Preparation And Nitrogen Fixation Performance Of Bi/order-disorder Homojunction Bi₂WO₆ Photocatalyst

Ammonia（NH₃）is an indispensable chemical substance for synthetic fertilizers and fibers in modern society.In recent years,due to its high hydrogen density（17.6 wt.%）and low liquefaction pressure（～8 atm）,it has also received extensive attention as a potential hydrogen carrier.Haber-Bosch industrial synthetic ammonia meets the needs of contemporary social development,but the environmental pollution（exhaust gas,waste water,waste residue）and energy consumption problems are exposed,so it needs to be resolved urgently.Photocatalytic fixation of N₂ to NH₃ in pure water in a gentle manner is the most interesting and environmentally friendly method to replace the traditional Haber-Bosch industrial synthesis of NH₃.However,the strong thermodynamic dissociation energy(410 k J mol^-1)of the first bond and low electron affinity（-1.9 e V）of the synthetic raw material N₂ make the cleavage of N≡N become a key rate-determining step in the reduction of N₂.The inevitable carrier recombination problem in the process decreases the photocatalyst activity.Therefore,the adsorption and activation of N₂ and the recombination of photogenerated carriers are the bottlenecks in the process of photocatalytic nitrogen fixation.Based on the above two bottlenecks,this paper takes Bi₂WO₆ as the research object,adopts the order-disorder homojunction strategy and the schottky junction strategy to construct the order-disorder homojunction Bi₂WO₆ photocatalyst and Bi/order-disordered homogeneous junction Bi₂WO₆ photocatalyst,which effectively adsorbs and activates N₂ and promotes the separation of photogenerated carriers.It provides a strategy for the rational design of photocatalysts with high efficiency and stable nitrogen fixation,and deepens the understanding of the regulation of disordered photocatalysts.The specific research contents of this paper are as follows:（1）This part focused on the adsorption and activation of N₂ by the disordered structure of Bi₂WO₆ amorphous state and the strategy of order-disorder homojunction to promote the separation of photogenerated carriers.A series of order-disorder homojunction materials（BWO,OD-1,OD-2,OD-3）were prepared by one-step hydrothermal method,and the nitrogen fixation performance and stability of the photocatalyst were tested.The results showed that OD-2 had the best nitrogen-fixing activity,its nitrogen-fixing activity was 1149.2μmol·L^-1·h^-1·g^-1,while its crystal counterpart BWO had no nitrogen-fixing activity.A series of characterization analysis showed that the disordered surface structure of OD-2 played a role in activating nitrogen in the photocatalytic nitrogen fixation.At the same time,the order-disorder interface effectively separated the photogenerated electron-holes parts,and the interface electrons were transferred to the OD-2.The surface had a disordered structure and reacted with N₂ adsorbed on the disordered structure to promote the smooth progress of the nitrogen fixation reaction.On the other hand,the photo-generated holes remained in the ordered part of OD-2,participating in the oxidation reaction of H₂O and converting it to O₂.（2）This part focused on the synergistic effect of metal Bi and disordered structure to adsorb and activate N₂ and the strategy of schottky junction to promote the separation of photogenerated carriers.On the basis of the aforementioned research,changing the solvent composition,a series of Bi/order-disorder homogeneous Bi₂WO₆ photocatalysts were synthesized by a one-step hydrothermal method,and the nitrogen fixation performance and stability of the photocatalysts were tested.The experimental results showed that the nitrogen fixation activity of BOD-2 was the best in the same series,up to 827.9μmol·L^-1·h^-1·g^-1,and can produce ammonia continuously and stabilize for 8 h,and its catalyst life was about 2.7 times longer than OD-2.When the solvent composition was changed,the morphology of the catalyst changed significantly,the disordered structure area increased,and the Bi elements were produced obviously.Based on a series of characterization analysis,the nitrogen fixation mechanism of BOD-2 was studied.The order-disorder homogeneous junction of Bi₂WO₆formed a schottky junction with the Bi on the catalyst surface,which made part of the electrons transferred to the Bi metal and activated the adsorbed N₂ on the Bi.The synergistic effect of the disordered structure and the metal Bi enabled the photogenerated electrons to be effectively utilized,thereby greatly improving the stability and lifespan of the BOD-2 photocatalyst.On the other hand,photogenerated holes remained in the ordered part of BOD-2,participating in the oxidation reaction of H₂O and converting it to O₂.