Surface Property Regulation Of TiO₂-based Nanomaterials And Mechanism Investigation For Electrocatalytic Nitrogen Reduction

Ammonia is one of the most basic chemical raw material for the modern industry and agricultural production.It is also regarded as a popular carbon-free hydrogen carrier due to various factors,e.g.green and high eneregy density and facile storage and transportation.In industry,N₂ and H₂ are usually used as the feeding gas to synthesize ammonia at high temperature and pressure,which requires 1%to 2%of the world's energy per year while adopting hydrogen as proton source.Conventionally produced hydrogen gas is mainly generated by the steam reforming of natural gas,leading to large amounts of CO₂ emissions.Electrocatalytic nitrogen reduction(NRR)can use nitrogen and water to synthesize ammonia under mild conditions in a carbon-neutral and sustainable manner.At present,various nanomaterials,such as noble metals,non-noble metals and non-metals,have been widely studied as NRR electrocatalysts.However,due to the high stability of nitrogen molecule,it is necessary to design high-efficient catalysts for enhancing the NRR activity.Based on the above facts,this thesis adopts a combined experiment and theory investigation to explore low-cost and stable high-efficiency transition metal-based NRR nanoelectrocatalysts by tuning defect,coordination structure and element composition.The main contents and conclusions are divided into the following sections.The cheapest and abundant Fe dopant is used to greatly enhance the NRR activity of TiO₂ nanoparticle.The experimental results show that Fe-doped TiO₂ nanoparticle can simultaneously obtain a large NH₃ yield rate with a high Faradic efficiency(FE)at?0.40 V vs.reversible hydrogen electrode,and the corresponding FE and NH₃ yield rate are 25.6%and 25.47?g h^?1 mg^?1_cat.,respectively,higher than that of pure TiO₂ and most of the reported Ti and Fe-based NRR electrocatalysts.This catalyst also presents strong electrochemical and structural stability.Density functional theory calculations show that the introduction of Fe can promote the formation of oxygen vacancy on the TiO₂(101)surface,and the induced bi-Ti³⁺defect structure is the main active site for N₂ activation and reduction.The whole NRR process follows the alternating mechanism,and the limiting-potential step occurs in both*N₂ to*NNH and*NH₃ desorption processes.Furthermore,the mixed-valent Cu is introduced into TiO₂ material(Cu-TiO₂)to construct abundant oxygen vacancies and Ti³⁺defects.Such catalyst exhibits a large FE of 21.99%and a high NH₃ yield rate of 21.31?g h^-1 mg^-1_cat.at?0.55 V,and has excellent electrochemical and structural stability.Density functional theory calculations reveal that compared to the Fe-TiO₂ system,the introduction of Cu dopant can more effectively reduce the formation energy of oxygen vacancy on the TiO₂(101)surface,while promoting the formation of multiple oxygen vacancies and Ti³⁺defects.And the defect structures of Ti³⁺-Ti⁴⁺and Ti³⁺-Ti³⁺are most valuable on Cu-TiO₂(101)surface for NRR.The Ti³⁺surface states with various concentrations and local coordination configurations are constructed,and the structure-activity relationship between Ti³⁺and NRR is analyzed through density functional theory calculation.Results show that Ti³⁺can significantly activate N₂ molecule only when it is unsaturated-coordination feature with the orbital splitting of e_g and t_2g defect states.Moreover,the synergy role between oxygen vacancy and Ti³⁺can greatly reduce the energy input of*N₂ to*NNH,thus facilitating the NRR process.Furthermore,the required maximum energy input in the whole NRR process is connected with the adsorption strength of*NNH for many NRR electrocatalysts,which shows a distinguished volcano shape.The new TiO₂-based candidates with an optimum NRR activity are successfully predicted by this key parameter.The role of the high-valent V dopant in TiO₂ on NRR activity is investigated.Firstly,the V-doped TiO₂ nanorod catalyst(V-TiO₂)is synthesized by the hydrothermal method.Such catalyst exhibits a maximum FE of 15.3%and a highest NH₃ yield rate of17.73?g h^-1 mg^-1_cat.at?0.40 V and?0.50 V,respectively,which are much higher than that of pristine TiO₂.Density functional theory calculations indicate that the whole NRR process follows the alternating mechanism,in which the limiting-potential step occurs at*N₂ to*NNH process.Particularly,the synergy role between the V⁴⁺dopant and Ti⁴⁺in the V-TiO₂ system can make the key intermediate of*NH₂NH₂ hydrogenated to NH₃,thus effectively promoting NRR.Above results can provide important guidance for quickly screening high-efficiency NRR electrocatalysts from engineering the surface geometry and electronic structures of transition metal oxides in the future.