Design And Synthesis Of Metal-based Photocatalytic Nanomaterials And Their Application In The Hydrogen Evolution Form Small Molecules Under Visible Light Irradiation

Ammonia borane?NH₃BH₃?and formic acid?HCOOH?with hydrogen content of19.6 and 4.4 wt%,are two promising chemical hydrogen storage materials with convenient transportation.However,there are some general scientific problems in the process of hydrogen evolution from the two hydrogen storage materials,namely,the low activity of catalysts containing cheap transition metals and the unclear reaction mechanisms.According to these scientific problems,we have introduced the photocatalysis and photothermal catalysis into the hydrogen evolution systems involving NH₃BH₃ and HCOOH and then have systematically studied the catalytic performance,the structure-function relationship and the hydrogen generation mechanism of the new photocatalysts in the thesis.The main research contents are listed as following:?1?From the viewpoint of regulating and enhancing the electron density of noble-metal-free nanoparticles?NPs?to remarkably enhance their catalytic performance,we have synthesized a series of Co and Ni NPs supported by semiconducting metal-organic frameworks?MOFs?,which were used to catalyze hydrogen evolution from aqueous NH₃BH₃ under visible light irradiation.At the same time,the photocatalytic reaction mechanism is also investigated.The results showed that the catalytic activity of all the catalysts is greatly improved under visible light irradiation and one catalyst Co/MIL-101?Cr?-NH₂ has the total turnover frequency?TOF?value of 117.7 min^-1,which is the highest activity among the non-noble metal catalysts reported at that time.The enhancement of photocatalytic hydrogen production activity of this type of catalysts is caused by the electron synergistic effect between photoactive MOFs and Co and Ni NPs with non-plasmon resonance?SPR?effect.Furthermore,the capture experiments confirmed that enhanced electron density of the catalysts indeed benefits the improved hydrogen evolution activity.?2?From the perspective of regulating the components and the band structures of semiconductors to remarkably enhance the photocatalytic performance of corresponding catalysts,firstly,we have synthesized a series of V_xO_y species?V₂O₅,VO₂ and V₂O₃?and calcined the pristine V₂O₅ in the H₂ atmosphere to generate three porous V₂O₅ nanosheets with narrow bandgaps and rich oxygen vacancies.Secondly,the as-synthesized V_xO_y species have been supported Co and Ni NPs.Finally,the visible-light-driven hydrogen evolution from NH₃BH₃ and reason mechanism were systematically investigated.The results show that the photocatalytic activity of the catalysts is greatly improved under visible light irradiation.Among all the catalysts,one Co catalyst containing porous oxygen-rich vacancies exhibits the highest activity with the total TOF value of 120.4 min^-1,which exceeds the values of all the reported noble metal-free catalysts.A series of structure characterizations show that the high photocatalytic activity of catalysts can be attributed to their porous nanosheets with tailored oxygen vacancies.In addition,it is experimentally found that the photogenerated electrons,holes and hydroxyl radicals,which were intermediates generated in the photocatalytic process,can effectively promote the hydrogen evolution,on the basis of which we present a new mechanism about the photocatalytic hydrogen evolution from aqueous NH₃BH₃.?3?From the perspective of regulating the components and morphologies of semiconductors to enhance their photocatalytic performance,we have synthesized three semiconducting nickel phosphides(Ni₂P,Ni₁₂P₅ and Ni₃P)and two Ni₂P species with different morphologies.Then we systematically study the photocatalytic hydrogen evolution from aqueous NH₃BH₃ over the single metal nickel phosphide catalysts under visible light irradiation and then present a new mechanism about the photocatalytic hydrogen evolution.The results show that the photocatalytic activity of all the catalysts is greatly improved under visible light irradiation.Among the Ni₂P,Ni₁₂P₅ and Ni₃P catalysts,Ni₂P has the highest photocatalytic activity.The enhanced activity of nickel phosphides can be attributed to the visible-light-driven synergistic effect of photogenerated electrons and hydroxyl radicals,which come from the oxidation of hydroxide anions by photogenerated holes.It shoud be noted that the Ni₂P with smallest size of NPs is much more active than Ni₂P with nano-flower shape,and its total TOF value reachs 124.2 min^-1.?4?From the perspective of that the photothermal synergistic catalysis can be used to improve the catalytic reaction efficiency,we have synthesized a series of Pd-based NPs supported by photoactive NH₂-functionalized metal-organic frameworks?MOFs?with different compositions and structures,which were used to catalyze hydrogen evolution from HCOOH under photothermal condition.The results showed that one catalyst Au_0.28Pd_0.47M_0.25/MIL101?Cr?-NH₂ has the hight activity with the TOF value of 2400 h^-1.A series of experiments verify that there is the synergy of thermal catalysis and semiconductor photocatalysis during the catalytic process,the electron density of catalytically active Pd centers in the heterometallic NPs is enhanced and the NH₂ groups benefit the activation of HCOOH molecule.These factors favor the high-performance hydrogen evolution from HCOOH.