Electrochemical Reduction of Bromate by a Pd Modified Carbon Fiber Electrode: Kinetics and Mechanism

• The BrO3− electroreduction proceeds with direct mechanism from 0 to -1.0 V.
• The BrO3− electroreduction proceeds via atomic H* at more negative than -1.0 V.
• Direct and indirect reduction result in strong and weak pH dependence respectively.
• Pd facilitates oxygen atoms removal by hydrogenation without palladium-bonding.

The electroreduction of bromate (BrO3−) was investigated at a Pd modified carbon fiber (Pd/C) electrode prepared from PdCl2 via electrodeposition method. Pd particles distributed on the carbon fiber substrate uniformly. Under various cathode potentials, electroreduction removal of BrO3− at pure carbon fiber electrode and the Pd/C electrode were firstly compared. It was observed that nearly no BrO3− removal was observed within the bias potential of -2.0 V at the carbon fiber electrode. At the Pd/C electrode, the removal efficiency of BrO3− was sharply increased from 24.0% to 58.4% at the cathode potential of -1.0 to -1.5 V, which was further increased to be 87.2% at -2.0 V. The electrochemical reduction of BrO3− was strongly pH-dependent at -0.5 V and the reduction rate could be enhanced at low pH. While at the potential of -2.0 V, a slight pH effect was observed. BrO3− electroreduction follows pseudo first-order kinetics; the rate constant k was firstly increased from 0.016 to 0.031 min−1 with the increase of the Pd loading amount from 0.31 to 0.73 mg/cm2, and then was decreased to 0.018 min−1 at a higher Pd loading amount of 1.05 mg/cm2. The Pd(0) nanoparticles played a significant role in forming atomic H* to realize indirect BrO3− reduction. The electrochemical reduction of BrO3− produces accumulated intermediates of HOBr and OBr−, which were subsequently reduced to Br− with the time evolution.

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