Preparation And Performance Optimization Of Nanoporous Mo-based Catalysts For Electrochemical Nitrogen Reduction Reaction

The most commonly industrial method of ammonia synthesis is Haber–Bosch technique.While Haber–Bosch technique not only consumes a lot of fossil fuels but also causes environmental problem through releasing CO₂.The electrochemical N₂reduction reaction（ENRR）not only reduces energy loss,but also belongs to green and renewable technique.There are main challenges for ENRR:（1）it is difficult to cleavage the N≡N due to the high dissociation enthalpy of N₂(～945 k J mol^-1);（2）since H⁺is more easily adsorbed than N₂,the competition of hydrogen evolution reaction（HER）limits the efficiency of ENRR.Therefore,reasonable design of electrocatalysts is important to break N≡N bond and keep a balance between HER and ENRR.In this thesis,Mo-based catalysts with high NH₃ yield rate and FE were controlled by designing different structures,morphologies,contents and phases.The followings show the main contents and the conclusions of the research:（1）The amorphous nanoporous CoMoO₄ catalysts were prepared by chemical dealloying of Al-Co-Mo precursor ribbons.After Al atoms dissolving instantly in a layer-by-layer way and forming vacancy,the newly exposed metal sites are rearranged and oxidized,which would disturb the original regular arrangement and result in the amorphization.The amorphous CoMoO₄ catalyst exhibits high NH₃ yield rate of 30.2μg h^-1 mg_cat.^-1and high FE of 3.8%in 0.1 M phosphate buffer saline（PBS）.The nanoporous structure with high specific surface exposes more active sites.In addition,amorphous structure increases the active sites in catalysts.Their combination contributed to the high ENRR performance.（2）To obtain highly ENRR catalytic activity and stability,the nanoporous MoO₃/NiO with oxygen vacancies（OVs）composite structure（np-OVs-NiO/MoO₃）was obtained through two-step method.The MoO₃/NiO with high-surface-area and bicontinuous nanoporous structure was synthesized by chemical dealloying method.OVs were introduced to the surface of catalyst by a solid-state reaction between np-NiO/MoO₃ and Na BH₄.The np-OVs-NiO/MoO₃ catalyst presents high NH₃ yield rate of 35.4μg h^-1 mg_cat.^-1and FE of 10.3%in 0.1 M PBS.The nanoporous structure exposes a large number of active sites and promotes diffusion of electrolyte and N₂.In addition to tailoring the electronic structures,OVs also enhance to capture and activate N₂,thereby improving the activity of ENRR.（3）In order to improve of conductivity and synthesize industrialized nanoporous Mo-based catalyst,the self-supporting nanoporous CoMoC catalyst was prepared by electrochemical dealloying method.During the electrochemical dealloying process,part of Co atoms dissolve instantly to form nanoporous structure.Due to lack of coordination,newly exposed Mo and C atoms experience diffusion and recombination to form Mo C in the nanoporous ribbons.The nanoporous CoMoC catalyst has high conductivity and low work function.The self-supporting nanoporous CoMoC catalyst shows that a highest FE of 22.4%and a largest NH₃ yield rate of 18.9μg cm^-2h^-1 at-0.05 V and-0.10 V,respectively.Metal Co enhances the electrons transfer between electron-rich Mo C and N₂,activating the N≡N bond.（4）In order to resolve the problem of HER competition and enhance catalytic activity,the self-supporting nanoporous Mo₄P₃（np-Mo₄P₃）electrode was modified by fluorosilane（FAS）to form FAS-np-Mo₄P₃ catalyst.The hydrophobic layer modifies the surface electronic structure of np-Mo₄P₃ catalyst and breaks the original absorption sequence（H⁺>N₂）,which significantly enhances FE.The FAS-np-Mo₄P₃ catalyst shows the NH₃ yield rate of 17.3μg h^-1 cm^-2 and the FE of 10.1%,besides FE is 3.9times of np-Mo4P3.The hydrophobic surface increases the number of three phase contact points（TPCP）and improves the interaction between active sites and N2,which promotes the FAS-np-Mo4P3 catalyst to endow high ENRR performance.