Design And Synthesis Of Metal/semiconductor Catalysts For Fullspectrum Photocatalytic Nitrogen Fixation

As one of the indispensable elements of amino acids and nucleotides(DNA/RNA),nitrogen plays an essential role in organisms.In nature,most of the nitrogen species exist in the atmosphere in the form of nitrogen molecules,accounting for about 78%of the total air quality.The nitrogenase being contained the nitrogen-fixing of microorganisms can convert nitrogen molecular into nitrogen-based compounds.The thermodynamically strong cleavage energy of N?N bond of the nitrogen(945k J/mol)manifests the critical challenge for the full dissociation of N?N.At the beginning of the 20th century,the iron-based catalysts successfully could be employed to convert the nitrogen(N₂)into the ammonia(NH₃),known as the Haber-Bosch process.The whole process contributed about 500 million tons fertilizer for the global agriculture every year,which caused high energy consumption and serious pollution at the same time.The utilization of solar energy to activate nitrogen has attracted widespread attention,which is expected to provide new possibilities for the realization of greener ammonia synthesis recent years.Therefore,in order to overcome the dynamics limitation and broaden the available spectrum,people are devoted to the structural design of photocatalyst,the surface of defect state regulation and electronic state regulation and the modification of co-catalysts,the key scientific problems are:(1)how to design the active site for the adsorption,activation and reduction;(2)how to efficiently transfer photogenerated electron from semiconductor to adsorbed nitrogen molecules through the interface;(3)how to design double active sites to realize the synergistic activation of nitrogen and hydrogen?To improve the efficiency of photofixation nitrogen,the semiconductor is modified by the structure design and the surface state through the following several aspects in this work:(1)building the surface metallization of TiO_2-x between surface terminal Ti-F groups and adjacent Ti³⁺,and realizing the efficient separation of photoproduction charge and interface transport via atomic-scale ohmic contacts;(2)improving the electron density of active sites and electronic feedback and realizing the full-spectrum nitrogen fixation at room temperature;(3)introducting the metalized energy level to broaden the range of light absorption and realizing the photocatalytic ammonia synthesis in the whole solar spectrum range at room temperature;(4)designing the catalyst to enable the activation of nitrogen and hydrogen at different active centers.Through the mechanism of"activated hydrogen transfer",the hydrogen poisoning is effectively reduced,and the efficiency of photocatalytic nitrogen fixation was further improved.The main research results are as follows:1.The electronic state of titanium dioxide was controlled by fluorine ion and the metallized titanium dioxide was successfully prepared.The surface metallization of reduced TiO_2-x was achieved through the interaction between terminal Ti-F groups and adjacent Ti³⁺under thermal annealing in vacuum.As was evidenced experimentally and theoretically,the surface conduction band of fluorinated TiO_2-x was significantly downward bent and partially filled with electrons,which eventually was leading to surface metallization.The metallic surface caused a strong magnetic frustration on the inner spins of Ti³⁺in the bulk phase,resulting in the quantum phase transition from the blocked superparamagnetic state to the quantum superparamagnetic state at ca.50 K.The fluorinated TiO_2-x with low work function and downward band bending possessed a unique electron-donating power to support metal active sites via atomic-scale ohmic contacts.Consequently,due to the superior optical properties and interfacial interactions,the efficient ammonia synthesis was achieved over the optimized Ru cocatalysts modified fluorinated TiO_2-x under visible and even the near-infrared light irradiation(400-1550 nm)at room temperature.An extremely high NH₃ generation rate of>1000?g g^-1 h^-1 was accessed at the normal condition.2.The low-valent molybdenum element was introduced into the lattice of titanium dioxide with oxygen defect used to regulate the electronic state of titanium dioxide.According to the experimental results and the theoretical calculation,coordination unsaturated molybdenum improves electron back-donation and enhances the adsorption and activation of nitrogen.At the same time,Pt nanoparticle would be loaded on Mo element doped TiO₂ to enhance the activation of hydrogen and reduce hydrogen poisoning on a single active site.It was found that the introduction of Pt nanoparticles can promote the activation and dissociation of hydrogen,which is making hydrogen atoms overflow from the surface of the catalyst.It can promote the hydrogenation and reduction of nitrogen molecules through the surface"activation hydrogen transfer"process.The multi-synergistic effect induced by Mo element doping and Pt nanoparticle loading significantly enhanced the photocatalytic nitrogen fixation performance.An extremely high NH₃ generation rate of>3000?g g^-1 h^-1 was accessed at the normal conditions.3.Black ZrO_2-x with oxygen defects was prepared by modification of commercial zirconia(c-ZrO_2-x)by thermal reduction method,and the semiconductor was loaded with ruthenium nanoparticles for the photocatalytic synthesis of ammonia at the room temperature.Due to different work functions,the interface of Ru/ZrO_2-x forms the Schottky barrier,which contributes to the unidirectional transmission of photoexcited electrons and Ru nanoclusters act as the electron trap with the enhanced feedback ability of electrons.At the same time,the introduction of defect state makes a good performance of visible light photocatalytic ammonia synthesis.An extremely high NH₃generation rate of>500?g g^-1 h^-1 was accessed at the normal conditions.