Study On Surface Modification Of Metal Tungstate And Its Application In Photocatalytic Nitrogen Fixation

With the rapid development of the industrial society and the great progress in the level of scientific and technological production,the crisis of depletion of fossil energy and environmental problems have become increasingly apparent.Ammonia（NH₃）is a widely used and irreplaceable important industrial and agricultural basic raw material.At present,the process of industrial ammonia synthesis in the world mainly adopts the Haber-Bosch reaction,but because nitrogen（N₂）molecules are very stable and difficult to activate,this reaction can only be realized under high temperature and high pressure（300-500℃,100-200 atmospheres）..This ammonia synthesis method has high energy consumption and is not environmentally friendly.The photocatalytic N₂reduction reaction（NRR）uses nitrogen and water（H₂O）as raw materials and utilizes the inexhaustible solar energy to cleanly and efficiently convert N₂to NH₃.Converting light energy into chemical energy,so as to realize the efficient use of renewable energy,is in line with the requirements of green chemistry in today’s world.The superior activity of transition metal oxides（TMO）towards NRR stems from the ability to facilitate the separation of charge carriers and the adsorption and activation of nitrogen molecules on photocatalysts.Therefore,engineering oxygen vacancies can effectively enhance the intrinsic photocatalytic activity of TMO.Heteroatom doping and nanostructure tuning are widely used to tune the oxygen vacancies of TMO,thereby providing more active sites for photocatalytic reactions.On this basis,to further improve oxygen vacancies,bimetallic oxides have been reported as efficient photocatalytic reaction catalysts.The enhanced reactivity is attributed to more oxygen vacancies compared to single metal oxides.Based on the above issues,in this thesis,we designed and synthesized double metal oxide metal tungstate,namely cobalt tungstate（Co WO₄）for photocatalytic NRR.Co WO₄-180-12（300Ar）catalyst was synthesized by a simple hydrothermal-calcination method.Through surface modification to create oxygen vacancies,the performance of the catalyst for photocatalytic nitrogen reduction can be significantly improved.Experimental results show that the catalyst display an ammonia production rate of 55.63μmol g^-1_cat.h^-1under the reaction conditions where 20%methanol is used as a sacrificial agent and a 300 W xenon lamp is added with a filter as a simulated visible light source.And good catalytic stability was maintained after recycled the catalyst for seven times.The calculated turnover frequency（TOF）value and apparent quantum yield of the catalyst were 0.171 h^-1and 0.93%,respectively,confirming its good photocatalytic nitrogen fixation performance.The visible light response,electron-hole separation and migration,and surface dynamics of the cobalt tungstate catalyst during photocatalytic nitrogen fixation were investigated.The results demonstrated that the cobalt tungstate catalyst after Ar annealing acquired more oxygen vacancies,thereby the absorption of visible light and the separation of carriers are effectively promoted.The density functional theory simulation results show that the increase of oxygen vacancy concentration can promote the active adsorption of nitrogen on the catalyst,thereby promoting the photocatalytic NRR.The active site of the reaction is the Co²⁺that appears on the（-111）crystal plane after the generation of oxygen vacancies.