Structure Regulation And Catalytic Performance Of Pd-based Heterometallic Nanocatalysts For High-efficiency Hydrogen Evolution From Formic Acid

Formic acid?HCOOH?with 4.4 wt%of hydrogen is a promising chemical hydrogen storage material because of its wide source,convenient storage and transportation as well as the realization of CO₂-HCOOH cycle.However,there are still some important problems in the catalytic hydrogen evolution from HCOOH?HCOOOH?H₂+CO₂?.That is,the catalysts only contain noble metals,the catalytic performance of the catalysts containing low-cost transition metals is much lower than that of the noble metal catalysts,and the mechanism of hydrogen evolution is not clear.In order to solve these problems,we have designed and synthesized a series of supported Pd-based hetero-metallic nanocatalysts on the basis of the key factors that restrict the efficient hydrogen evolution from HCOOH,and then have systematically investigated their room-temperature catalytic performance and the hydrogen evolution mechanism in the thesis.The main research contents are listed as following:?1?From the viewpoint of precisely regulating the chemical environment around metal nanoparticles in the supported catalysts at the atomic/molecular scale,we have firstly functionalized the unsaturated metal ions and organic ligands of MIL-101 in porous metal-organic frameworks?MOFs?using NH₂,NO₂ and SO₃H groups,which have electron-donating or electron-accepting characteristics.Then we have synthesized a series of noble metal AuPd nanoparticles and low-cost transition metal-substituted AuPdM?M=Fe,Co and Ni?nanoparticles supported by these functional MIL-101 species.Finally,we have investigated the catalytic performance of as-synthesized catalysts and the corresponding catalytic mechanisms.The results show that the different catalysts have remarkably different catalytic performance.The electronic structure,steric effect and catalytic intermediates of different structures related to the chemical environment of hetero-metallic nanoparticles are responsible for the different catalytic performance.Among these catalysts,the AuPdCo nanoparticles supported by MIL-101?Cr?-NH₂ have 100%of H₂ selectivity and high activity with the initial TOF value of 347 h^-1.?2?From the viewpoint of enhancing the adsorption and activation ability of the catalytic substrates,we have synthesized a series of AuPdM?M=Fe,Co and Ni?nanoparticles supported by the activated carbon XC-72,which framework is co-modified by NH₂ and N using aminosilane.Then we have studied the catalytic performance and the structure-performance relationship of these catalysts.The results show that the bifunctional group-modified activated carbon supported trimetallic catalysts exhibit higher hydrogen evolution activity than the single group-functionalized catalysts.Especially,one catalyst has the total TOF value of 1600 h^-1,which is higher than the values of the same type of all the reported catalysts.The content of graphitic nitrogen,synergistic effect of NH₂ and graphitic nitrogen,and the adsorption and activation of HCOOH by NH₂ and N in the activated carbon are the main reasons for the high activity of the bifunctional trimetallic catalysts.?3?From the viewpoint of regulating the electronic properties of the catalytically active centers in photocatalysts,we firstly have synthesized two types of photoactive supports of graphitic carbon nitrides?C₃N₄?with tunable band structures and rich N defects and C₃N₄modified by conductive polymer polyaniline.Then we have synthesized a series of AuPdM?M=Fe,Co and Ni?nanoparticles supported by these C₃N₄ species,which are modied by NH₂ using aminosilane.Finally,we have investigated the photocatalytic performance of as-synthesized photocatalysts and the corresponding photocatalytic hydrogen evolution mechanisms.The results show that the photocatalytic activity of all the catalysts under visible light irradiation is much higher than that in the dark.Among these catalysts,the trimetallic catalyst containing C₃N₄ with N-rich defect and narrow bandgap has the highest activity with the total TOF of 1200 h^-1,which is also higher than the values of same type of all the reported catalysts.Also,the photocatalytic activity of the polyaniline-modified catalyst is 2.04 times of that of the catalyst without polyaniline.A series of characterizations show that the different C₃N₄-based supports have different electronic structures,which determine the electronic properties of catalytically active centers and the corresponding photocatalytic hydrogen evolution performance.?4?From the viewpoint of controlling the crystal structures of photoactive supports and adsorbing and activating catalytic substrate molecules through multi-sites,we have firstly synthesized two kinds of supports,namely,MIL-101?Cr?-NH₂,MIL-101?Al?-NH₂,MIL-53?Cr?-NH₂ and MIL-101?Al?-NH₂ as well as MIL-101?Cr?-NH₂ modified by graphene containing bifunctional NH₂ and N.Then we have synthesized a series of AuPdM?M=Fe,Co and Ni?nanoparticles supported by these supports and have studied their catalytic performance and the structure-performance relationship.The results show that MOFs with different compositions and structures have different basicity,which leads to the different photocatalytic activity of corresponding supported trimetallic catalysts.Among them,the catalyst containing MIL-101?Cr?-NH₂ has the high photocatalytic activity.Especially,the catalyst modified by graphene with high conductivity has the high photocatalytic hydrogen evolution activity with the total TOF of 1622 h^-1.In view of the porosity of MOF containing NH₂ and the coexistence of NH₂ and N in the graphene/MOF support,the HCOOH molecule is absorbed and activated by these multi-sites.This leads to the high photocatalytic hydrogen evolution activity of the corresponding catalysts.