Effect of the catalyst composition in the Pt x(Ru–Ir) 1− x/C system on the electro-oxidation of methanol in acid media

Abstract

The effect of variations in the composition for ternary catalysts of the type Pt x (Ru–Ir) 1− x /C on the methanol oxidation reaction in acid media for x values of 0.25, 0.50 and 0.75 is reported. The catalysts were prepared by the sol–gel method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption spectroscopy (AAS) and energy dispersive X-ray (EDX) analyses. The nanometric character (2.8–3.2 nm) of the sol–gel deposits was demonstrated by XRD and TEM while EDX and AAS analyses showed that the metallic ratio in the compounds was very near to the expected one. Cyclic voltammograms for methanol oxidation revealed that the reaction onset occur at less positive potentials in all the ternary catalysts tested here when compared to a Pt 0.75–Ru 0.25/C (E-Tek) commercial composite. Steady-state polarization experiments (Tafel plots) showed that the Pt 0.25(Ru–Ir) 0.75/C catalyst is the more active one for methanol oxidation as revealed by the shift of the reaction onset towards lower potentials. In addition, constant potential electrolyses suggest that the addition of Ru and Ir to Pt decreases the poisoning effect of the strongly adsorbed species generated during methanol oxidation. Consequently, the Pt 0.25(Ru–Ir) 0.75/C composite catalyst is a very promising one for practical applications.