oxidation of glucose has been the subject of only a few investigations. The chemical oxidation of sucrose was first mentioned in [1-3]. The analysed the oxidation products of fructose, glucose, glucono-γ-lactone and sucrose in 0.5 M NaHCO3. The author concluded that the main reaction products were CO2 and H2O. Bockris et al. [4] investigated the electrochemical oxidation of different carbohydrates at platinum electrodes for their possible use in fuel cells. They noticed that the electroactivity was better in alkaline medium than in acidic medium, and that the reactivity of the molecule decreases with increasing molecular weights. Court [5] used cyclic voltammetry for comparing the activity of different catalytic metals towards the electrooxidation of sucrose. However, in spite of some attemps, the analytical techniques available at that time did not allow the identification of the reaction products. Other studies [6, 7], although they were also carried out by cyclic voltammetry on noble metal electrodes (Pt and Au) and on nickel electrodes, aimed to improve the amperometric detection of carbohydrates and were thus not directly related to our purpose. Shabd and al. [8] compared the hydrodynamic properties and the electroosmotic permeability of D-fructose,

sucrose and urea through Pyrex sinters. They found a relation between the phenomenological coefficients and the structure of the molecule. The carboxylic acids derived from sucrose may find some use in pharmaceutical and agricultural chemistry. The uronic and 2-keto-aldonic acids obtained by hydrolysis of these compounds represent a great industrial interest for the production of detergents, foods, emulsifiers and pharmaceuticals. Edye and al. [9] stated that with a catalyst consisting of platinum deposited on carbon, the chemical oxidation by oxygen, at 100°C and at a constant neutral pH, was highly specific for the production of carboxylic acids at the 6-and 6’-positions of sucrose. The reaction kinetics of the glucose oxidation is known to be sensitive to electrode materials and crystallographic orientations of electrode surface [10, 11]. Gold displays intersting reactivity towards electrocatalytic oxidation of glucose in alkaline solution [12, 13]. Gold electrodes modified by submonolayer silver underpotential deposition display higher electrocatalytic activity than bare gold electrodes for the glucose oxidation, resulting in about 0.1 V negative shift in oxidation potential [14, 15]. On the other hand, platinum electrodes show serious self-poisoning in the glucose oxidation [16, 17]. Surface modification was carried out

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by depositing a second metal on the platinum electrodes to decease sensitivity of the electrodes towards the poisoning, and bismuth modification was reported effective, but the glucose oxidation on the electrodes showed peaks at much higher potentials [18]. This work aims to improve electrocatalytic oxidation of glucose on the natural phosphate modified iron electrode (NPI- electrode), investigated by cyclic voltammetry.. 2. Experimental 2. 1. Reagents A natural phosphate (NP) used in this work was obtained in the Khouribga region (Morocco) [19]. Prior to use this material was treated by techniques involving attrition, sifting, calcination (900 °C), washing, and recalcination [20]. 2.2. Preparation of the NPI-electrode The modified NP-electrodes were obtained by electrodepositing of natural phosphate (NP) into iron plate (1 cm2). Prior to use, the iron plate was polished by smooth paper. Electrical contact was established by a wire of cooper. The resulting electrode is hereby denoted as NPI-electrode. 2.3. Instrument Cyclic voltammetry, chronoamperommetry and colommetry were carried out with a voltalab potentiostat (model PGSTAT 100, Eco Chemie B. V., Utrecht, the Netherlands) driven by the general purpose electrochemical systems data processing software (voltalab master 4 software). The electrochemical cell was configured to work with three electrodes; using NPI-elecytrode as the working, platinum plate (1 cm2) for counter and saturated calomel (SCE) as reference electrodes. The pH-meter (Radiometer Copenhagen, pH-M210, Tacussel, French) was used for adjusting pH values. 2.4. Procedure The glucose oxidation on the prepared electrode was performed in sodium chloride containing glucose by using platinum counter electrode and calomel

reference electrode at a scan rate of 10 mV/s. The scan was started at – 1V in the positive direction and retraced at 0.6 V. All potentials were referred to calomel electrode. 3. Results 3.1. Natural phosphate characteristics The surface structure of natural phosphate was observed using scanning electron microscopy (Fig. 1). The treatment of NP descibes above lead to a fraction between 100 μm and 400 μm that is rich in phosphate and as can be seen thate compact natural phosphate appearance was evident. The treated NP has following chemical composition: CaO (54.12%), P2O5 (34.24%), F- (3.37%), SiO2 (2.42%), SO3 (2.21%), CO2 (1.13%), Na2O (0.92%), MGO (0.68%), Fe2O3 (0.36%), K2O (0.04%) and several metals in the range of ppm.

Fig. 1. Scanning electron micrograph of natural phosphate modified iron electrode 3.2. Glucose oxidation on the NPIelectrode Figure 2 shows cyclic voltammogram enregistred for natural phosphate covered iron electrode (NPI-electrode ), a reduction peak is observed at -0.6V. The glucose oxidation on the NPI-electrode is shown in

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Fig. 3, The cyclic voltammogram shows that the oxidation process is very weak and no notable oxidation peak does appear.

Fig.4. Cyclic voltammogram of natural phosphate covered iron electrode modified by lead. Scan rate 10 mV/s. In sodium chloride solution.

In the case of the electrocatalytic oxidation of glucose at a NPI-electrode, the presence of lead adatoms greatly enhances the oxidation current leading to a well defined oxidation peak starting from -0.2 V (Fig. 5). The electro-oxidation of glucose are carried out with different concentrations of the Pb precursor salt ranging from 0.01 M to 0.8M. The best result was obtained with 0.5M Pb2+. Fig.2. Cyclic voltammogram of natural phosphate covered iron electrode. Scan rate 10 mV/s. In sodium chloride solution.

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Current density [mA/cm²]

60 40 20 0 -20 -40 -60 -80 -100 -1.5

-1

-0.5

0

0.5

1

Potential [mV]

Fig.3. Cyclic voltammogram of natural phosphate covered iron electrode. Scan rate 10 mV/s. In sodium chloride solution contenent glucose.

3.3. Electrocatalytic oxidation of glucose on lead modified NPI-electrode In cyclic voltammetry, the underpotential deposited of lead occurs during the negative sweep at potentials below 0.4V. The lead adatoms involve a considerable change in the pace of the NPI-electrode voltammogram (Fig. 4).

Fig.5. Cyclic voltammogram of natural phosphate covered iron electrode modified by lead. Scan rate 10 mV/s. In sodium chloride solution contenent glucose.

The current density of oxidation reaction increases with the concentration of glucose (Fig. 6). This suggests that the glucose oxidation may be facile on the lead modified NPI-electrode, who probably ensures a very important surface activates.

Fig.6. Cyclic voltammogram of natural phosphate covered iron electrode modified by lead. Scan rate 10 mV/s. In sodium chloride solution contenent glucose. a- 0.01, b- 0.05, c- 0.07, d- 0.1 and e- 0.5M.

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The stability of electrocatalytic activity is another important factor for an electrode. The glucose oxidation on the lead NPIelectrode showed a positive shift in peak potential and current up fourth, but peak potential and current remained stable in the following as seen in Fig. 7. The positive

Fig. 7. Prak change in cyclic voltammograms of the lead modified naturalphosphate/iron electrode with the scan cycles in 0.05M glucose + NaCl solution. Scan rate 10 mV/s.

shift in axidation potential and the drop in current are due to the blocking of the active sites in the electrode surface. Figure 8 shows impedance spectra of lead NPI-electrode, recorded with various concentrations of glucose. The variation of the concentration of glucose does not have an influence on the pace of the Nyquist diagrams. These diagrams consist of an inductive loop at high frequency, characteristic of the adsosption phenomenon. At low frequency, the diagrams EIS produce a line of Warburg which suggests that the glucose oxidation References [1] M.A. Bresler, Bull. Chem. Rev., 8(1866)23. [2] K. Useh, Z. Elektrochem., 6(1883)539. [3] J.U.Karabinos, Revista Euclides, vol. Xiv, (1954). [4] J.O’M. Bockris, K.J. Piersma, E. Gieleadi, Electrochem. Acta, 15(1964)1329. [5] D.E. Court, Ph. D. Thesis, University of Southampton, England, (1984).

reaction on lead NPI-electrode is controlled by the diffusion stage. The increase in the glucose concentration moves the diagram EIS towards the low frequencies, which shows that the reaction is fast.

Fig. 8. Electrochemical impedance spectraof natural phosphate covered iron modified by lead in NaCl solution contenenet glucose. a- 0.01M, b-0.05M and c- 0.1M.

4. Conclusions The application of natural phosphate in the preparation of iron modified electrodes showed to be an interesting support who ensures a high active surface. Electrocatalytic activity of lead modified NPI-electrode towards the glucose oxidation is mainly reflected by oxidation potential. The presence of lead adatoms leads to considerable increase of the activity of the modified electrode. The best result is obtained for 0.5 M Pb2+. Current density of glucose oxidation decreases after the first cycle owing to poisoning of the electrode.. [6] J. Hugler, D.C. Johnson, Anal. Chim. Acta, 132(1987)11. [7] L.M. Santos, R.D. Balduin, Anal. Chim. Acta, 206(1987)11. [8] R. Shabd, B.M. Upadhay, Carbohydr. Res., 90(1981)187. [9] L.A. Edye, G.U. Meechan, G.N. Richards, Carbohydr. Res., 10(1991)11. [10] Y. B. Vassilyev, O.A. Khazova, N.N. Nikolaeva, J. Electroanal. Chem., 196(1985)127.

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[11] R.R. Adzic, N.W. Hsiao, E.B. Yeager, J. Electroanal. Chm. 260(1989)475. [12] A.M.C. Luna, M.F.L. de Mele, A.J. Arvia, J. Electroanal. Chem. 323(1992)149. [13] M.W. Hziao, R.R. Adzik, E.B. Yeager, J. Electrochem. Soc. 143(1996)759. [14] S.B. Aoun, Z. Dursun, T. Koga, G. Bang, T. Sotomura, I. Taniguchi, J. Electroanal. Chem. 567(2004)175. [15] I. Taniguchi, Y. Nonaka, Z. Dursun, S.B. Aoun, C. Jin, G. Bang, T. Koga, T. Sotomura, Electrochemistry, 72(2004)427.

[16] I.T. Bae, X. Xing, C.C. Liu, E. Yeager, J. Electroanal. Chem., 284(1990)335. [17] I.T. Bae, E. Yeager, X. Xing, C.C. Liu, J. Electroanal. Chem. 309(1991)131. [18] G. Hittstock, A. Strubing, R. Sargan, G. Werner, J. Electroanal. Chem. 444(1998)61. [19] Natural Phosphate (NP) Comes Khouribga, Region (Morocco). It is readily available (raw or treated) from CERPHOS 37, Bd My Ismail, Casablanca, Morocco. [20] M.A. El Mhammedi, M. Bakasse, A. Chtaini, J. Hazardouz. Mat., 145(2007)1-7.

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