Study On The Preparation Of TiO₂ Nanocomposite And Its Electrocatalytic Nitrogen Reduction Performance

Ammonia（NH₃）is a crucial component in contemporary social life and industrial production.Because its large hydrogen capacity（17.6%）and high energy density(4.3kWh h^-1),NH₃ is a promising alternative energy source for chemical fuels in the hydrogen economy in the future.In the past century,the ever-increasing social demands have stimulated in-depth research on artificial large-scale production of ammonia.At present,the industrial large-scale synthesis of NH₃ mainly relies on the Haber-Bosch process,which uses purified hydrogen（H₂）and nitrogen（N₂）as raw materials through iron or ruthenium-based catalysts at high temperature and pressure（about 500℃and200 atm）.In order to maintain a high temperature and pressure reaction atmosphere,it will inevitably consume a large amount of fossil fuels such as coal,natural gas and release vast quantities of carbon dioxide into the atmosphere,which result in warming up constantly.Therefore,it is necessary to explore a green and sustainable method to synthsize ammonia.The synthesis of NH₃ by electrochemical technology under ambient conditions has become an attractive alternative method,but the stable N≡N triple bond makes nitrogen difficult to crack,so the nitrogen reduction reaction（NRR）requires efficient and durable catalysts to promote the progress of the reaction.The noble-metal NRR catalyst has high electrochemical activity,but they are too expensive and scarce to be used on a broad scale.Therefore,it is very important to design and develop a large number of catalysts that exist on earth for nitrogen reduction.TiO₂ has aroused extensive research attentions due to its abundant,non-toxic,high stability in recent years.This dissertation focuses on the preparation of TiO₂ nanomaterials and their electrocatalytic nitrogen reduction performance.The main research contents are as follows:（1）Ag@TiO₂ as an Efficient Electrocatalyst for N₂ Fixation to NH₃ under Ambient Conditions.Among metal oxides,TiO₂ has aroused our great research interest due to its chemical stability and thermal stability,good biocompatibility and environmentally friendly nature.At the same time,compared with other noble-metals,Ag has more abundant reserves on the earth,so it is expected to be developed for wider applications.In recent years,a large number of research results have confirmed that Ag in Ag/TiO₂can increase the electron transfer rate of hybrid materials,which is beneficial to its application in photocatalysis（degradation of organic pollutants,energy production of hydrogen）and supercapacitors.But so far,the research of this hybrid complex in electrocatalytic NRR has not been reported.We synthesized nano-scale Ag and TiO₂composites（Ag@TiO₂）by a simple hydrothermal method.The nanoparticle structure with a smaller size has a larger specific surface area,which can provide more active sites during the catalytic reaction process,so that the catalytic activity of the catalyst is enhanced.The NRR catalytic performance of Ag@TiO₂ was studied by electrochemical testing techniques.The data showed that the catalyst exhibited excellent catalytic performance when the overpotential was-0.70 V in a neutral electrolyte of 0.1 M Na₂SO₄.The ammonia rate is 14.88μg h^-1 mg^-1_cat,and the Faraday efficiency is 6.2%.In addition,the electrode also has excellent stability and selectivity.The results of these experimental data all show that Ag@TiO₂ is an excellent NRR catalyst.This is because its smaller particle size leads to a larger active surface area and the synergy between the two metals.The above research contents not only provide us with a fascinating electrochemical NH₃ synthesis catalytic material,but also stir up new ideas for exploring the design of TiO₂-based as NRR electrocatalysts.（2）CoS₂@TiO₂ nano bead:A hierarchical structure for efficient electrocatalytic nitrogen reductionAmong the NRR catalysts that have been reported,transition metal-based compounds have received widespread attention because of their high abundance,low cost,and promising prospects for catalyzing the synthesis of NH₃.In the past few years,people have conducted rigorously research on metal sulfides in many different fields.However,due to the instability of the chalcogenide catalysis,durability problems will occur during the test.The nanoarray structure can improve the catalytic performance of the catalyst.To this end,we used a two-step hydrothermal method to synthesize CoS₂nanosquares in situ on the TiO₂ nanowire array（CoS₂@TiO₂）.The TiO₂ nanoarray has a linear structure to increase the contact area between the electrode surface and the electrolyte,and it also provides a fast and efficient transmission channel for charges and electrolyte ions.Through coordinated-driven epitaxial growth,the CoS₂ nano-square particles are uniformly confined to a single TiO₂ nanowire,forming a double-active hierarchical structure.This structure creates an intimate coupling between CoS₂ and TiO₂,which makes charge to transfer easily in between.In addition,it effectively prevents the clusters of CoS₂ nanosquares,thereby preventing the loss of active sites.As a catalyst electrode for NRR,in a 0.1 M Na₂SO₄ neutral electrolyte,CoS₂@TiO₂ has an obvious high ammonia yield(9.22×10^-11 mol s^-1cm^-2)at an overpotential of-0.40 V due to its novel hierarchical structure and a high Faraday efficiency at-0.35 V（6.3%）,and can maintain its stability for at least 24 hours.This study once again proves the great potential of effective TiO₂ modification as a NRR catalyst for nitrogen fixation in neutral media.In summary,TiO₂-based nanomaterials have excellent performance when used as NRR electrocatalyst,and provide novel ideas for the design of high-performance NRR electrocatalysts.