| The development of agriculture is greatly promoted with the world population growing, and the use of pesticides in agriculture is essential.2,4-dichlorophenoxyacetic acid (2,4-D), as a herbicide, can remove a variety of weeds to protect the normal growth of crops. Its excellent performance makes it one of the most commonly used herbicides around the world, while the consenquent pollution problems occurred.2,4-dichlorophenol (2,4-DCP) is a synthetic intermediate of 2,4-D. It is a kind of chlorinated organic compounds with anti-degradation ability, and it is highly toxic and carcinogenic. Its extensive use in chemical production caused severe pollution. It is one of priority environmental pollutants listed by USEPA. Therefore, how to achieve their efficient removal is currently the focus at home and abroad.Compared with traditional removal technologies, liquid catalytic hydrodechlorination has many advantages, such as fast reaction rate, mild operation conditions, high product selectivity, and no secondary pollution; it exhibits great potential in water treatment. In this paper, graphite carbon nitride was selected as the support, and Pd-loaded catalysts were prepared in different methods, the as-prepared catalysts were characterized by X-ray diffraction, transmission electron microscopy, zeta potential, X-ray photoelectron spectroscopy, N2 adsorption-desorption, inductive coupled plasma emission spectrometer, CO chemisorptions, elemental analysis. Meanwhile, their catalytic hydrodechlorination efficiency of 2,4-DCP and 2,4-D were investigated, and the impact of preparation and reaction conditions on catalytic efficiency was discussed. The main research contents are listed as follows:1) The Pd/g-C3N4 catalysts prepared by different methods were used in liquid catalytic hydrodechlorination of 2,4-DCP. Catalysts prepared by photo-deposition method had higher catalytic activity than those prepared by impregnation method. As can be seen from the TEM images, Pd particles loaded on the surface of catalysts had more uniform size and dispersion. But agglomeration occured in catalysts prepared through impregnation method inevitablely, which caused differences in catalytic activities. Increasing Pd loading amount could improve catalytic hydrodechlorination efficiency per unit mass of catalyst, but not for the unit mass of Pd. it was found that there was a substantial decline in utilization of unit mass of Pd with the increasing of Pd loading. The phenomenon is also relevant to the dispersion of Pd on the catalysts. In the experiments of reactions under different pH, it was found that acidic condition was benefit to catalytic hydrodechlorination reactions, but not to the reuse of catalyst. This is relevant to the adverse effect of HCl produced on the active sites during the reaction. However, alkaline conditions can quickly neutralize HCl congested on the surface of catalyst and therefore more conducive to protect the stability of the catalyst.2) Pd/g-C3N4 catalysts were prepared by deposition precipitation method, and NaBH4 was used as the reducing agent. Changing Pd2+/(Pd2++Pd0) ratio through adding different amount of reducing agent was used to understand the roles Pd2+ and Pd0 played during catalytic hydrodechlorination. The results showed that Pd0 in the Pd/g-C3N4 catalysts could convert hydrogen into active H, which had strong reduction ability. Meanwhile, the presence of Pd2+ favored the C-Cl bond cleavage. Moreover, increasing Pd loading amount could alter the Pd particles size on the surface of catalysts. The larger Pd particles could facilitate the formation of β-PdH, so as to enhance the efficiency of the catalytic hydrodechlorination. In addition, in comparison of with different reaction substrate (monochlorophenoxyacetic acid, dichlorophenoxyacetic acid and trichlorophenoxyacetic acid), it was found that more chlorine atoms made it more difficulty to be dechlorinated, and the efficency was also relevant to the position substitution positions of Cl. |
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