Journal of Applied Sciences and Environmental Management World Bank assisted National Agricultural Research Project (NARP) - University of Port Harcourt ISSN: 1119-8362 Vol. 12, Num. 4, 2008, pp. 103-110

Journal of Applied Sciences and Environmental Management, Vol. 12, No. 4, 2009, pp. 103-110

Characterization of Thermal Prepared Platinized Tin Dioxide Electrodes: Application to Methanol Electro-Oxidation

BENIE KOUAKOU ; *LASSINE OUATTARA ; ALBERT TROKOUREY ; YOBOU BOKRA

22 BP 582 Abidjan 22, Laboratoire de chimie physique, UFR SSMT, Universite de cocody Abidjan-Côte d’Ivoire Corresponding author e-mail : ouatlassine@yahoo.fr
* Corresponding author : Lassine Ouattara

Code Number: ja08074

ABSTRACT:

Different platinized tin dioxide electrodes were prepared by the thermal decomposition procedure on titanium substrate. They were characterized physically and electrochemically. The platinum modified tin dioxide electrodes developed in this work present rough surface with small cracks. Amorphous SnO₂ and polycrystalline Pt were identified by X-Ray pattern analysis. The platinum particles size vary from 6,6 to 8,9nm indicated their nanostructure. XPS analysis indicates that pure deposits are prepared and their surface is composed of Pt, O, Sn. Deconvolution of the Pt, O and Sn signals revealed that the deposit is composed of SnO₂, zero-valence and oxidised form of Pt. The electrochemical investigation with ferri-ferrocyanide redox couple showed a quasireversible behaviour on the electrodes. Their electrocatalytic activity increases to reach a maximum for a composition between 40%mol Pt and 70%mol Pt in the deposit. Then after, this activity decreases to reach that of pure platinum. That was related to the distribution of platinum and its size in the deposit. The electrochemical oxidation of methanol on the platinum modified tin dioxide electrodes led to a decrease of its oxidation overpotential in comparison to that of pure platinum. This oxidation begins at potential of about 220mV lower than the pure platinum. The decrease of the overpotential of methanol oxidation is due to the involvement of platinum and tin dioxide in the oxidation process. A mechanism of methanol oxidation has been proposed.

In recent years, there has been increasing interest in environmental damage and human injury by industrial pollution. In this framework, electrochemical methods offer a good opportunity to prevent and remedy pollution problems due to the discharge and sewage effluents. Thus electrochemical technologies have attracted a great deal of attention because of their versatility which makes the treatment of liquids, gases and solids possible. As a consequence, the electrochemical methods find several applications such as metal ion removal and recovery and destruction of toxic and non-biodegradable organics by direct or indirect anodic oxidation. Electrochemical techniques find also application in fuel cell when small organic compounds like methanol are concerned. In this technique, one of the problems was to find the best anodes that could be stable and present good electrocatalytic properties. Thus many studies have been carried out on electrochemical treatment of organic compound and several anode materials have been tested. For example, Sb doped tin dioxide (SnO₂) was tested for organic compounds oxidation. The oxidation occurs at high anodic overpotential in the oxygen evolution domain involving hydroxyl radicals( Comninellis, et al., 1991, p: 703,Comninellis, et al., 1993, p: 108,Kötzl, et al., 1991, p: 14) but in that domain, it undergoes dissolution. Platinum (Pt), one the best electrocatalyst, was also used. With Pt, the oxidation of organic compound occurs at lower potential compared to SnO₂. But Pt presents some drawbacks. Indeed, in the case of methanol oxidation for example, platinum presents surface poisoning by strongly adsorbed species (CO_ads)( G. Foti, 2000, p: 147,Scott, et al., 1999, p: 43,Vielstich, 2003, p: 503) decreasing its activity. In order to improve the electrocatalytic activity of platinum, several different catalysts containing Pt with other metals such as Pt-Ru, Pt-Sn, Pt-W and Pt-Mo have been studied( Arico, et al., 1995, p: 159,Delime, et al., 1999, p: 1249,Fujiwara, et al., 1999, p: 120,Gasteiger, et al., 1993, p: 12020,Gotz, et al., 1998, p: 3637,Krausa, et al., 1994, p: 307,Oliveira-Neto, et al., 2000, p: 39,Vigier, et al., 2004, p: 439,Volkmar, et al., 1996, p: 17901,Wang, et al., 1996, p: 2587). Unfortunately short electrocatalytic effect of those modified electrodes have been observed by Morimoto and Yeager( Morimoto, et al., 1998, p: 77,Morimoto, et al., 1998, p: 95). Fortunately, oxides were found to improve the electrocatalytic activity when associated to metals ( Iwasita, et al., 2000, p: 2000,Katayama, 1980, p: 376,Wang, et al., 2001, p: C222). In fact, oxides are capable of adsorbing large quantities of OH species, which are involved in the oxidation/reduction mechanism taking place between the different possible oxidation states of the metal oxides( Burke, et al., 1979, p: 351,Rolewicz, et al., 1988, p: 573).

In this work, platinum modified tin dioxide thin films were prepared by thermal decomposition techniques. They were subjected to physical and electrochemical characterization. Their electrocatalytic activities towards methanol oxidation were examined by cyclic voltammetry and a mechanism of the reaction is proposed.

MATERIALS AND METHODS

Platinum modified tin dioxides electrodes were prepared by the thermal decomposition method on titanium substrates. The precursor solution is made by the dissolution of SnCl2, 2H₂O (Merck) and H₂PtCl₆,6H₂O (Fluka) in pure isopropanol. Prior to the deposition and to ensure maximum adhesion of the coating, the substrate (titanium) was sandblasted. Electrodes of different xPt-(1-x)SnO₂ percentage molar were prepared. X represents the molar percentage of platinum in the precursor solution. For the prepared electrodes x corresponds to: 0%, 17%, 40%, 70%, 100%.

The precursor was applied by a painting procedure on the titanium (Ti) substrate then put in an oven during 10mn at 80°C to allow the solvent evaporation. Then after, it is put in a furnace at 400°C during 15mn to allow the decomposition of the precursor. These steps were repeated until the desired mass of the coating is reached. A final decomposition of 1h was done at 400°C. The loading of the deposits are around (5,0±0,3) g/m².

The morphology of the electrodes was made by Scanning Electron Microscopy (SEM, JEOL 6300F). The crystalline structure was examined by X-Ray diffraction (XRD) using a Siemens equipment with a Cu Kα cathode. The XPS analyses were carried out with a Kratos Axis-Ultra Spectrometer using a monochromatic Al Kα X-Ray source, operated at 15 kV and pass energy of 20 eV. Sn, Pt and O signals were deconvoluted using the CasaXPS computer program.

The electrochemical measurements were performed in a three-electrode electrochemical cell using an autolab PGStat 20 (Echochemie). The counter electrode was a platinum wire and the reference electrode was a saturated calomel electrode (SCE). The apparent exposed area of the working electrode was 0,78cm². All the potential are referred to a normal hydrogen electrode (NHE). The current density was normalized according to the electrode geometric area.

All the solutions were made with distilled water with analytical grade reagents.

RESULTS AND DISCUSSION

The surface morphology of the electrodes layers was examined by SEM. Figure 1a-c shows the microphotographs obtained with the 40%mol Pt, 70%mol Pt and 100%mol Pt electrodes respectively. The surfaces do not have the mud cracked dried structure often observed for doped tin dioxide films( Rodrigues, et al., 2003, p: 1105). The films present almost the same feature. They are relatively compact, rough and present some small cracked regions.

The X-Ray diffraction pattern of all the prepared electrodes is presented in figure 2. The broad peak around 2θ=28° and the small sharp peak at 2?= 57,7° are assigned to an amorphous structure of SnO₂. Peaks at 2?=34,8°; 38,2°; 40°; 43°; 53°; 65° and 71° are characteristic of Ti( Lee, et al., 2002, p: 445). The peaks corresponding to polycrystalline Pt are present at 2θ=40°; 46,5° and 68°. The mean sizes (d) of the platinum particles were determined from the X-Ray pattern using the Scherrer equation (1) and assuming that the particles are spherical( Krumm, 1999, p: 489).

d = 0 , 9 λ / (B cos θ) ... 1

where λ is the X-Ray wavelength (1,54Å for the Cu Kα radiation), B is the width of the diffraction peak at half-height and ? is the angle at the diffraction peak position. The values calculated at the Pt diffraction peak located at 2θ=46,5° are shown in table 1. These values vary between 6,6 and 8,9nm. These values indicated that the platinum exist in the tin dioxide lattice as nanoparticles. All the XPS analyses were performed on as-prepared electrodes. Figure 3 shows the wide range XP spectra for all the electrodes developed. These spectra indicate that the surfaces are almost free of impurity regardless of the presence of C1s. Its amount seems to be the same in all the surfaces analysed. Detailed scans were recorded in the Sn 3d, Pt 4f and O1s regions. The obtained results for the electrode containing 40%mol Pt are presented in figure 4a-c. The same technique was employed on the other spectra and the results are reported in table 2.

Table 2 indicates three contributions for Sn 3d, two contributions for O 1s and five contributions for Pt 4f. This result indicates that Sn and Pt exist in the deposit in different oxidised states( Moulder, et al., 1992). Sn exits in the form of Sn2+ (very low binding energy i.e. < 486,02eV) and Sn4+ (high binding energy i.e. >486,02eV). Deconvolution of the Pt 4f spectrum resulted into several contribution divided in four components with binding energies that are in agreement with the literatures values for Pt(0) (71,2±0,2)eV, for Pt(OH)2 (72,1±0,4) eV, for PtO between (74,1±0,2) and (76,1±0,2) eV and for PtO2 (>76 eV). The XPS results for the Sn 3d and Pt 4f lines combined with the deconvoluted components of the O 1s suggest that Sn is present mainly as SnO₂ while Pt is found in the zero-valence and oxidised states. The electrochemical characterizations of the electrodes in supporting electrolyte were performed in 1M HClO₄ . In this experiment, the as grown electrodes were submitted to 20 cycles in order to obtain reproducible voltammograms. This procedure was necessary for progressive hydration of internal sites which lead to an increase in the available surface area ( Trasatti, et al., 1981). In Figure 5, the voltammograms obtained on the electrodes are presented. The voltammograms of the platinum modified tin dioxide electrode were compared to that of pure platinum. It is possible to observe the hydrogen adsorption/desorption peaks characteristic of polycrystalline Pt at low potential. Hydrogen adsorption/desorption allows the determination of the platinum active area to be 27; 23,7 and 20,6 cm² for the 17%mol Pt, 40%mol Pt and 70%mol Pt respectively. The peaks related to the formation and reduction of Pt oxides can also be noticed at high potential. The charge transfer processes associated to platinum and tin dioxide show a greater overlap. Thus the currents measured between 0,5V and 0,9V are due to redox process in tin dioxide. The voltammetric charges were determined and plotted against the potential scan rates (figure 6a). It appears in figure 6a that the voltammetric charge of the 17%mol Pt electrode is lower to that of pure platinum contrary to those of 40%mol Pt and 70%mol Pt. In the same figure, one observes that the voltammetric charge is higher at lower potential scan rates for all the electrodes investigated. This charge decreases to reach to an almost steady state after 100mV/s (figure 6a). That observation could be related to the fact that at lower scan rates the total voltammetric charge is the sum of the inner and the outer voltammetric charge but at higher scan rates, only the outer voltammetric charge is concerned. In figure 6b, the voltammetric charge of the electrodes was examined according to the amount of platinum in the deposit for two fixed scan rates. The curves present the same feature. The voltammetric charge increases as the platinum amount increases in the deposit to reach to a maximum value between 40% and 70% mol of platinum. After that, the charge decreases tending to that of pure platinum. Indeed, the obtained results could be related to the distribution of Pt in the deposit. In fact, for 17%mol Pt, most of the Pt particles seem to be inside the deposit lattice with few highly separated platinum nanoparticles on its surface dominated by SnO₂. Thus the electrolyte does not access easily to their active sites. For the 40%mol Pt and 70%mol Pt, the surface of the electrode is enriched with nanoparticles of platinum. For a platinum composition higher than 70%mol Pt, the decrease of the voltammetric charge could be explained by the fact that the surface of the deposit could be composed of high platinum agglomerates decreasing the active sites.

The electrochemical characterization of the electrodes with ferri-ferrocyanide redox couple was made. The results obtained on pure platinum are presented in figures 7a,b. The same feature is observed on the other electrodes (figure 8). All of the voltammograms present an oxidation and reduction peaks of the redox system. Detailed analysis led to a difference between the peak potential to be higher than 60mV at 25°C indicated that the redox process is quasireversible. The voltammograms of the electrodes containing more than 17%mol Pt present the same height of the peak current density. But that of the 17%mol Pt electrode is slightly low. That indicates that the platinum modified tin dioxide electrodes seem to have the same conductivity as the platinum electrodes. That conductivity is slightly low for the 17%mol Pt electrode because the electrode surface which is involved in the redox process is dominated by the semiconducting effect of SnO₂.

The evolution of the peak current density against the square root of the scan rate and the concentration were examined. The obtained results were illustrated in figure 9a and in figure 9b respectively. A linear evolution is observed in both cases for all the electrodes indicated that the process is diffusion controlled at the electrodes. The low conductivity of the 17%mol Pt electrode led to a straight line to have a low slope compared to the others.

The electroactivity of the electrodes towards the oxidation of methanol were investigated by cyclic voltammetry. The obtained results are presented in figure 10. As it can be seen, all the voltammograms present the same feature. They are similar to that obtained with pure platinum (figure 10a). During the scan towards more positive potentials the current starts to increase at a potential about 0,45V on the platinum containing tin dioxide electrodes. It increases rapidly until a first current peak (I) is seen at potential around 0,87V. At approximately 1,15V, the current passes through a minimum and then starts to increase again. An anodic peak (II) is also observed at the potential about 0,70V in the reverse potential scan. On the pure platinum electrode, the onset of methanol oxidation potential is about 0,67V. Thus the electrooxidation of methanol on platinum modified tin dioxide electrodes begins at potential of about 220mV lower than the pure platinum electrode. Platinum modified tin dioxide electrodes present the highest current density at potential about 0,87V (figure 11).

In fact, it is well known that methanol oxidation involves generation of chemisorbed carbon monoxide (CO) on the Pt phase of the catalyst in the reaction:

Pt + CH₃OH → Pt-CO + 4H⁺ + 4e^-

The strongly bond formed between Pt and CO led to its deactivation. As has been shown in figure 10, the kinetic of methanol oxidation on bare Pt is low. But coupling SnO₂ and Pt forming the platinum modified tin dioxide electrode increases the kinetic of methanol oxidation leading to a decrease of its oxidation potential. In fact, a continuous removal of adsorbed CO from the Pt active sites occurs favouring continuous oxidation of methanol. Indeed, during the electrochemical process, discharge of water leads to chemisorbed OH* radicals on SnO₂ at low potential( Licazo-Valbuena, et al., 2003, p: 3869). Reaction between chemisorbed species CO and OH* occurs to produce CO₂ that is removed from the electrode surface according to the following scheme:

Pt-CO + SnO₂-OH*→ Pt + SnO₂ + CO₂ + H⁺ + e^-

Conclusion: The platinum modified tin dioxide electrodes developed in this work present rough surface with small cracks. The platinum particles size are between 6,6 and 8,9nm. Platinum modified tin dioxide electrodes tend to have the same conductivity as Pt. Pure deposits are prepared and their surface is composed essentially with Pt, O, Sn elements as indicated by XPS measurements. Deconvolution of the Pt, O and Sn signals indicated that the deposit contains mainly SnO₂, zero valence and oxidised form of Pt. They present a quasi-reversible behaviour with ferri-ferrocyanide redox couple. The electrochemical oxidation of methanol is fast on the platinum modified tin dioxide electrode than on bare Pt. This enhancement of the electrode catalytic activity is due to the involvement of platinum and tin dioxide in the oxidation process. Hydroxyl radicals that are adsorbed chemically on SnO₂ react with CO which is chemisorbed on Pt sites leading to CO₂. CO₂ is then removed from Pt active sites leading to continuous oxidation of methanol. That favours the methanol oxidation to begin at potential of about 220mV lower on platinum modified tin dioxide electrodes than the pure platinum electrode.

Acknowledgement: The authors greatly thank Professor Christos Comninellis and his collaborators at the Ecole Polytechnique Féderale de Lausanne (Switzerland) for their help in the development of that project.

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