Study On Electrochemical Synthesis Of Ammonia With Titanium Dioxide-based Materials

Ammonia is an important chemical raw material.Nitrogenous fertilizer made from ammonia increased the global crop yields and fed billions of people over the past century.At present,the industrial ammonia synthesis process still relies on the Haber-Bosch method.However,this method is capital-and energy-intensive,accompanied by a large amount of greenhouse gas emission,which does not meet the requirements of green and sustainable development.Therefore,researchers are looking for new ways to synthesize ammonia under mild conditions.Electrochemical synthesis of ammonia(EAS)driven by renewable electricity is one of the most promising technologies,which has attracted wide attention.Due to the extremely stable chemical properties of the N2 molecule,catalysts for EAS should exhibit a strong activation ability of N2.Constructing specific active sites for N2 adsorption or increasing the number of active sites by appropriate catalyst design is the research focus in this field.TiO2-based materials have the advantages of high stability and controllable preparation process and they have been widely used as the catalysts in the field of heterogeneous catalysis,thereby they are the ideal candidates for EAS.In this paper,A-TiO(OH)2 prepared by hydrolysis method and metal-doped titanium dioxide(TiO2-M)prepared by sol-gel method were used as catalysts for EAS in aqueous solution system.A strict detection method for the produced ammonia after EAS experiments was established based on the ion chromatography.The EAS performance of these two catalysts were evaluated based on the formation rate of ammonia(RNH3)and faraday efficiency(FE).Further,the EAS mechanism was also explored.The major contents and conclusions are as follows:(1)A-TiO(OH)2 was prepared via a simple hydrolysis method and it was used to realize EAS.In the lattice of A-TiO(OH)2,a lot of Ti3+ species and oxygen vacancies which were conducive to the adsorption and activation of N2 were confirmed by characterizations.The results of electrolysis experiments showed that the electrolysis conditions,such as the catalyst loading and the cathode potential,could affect the EAS performance.The highest RNH3 Of 1.27×10-11 mol s-1 cm-2 was obtained at the cathodic potential of-1.55 V(vs.Ag/AgCl,the same below)in 0.01 mol L-1 K2SO4 solution;the highest FE of 35%was achieved at the cathodic potential of-0.55 V.(2)TiO2-M(M=Cu,Mn,Pd)was prepared via a sol-gel method and it was used to realize EAS.Characterizations demonstrated that these doped metals were embedded on the surface of TiO2 in the unsaturated-coordination modes(-O-M-O-).A series of EAS experiments showed that the order of RNH3 was TiO2-Pd>TiO2-Cu>TiO2-Mn,while the order of FE was TiO2-Cu>TiO2-Mn>>TiO2-Pd.The results of linear sweep voltammetry(LSV)tests and density functional theory(DFT)calculations showed that TiO2-Pd possessed a strong adsorption activity of N2;however,it also exhibited high catalytic activity for the hydrogen evolution reaction(HER).The HER activity of TiO2-Cu and TiO2-Mn were demonstrated to be relatively weak which could be used to explain their higher FEs.(3)TiO2-Cu was further systematically investigated.The highest RNH3 Of 1.68×10-11 mol s-1 cm-2 with an FE of 5.51%was demonstrated at the cathodic potential of-0.75 V in 0.01 mol L-1 K2SO4 solution.When the cathode potential decreased to-0.55 V,a high FE of 15.33%with an RNH3 of 9.77×10-12 mol s-1 cm-2 was achieved.DFT calculations disclosed that TiO2-Cu could effectively adsorb and activate the N2 molecule in both the "end-on" and "side-on"configurations.Further,it was confirmed that the most possible EAS pathway on TiO2-Cu was the alternative enzymatic pathway with the "side-on" initial configuration based on the calculations of free energy change of elementary reactions.