Study On Hydrogenation And Dechlorination Of Chlorinated Organic Pollutants In Water By Pd-based Catalysts

Chlorinated organic compounds are widely used in chemical,pharmaceutical,pesticide,tannery and other industries,making them one of the most common organic pollutants,not only because they have wide variety,but also because they are important chemical raw materials,intermediates and organic solvents.Chlorophenol is one of the most commonly used chemical raw materials and chemical products,the discharge of industrial wastewater and the extensive use of pesticides have made chlorophenols gradually into the natural water,and have caused serious water pollution problems.Chlorophenoxyacetic acid includes 4-chlorophenoxyacetic acid?4-CPA?,2,4-dichlorophenoxyacetic acid?2,4-D?and 2,4,5-trichlorophenoxyacetic acid?2,4,5-T?,which are often used as pesticides to control crop pests.Due to the intensive use of these components in agriculture,they are often present in surface water and ground water at relatively high concentrations.The pollution caused by its residue is highly toxic and carcinogenic,which has aroused widespread concern in society.In this paper,the common chlorinated organic pollutants and treatment methods were introduced systematically,and the hydrodechlorination method was analyzed.Further more,this paper introduced the common types,preparation methods and characterization methods of the catalysts.On this foundation,on the one hand,in order to study the effect of catalyst carrier and loading method on the catalytic activity,graphene?GE?and nitrogen-containing graphene?N-GE?were used as carriers,and Pd-GE,dp-Pd/GE,im-Pd/N-GE and dp-Pd/N-GE were prepared by uniformly dispersing the metal Pd onto the carriers by the impregnation method?denoted as im-?and the precipitation-deposition method?denoted as dp-?.The materials were characterized by elemental analyzer,transmission electron microscopy and X-ray photoelectron spectroscopy.The prepared catalysts were used for the hydrodechlorination of 2,4-dichlorophenol in water,in order to examine the active performance of carbon-based catalysts in the HDC process.The results showed that the nitrogen content of the catalyst was 11.3%.Compared with Pd/GE,the catalyst with nitrogen-containing graphene as the carrier has higher cationic Pd?Pdn+?content and smaller Pd particles,and showed high catalytic activity.Using the precipitationdeposition method further enhanced the catalyst activity.In addition,the dechlorination of 2,4-dichlorophenol is carried out in two ways by gradual dechlorination and simultaneous dechlorination at the same time,and the existence of Pdn+ in the catalyst is favorable to the simultaneous dechlorination.On the other hand,since the hollow materials of nano or micron scale have the characteristics of large specific surface area,low density,good stability and good filterability,and their hollow parts can accommodate a huge number of object molecules or large-sized object molecules,making the hollow materials have attracted much attention in recent years.In order to study the effect of catalyst morphology on the catalytic activity,Ti O₂ was used as the catalyst carrier,and the noble metal Pd was used as the active metal,and the surface supported Ti O₂-based catalysts and the hollow Ti O₂ ball-coated active metal catalysts were synthesized.The structures were characterized by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.The catalysts were used for the hydrodechlorination of 2,4-dichlorophenoxyacetic acid?2,4-D?in water,in order to examine the active performance of metal oxide-based catalysts in the HDC process.The results showed that,if the concentration of lye is too high or the lyeing time is too long,the exine structure of the hollow Ti O₂ ball-coated active metal catalysts would be destroyed.In the process of dechlorination of 2,4-dichlorophenoxyacetic acid,whose concentration is 0.2 mmol/L,the removal rate of Pd/Ti O₂ was about 55%,while Pd@Ti O₂ could completely degrade the chlorinated organic pollutants.After bring humic acid into the reaction system as a interference factor,the interference factor,the catalytic ability of Pd/Ti O₂ and Pd@Ti O₂ were greatly inhibited,but Pd@Ti O₂'s pollutants removal rate were still higher than 50%,the degradation ability of chlorinated organic pollutants performed relatively well.It can be obtained that when the active noble metal is loaded on the inside of spherical shell of the carrier,catalyst's catalytic activity will be optimized,its catalytic efficiency will be enhanced,and to a certain extent,it is less affected by external factors.