Complete dehalogenation of bromochloroacetic acid by liquid phase catalytic hydrogenation over Pd/CeO catalysts.

Bromochloroacetic acid is classified as one of the typical disinfection byproducts (DBPs). In this work, supported palladium catalysts on different supports (CeO AlO, SiO and activated carbon (AC)) (labelled as Pd/support) were synthesized via the deposition-precipitation method (D-P method) and their activities for the complete dehalogenation of bromochloroacetic acid by liquid phase catalytic hydrogenation were evaluated. Comprehensive characterizations of the catalysts were conducted by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), point of zero charge (PZC), X-ray photoelectron spectroscopy (XPS) and CO chemisorption. Results indicated that the PZCs of the supports varied with each other. The stronger Pd-support interaction and higher Pd dispersion of Pd/CeO and Pd/AlO than those of Pd/AC and Pd/SiO were confirmed by X-ray photoelectron spectroscopy and CO chemisorption. Pd/CeO had a higher ratio of positively charged Pd to metallic Pd (Pd/Pd) than Pd/AlO and Pd/AC due to a stronger metal-support interaction. Accordingly, a negligible bromochloroacetic acid conversion was observed on Pd/SiO, whereas bromochloroacetic acid was found to be readily decomposed on Pd/CeO, Pd/AlO and Pd/AC. However, the dechlorination reaction could not further proceed on Pd/AlO and Pd/AC catalysts after the bromine functionality was removed from bromochloroacetic acid. A complete dehalogenation of bromochloroacetic acid occurred only on Pd/CeO. Furthermore, the dechlorination rate constants of monochloroacetic acid and bromochloroacetic acid over Pd(1.40)/CeO were 0.018 and 0.031 min respectively, confirming an induced synergistic effect due to the existence of bromine atoms. It was worth noting that a stepwise-concerted pathway was verified during the liquid phase catalytic hydrodehalogenation of bromochloroacetic acid.