Preparation And Catalytic Performances Of Coated Catalysts For Liquid Phase Hydrogenation Reduction Of Typical Pollutants

Hexavalent chromium?Cr????,copper ions?Cu????and bromate?BrO₃^-?are commonly identified heavy metal pollutants and disinfection byproduct in water.Due to their potential carcinogenic nature to humans,these pollutants have been strictly regulated.The United States Environmental Protection Agency?EPA?regulated BrO₃^-level below 0.01 mg/L in drinking water.The World Health organization?WHO?regulated Cr???level below 0.05 mg/L and Cu???level below1.5 mg/L.Therefore,a variety of treatment methods have been developed to eliminate these pollution in water.Liquid phase catalytic hydrogenation provides a simple,green and cost-effective method to remove pollutants in water.For liquid phase catalytic hydrogenation reduction,supported noble metal such as Pt,Pd,Ru,Rh are commonly used as active catalysts due to their very strong capabilities in activation of H₂.For conventional supported noble metals,however,catalyst deactivation is inevitable due to metal leaching,metal particle aggregation and surface contamination.Considering these factors,we developed highly stable and reusable coated supported noble catalysts using different overcoatings for liquid phase catalytic hydrogenation and the catalytic activity and stability were investigated.In addition,we investigated the impact of shell properties of Pd catalysts supported on magnetites with core-shell structures on the liquid phase catalytic hydrogenation of BrO₃^-.The catalysts properties were characterized by X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,N₂ adsorption-desorption and CO chemisorption and so on.The main research contents and results are summarized as follows:1.The stable coated Pd/CNT catalysts were prepared using SiO₂,carbon and N-doped carbon as overcoatings?denoted as Pd/CNT@SiO₂,Pd/CNT@C,Pd/CNT@CN?and used for liquid phase catalytic hydrogenation of Cr???.The catalysts were fully characterized.Characterization results showed that Pd/CNT could be coated by SiO₂,carbon and N-doped carbon and Pd particles were effectively blocked.For catalytic Cr???reduction,the reactions mechanisms on Pd/CNT and coated Pd/CNT catalysts were different.The Cr???reduction on Pd/CNT followed an adsorption-suppressed mechanisms,while an adsorption-controlled mechanism was observed on coated catalysts.For the three coated Pd/CNT catalysts,Pd/CNT@CN exhibited the highest catalytic activity,due to its high electrical conductivity and adsorption affinity to reactant of CN overcoating.The lower activity of Pd/CNT@CN than that of Pd/CNT was due to the blocking of Pd sites by the overcoating.However,Pd/CNT@CN had a higher catalytic stability than that of Pd/CNT.After ten consecutive reaction cycles,Pd/CNT lost more than91.8% of its initial catalytic activity.On the contrary,negligible deactivation was observed on Pd/CNT@CN.Notably,catalytic activity of Pd/CNT@CN was higher than that of Pd/CNT after two consecutive reaction cycles.The results suggested that Pd/CNT@CN catalyst would be a promising choice for the reduction of Cr???by liquid phase catalytic hydrogenation.2.The supported Pt/CNT was coated using carbon overcoating?denoted as Pt/CNT@C?and further functionalized by oxidation treatment?denoted as Pt/CNT@Oxi-C?.The catalysts were fully characterized and the liquid phase catalytic reduction and recovery of Cu???and Cu???-EDTA complexes were investigated.The results show that Pt particles were completely blocked by carbon shells without any exposed Pt particles.Despite of embedment of Pt particles,the coated Pt/CNT catalysts were still active for catalytic reduction Cu???,suggesting that H₂ could be activated and Cu???could be converted on the carbon overcoating due to the existence of metal-carbon heterojunction,which was capable of evoking Mott-Schottky effect.Accordingly,the increased graphitization degree and conductivity of carbon overcoatings was obtained with the increasing carbonization temperature from 500 ? to 600 ?,resulting in enhanced catalytic activity for Cu???reduction,however,further increasing carbonization temperature to 800 ? led to high surface hydrophobicity,gradually inhibiting Cu???reduction.In parallel,surface oxidation improved surface hydrophilicity,giving rise to enhanced Cu???affinity and reduction.The regenerated Pt/CNT@Oxi-C catalyst could be obtained by removing deposited metallic Cu via HNO₃ treatment.As for stability,Pt/CNT@Oxi-C still retained high catalytic activity after five catalyst cycles.In contrast,Pt/CNT lost 92% of its initial activity due to marked loss of Pt particles during the reaction and regeneration.The findings clearly suggested that Pt/CNT@Oxi-C catalysts could be used for removal and recovery of heavy metal ions from water.3.Pt-based catalyst with fine Pt particles embedded in carbon rods of N-doped CMK-3?Pt@N-CMK-3?was prepared by a two-step infiltration method using SBA-15 as the template and quinoline as nitrogen-doped carbon precursor.For comparison,commercial Pt/C,Pt/CNT@C,Pt@CMK-3 and Pt/N-CMK-3 were prepared.The catalysts were fully characterized and the liquid phase catalytic reduction of BrO₃^-was investigated.Results showed that fine Pt particles and even Pt particle distribution were obtained resulting from the confinement effect of the ordered mesoporous structure of CMK-3/N-CMK-3.N-doping and a high surface area of the carbon matrix provided more active sites for the adsorption and reduction of BrO₃^-.In addition,the effective embedment of Pt particles in N-CMK-3 matrix increased the interface areas between Pt and carbon matrix and meanwhile suppressed Pt particles leaching and aggregation.Therefore,Pt@N-CMK-3displayed a superior catalytic performance in the liquid catalytic hydrogenation reduction of BrO₃^-to Pt/N-CMK-3,Pt@CMK-3 and Pt/CNT@C.This findings demonstrated that embedding of acitive metal in a porous carbon matrix could enhance the utility of noble metals in heterogeneous catalysis.4.Magnetic separation was proven to be a convenient way to recover catalysts and the properties of magnetic supports also strongly influence the catalytic activity.In this study,Pd nanoparticles supported on core-shell structured magnetites with varied shells?e.g.,carbon?C?,SiO₂,polypyrrole?PPy?,polyaniline?PANI?,polydopamine?PDA?and chitosan?CHI??were prepared and denoted as Pd/?Fe₃O₄@C?,Pd/?Fe₃O₄@SiO₂?,Pd/?Fe₃O₄@PPy?,Pd/?Fe₃O₄@PANI?,Pd/?Fe₃O₄@PDA?and Pd/?Fe₃O₄@CHI?respectively.These catalysts were characterized and catalytic reduction of BrO₃^-on the catalysts were investigated.Characterization results indicated that Pd/?Fe₃O₄@SiO₂?and Pd/?Fe₃O₄@C?had very low PZCs below 3,while Pd/?Fe₃O₄@PPy?,Pd/?Fe₃O₄@CHI?,Pd/?Fe₃O₄@PDA?and Pd/?Fe₃O₄@PANI?have higher PZCs because of the presence of amino-groups.The Pd/?Fe₃O₄@CHI?and Pd/?Fe₃O₄@SiO₂?catalysts had stronger hydrophilicity,whereas other shells have higher hydrophobicity.Although magnetic supports with different shells had decreased magnetization,the catalysts could be feasibly separated and recovered in an external magnetic field within 3 min.As for catalytic reduction of BrO₃^-,Pd/?Fe₃O₄@C?and Pd/?Fe₃O₄@SiO₂?exhibited low catalytic activities due to their low PZCs,which invoked electrostatic repulsion and suppressed BrO₃^-adsorption.In contrast,Pd/?Fe₃O₄@CHI?,Pd/?Fe₃O₄@PDA?and Pd/?Fe₃O₄@PANI?exhibited high catalytic activities due to the enhanced BrO₃^-adsorption.Notably,despite high PZCs,Pd/?Fe₃O₄@PPy?displayed a negligible catalytic activity because of its strong surface hydrophobicity,which suppressed the access of BrO₃^-to Pd sites.For Pd/?Fe₃O₄@PANI?,BrO₃^-reduction followed the Langmuir-Hinshelwood model,indicating that the adsorption of BrO₃^-on catalyst surface was the rate-controlling step.Furthermore,the TOF value of Pd site gradually decreased with the increase of Pd particle size in Pd/?Fe₃O₄@PANI?,indicative of the critical role of BrO₃^- activation on positively charged Pd sites for BrO₃^-reduction.p H had a marked influence on catalytic activity and BrO₃^-reduction could be enhanced at low p H.As for the catalytic stability,Pd/?Fe₃O₄@PANI?maintained 87% of its initial activity after five catalytic reaction recycles,reflecting that Pd/?Fe₃O₄@PANI?was a highly active and stable catalyst in the liquid phase catalytic reduction of pollutants.