Liquid Phase Catalytic Hydrodehalogenation Of Halogenated Pollutants Over Porous Materials Supported Noble Metal Catalysts

In recent years,2,4-dichlorophenoxyacetic acid(2,4-D)and iodinated X-ray contrast agents(ICMs)with high water solubility and strong biochemical stability have been frequently reported as the major contaminants in both surface water and groundwater with high concentrations.Liquid phase catalytic hydrodehalogenation produces less toxic intermediates and products that has been recognized as an effective and green technique for the removal of halogenated organic compounds.In general,supported noble metal catalysts are commonly used for liquid phase catalytic hydrodehalogenation and the structural properties of the catalysts directly affect the catalytic activity and stability.Therefore,to design a catalyst with high efficiency and stability is an important research direction.In this study,noble metal(Pd,Pt and Rh)catalysts supported on porous materials(porous silica or porous carbon)were prepared and their catalytic performance for liquid phase catalytic hydrodehalogenation of ICM and 2,4-D was investigated.The catalysts were characterized by X-ray diffraction,N2 adsorption-desorption isotherms,Fourier transform infrared,transmission electron microscopy,scanning electron microscope,elemental analysis,X-ray photoelectron spectroscopy,zeta-potential measurement and potentiometric mass titration.The main research contents and research results are summarized as follows:(1)Pd catalysts supported on mesoporous carbon nitride(MCN)were prepared by the deposition-precipitation method.The liquid phase catalytic hydrodeiodination of two typical ICM was investigated on the catalysts.For comparison,Pd catalysts supported on ordered mesoporous carbon(CMK-3)were prepared using the same method.The results showed that Pd/MCN catalysts had a higher Pd dispersion and more homogeneous Pd distribution than that of Pd/CMK-3.The correlation between carbonization temperature and Pd particles size followed a concave-type relationship and Pd(1.6)/MCN-500 presented the smallest Pd particles size and the highest Pd dispersion,which exhibited the highest catalytic activity for ICM hydrodeiodination.The catalytic hydrodeiodination of ICM followed the Langmuir-Hinshelwood model,reflecting that the conversion of adsorbed ICM was the rate-controlling step.Compared with H2 dissociation,the activation of C-1 over Pdn+ played a more important role in ICM hydrodeiodination.Solution pH had little influence on hydrodeiodination when pH was above 5,while a decreased catalytic activity was observed at pH below 5 likely due to the adsorption of iodine ion(I-1)on the surface of catalyst that blocked the active sites of the catalysts.(2)Pd selectively binding to N-doped carbon quantum dots(NCQDs)were supported on SB A-15,Pd-N/SBA-15.For comparison,Pd catalysts supported on SBA-15 and NCQDs modified SBA-15 were prepared using the impregnation method.The liquid phase catalytic hydrodechlorination(HDC)of 2,4-D was investigated on the catalysts.The results showed that the catalysts with Pd binding to NCQDs had smaller Pd particle size,a higher Pd dispersion and stronger metal-support interaction than that prepared by the impregnation method.Additionally,the correlation between NCQDs loading amount and Pd particles size followed a concave type relationship and Pd(1.5)-N(2)/SBA-15 presented the smaller Pd particles size and the higher Pd dispersion,which exhibited a much higher catalytic activity for the HDC of 2,4-D compared with the catalysts prepared by the impregnation method.The catalytic HDC of 2,4-D followed the Langmuir-Hinshelwood model,reflecting that the conversion of adsorbed 2,4-D was the rate-controlling step.Furthermore,the HDC of 2,4-D processed via both stepwise and concerted pathways while the concerted dechlorination pathway played a dominant role.Notably,the Cl atom at para-position was more vulnerable to be removed than ortho-position Cl of 2,4-D because of the steric hindrance.The TOF value defined as the initial catalytic activity per Pd active site increased with the Pdn+content and then kept stable as the value of Pdn+/Pd0 higher than 1.20,reflecting that the activation of C-Cl could be played a more important role than H2 dissociation.The catalytic activity of HDC of 2,4-D decreased with the solution pH because of the enhanced electrostatic repulsion between the catalysts and 2,4-D.(3)Pt catalysts supported on zeolite templated carbon(Pt/ZTC)were prepared by the in-situ chemical vapor deposition(CVD)using Pt exchanged Y zeolite as the template.For comparison,Pt catalysts supported on zeolite templated carbon(ZTC)and commercial activated carbon(AC)were prepared using the impregnation method.The liquid phase catalytic hydrodeiodination of two ICM on the catalysts was investigated.The results showed that the Pt/ZTC catalysts prepared by in-situ CVD under varied reaction conditions were perfect replicas of Pt exchanged Y zeolite.Compared with the catalysts prepared by the impregnation method,Pt/ZTC had a higher Pt dispersion and more homogeneous Pt distribution.Increasing CVD temperature resulted in Pt particle growth in Pt/ZTC.The catalytic hydrodeiodination of ICM proceeded via a sequential deiodination pathway,and followed the Langmuir-Hinshelwood model,reflecting that the conversion of adsorbed ICM was the rate-controlling step.Furthermore,Pt/ZTC prepared at higher CVD temperature exhibited lower catalytic activities for ICM the hydrodeiodination.pH markedly impacted the catalytic ICM conversion,and enhanced hydrodeiodination was observed with the increase of pH.As for catalyst reuse,Pt/ZTC remained about 80%of initial activity after 4 reuse cycles,exhibiting much higher stability than the catalyst prepared by the impregnation method.(4)Uniform Rh particles encapsulated within mesoporous silica sphere(mSiO2)were prepared by self-assembly method.For comparison,Rh supported mSiO2 catalysts were prepared using the wetness impregnation method and co-impregnation method.The liquid phase catalytic hydrodeiodination of iohexol was investigated on the catalysts.The results showed that the Rh particles of Rh@mSiO2 were well confined in the inside of mSiO2.Compared with the catalysts prepared by the impregnation and co-impregnation method,Rh@mSiO2 catalysts exhibited more homogeneous Rh dispersion with higher catalytic activity for iohexol deiodination.The catalytic hydrodeiodination of iohexol proceeded through a sequential mechanism and followed the Langmuir-Hinshelwood model,reflecting that the adsorption of iohexol on catalysts surface was the rate-controlling step.The initial catalytic activity increased with Rh particle size because smaller Rh particles exhibited a stronger adsorption affinity to iohexol,which block the active sites of the catalysts However,a larger Rh particle size had smaller specific surface area and fewer active sites,leading to a decreased catalytic activity.Moreover,the catalytic activity for iohexol hydrodeiodination increased with the solution pH due to the suppression of I-1 adsorbed on catalysts surface.As for the reusability of catalyst,Rh(0.42)@mSiO2 catalyst exhibited a high catalytic stability and remained 86%of initial activity within 6 reuse cycles.