New pattern of Pd-Catalyzed electrochemical hydrodechlorination in conversion of chlorinated aromatic pollutants to Value-Added chemicals

Pd-catalyzed electrochemical hydrodechlorination (HDC), as a key process before discharge and biological treatment, is an effective strategy for the in-situ conversion of low concentration chlorinated aromatic pollutants (CAPs) in wastewater or polluted water. However, its application has been greatly hindered by the low Pd utilization and operation efficiency. Herein, we developed an electrocatalytic HDC method using Pd nano-particles modified nickel foam (Pd NPs/Ni) as cathode and alkaline aqueous solution as catholyte, and applied it to convert high concentration CAPs to value-added chemicals. The results showed that the reaction efficiency and selectivity of the developed HDC method mainly depend on the cathode material, pH and cathode potential. The HDC method can convert CAPs (4 categories) with various high concentrations (0.05 -1 M) into a single value-added chemical (picolinic acid, phenoxyacetic acid, benzoic acid or phenol) with very high yield (>95 %) and chemo-selectivity (>97 %). Compared with the conventional Pd-catalyzed HDC methods, the new patterned one can greatly improve Pd utilization and operation efficiency (The Pd mass activity and the area activity of cathode increased by 16 -685 times). In addition, the underlying reaction pathway and catalytic mechanism of the developed HDC system has also been studied using 3,6-dichloropicolinic acid (3,6-D) as the model CAPs. The HDC of 3,6-D on the Pd NPs/Ni in a stepwise fashion with 3-chloropicolinic acid (3-ClPA) as the main inter-mediate product is suggested, and it follows the indirect HDC mechanism with electrochemically adsorbed H as the reductant. The end product (picolinic acid) of the HDC could be further reduced to pipecolinic acid at very negative potentials following direct hydrogenation mechanism, which would significantly lower the selectivity of HDC.