Electrocatalytic Dechlorination Of 2,4-dichlorophenoxyacetic Acid By Nanosized Titanium Nitride Doped Palladium/Nickel Foam Electrodes

Organic chlorine pesticides are one kind of chlorinated organic pollutants with the characteristics of long-term residue, bioaccumulation, biological toxicity and semi-volatility. The pollution control technologies of organic chlorine pesticides have been paid more attention to by the researchers. Electrochemical reductive treatment is recognized as a promising approach due to its unique features such as high efficiency and mild reaction conditions. The development of an efficient catalytic dechlorination electrode has always been a research focus on electrochemical techniques. In this paper, nanosized titanium nitride (nTiN) was employed as dopant for the modification of electrocatalytic dechlorination electrodes. NTiN doped palladium/nickel (Pd/Ni) foam electrode were successfully prepared via electroless deposition method, which were further applied in the electrocatalytic reductive dechlorination of a typical organic chlorine pestice 2,4-dichlorophenoxyacetic acid (2,4-D).Different techniques including field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and linear sweep voltammetry (LSV) were used for the characterization of nTiN doped Pd/Ni foam electrode surface morphology, element types, valence state and hydrogen evolution reaction capability. It was found that Pd layer on the composite electrode was predominated by the metallic palladium Pd0. NTiN was successfully doped into Pd layer, which changed the electrode surface micro-morphology and improved the hydrogen evolution reaction capability.The nTiN doping content and Pd loading had a synergistic effect on the enhancement of the composite electode dechlorination efficiency. When Pd loading was 0.44 mg cm-2, the optimal dechlorination capability improvement was achieved by the nTiN doping content of 2 mg. In this case, dechlorination efficiency of more than 99% was achieved by nTiN doped Pd/Ni foam electrode, which was only 57.13% when Pd/Ni foam electrode was used. The 2 h average current efficiency of the dechlorination system was 6.7%.The impact of environmental factors including 2,4-D initial concentration, current density, reaction temperature and dissolved anions on the electrode dechlorination efficiency was investigated. Higher 2,4-D initial concentration increased the efficient utilization of active hydrogen atom [H] by nTiN doped Pd/Ni foam electrode. Improving current density achieved higher electrode dechlorination efficiency, while the hydrogen evolution side reaction was undesirably increased, leading to a lower 2 h average current efficiency. Higher reaction temperature was favorable for better dechlorination efficiency. NO3- and reduced sulfur compounds including S2- and SO32-showed negative impact on Pd catalytic capability, while CO32- and Cl- exhibited less adverse effect on dechlorination efficiency.Two typical stages existed in the process of 2,4-D electrocatalytic dechlorination by nTiN doped Pd/Ni foam electrode. The first 30 min of dechlorination experiment was electrode activation procedure. In this stage, H2 entered into Pd lattice and formed hydrogen solid solution, which further reacted to form metal hydride. The [H] could not be utilized to dechlorinate 2,4-D because it was fixed by metal hydride, leading to low dechlorination efficiency. After 30 min, an equilibrium was reached between hydrogen solid solution and metal hydride phase. The 2,4-D dechlorination rate increased significantly. According to Arrhenius theorem, the activation energy for 2,4-D dechlorination by nTiN doped Pd/Ni foam elecotrde was 32.06 kJ mol-1, which for electrode activation procedure and 2 h whole experiment was 55.92 kJ mol-1 and 34.21 kJ mol-1, respectively.2,4-D was dechlorinated on the surface of nTiN doped Pd/Ni foam electrode via indirect reductive dechlorination. H2, produced by water electrolysis, was dissociated into [H] by catalyst Pd. [H] would attack C-Cl bond in 2,4-D molecure structure, leading to 2,4-D dechlorination. A successive 2,4-D reductive dechlorination process was observed.2,4-D was first dechlorinated to inermediate products including o-CPA and p-CPA, then to final product PA. Because the steric hindrance of functional group (-OCH2COOH) in 2,4-D structure played a leading role in dechlorination process, the concentration of o-CPA was apparently higher than p-CPA.