Electrocatalytic hydrodechlorination of 4-chlorophenol on Pd supported multi-walled carbon nanotubes particle electrodes

A series of Palladium (Pd) supported multi-walled carbon nanotubes (Pd/MWCNTs) were synthesized via modified impregnation of Pd2+ followed by in situ chemical reduction with ethanol, NaBH4, and H2 as the reducing agent (referred as Pd/MWCNTs-E, Pd/MWCNTs-B, and Pd/MWCNTs-H, respectively). The electrocatalytic hydrodechlorination of 4-chlorophenol (4-CP), a highly toxic, cancerigenic, and bio-refractory contaminant, was investigated in a three-dimensional electrochemical reactor with Pd/MWCNTs as the particle electrodes. Nearly 100% of 4-CP could be efficiently dechlorinated and completely converted into phenol within 30 min under optimized conditions. Transmission electron microscope (TEM) and X-ray diffraction (XRD) results indicated that the small Pd nano-particles (6.4–13.1 nm) were uniformly supported on the surface of MWCNTs and formed face centered cubic (fcc) structure in all as-prepared catalysts. The removal efficiency of 4-CP was significantly affected by the size of loaded Pd nano-particles, where Pd/MWCNTs-B (100%, 6.4 nm) > Pd/MWCNTs-E (60%, 9.5 nm) > Pd/MWCNTs-H (29%, 13.1 nm). Effects of current density, initial pH, and initial dissolution oxygen (DO) on the 4-CP removal were also investigated. Scavenger experiments confirmed that indirect reduction by atomic H∗ was responsible for the reductive dechlorination of 4-CP. The stability of Pd/MWCNTs-B for the 4-CP dechlorination was also exhibited in repetitive experimental cycles.