Preparation Of Metal-organic Framework-based Composites And Regulation Of Their Photocatalytic Performances For Hydrogen Evolution Under Visible Light

Hydrogen is a kind of renewable energy,which has the advantages of clean,pollution-free,high energy density,etc.In the current hydrogen evolution technology,photocatalytic water splitting for hydrogen evolution is a green and environment friendly way.In recent years,Metal-Organic Framework materials?MOFs?have been widely used in the field of photocatalysis due to their advantages of high porosity,low density,high specific surface area,adjustable structure and composition.Among many MOF compounds,Zr-MOFs and Zeolite-like Imidazole Framework materials?ZIFs?with high stability are regarded as the promising MOFs materials for practical application.However,a single photocatalyst is difficult to meet the demand of industrial hydrogen evolution due to poor visible light response,low activity and limited stability.Therefore,it is necessary to fabricate composite photocatalyst and study its catalytic performance based on the MOFs and their derivatives.Herein,a series of novel MOFs based composite photocatalysts were prepared using containing anthracene based Zr-MOF?Zr-ADBEB?with high efficient visible light absorption properties as the support by surface modification and the heterogeneous junction structure construction,meanwhile the relationship between their structure and photocatalytic activity was studied.We also studied the synthesis of cocatalyst by calcination with ZIF as precursor and the construction of composite catalyst for photocatalytic H₂ evolution on the basis of cocatalyst.The formation,recombination,separation and transmission of photogenerated charge carriers and the possible mechanism of photocatalytic hydrogen evolution were discussed systematically through absorption spectrum,surface photovoltage,steady state photoluminescence spectroscopy,the time-resolved emission decay spectra and electrochemical measurement.The results mainly include the following four parts:1.Zr-ADBEB,a novel metal-organic framework compound with visible light response and good chemical stability,was used to construct Zr-ADBEB/Pt photocatalyst.The platinum nanoparticle on Zr-ADBEB was prepared by chemical reduction of chloroplatinic acid and sodium borohydride.Studies showed that the Schottkey barrier between Zr-ADBEB and Pt nanoparticles in the composite effectively promoted the separation of photogenerated carrier of Zr-ADBEB,and the electrons transferred to the active center of Pt and reduced proton to hydrogen.The quantum efficiency of the optimal amount of Pt cocatalyst in Zr-ADBEB photocatalyst was 0.8%at 450 nm.2.The Zr-ADBEB/g-C₃N₄ heterojunction was constructed to regulate the photogenerated charge separation and transfer.The results show that Zr-ADBEB/g-C₃N₄ heterojunction promotes the charges separation and transfer,and the composite is a Z type heterojunction.Under the excitation of visible light,photogenerated electrons in the conduction band of Zr-ADBEB flow into the valence band of g-C₃N₄ and recombination with holes,and the electrons in conduction band of g-C₃N₄ participate in the proton reduction reaction.Under the condition of 420 nm,the quantum efficiency of the optimized Zr-ADBEB/g-C₃N₄heterojunction was 1.2%.3.In order to further improve the performance of Zr-ADBEB/g-C₃N₄photocatalyst,we tried to use Ni₃C,a non-noble metal cocatalyst,to modify Zr-ADBEB/g-C₃N₄ through grinding and calcination to achieve efficient visible photocatalytic hydrogen evolution.The research shows that the introduction of Ni₃C further improves the efficiency of photogenerated charge separation and transfer of Zr-ADBEB/g-C₃N₄ photocatalyst,and can effectively reduce the hydrogen evolution overpotential of Zr-ADBEB/g-C₃N₄ heterojunction.The optimized Zr-ADBEB/g-C₃N₄/Ni₃C catalyst has a quantum efficiency of 2.3%measured under420 nm light.4.In addition to being the main photocatalyst,the derivative of MOFs can also be used as a hydrogen evolution cocatalyst.In this chapter,MOF?Co Zn-ZIF?was used as the precursor to obtain cobalt,nitrogen-doped graphite carbon?Co,N-GC?hydrogen evolution cocatalyst by calcining in high temperature inert atmosphere.Then Cd_0.5Zn_0.5S photosensitizer was introduced to construct Cd_0.5Zn_0.5S/Co,N-GC composite photocatalyst.The test results showed that Cd_0.5Zn_0.5S nanoparticles loaded on Co,N-GC were small in size and well dispersion.The presence of Co,N-GC inhibited the photogenerated charge recombination of Cd_0.5Zn_0.5S,promoted the interfacial charge separation and transfer,and effectively reduced the photocatalytic hydrogen evolution overpotential of Cd_0.5Zn_0.5S.The optimized Cd_0.5Zn_0.5S/Co,N-GC photocatalyst achieved 8.7%quantum efficiency under 420nm light irradiation.