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Ammonia is one of the most important inorganic chemical products. The ammonia industry occupies an important position in the national economy. Liquid ammonia can be directly used as a fertilizer. Nitrogen fertilizers used in agriculture such as urea, ammonium nitrate, ammonium phosphate, ammonium chloride, and various nitrogen-containing compound fertilizers use ammonia as a raw material. Synthetic ammonia is one of the largest chemical products. The world's annual output of synthetic ammonia has reached more than 100 million tons, of which about 80% is used for chemical fertilizers and 20% is used as raw materials for other chemical products. At the same time, all organisms need nitrogen to build proteins, nucleic acids, and many other biological molecules.
Nitrogen accounts for about 78% of the earth's gas, but it cannot be directly absorbed by most organisms, because NN's covalent triple bond high ionization energy causes it to be very stable. The fixation of N2 on NH3 by the classical Haber-Bosch process must be carried out in the presence of an iron-based catalyst under harsh reaction conditions (15-25 MPa, 300-550°C). The provision of reaction conditions consumes 1-2% of the world's energy supply and produces about 2.3 tons of carbon dioxide per year. At present, the raw material hydrogen used for the synthesis of NH3 is mainly from the reformation of methane, which consumes about 3-5% of the world's natural gas production each year and releases a large amount of carbon oxides.
Given the shortage of fossil fuels and global climate change, nitrogen fixation through the reduction of energy demand is a challenging and long-term goal. The use of solar photocatalytic technology to convert solar energy into chemical energy has been considered as one of the best ways to solve future renewable energy sources.
Due to its unique laminar structure and large proportion of exposed active sites, two-dimensional nanomaterials exhibit superior performance in photoelectrocatalysis, which has attracted extensive attention of researchers. Lamellar double hydroxides (hydrotalcite, LDH) exhibit regulatable catalytic properties because of their advantages in that the laminate consists of various components and the thickness of the laminate is adjustable. Zhang Tierui, a researcher at the Institute of Physics and Chemistry, Chinese Academy of Sciences, prepared a series of MIIM III-LDH (MII = Mg, Zn, Ni, Cu; MIII = Al, Cr) nanosheet photocatalysts through a simple co-precipitation method. The X-ray absorption fine structure, low-temperature electron paramagnetic resonance and positron annihilation lifetime measurements show that the ultra-thin LDH nanosheets enhance the adsorption of N2 molecules due to oxygen-rich defects, structural deformation and compressive strain, and transfer of photogenerated electrons from LDH photocatalysts N2, which promotes the effective synthesis of NH3 (especially CuCr-LDH nanosheets, whose quantum yield at 500nm can still reach ~0.10%). Research shows that the reduction of N2 to NH3 based on LDH is a new potential and promising approach under normal temperature and pressure, visible light or direct solar radiation.
The results of the study were published in "Advanced Materials". The relevant research work was supported by the National Key Basic Research Program of the Ministry of Science and Technology, the National Natural Science Foundation of China's Outstanding Youth Science Fund Project, the National Natural Science Foundation of China Youth Fund Project, and the National Million Plan - Youth Top Talent support program, support of the Chinese Academy of Sciences' strategic pilot technology project (Class B).
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