Temperature-programmed reduction and cyclic voltammetry of Pt/carbon-fibre paper catalysts for methanol electrooxidation

Abstract

Temperature-programmed reduction (TPR) and cyclic voltammetry (CV) studies of platinum catalysts supported on pyrographite-coated carbon-fibre paper, and prepared by either ion exchange or impregnation, clearly demonstrate the nature of the interactions between the platinum species and the support. After drying the above catalysts at 120 °C, the ion-exchanged preparation exhibits the stronger interaction with the carbon support, as might be expected since a chemical interaction with carbon surface groups is known to occur in such catalysts. The presence of a fraction of bulk Pt(NH 3) 4(OH) 2 impregnating salt in the impregnated catalyst has been detected using TPR. After air activation at 300 °C, subambient reduction peaks were observed and the strength of binding of Pt in the ion-exchanged catalyst was reflected by its increased difficulty of reduction in comparison with that of the impregnated catalyst. The stoichiometry of reduction in ion-exchanged catalysts corresponds to Pt 2+ → Pt 0 ° in both dried and activated catalysts, with a small amount of Pt 4+ present in the latter. Upon activation the impregnated catalyst showed the presence of some Pt metal, which was thought to arise from the decomposition of the fraction of bulk Pt(NH 3 4(OH) 2 in the dried catalyst. Activation of ion-exchanged catalysts at temperatures higher than 300 °C led to a progressive weakening of the Pt-support interaction and consequent smaller Pt surface areas. Activation at 500 °C in air produced Pt metal exclusively and very low Pt surface areas. The strong interaction between Pt and the carbon support upon activation of the ion-exchanged catalyst at 300 °C is thought to be the origin of the large metal surface area and the high catalytic activity for methanol electrooxidation found upon reduction.