Design And Preparation Of Schottky Photocatalysts For Visible-light Driven Ammonia Synthesis At Room Temperature

Nitrogen is the basic element of amino acids and nucleic acids?DNA/RNA?,which is one of the most important elements for organisms.In nature,nitrogen element mainly exists in the form of nitrogen molecule?N₂?that is difficult to be directly applicable to organisms due to inert chemical properties.In the past 100 years,the main synthetic ammonia method is the traditional Haber-Bosch process,which consumes more than 1%of the global energy and releases more than 300 million tons of carbon dioxide,leading to increasingly serious energy and environmental problems.Therefore,it is well worth the effort to develop innovative method and technology for ammonia synthesis.Since the initial report of reduction of nitrogen to ammonia under ultraviolet irradiation in 1977,researchers have devoted increasing attention to activate nitrogen molecules by clean light energy and achieve efficient ammonia synthesis under mild conditions.After several decades of development,there are still two major difficult problems in the field of photocatalytic ammonia synthesis.One is the difficulty of N?N triple bond dissociation at low temperature and the other is the low efficiency of electron transfer between photocatalyst and nitrogen.Therefore,the researchers expected that the energy band structure,surface electronic states and catalytic active sites of photocatalysts can be designed to improve the efficiency of photocatalytic ammonia synthesis.To address the above two major difficult problems,these measures are adopted in our work,including by:1)adjusting the band structure of semiconductor,and building Schottky barrier on the surface of catalysts to promote the effective separation of photogenerated electron-hole pairs and the interface transfer of photogenerated electron;2)building appropriate catalytic active center?such as metal or lattice oxygen defect?to further enrich photogenerated electron and enhance electron-donating powder of the catalytic active center,which can transfer electrons to the anti-bonding orbitals of nitrogen molecule to break N?N bond at room temperature,and improve the efficient photocatalytic synthesis of ammonia.This paper explored the use of transition metals/nonmetals to replace precious metals for visible light catalytic activation of nitrogen under room temperature,and the following research results have been achieved:1.Metal iron was used as the active site for activating nitrogen,but it was easy to agglomerate due to its magnetic properties.In order to regulate the electronic state and magnetic properties of iron active sites and obtain ultra-small metal Fe,a small amount of Pt²⁺ions were added to successfully prepare superparamagnetic Fe Pt clusters nanoclusters.The nanoclusters were loaded on a series of semiconductors including graphite phase carbon nitride,gallium nitride nanowires,molybdenum dioxide,zinc oxide and monocrystalline silicon,which were applied to the visible-light catalytic ammonia synthesis.It was found that a small amount of Pt²⁺dopant could significantly change the magnetic properties,electronic states and enthalpies of zero valent iron nanoclusters.Besides,the electrons near the surface of semiconductors transferred to the Fe Pt clusters,resulting in the upward bending of semiconductor band and the formation of Schottky barrier.The Schottky barrier could effectively inhibit the recombination of photogenerated electron-hole pairs,and photo excited high-energy electrons were enriched at the catalytic sites,which were further transferred to adsorbed N₂to realize visible-light fixation of nitrogen.The results showed that the activation of nitrogen molecules was very sensitive to the electron density of metal active sites and the surface energy band bending of semiconductors.The Schottky photocatalysts with high electron-donating powder provided a new way for nitrogen reduction under visible-light irradiation at room temperature.2.The electronic state of surface defects on polar facets of zinc oxide rich in oxygen vacancies was controlled by titanium dopant to prepare the low-valence titanium-doped zinc oxide rich in oxygen vacancies catalyst,which were applied to the visible-light catalytic ammonia synthesis at room temperature.The doped Ti species nearby the oxygen vacancies were coordinatively unsaturated and played multiple roles in N₂ photoactivation:?1?trapping and localizing the electrons of neighboring oxygen vacancy to form low-valence Ti species,which enabled the efficient chemisorption and dissociation of N₂ with promoted adsorption energy and electron-donating powder;?2?dissolving extra valence electrons into the Zn O,resulting in the reduction in the electrical resistivity and the elevation in the Fermi level of Zn O host;?3?making Zn O an efficient electron donor for the Ru photocatalysts.The formed Schottky barrier at Ru/semiconductor interface built up a one-way channel for the efficient transfer of photoexcited electrons to the N?N bond.The catalytic test showed that the low-valent titanium-doped zinc oxide rich in oxygen vacancies catalyst had visible light catalytic synthesis of ammonia under room temperature,and the metal ruthenium nanoclusters could further improve the nitrogen fixation performance of the catalyst.3.The oxygen vacancies on the surface of ceria were used as the active sites for activating nitrogen,and the excess electrons of the oxygen vacancies were filled into the anti-bonding orbitals of nitrogen molecules to promote the activation of nitrogen molecules under visible light irradiation at room temperature.In order to broaden the light absorption range of the catalyst,Au nanoparticles were loaded on the surface of cerium oxide rich in oxygen vacancies by the method of photo-deposition.Experiments results showed that the local surface plasmon resonance of Au could enhance the absorption and utilization of light energy.Au nanoparticles generated hot electrons under visible light irradiation,which could not only dissociate hydrogen,but also could be injected into the conduction band of the semiconductor,thereby promoting the reduction of nitrogen molecules on the oxygen vacancies on the ceria surface.