Supported Pd-Based Bimetallic Nanoparticles:synthesis And Catalytic Reduction Of Hexavalent Chromium

Hexavalent chromium(Cr(VI))as a common pollution has high environmental toxicity and carcinogenicity,while trivalent chromium(Cr(III))shows low toxicity and can be converted to chromium hydroxide and readily separated from water.The reduction of Cr(VI)to Cr(III)has considered as one of the most effective methods to eliminate the chromium pollution.In this dissertation,we have synthesized two kinds of carbon-supported metal catalysts through the solution-phase method,and explored their catalytic activities and mechanism for the reduction Cr(VI)to Cr(III)using formic acid as reductant.Firstly,PdCu/NG nanohybrids were synthesized via a facile co-reduction method using N-doped reduced graphene oxide(NG)as support.The phase,structure,morphology,composition,and chemical states of the catalyst were characterized using XRD,HRTEM,EDX,ICP-OES and XPS,respectively.It is found that the NG nanosheets are deposited by numerous nanosized homogeneous PdCu alloy(not two-phase mixture of Cu and Pd).The loading amount of metals is 35 wt.% and the average diameter of PdCu nanoalloy is 4.7 nm.The PdCu/NG catalyst shows an excellent catalytic activity for the reduction of Cr(VI)by formic acid,the catalytic reaction rate is as high as 0.406 min-1,the activation energy is as low as 34.9 kJ/mol.The promising catalysis is due to the strong metal/support interaction(SMSI)effect and the synergistic effect between Cu and Pd.Secondly,bimetallic NiPd/ZC nanohybrids were synthesized via a hydrothermal method using ZC as support,where ZC is obtained through the carbonization of metal-organic framework compound ZIF-8.The resultant Ni Pd/ZC was characterized by XRD,HRTEM,EDX,ICP-OES and XPS,respectively.It is found that the carbonized ZIF-8(ZC)maintains a porous carbon skeleton structure,and the NiPd bimetallic nanoparticles are uniformly loaded in the pores.Bimetallic NiPd/ZC shows a superior catalytic reduction of Cr(VI)over its monometallic counterparts.The catalytic rate constant is estimated to be 0.396 min-1 and the activation energy is as low as 28.7 kJ/mol.After five cycles of use,the catalytic activity decreased only by 2.7 %.Compared with PdCu/NG catalyst,the NiPd/ZC has a larger specific surface area and unique spatial framework,which increases the adsorption sites and favors the catalysis.The NiPd/ZC shows a good recoverability and recyclability due to the magnetic attraction of magnetic NiPd particles.Finally,the whole dissertation is summarized and the future research is prospected.