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The mechanism of electrochemical reactions of rhenium species at stainless steel 316 electrodes and the effect of the pH in 0.5 M (NH 4 ) 2 SO 4 solutions containing 5 mM HReO 4 (pH 1, 3, and 10) were investigated by electrochemical measurements and surface analyzes of the rhenium-deposited electrodes. From the results of cyclic voltammetry, we found that ReO 4 − anion was electrochemically active and the electrodeposition of rhenium species was accelerated after hydrogen evolution occurred, whereas ReO 4− anion was inactive before hydrogen evolution occurred. This indicates that adsorbed hydrogen atom on the electrodes is needed to reduce ReO 4 − anion. The deposits obtained by potentiostatic electrolysis at a potential lower than that of the hydrogen evolution reaction were analyzed by XPS and SEM. In the pH 1 solution, the deposit was metallic Re, whereas in the pH 3 and 10 solutions, the deposit was a mixture of metallic Re, ReO 2 , and ReO 3 . From the results of anode stripping voltammetry, it was found that the metallic Re was oxidized to ReO 4 − via ReO 3 or ReO 2 .

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