EFFECT OF PREPARATION CONDITIONS OF PT ALLOYS ON THEIR ELECTRONIC, STRUCTURAL, AND ELECTROCATALYTIC ACTIVITIES FOR OXYGEN REDUCTION-XRD, XAS, AND ELECTROCHEMICAL STUDIES

The effect of different alloying conditions (alloying temperature, ann ealing period) on the electrocatalytic activities for the oxygen reduc tion reaction (ORR) by three carbon-supported Pt alloy electrocatalyst s (Pt/Cr, Pt/Co, Pt/Ni) was investigated and correlated with electroni c and structural parameters determined by in-situ XAS. The results ind icate that all the Pt alloys show enhanced ORR activities relative to a Pt/C electrocatalyst. However, the electrocatalytic activity and act ivation energy for ORR in the case of Pt/Ni and Pt/Co alloys show mark ed effect due to different alloying conditions. This was in contrast t o Pt/Cr alloy, where both parameters remained unchanged over the range of alloying conditions. Those electrochemical results were correlated with those obtained from in-situ X-ray absorption spectroscopic (XAS) investigations, which provided information on the electronic (Pt Sd-o rbital vacancy, from the X-ray absorption near-edge structure) and geo metric (Pt-Pt bond distances, from the extended X-ray absorption fine structure) factors. In-situ XAS results indicate that the supported al loys possess higher Pt Sd-orbital vacancies and shorter Pt-Pt bond dis tances. In addition, the XAS results showed that alloying inhibited ch emisorption of oxygenated species (OH) on the Pt at potentials above 0 .8 V vs RHE. Correlation of electrocatalytic activities and activation energies for ORR with parameters obtained from in-situ XAS studies in dicates that, in the case of Pt/Ni and Pt/Co alloys, higher alloying t emperature and longer annealing periods result in higher Pt Sd-orbital vacancies with the geometric parameters remaining unchanged. The Pt/C r alloy on the other hand revealed no dependence of either the Pt d-or bital vacancies or the geometric parameters on alloying temperature. T hese observations indicate that the dependence of electrocatalytic act ivities and activation energy for Pt/Co and Pt/Ni alloys on the therma l history and the absence of such an effect in the Pt/Cr alloy could b e related to the differences in the Pt Sd-orbital vacancies.