Photo (Electro) Catalytic Nitrogen Reduction At Ambient Conditions

Ammonia is one of the most highly produced chemicals because it is a source of nitrogen for fertilizer and a novel hydrogen storage material.Up to now,energy and capital intensive Haber-Bosch process still remains the dominant route for ammonia synthesis.In view of the fossil fuels shortage and global climate change,nitrogen fixation through less energy-demanding process is therefore a challenging and long-term goal.The development of a new,scalable ammonia synthesis technology to reduce the harsh condition requirements and to improve security is an urgent need.Utilizing renewable solar energy or electricity to convert water and N2 into ammonia under ambient conditions is very promising for the sustainable development.The main work of this thesis is to explore the technique of ammonia synthesis from water and N2 under room temperature and atmospheric pressure and to develop high-efficiency photocatalysts,electrocatalysts and photoelectrocatalysts.Combining the research results reported in the existing literatures with the research basis of our group,the plasmonic catalyst and metal catalyst were prepared to solve the problem of the low yield of ammonia synthesis at ambient conditions and the low utilization of visible light and near-infrared light in sunlight.Based on the above reasons,this paper carries out the following three parts of research work:(1)A plasmonic photocatalyst Au/MIL-100(Cr)was prepared by solution immersion reduction method for photocatalytic nitrogen reduction using water and nitrogen at room temperature and atmospheric pressure.Control experiments show that the ammonia synthesis is a process of photocatalytic nitrogen fixation.The nitrogen element in ammonia is derived from nitrogen,and the hydrogen element is derived from water.The activity results of the plasmonic photocatalyst illuminated by different monochromatic light show that the apparent quantum efficiency reaches the maximum at the wavelength of the incident light of 550 nm,which is attributed to the surface plasmon resonance effect of Au.Compared to pure MIL-100(Cr),Au/MIL-100(Cr)exhibits significantly improved activity of ammonia synthesis under visible light.Synergistic effect of the adsorption of nitrogen by MIL-100(Cr)and the activation and dissociation of nitrogen by hot electrons improves the photocatalytic nitrogen reduction activity.(2)The Au nanorods were synthesized by seed-induced growth method,the aspect ratio of Au nanorods was adjusted by changing the amount of reducing agent and directing agent.Ru nanoparticles were loaded on Au nanorods for photoelectrocatalytic nitrogen reduction.To maximize the use of sunlight,the material was designed to absorb both visible light and near-infrared light by controlling the peak position of the longitudinal plasmon vibration peak of the Au nanorod.The loading of Ru changes the optical properties and electronic structure of the Au nanorods,reduces the initial potential of the catalytic reaction,and significantly increases the activity of electrocatalytic nitrogen reduction.The addition of light reduces the energy barrier of the electrochemical reaction and increases the yield of the ammonia produced by electrocatalysis.The activity of the photoelectrocatalytic nitrogen reduction to ammonia is higher than that of synthesizing ammonia only by electrocatalytic technology under either visible light or near-infrared light.(3)The Au-Cu bimetal was synthesized by a displacement reaction method for electrocatalytic nitrogen reduction reaction.To select the optimal Au-Cu bimetal catalyst,different ratios of Au-Cu bimetals were prepared by adjusting the addition of Au precursor solution and their electrocatalytic activities were also measured.The activity test was carried out at different voltages to select the optimum reaction voltage.The experimental results show that the catalytic activity of bimetals is better than that of single metals.This is attributed to the fact that bimetals usually have different surface and interface properties than single metals,the electronic structures of the two metals interact with each other to promote the adsorption and activation of nitrogen on the catalyst surface.