Promoted Photocatalytic Ammonia Synthesis Over Tungsten Oxide And Molybdenum Oxide With Lanthanum Modification

Since human society paced into modern civilization,with the rapid development of economy and industry,human beings have continuously enriched material civilization.However,it is based on the large-scale exploitation and utilization of fossil energy,which not only directly leads to energy depletion,but also causes a series of environmental problems.The serious consequences of energy shortage are becoming increasingly apparent,and scientists have to accelerate the pace of exploring new energy.There are varieties of new energy sources currently available,including solar energy,geothermal energy,wind energy,water energy,etc.Among them,solar energy has become the most promising renewable energy source due to its advantages of clean,widely distributed,huge reserves and so on,attracting widespread attention and research from scientists.Therefore,the development of science and technology that converts solar energy into chemical energy is of great significance for improving earth resources and environmental issues.Semiconductor photocatalytic technology is a major breakthrough in the conversion of solar energy to chemical energy,which originated from catalyzing water splitting into hydrogen.In the past few decades,semiconductor photocatalytic technology has been widely used in the fields of environmental pollution control and photocatalytic organic synthesis.In addition,researches related to photocatalytic nitrogen fixation have increased dramatically in recent years,showing that the photo-generated electrons generated by semiconductors after light absorption during photocatalysis can reduce the main component of the air,nitrogen,and subsequently obtain ammonia,which is essential for human life.The photocatalytic nitrogen fixation technology is inspired by the life phenomenon of nitrogen fixation in nature.Legumes rhizobium can fix nitrogen in the air to synthesize organic ammonia as a raw material for synthesizing its own nutrients through nitrogen fixation enzymes,but the biological nitrogen fixation is rarely utilized in industry.In the photocatalytic nitrogen fixation reaction,sunlight stimulates electron transitions,water provides protons,and the catalyst serves as the adsorption site for nitrogen.The nitrogen fixation process is cycled with normal temperature and pressure conditions,breaking the restrictions of harsh conditions such as high temperature and pressure of traditional industrial ammonia synthesis technology.However,the non-polar N?N bond is hard to activate and the affinity of proton or electron is weak,which leads to the low efficiency of photocatalytic ammonia synthesis,and it is difficult to achieve large-scale practical application in the short term.Therefore,designing and synthesizing photocatalysts with excellent photocatalytic nitrogen fixation performance is very important for building an efficient photocatalytic ammonia synthesis system.Based on semiconductor defect engineering and modification of photocatalytic materials,rare earth metal lanthanum modified tungsten oxide and molybdenum oxide with rich oxygen vacancies were prepared.It was found that lanthanum doping or single-atom lanthanum loading can enhance the performance of photocatalytic ammonia synthesis by tungsten or molybdenum oxide.And the possible mechanism of photocatalytic ammonia synthesis with lanthanum modification was explored.The purpose of this study is to reveal the influence of rare earth metal modification on the structure of semiconductor materials,its relationship with oxygen vacancies and its function in the photocatalytic ammonia synthesis,providing a unique perspective for the design of new efficient photocatalysts.It mainly includes the following two parts:1.We chose the widely used catalyst tungsten oxide as a model,and oxygen vacancies were built on the surface,so that its performance of nitrogen adsorption was promoted and can be used as the active site of the reaction.At the same time,there are many unpaired electrons on the rare earth metal lanthanum,valence electrons need to be lost when participating in the chemical reaction,and its active d-orbital electrons can be used to enhance the activation of nitrogen.We attempted to combine the rare earth metal lanthanum with defective tungsten oxide to achieve an efficient photocatalytic ammonia synthesis reaction.We prepared a defective tungsten oxide nanowire with lanthanum doping and found that its photocatalytic activity of ammonia synthesis was much higher than that of pure defective tungsten oxide nanowire samples.The kinetics and active species of the photocatalytic ammonia synthesis were experimentally explored.Combined with the related studies on the kinetics of photo-generated carriers,the enhancement of nitrogen fixation activity due to the change in energy band structure of materials and then promoting light absorption was excluded.The addition of lanthanum can effectively promote the transfer of photo-generated charges to oxygen vacancies,strengthen the adsorption and activation of nitrogen molecule,and thereby improve the efficiency of photocatalytic ammonia synthesis.Finally,a possible reaction mechanism of lanthanum doped defective tungsten oxide nanowire photocatalytic ammonia synthesis was proposed based on the nitrogen temperature-programmed desorption test and in-situ infrared spectroscopy of the nitrogen fixation reaction under light.2.Based on the research in the previous section,we found that the introduction of rare earth metal lanthanum can effectively improve the efficiency of the photocatalytic synthesis on transition metal oxides.In order to build a more efficient photocatalyst we have made further attempts.Inspired by the molybdenum-iron proteinase in nitrogenase,which is the site of nitrogen adsorption and activation,the defective molybdenum oxide was selected as the research carrier.Based on the advantages of high surface free energy and excellent catalytic performance of single atom,defective molybdenum oxide photocatalyst supported with single-atom lanthanum was designed and synthesized,and then was applied in photocatalytic nitrogen fixation.It was found that the loading of single-atom lanthanum greatly improved the photocatalytic efficiency.Incidentally,oxygen vacancies on the pure defective molybdenum oxide had insufficient to adsorb and activate nitrogen.We proved the morphology and the unsaturated defect state of the material,and the single-atom loading state of lanthanum was also clarified by corresponding characterization.The influence of single-atom rare earth metal loading on the mechanism of oxygen vacancy was revealed.And we explored the role of the loaded single-atom in improving the efficiency of the photocatalytic ammonia synthesis,finally proposed a possible nitrogen fixation mechanism of defective molybdenum oxide supported with single-atom lanthanum.This work explores the mechanism of single-atom loading on defective metal oxides and its effect on the photocatalytic ammonia synthesis,and provides inspiration for the design of single-atom catalysts.