Reductive removal of chloroacetic acids by catalytic hydrodechlorination over Pd/ZrO2 catalysts

Abstract

Chloroacetic acids are common disinfection byproducts in drinking water and are of significant concern due to their strong carcinogenic, mutagenic and hepatotoxic effects. In the present study, the catalytic hydrodechlorination of chloroacetic acids (monochloroacetic acid, dichloroacetic acid and trichloroacetic acid) was investigated over supported Pd catalysts on SiO2, ZrO2, and activated carbon (AC) prepared by the impregnation method (denoted as im-Pd/support) and/or the deposition-precipitation method (denoted as dp-Pd/support). The catalysts were characterized by X-ray diffraction, transmission electron microscopy, measurement of point of zero charge, N2 adsorption–desorption isotherm, H2 chemisorption, and X-ray photoelectron spectroscopy. Characterization results showed that the points of zero charge of the supports varied. In contrast to im-Pd/ZrO2, strong metal–support interaction was identified in dp-Pd/ZrO2. Accordingly, im-Pd/ZrO2 was found to be more active than im-Pd/AC and im-Pd/SiO2, and dp-Pd/ZrO2 exhibited higher catalytic activity than im-Pd/ZrO2. Complete dechlorination of chloroacetic acids to acetic acid could be achieved on dp-Pd/ZrO2 within 120min of hydrodechlorination. The hydrodechlorination rate constants of trichloroacetic acid, dichloroacetic acid, and monochloroacetic acid over dp-Pd(1.74)/ZrO2 were 0.22, 0.16, and 0.044min−1, respectively, reflecting an increase in dechlorination activity with increasing number of chlorine atoms in chloroacetic acids. It was further demonstrated that the catalytic hydrodechlorination was accomplished via a combined stepwise and concerted pathway for both trichloroacetic acid and dichloroacetic acid.