Design And Catalytic Performance Of Metal-based Photocatalytic Materials For The High-efficiency Hydrogen Generation From Small Molecules

Ammonia borane(NH₃BH₃)and formic acid(HCOOH)are two valuable chemical hydrogen storage materials and their hydrogen contents are 19.6 and 4.4 wt%,respectively.They also have a wide range of sources and are easy to be stored and transported.However,there are still some important problems in the catalytic hydrogen generation from both small molecules(NH₃BH₃+2H₂O?NH₄BO₂+3H₂;HCOOH?H₂+CO₂).Noble metals are the main catalytically active components,which are expensive and difficult to be obtained.However,the catalysts containing low-cost transition metals have lower activities than the noble metal catalysts,and the hydrogen production mechanism is still unclear.Based on the above problems and according to the hydrogen production characteristics of ammonia borane and formic acid,a series of Mott-Schottky-type monometallic,bimetallic and trimetallic nanocatalysts are designed and synthesized.Furthermore,the catalytic performance,the relationship between the structures and catalytic performance of as-synthesized catalysts as well as the mechanism of their room-temperature photocatalytic hydrogen generation are systematically studied.The main results are as follows:(1)To solve the problem of low surface electron density of non-noble metal nanoparticles(NPs)in the traditional catalytic systems,a series of Mott-Schottky catalysts,which catalytze the hydrogen generation from NH₃BH₃,are constructed by adjusting the compositions,morphologies and structures of metal NPs supported by visible-light responsive carbon nitrides(C₃N₄).The results show that the electron density of catalytically active metal NPs in these catalysts is significantly increased by using visible light as a driving force.At the same time,by adjusting the compositions,morphologies and structures of metal NPs,their electronic properties are further tuned and thus their catalytic hydrogen generation activity is regulated.Among the as-synthesized catalysts,the catalyst containing amorphous bimetallic Cu Co NPs has the highest photocatalytic activity with the total turnover frequency(TOF)value of 75.1 min^-1.(2)In order to solve the problems of weak light absorption and low electron-hole separation efficiency of semiconducting supports in the supported metal catalsyts,three different strategies(secondary heat treatment,amorphous structure construction and the synergistically introduction of B doping and N defects)are used to regulate the band structures,electronic structures and nanoscale structures of carbon nitrides,which are selected as supports of Co and Ni NPs for the photocatalytic hydrogen generation from NH₃BH₃.The results show that the as-synthesized carbon nitrides have strong light adsorption ability and enhanced efficiency of photogenerated carriers.This leads to the remarkable enhancement of photocatalytic hydrogen generation activity of the as-synthesized catalysts,among which the TOF value of Co catalyst supported by the carbon nitrides regulated by synergistically introduction of B doping and N defects is enhanced to be 301.1 min^-1,which exceeds the values of some noble metal catalysts.(3)In order to maximize the utilization of solar energy,the photoresponsive Cu-Cr layered double hydroxide(Cu Cr-LDH)is used as precursor to construct the heterojunction of Cu O and Cu Cr-LDH as well as Cu-Cr oxide with super-wide spectral response,which are selected as supports of Co and Ni NPs for the photocatalytic hydrogen generation from NH₃BH₃.The results show that through remarkably enhancing the photoresponsive range of composite supports,the photocatalytic activity of as-synthesized catalysts is greatly improved.Especially,the Co-based catalyst has the highest activity with the TOF value of276.9 min^-1.(4)Aiming at how to regulate the chemical environment around the catalytically active metal components and enhance the adsorption and activation ability of the catalyst for substrate,a series of heterometallic catalysts with Pd and Co(Ni)are constructed using the functional and structure-tuned carbon nitrides as electron promoters.Then their photocatalytic hydrogen generation from HCOOH is systematically studied.In the synthetic process,three kinds of carbon nitrides with different molecular structures are prepared by changing the basic structural units such as heptazine ring or triazine ring and the intermediate connecting groups such as nitrogen atom or imide group-NH.Simultaneously,the aminosilane with donating group is grafted onto the surface of carbon nitrides by a hydroxyl condensation.The catalytic results show that the Au Pd Co and Au Pd Ni NPs in the catalysts exhibit unique chemical environments and electronic properties.This leads to the super-high hydrogen generation activity and 100%of H₂ selectivity.Among the as-synthesized catalysts,one Au Pd Ni catalyst had the TOF value of 1214.6 h^-1.