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Co-Pt alloys were studied in detail by a corrosion test under phosphoric acid fuel cell (PAFC) conditions on the well-defined crystallographic structures for a typical combination of the alloy catalysts used in PAFCs, examining the long-life stabilities of the structures and the catalytic a tivities for O2 electroreduction. The ordered (O) and disordered (D) alloys at the same particle sizes can be obtained by heat-treating the mother alloy in different temperature s quences. The O-alloy exhibits a specific activity, an electrocatalytic a tivity based on the catalyst surface area, L35 times higher than the D-alloy before the corrosion test, but shows less activity (0.73 times) after the corrosion test, due to a higher degradation (47%) in the O-alloy activity as compared with that of the D-alloy (1%). It was found that the Co atoms on particle surfaces of both alloys dissolve easily in the acid. This is followed by a second slow dissolution from inside the alloy particles probably due to the protective action by a monolayer thickness of Pt remaining on the alloy surfaces, but the loss of Co in the second stage dissolution for the O-alloy ishigher by several percentage points compared to that of the D-alloy. It was also found that the Pt content does not change on the catalyst support even after 50 h of corrosion test, but the pure Pt phase is formed in the corrosion product, where the phase for the O-alloy grows faster than that for the D-alloy with corrosion time. Based on these results, obtained by chemical, x-ray diffraction, and transmission electron microscopy with energy dispersive spectroscopy analyses, the corrosion for Pt alloy catalysts i clearly explained, i.e., after the dissolution
Watanabe et al. (Sat,) studied this question.