Catalytic Hydrogenation Reduction Of Disinfection Byproducts Over Supported Pd Catalysts In Water

During the advanced drinking water treatment, chlorination inevitably leads to the formation of disinfection byproducts (DBPs), e.g. chloroacetic acids, bromate, chlorophenol, etc., which cause the potable water pollution and serious harm to human health. Therefore, development of effective treatment methods to eliminate the DBPs in the drinking water is highly demanded. Liquid phase catalytic hydrogenation has been recognized as a low consumption, environmentally friendly water treatment technique, which can eliminate the toxicity of many organic and inorganic compounds effectively.Exploring effective catalysts is a very important aspect in catalytic hydrogenation. For supported catalysts, the catalytic activity was affected by the properties of the supports remarkably. In the present study, we prepared different supports to improve the catalytic activities of the supported catalysts, and studied the catalytic hydrogenation mechanism of chloroacetic acids, bromate and2,4-chlorophenol systemically.For the catalytic reduction of monochloroacetic acid (MCAA), supported noble metal catalysts with different supports of activated carbon (AC), SiO2and ZrO2were prepared via the impregnation and deposition-precipitation (DP) methods. The catalysts were further characterized and used to remove MCAA. Characterization results showed that the points of zero charge (PZCs) of ZrO2was around6.1, whereas the PZCs of AC and SiO2were lower than3.0. The XPS results reflected that Pd/ZrO2prepared by the deposition-precipitation method (dp-Pd/ZrO2) had stronger metal-support interaction than that of impregnation method, and the metal-support interaction was attenuated with the increase of Pd loading. In parallel, higher Pd dispersion and smaller Pd particle size were identified in dp-Pd/ZrO2catalyst by H2chemisorption and TEM characterization, and the metal particles assembled with Pd loading. Different catalysts showed varied catalytic activities. For the catalysts prepared by the impregnation method, Pd/ZrO2showed the highest activity, and the catalytic activity was further enhanced when synthesized by the deposition-precipitation method. For dp-Pd/ZrO2, the MCAA reduction followed the Langmuir-Hinshelwood model. In addition, the turn over frequencies (TOF) first increased then decreased with Pd loading amount. The kinetics demonstrated that dichloroacetic acid and trichloroacetic acid were proposed to be dechlorinated via a combined stepwise and concerted pathway, and dechlorination rate increased with the chlorine amount.For the simultaneous reduction of MCAA and bromate, supported Pd catalysts with CeO2and ZrO2solid solution of different Ce/Zr ratio as the supports were synthesized. XPS characterization results displayed that the metal-support interaction increased with the CeO2amount, but this interaction decreased slightly for Pd/CeO2. CO chemisorption results exhibited similar results. For the individual reduction of MCAA and bromate, compared with Pd/ZrO2, the catalyst with a small amount of CeO2showed much higher activity, and Pd/Ceo.8Zro.202had the highest reaction rates for both MCAA and bromate. For the simultaneous reduction of MCAA and bromate, bromate exhibited much stronger reaction competitiveness than MCAA. In addition, the reduction of reactants on Pd/Ceo.8Zro.202can be well described by the Langmuir-Hinshelwood model, reflecting a competitive adsorption controlled reaction mechanism.For carbon material supported catalyst, superficial metal tends to aggregate on the support due to the weak metal-support interaction. The catalytic activity of the carbon supported catalyst may be improved by doping some other atoms. In this research, the noble metal catalysts with mesoporous carbon as the support were decorated by boron doping. The supported catalysts were prepared by complexing-reduction method and were used in the hydrodechlorination (HDC) reaction of2,4-dichlorophenol (2,4-DCP). The characterization results showed that the catalysts with boron-doping mesoporous carbon as the support had higher Pdn+amount and smaller Pd particle size. Additionally, the decoration of boron doping improved the activity of the catalyst. The HDC of2,4-DCP controlled by the adsorption between the reactant and catalyst surface, and the reaction rates influenced by the Pd particle size remarkably.