Photocatalytic Reduction Of Nitrogen And Carbon Dioxide By Niobium-based And Titanium-based Oxides Through Point Defect Engineering

Gaseous nitrogen and carbon dioxide are abundant in the atmosphere and their efficient utilizations are vital to the future of the industrial world.Nitrogen is the main raw material for the industrial Haber-Bosch ammonia synthesis.In the Haber-Bosch process,ammonia is synthesized from N2 and H2 using iron as catalyst.The reaction is typically carried out at high temperatures(400℃-500℃)and high pressures(150 atm-250 atm).Because the reaction conditions are very harsh and the energy consumption is high,a green method for synthesizing ammonia needs to be developed.Photocatalytic nitrogen fixation technology,which uses clean and renewable solar energy as the driving force and abundant water and N2 as raw materials to convert solar energy into chemical energy under normal temperature and pressure,has received extensive attention in recent years as an alternative new method for ammonia synthesis.Since the N-N covalent triple bond is difficult to break,the current technology is limited by the low photocatalytic nitrogen fixation efficiency.Therefore,the development of more efficient photocatalysts for nitrogen fixation is crucial to future development of the photocatalytic nitrogen fixation technology.Carbon dioxide,the primary carbon source for life on earth,is also a major greenhouse gas,which must be well controlled.One of the most effective ways to reduce the carbon dioxide is to recycle it to a valuable product such as fuel.Photocatalytic reduction of CO2 has drawn much attention recently largely because the reaction may yield useful energetic products and it can be driven by clean and renewable energy.However,since the CO2 is relatively stable like N2,industrial application of the photocatalytic technology has been limited by the low photocatalytic efficiency.In this study,taking niobic acid as the starting point,point defects were added by hydrothermal reduction,doping and thermal reduction to promote the adsorption and activation of N2 on the catalyst surface.The photocatalytic reduction of CO2 was further studied by doping TiO2 with Nb element.The main contents of the work are as follows:1.Nb2O5·nH2O nanosheets were prepared by the hydrothermal method,through hydrothermal reduction by weak reducing agent glyoxal and subsequently heat-treating,the effects of oxygen vacancy(VO)and acid sites of Nb2O5·nH2O on photocatalytic nitrogen fixation were studied.Experimental results showed that the synergy of VO point defects and acid sites was not only beneficial to the adsorption and activation of N2,but also improved the separation and migration efficiency of photogenerated charge carriers.The photonitrogen fixation reached a high yield of 173.7 μmol g-1 h-1 without sacrificial agent and cocatalyst,which offers good potential for a wide field of applications.2.Similar results were obtained in N-doped H3ONb3O8 nanosheets,which were prepared by in-situ doping in comination with freeze-drying.In this case,N-doping created a significant number of the VO point defects,which benefitted the adsorption and activation of N2,as well as the separation and migration efficiency of the charge carriers.As a result,the yield of ammonia synthesis was increased by ten folds as compared to that on the pristine H3ONb3O8 nanosheets.3.The niobic acid made earlier was ground with Na2CO3,Na2CO3+H3BO3 and strong reducing agent NaBH4 and subsequently heat treated to produce NaNbO3 nanocubes,B-NaNbO3 nanocubes,VO-NaNbO3 nanoparticles and B-Vo-NaNbO3 nanoparticles.The presence of B dopant and VO enabled NaNbO3 to have visible-nearinfrared photoresponse,widening the spectrum range for photocatalytic reactions.At the same time,these point defects also promoted the adsorption and activation of N2 molecules,desirable for N2 fixation.Upon light illumination,the same defects were shown to benefit the photogenerated charge separation and accelerate the migration efficiency of those charge carriers.Consequently,a very high photocatalytic N2 fixation efficiency was achieved at ammonia production rates of 47.56 and 26.99 μmol g-1 h-1,under simulated sunlight and visible light respectively.4.For point defect engineering in TiO2,Nb was chosen as dopant to fabricate Nbdoped TiO2 nanotube arrays by anodizing Ti-Nb alloys in acidic solutions.Theoretical calculations and experimental characterization indicated that Nb dopant created the acid ceIters and Ti3+ point defects in Nb-doped TiO2 nanotubes,which enhanced both the adsorption and activation of CO2 on the catalyst surface,leading to high-efficiency and high-selectivity photocatalytic CO2 reduction.Under simulated sunlight,Nb-doped TiO2 nanotubes exhibited a CH3CHO yield of 572 μmol g-1 h-1 and 99%selectivity.