Optimisation of electrocatalytic dechlorination of 2,4-dichlorophenoxyacetic acid on a roughened silver–palladium cathode

Electrocatalytic hydrogenolysis (ECH) dechlorination of 2,4-dichlorophenoxyacetic acid (2,4-D) in an aqueous solution was investigated at room temperature using a roughened silver–palladium cathode (Pd/Ag(r) cathode) in batch-mode electrolysis experiments. The Pd/Ag(r) cathode was prepared by galvanic replacement reaction (GRR) of a roughened silver (Ag(r)) electrode with PdCl2 solution. The effect of preparation conditions on the catalytic activity and stability of the Pd/Ag(r) cathode and of operating parameters on the rate and current efficiency (CE) of the ECH dechlorination reaction were evaluated. In particular, the ECH dechlorination mechanism of 2,4-D was analysed with regard to the dependence of dechlorination efficiency on the different operating parameters. Moreover, preliminary assessments of product selectivity and carbon mass balance of the dechlorination reaction were carried out. The results demonstrate that a moderate GRR time and GRR temperature favoured the catalytic activity and cathode stability and that a basic aqueous solution without ethanol, high 2,4-D concentration, and moderate current density had the most beneficial effects on the dechlorination process. Under the optimised conditions, 25 mM of 2,4-D could be selectively dechlorinated to phenoxyacetic acid with 85% yield and 66% CE at 298 K after 6 h electrolysis. The only products generated during the electrolysis process were phenoxyacetic acid, 2-chlorophenoxyacetic acid, and 4-chlorophenoxyacetic acid.