Simultaneous removal of monochloroacetic acid and bromate by liquid phase catalytic hydrogenation over Pd/Ce1−xZrxO2

Abstract

Palladium catalysts supported on ZrO2, CeO2 and CeO2–ZrO2 solid solutions were prepared using the deposition–precipitation method, and individual and simultaneous reduction of monochloroacetic acid (MCAA) and bromate by liquid-phase catalytic hydrogenation on these catalysts was investigated. The catalysts were characterized by N2 adsorption–desorption, X-ray diffraction, measurement of point of zero charge, X-ray photoelectron spectroscopy and CO chemisorption. Characterization results showed that CeO2–ZrO2 solid solutions with varied Ce/Zr ratios had nearly identical points of zero charge to those of ZrO2 and CeO2. For supported Pd catalysts, the binding energy of Pd 3d5/2 of Pd/CeO2 was found to be higher than that of Pd/ZrO2, indicative of a stronger metal-support interaction in the former. Given a similar Pd loading, Pd dispersion of Pd/Ce1−xZrxO2 first increased and then decreased with CeO2 content, and the highest Pd dispersion was observed on Pd(0.86)/Ce0.8Zr0.2O2. Accordingly, Pd/Ce1−xZrxO2 exhibited much higher catalytic activities than Pd/ZrO2 at a Pd loading amount of around 0.85wt.%, and the highest activity was identified on Pd(0.86)/Ce0.8Zr0.2O2 for individual reduction of bromate and MCAA. The turnover frequencies of Pd(0.86)/Ce0.8Zr0.2O2 for the reduction of bromate and MCAA were 0.69 and 0.23s−1, respectively, which were one order of magnitude higher than the supported Pd catalysts previously reported. For the simultaneous reduction of MCAA and bromate, the presence of bromate completely suppressed MCAA reduction on Pd(0.85)/ZrO2, while both MCAA and bromate could be effectively reduced on Pd(0.86)/Ce0.8Zr0.2O2. Additionally, the simultaneous reduction of bromate and MCAA proceeded via a competitive reaction mechanism, and the reduction rate of bromate or MCAA was negatively correlated to the competitor concentration.