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Synthesis and Characterization of Carbon Supported Pt-Sn and Pt-SnO2 Catalysts and Investigation the Effect of Oxide Phase in Ethanol Electro-Oxidation Reaction

Hilal Kivrak1, Sadig Kuliyev2, and Deniz Uner1. (1) Chemical Engineering, Middle East Technical University, Ankara, 06531, Turkey, (2) Vestel Defence Industry, Ankara, 06450, Turkey

Direct use of alcohols, such as ethanol and methanol in fuel cells are currently being investigated in broad extent. Ethanol is non-toxic and has higher hydrogen content when it is compared with methanol. Pt is the best known electro-oxidation catalyst used in the fuel cells. However, pure Pt is poisoned by the strongly adsorbed intermediates especially CO. Therefore, Pt is not a very good catalyst for ethanol or methanol electro-oxidation reaction. Current solution to the problem is to develop Pt alloys resistant to CO poisoning. The most suitable alloy as a Direct Ethanol Fuel Cell anode catalyst reported in literature is Pt-Sn alloy.

In the present study electrochemical oxidation of ethanol by using of cyclic voltammetry method on 20%Pt-Sn/C and 20%Pt-SnO2/C was investigated. XRD and microcalorimetric measurements of carbon monoxide and hydrogen were used to characterize the catalysts. The catalysts prepared by polyol method were performed to investigate how tin addition affects electronic and geometric structure of catalysts and how the chemical state of Sn affects the mechanism of ethanol electro-oxidation reaction.

Two sets of catalysts were prepared. Carbon supported Pt-Sn catalysts were prepared by dissolving H2PtCl6.6H2O, SnCl2.2H2O, and XC72-R in ethylene glycol, pH was controlled and then refluxed under Ar atmosphere. Following these steps, the mixture was filtrated, washed, and dried. Pt: Sn atomic ratios were changed in the range 1:0 to 15:1.

Carbon supported SnO2 promoted catalysts were also prepared by polyol method. In a typical procedure appropriate amount of SnCl2•2H2O was dissolved in ethylene glycol and refluxed under atmosphere to oxidize tin. Then the clear solution turned into light-blue, indicative of colloidal tin hydroxide formation. After cooling down to room temperature under continuous stirring the resulting colloid, H2PtCl6.6H2O ,and XC72-R were mixed in the 1:0 to 15:1 Pt: Sn atomic ratios and stirred for 1 more hour to ensure the formation of Pt/SnO2 phases under Ar atmosphere. Carbon was added this mixture and black slurry was obtained. This black slurry was cooled down, filtrated, washed and dried.

XRD patterns of all samples were obtained with a Rigaku X-Ray powder diffractometer using Cu-K&alpha radiation (lCu-K&alpha =1.54). The angular resolution in the 2Θ scans was 0.05o for the wide angle 2Θ scans. The scan range is from 20o to 90o, and the scan rate was 2o/min.

Differential heats of carbon monoxide and hydrogen adsorption were measured at 323 K by using a Tian-Calvet type heat flow calorimeter (Seteram C-80) connected to a gas handling system and a volumetric adsorption apparatus employing Baratron capacitance manometers in the range of 10-4-10 Torr for precise pressure measurements. Prior to the introduction of doses of gas, the catalyst was reduced in-situ by hydrogen at 523 K. After the reduction process, the catalyst was evacuated for approximately 10 h, while the catalyst bed cooled down to 323 K. The measurements of differential heats were conducted by introducing small doses of gas on to the samples at 323 K. This procedure was repeated until the surface was saturated, i.e., no detectable heat signal was observed.

Electrochemical measurements on these catalysts were performed in a conventional three-electrode cell at room temperature using of Iviumstat potentiostat. A Pt wire was used as the counter electrode. The working electrode was a glassy carbon disk with a diameter of 3 mm held in a Teflon cylinder. About 5mg of the powdered catalyst was suspended in 0.7 ml deionized water and 0.225 ml Nafion® solution for about 1 h to obtain the catalyst ink, then 5µl of the ink was spread on the surface of the glassy carbon electrode using a micropipette and dried at room temperature to eliminate the solvent. The glassy carbon electrode was polished with alumina before deposition. All potential of working electrodes was measured by normal hydrogen reference electrode (NHE). This reference electrode was prepared by electrodeposition of palladium on platinum foil. This foil was inserted in a special glass cell with containing base solution. Before each experiment this electrode was saturated with hydrogen by cathodic polarization in 0.5 M H2SO4 .

XRD patterns on these catalysts showed that the diffraction peaks around 39o, 46o, 68o, and 81o were due to the diffraction at the Pt(111), (200), (220), and (311) planes, respectively. Furthermore, peaks at around 34o and 52o were related to tin oxide. The diffraction peaks of Pt were clearly broadened with increasing tin content indicating intimate interaction between Pt and Sn.

Differential heats of CO and H2 adsorption data was used to determine the population and the strength of the defect-like sites present in the catalysts. Results on gas phase adsorption showed that when tin amount increased, the saturation coverage of CO and H2 decreased and low energy sites were lost.

Ethanol electro-oxidation was performed in 0.5 M H2SO4 on C, Pt-Sn/C, Pt-SnO2/C, Pt/C, and Pt/C (E-tek) catalysts. Present results indicated that per gram of Pt activity for a commercial 20% Pt/C (E-Tek) catalyst and homemade Pt/C catalyst were approximately same. Cyclic voltammetry results on these catalysts revealed that by increasing atomic ratio of tin, current values for the ethanol oxidation increased. It was observed that oxide phase of tin improves the ethanol electrochemical oxidation activity. The catalyst with the highest Sn content revealed only adsorption states for the defect like sites. Ethanol electrooxidation over this catalyst exhibited the highest amount of hydrogen evolution indicating the role of the defects. Work is in progress to determine the role of defect sites on hydrogen evolution, the role of terraces for poisoning and the effect of oxide phase involving in ethanol electro-oxidation reaction.