The current industrial ammonia synthesis technology is based on the Haber method using iron-based catalysts. The reaction conditions are very demanding (250 atmospheres, 400 degrees Celsius) and require huge energy consumption. Photocatalytic technology can directly convert solar energy into chemical energy, The energy consumption of ammonia provides a very promising method. However, the ultra-high bond of the nitrogen-nitrogen bond makes the nitrogen molecule exhibit stable chemical properties, which makes it difficult for conventional photocatalytic materials to activate nitrogen molecules. The development of high-efficiency nitrogen-fixing ammonia photocatalyst still faces enormous challenges. Recently, Professor Xiong Yujie from the University of Science and Technology of China and Professor Wu Xiaojun's theoretical research group, based on the defect engineering control of metal oxide photocatalysts, found that the catalyst was refined by doping. The defect state can promote the efficient activation of nitrogen molecules by defect sites, and effectively improve the efficiency of photocatalytic nitrogen fixation of ammonia. The work was published online in the important journal of international chemistry, J. Am. Chem. Soc. DOI: 10.1021/jacs.8b02076), the co-first author is Dr. Zhang Ning, Ph.D. student Abdul Jalil and
Schematic diagram of W18O49 catalyst based on molybdenum doped finishing defect state for light-driven nitrogen fixation
From the kinetic point of view, in view of the high chemical stability of nitrogen molecules, the activation of nitrogen molecules is generally considered to be a prerequisite for nitrogen reduction. For photocatalytic materials, surface defect sites can serve as active sites for chemical adsorption of nitrogen molecules. The localized electrons at the defect can be transferred into the anti-bond π orbital of the adsorbed nitrogen molecule, thereby achieving the weakening of the nitrogen-nitrogen bond. Although the related literature has reported that the catalyst material based on defect construction can be used for photocatalytic nitrogen fixation. Synthetic ammonia reaction, but its activity still needs to be further improved. The bottleneck comes from many aspects: Firstly, it is necessary to further regulate the adsorption of nitrogen sites by catalytic sites, and promote the transfer of photogenerated electrons from catalyst to adsorbed nitrogen molecules to improve nitrogen. The activation ability of the molecule; secondly, it is necessary to suppress the energy relaxation process of photogenerated electrons at the defect to reduce the energy loss during electron transfer.
In response to this series of challenges, Xiong Yujie team implanted molybdenum atoms at the defect sites of W18O49 catalysts to achieve efficient activation of nitrogen molecules in photocatalytic systems. The researchers combined with synchronous radiation characterization, in-situ infrared spectroscopy and theoretical calculations. , revealing the refinement effect of doped molybdenum atoms on the defect state. On the one hand, molybdenum doping increases the defect level of the catalyst and reduces the energy loss caused by the electron energy relaxation process; on the other hand, the formation of molybdenum doping The molybdenum-tungsten heterogeneous site regulates the charge state of the adsorbed nitrogen molecules, increases the charge difference between the nitrogen atoms, increases the covalentity of the metal-oxygen bonds, and promotes the photogenerated electron transfer process. The synergistic effect between the different effects of the impurities effectively promotes the activation of the nitrogen molecules at the catalytic sites, and achieves a significant increase in the efficiency of the catalyst-driven nitrogen-fixing ammonia synthesis. The progress is to develop efficient nitrogen-fixing photocatalysts and to modulate catalyst defects. The state provides a new idea and demonstrates the importance of the regulation of the electronic structure of the catalytic site for the catalytic reaction.
Synchrotron radiation X-ray absorption spectroscopy, photoelectron spectroscopy and infrared spectroscopy in situ detection of this work were respectively supported by Professor Song Li, Prof. Zhu Junfa and Associate Researcher Yan Zeming of the University of Science and Technology of China. The research work has been developed by the state. The plan, the National Science Fund for Distinguished Young Scholars, the Key Research Project of the Frontier Science of the Chinese Academy of Sciences, and the Innovation Crossing Team of the Chinese Academy of Sciences.
