Electrocatalytic dechlorination of 2,4-dichlorophenoxyacetic acid using nanosized titanium nitride doped palladium/nickel foam electrodes in aqueous solutions

•Effective electrocatalytic dechlorination of 2,4-D was achieved.•Dechlorination efficiency was enhanced by nTiN doped in Pd layer on the electrode.•nTiN doping content and Pd loading influenced the dechlorination efficiency.•Hydrogen adsorption balance existed between two phases in Pd layer.•Active hydrogen atom [H] could be stored in Pd lattice for a period of time.

Nanosized titanium nitride (nTiN) doped palladium/nickel (Pd/Ni) foam electrodes were successfully prepared via electroless deposition method. The electrodes were evaluated by different techniques including field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and linear sweep voltammetry (LSV). FE-SEM images demonstrated that two different morphologies existed in the Pd layer after nTiN doping. The nTiN doped Pd/Ni foam electrodes were found to be highly effective for electrochemical reductive dechlorination of 2,4-dichlorophenoxyacetic acid (2,4-D) with excellent activity and stability, as revealed by batch experiments. Under the conditions of nTiN doping content of 2 mg, Pd loading of 0.44 mg cm−2, the energetic electrode achieved nearly 100% removal of 2,4-D (0.226 mmol L−1) within 2 h at a current density of 1.667 mA cm−2. However, the removal efficiency reached only 57.13% when a normal Pd/Ni foam electrode with identical Pd loading and current density was utilized. A successive 2,4-D reductive dechlorination process was observed. o-Chlorophenoxyacetic acid (o-CPA), p-chlorophenoxyacetic acid (p-CPA) and phenoxyacetic acid (PA) were detected and identified as transformation products. Regression analysis proved that the pseudo-first order kinetic model was not suitable to describe the dechlorination process on nTiN doped Pd/Ni foam electrodes due to the appearance of a plateau in the beginning of the curve. Moreover, an adsorption theory for Pd chemisorption of active hydrogen atom was proposed to better explain the phenomenon. An adsorption equilibrium of hydrogen existed in the Pd lattice between hydrogen in Pd solid solution and hydrogen in metal hydride, which would influence the effective utilization of active hydrogen atom [H] for dechlorination treatment. An indirect reduction mechanism of the 2,4-D dechlorination on the as-prepared electrodes was also elucidated.

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