THE MORPHOLOGICAL AND SURFACE-PROPERTIES AND THEIR RELATIONSHIP WITH OXYGEN REDUCTION ACTIVITY FOR PLATINUM-IRON ELECTROCATALYSTS

A series of iron-platinum bimetallic and pure platinum catalysts suppo rted on carbon were prepared and characterized by means of H-2-O-2 tit ration, X-ray diffraction, TEM, XPS and cyclic voltammetry. Their cata lytic activities for the electrochemical reduction of oxygen in 100% p hosphoric acid were related to their physical properties. When the bim etallic catalysts were heated at 750 degrees C, a disordered alloy pha se was formed. An almost completely ordered superlattice structure, Pt ,Fe, was achieved when heated at 900 degrees C. The mass activity show ed a maximum value as a function of Pt particle size for both pure pla tinum and platinum-iron catalysts, and no noticeable enhancement in th e activity was found by alloying platinum with iron. The specific acti vity increased as the particle size was increased for pure platinum ca talysts, whereas it was nearly independent of the particle size for Pt -Fe alloy catalysts. The specific activities of Pt alloys were much hi gher than those of pure platinum over the tested range of particle siz e. Therefore, it is expected that a Pt alloy catalyst having a superio r mass activity can be made only if metal particles are kept extremely small; ie appreciately smaller than 3 nm. The increase in the specifi c activity is caused by increased chemisorption strength of hydrogen a nd simultaneous decrease in the oxygen chemisorption strength in the c athode.