Detection of an Unstable Intermediate in Br− Electro-oxidation to Br3− on a Platinum Electrode in Nitrobenzene by Scanning Electrochemical Microscopy

We report the detection of an unstable intermediate generated during Br− oxidation to Br3− in nitrobenzene by scanning electrochemical microscopy (SECM), and we attempt to simulate the collection of the intermediate using a proposed mechanism of Br− oxidation to Br3−. At a distance of ∼ 3.5 μm between the tip and the substrate electrode in the SECM experiment, we observed two waves as we measured the collection efficiency, Nss, while holding the tip at an anodic potential and scanning the substrate toward cathodic potentials. The second wave obtained at more negative substrate potentials was associated with the collection of Br3−, and the first wave was associated with the collection of an intermediate generated in Br− oxidation to Br3−. The Nss as a function of d, estimated at the constant substrate potential of 0.27 V vs. TMPD/TMPD+·, abruptly increased as d was brought below 2.5 μm. We simulated this approach curve by using the following mechanism of Br− oxidation to Br3−: (1) Br− + e ⇋ Br·, (2) 2Br· ⇋ Br2, (3) Br· + Br− ⇋ Br2−·, (4) Br2−· + e ⇋ Br2, and (5) Br2 + Br− ⇋ Br3−.