The Modified Synthesis Of UiO-66 And Their Performance On Ammonia Electrosynthesis

At present,NH₃is predominantly produced via Haber-Bosch process at high temperature and pressure.This method suffers from huge CO₂generation and huge energy consumption.Such method runs counter to the green chemical concept.Compared with the Haber-Bosch process,ammonia electrosynthesis can occur under mild environmental conditions,with low energy consumption and no carbon emission,which is considered as a hopeful technology for ammonia synthesis.However,it is urgent to develop an efficient electrocatalyst to improve the NH₃yield rate and Faradaic efficiency(FE)of ammonia electrosynthesis.Metal organic framworks(MOFs)with high surface area,adjustable structure channels and abundant active sites,have shown excellent activity in the field of electrocatalysis,which makes them potential electrocatalysts for nitrogen reduction reaction(NRR).This paper focuses on the study of Zr-based UiO-66 and its NRR performance is improved through modification synthesis.Besides,the application of Fe OOH in NRR is preliminarily explored.After a series of control experiments and isotopic labeling experiments,the detected ammonia completely comes from the reduction of N₂.The specific research contents include six parts as follows:1.Defects in UiO-66 are constructed by adding different concentrations of formic acid or acetic acid during the synthesis process.The results demonstrate that the addition of acid induces the formation of more defect sites,which can enhance the chemical adsorption of N₂molecules on the surface of the electrocatalyst.Besides,after the addition of acid,the particle size becomes smaller and more uniform and the electrochemical active surface area(ECSA)increases,which means the exposure of more active sites,thus beneficial for NRR.However,when the added acid is too strong,too many defects are formed,resulting in frame collapse and pore blockage,which is unfavorable for the exposure of active sites.The optimized UiO-HAc-0.75 M has the best NRR activity at-0.3 V,showing the NH₃yield rate of 4.62×10^-11mol cm^-2s^-1,and the FE of 48.06%.2.BDC ligands with different functional groups including electron-withdrawing groups of-NO₂and-SO₃H,and electron-donating groups of-OH and-NH₂,are used to synthesize UiO electrocatalysts to modify the NRR activity with the electronic effect of the ligand group.It is found that the electron-withdrawing groups have little effect on NRR activity,while the electron-donating group could significantly enhance the NRR activity.The results of in situ FT-IR demonstrate that UiO-NH₂catalyzes NRR following an associative mechanism,in which the hydrogenation step was the rate-determined step.Therefore,the electron-donating group-NH₂can push electrons to the adjacent metal centers and modify their charge density,which can facilitate the hydrogenation step,thus enhancing the NRR activity.Among the prepared electrocatalysts,UiO-NH₂has the best NRR activity at-0.4 V,achieving the NH₃yield rate of 6.07×10^-11mol cm^-2s^-1,and the FE of 59.15%.3.The bimetal UiO-Zr-Ti is prepared to introduce the additional active Ti species for NRR through the post-synthetic exchange method.The introduced Ti can not only serve as the active site of NRR to improve ECSA,but also improve the charge transfer efficiency,which is favorable for the improvement of NH₃yield rate.Besides,both Zr and Ti species can suppress the competing HER during NRR process,which benefits the high FE.The characterization results show that when Zr⁴⁺ions with a larger ion radius are replaced by Ti⁴⁺ions with a smaller ion radius,the lattice of UiO shrinks.After the introduction of Ti,the crystallinity becomes lower,and the BET specific surface area becomes smaller.The modified UiO-Zr-Ti-5d exhibits the highest NH₃yield rate of 1.16×10^-10mol cm^-2s^-1and the highest FE of80.36%at-0.3 V.4.N₂H₄and Na BH₄are used to reduce Au nanoparticles loading on UiO.Au with high NRR activity is used as the additional active species to enhance the NRR performance of the whole electrocatalysts.Due to the porous structure of the substrate UiO,the loaded Au nanoparticles with uniform size show excellent dispersion,which is favorable for the exposure of active Au sites and the improvement of Au utilization.Besides,Au with good electrical conductivity can enhance the charge transfer efficiency of the electrocatalysts,which is in favor of the NRR activity,but it also has a negative effect on FE.The effect of the reductant is also studied.As Na BH₄is more reductive than N₂H₄,the Au nanoparticles reduced by Na BH₄achieve more amounts,smaller particle size and better dispersion.However,excessive Au particles will lead to agglomeration,thus unfavorable for NRR.As a result,the sample 5Au@UiO-Na BH₄shows the highest NH₃yield rate of 2.14×10^-10mol cm^-2s^-1and FE of 46.27%at-0.3 V.5.The conductivity of UiO electrocatalysts is enhanced by compounding carbon nanotubes with UiO.The addition of CNTs indeed enhances the charge transfer efficiency of the electrocatalyst and greatly increases the NH₃yield rate,but it is unfavorable for FE.In addition,the existence of CNTs provides the substrate for the growth of UiO crystals,which is beneficial to inhibit the agglomeration of UiO crystals and improve their dispersion.However,excessive CNTs also result in the dilution of the active species,and reduces the activity per unit mass of the electrocatalyst.Therefore,when the CNT feeding amount is 0.15 g,the highest NH₃yield rate of UiO/CNT-0.15g is obtained,which is 1.85×10^-10mol cm^-2s^-1at-0.3V,but its FE is only 36.78%.6.The application of Fe OOH on NRR is preliminarily explored,and the Zr dopeda-Fe OOH electrocatalyst is synthesized by a facile one-step hydrothermal method.Due to the heteroatom doping,more vacancies and defects are generated,which can facilitate NRR.The presence of defects is found to weaken the charge-transfer efficiency,which is favorable for improving FE.In addition,doped Zr not only plays the role of active sites,but also effectively inhibits the competitive HER,leading to the high activity and selectivity.XRD refinement results demonstrate that Zr doping with a larger ion radius would lead to the lattice expansion of Fe OOH,inhibit grain growth,reduce crystallinity,and make particle surface rough.Therefore,4%Zr/a-Fe OOH has the highest ammonia yield and FE at-0.5 V,which are1.39×10^-10mol cm^-2s^-1and 35.63%,respectively.Moreover,in situ FT-IR results demonstrate that the NRR reaction on Zr/a-Fe OOH follows an associative mechanism.