| Halogenated organic compounds have been widely used in industry,agriculture and pharmacy.For example,perfluorooctanoic acid is an important processing aid;poly brominated diphenyl ethers(PBDEs)are the main ingredients of fire retardants;and chlorinated organic compounds are chemical production materials with wide applications in the field of organic pesticides.Halogenated organic contaminants(HOCs)are also well known as typical environmental pollutions due to their bioaccumulation and carcinogenicity.Recently,a lot of efforts have been devoted into the removal and degradation of HOCs.Because the toxicity of HOCs is mainly derived from halogens,dehalogenation is the key step of pollutant treatment.Among the disposal stratergies,liquid phase catalytic hydrodehalogenation has attracted tremendous interests as a highly efficient,energy-saving water treatment technique.In this work,two typical chlorinated organic compounds,2,4-dichlorophenoxyacetic acid(2,4-D)and monochloroacetic acid(MCAA),were selected as the target pollutants,and the prepared catalysts were used for catalytic hydrodehalogenation of the pollutants under different reaction conditions.The catalysts were characterized by X-ray diffraction(XRD),transmission electronic microscope(TEM)and X-ray photoelectron spectroscopy(XPS)to investigate their structural properties.The products were identified using high performance liquid chromatography(HPLC)and ion chromatography(IC),and the reaction path were thus deduced.Additionally,we systematically studied the influence of catalyst supports,active metals,catalyst dosages,the initial concentrations of reactants and solution pH on the catalytic activities.In the catalytic reduction of 2,4-D,supported catalysts with different supports of AC,CMK-3 and MCN were prepared via the impregnation method.The experimental results showed that Pd/MCN exhibited higher catalytic activity in comparison with that of Pd/AC and Pd/CMK-3,due to the stronger metal-support interactions,higher Pdn+amount and smaller Pd particle size of Pd/MCN.For further inverstigation,Pd/MCN were prepared at different carbonization temperatures(400,550,700℃).Elemental analysis and XPS results showed that the total nitrogen content,pyridine nitrogen content,and Pdn+ content of Pd/MCN gradually decreased with carbonization temperature,while Pd particle size showed different trend.Pd/MCN-550 had the highest catalytic activity due to its smallest Pd particle size and highest Pdn+amount.Additionally,it is found that the catalytic selectivity to 2-CPA was much higher than to 4-CPA,as a result of steric hindrance effect.In the catalytic reduction of MC AA,Pd catalysts and Rh catalysts supported on CeO2,AlO3,SiO2 and CNT were prepared using the deposition-precipitation method,among which Pd/CeO2 had the highest activity.Catalytic hydrodehalogenation of monobromoacetic acid(MBAA)and monoiodoacetic acid(MIAA)over catalysts were also studied.All the catalysts performed higher activities for the catalytic hydrodehalogenation of MBAA than MCAA.However,suppressed activities were observed for the catalytic hydrodehalogenation of MIAA.The variation of catalytic behavior associated with halogen was because of the different strength of C-X bond.Additionally,the high adsorption of I-on Pd might also responsible for the decline of the catalytic activity.Furthermore,with the change of the halogen from Cl to I,the active order of Pd/CNT was continuously enhanced.It may be because that the adsorption capability of carbon materials could weaken the toxic effect of halogen ions on the active sites derived from enhanced polarity of haloacetic acid. |
