Bismuth-based Composite Oxides With Broad Spectrum Response For Photocatalytic NO Removal

The excessive emission of nitrogen oxides(NOx)disrupts the natural nitrogen cycle balance,leading to serious environmental pollutions such as acid rain,smog,and greenhouse effect.Moreover,nitrogen oxides(NOx)undergo photochemical reactions with volatile organic compounds and hydrocarbons under sunlight,producing irritating and toxic secondary pollutants such as O3 and peroxyacetyl nitrate,posing a serious threat to human health.Traditional control technologies such as physical/chemical absorption,thermal catalytic oxidation and reduction methods are not suitable for removing low-concentration NOx(<1 ppm)in residential areas,roads,tunnels,underground garages,and other areas.Therefore,developing efficient and environmentally friendly low-concentration NOx conversion methods remains a significant challenge in the field of air pollution prevention and control.Photocatalytic technology exhibits a great potential in environmental remediation,with the characteristics of low cost and environmental friendliness.It is a highly promising NOx pollution prevention and control technology.Numerous photocatalytic materials have been discovered and employed in the field of photocatalytic NOx removal.Bismuth-based composite oxides are highly regarded due to their visible light response ability and easily adjustable structure.However,the practical application of photocatalytic NOx conversion technology is limited by low photocatalytic efficiency.The formation of surface defects,the load of the plasma metal,and the construction of heterojunction are employed to improve the efficiency of photocatalytic NOx removal.In addition,H2O molecules in the actual environment are much more concentrated than NO and can compete with the target reactants for adsorption.Therefore,the starting point of this thesis is to improve the efficiency of bismuth based photocatalytic materials in photocatalytic conversion of NO in high humidity environments.The modification of surface defects and plasma Bi were employed to broaden the light absorption range,optimize the photogenerated charge carrier dynamics,promote surface chemical reactions,and thereby improve photocatalytic efficiency.The main works of the thesis are as follows:(1)Oxygen vacancies promoted ferroelectric polarization improved the photocatalytic NO conversion efficiency of over SrBi4Ti4O15 nanosheets.Based on the influence of defects on ferroelectric polarization of SrBi4Ti4O15,at the same time,in order to broaden the light response range of SrBi4Ti4O15 and improve the efficiency of photocatalytic conversion of NO.Defective SrBi4Ti4O15 nanosheets with exposed {001} facets were successfully prepared by a molten salt method,showing 49%photocatalytic activity under visible light,which is 2.06 times that of SrBi4Ti4O15 prepared by solid-phase synthesis.Ferroelectric polarization characterizations and DFT calculations revealed the structure-property relationship of SrBi4Ti4O15 polarization intensity with crystal orientation and surface defects.The results showed that the polarization direction of SrBi4Ti4O15 on the {001} plane is mainly along the[110]direction,and the formation of oxygen vacancies in the titanium-oxygen polyhedron contributes to improve the polarization intensity of SrBi4Ti4O15,and the concentration of oxygen vacancies is positively correlated with the polarization intensity.The synergistic effect of the formation of oxygen vacancies and the improvement of spontaneous polarization intensity expands the light response range,accelerate the migration and separation of photoinduced carriers,and thereby the activity of SrBi4Ti4O15 nanosheets in photocatalytic conversion of NO was improved.(2)Oxygen vacancies enhanced ferroelectric polarization improved the photocatalytic conversion of NO by Bi4Ti3O12 nano wires.Considering the anisotropy of ferroelectric polarization in {001} plane and the improvement of polarization intensity by oxygen vacancies,[010]preferred growth Bi4Ti3O12 nanorods and Bi4Ti3O12 nanowires were prepared via a hydrothermal method with the addition of structure-directing agents.Under visible light irradiation(λ>420 nm),Bi4Ti3O12 nanowires exhibited a photocatalytic NO removal efficiency of up to 67.5%,much higher than that of Bi4Ti3O12 synthesized by solid-phase method(3%).DFT theoretical calculations revealed the influence of oxygen vacancies on Bi4Ti3O12 in the[010]and[100]directions,which indicated that the polarization intensity of oxygen vacancy-modified Bi4Ti3O12 is mainly concentrated in the[010]direction and positively correlated with the concentration of oxygen vacancies.Therefore,the improvement of photocatalytic activity of Bi4Ti3O12 nanowires is due to the improved spontaneous polarization,which promotes the separation and migration of photogenerated charge carriers.Additionally,the formation of oxygen vacancies widened the light response range of Bi4Ti3O12 nanowires,thus improves the photocatalytic activity.(3)Synergistic effect of plasma Bi and oxygen vacancies enhanced full spectral photocatalytic NO conversion of Bi2Ti2O7.To further broaden the light response range of bismuth-based photocatalytic materials and reduce the yield of NO2 during the photocatalytic conversion of NO,the defective full-spectrum responsive Bi@Bi2Ti2O7 were prepared by a one-step hydrothermal method.The optimal sample showed a photocatalytic NO conversion efficiency of 79%under visible-near infrared light(λ>420 nm),more than Bi2Ti2O7(31.79%),reference samples Bi@Bi2Ti2O7(57%)and Bi2Ti2O7-OV(70%).Bi and oxygen vacancies not only broadened light absorption to near infrared,but also inhibited the generation of toxic intermediates and alleviates the inactivation of Bi@BT-OV-4.Moreover,the formation of oxygen vacancies widened the light response range to near-infrared,and promoted the adsorption of NO on the surface of Bi@Bi2Ti2O7 samples.The adsorption and photocatalytic conversion of NO over optimized Bi@Bi2Ti2O7 was explored by in situ DRIFTS,and found that the final oxidized product of NO was mainly nitrate.The co-modification of plasma Bi and oxygen vacancies can broaden the light absorption range of bismuth-based photocatalytic materials and promote the conversion of NO→NO3-.(4)Photothermal effect improved the performance of photothermal catalytic conversion of NO over Bi and O dual defects modified by Bi@Bi4Ti3O12.To explore the influence of environmental factors(relative humidity,reaction temperature)on the performance of photocatalytic conversion of NO,a two-step method was used to prepare Bi@Bi4Ti3O12 with Bi and O dual defects.The optimal sample achieved a photocatalytic conversion efficiency of 75%for NO removal under visible-near-infrared light,which was 2.05 times higher than that of the initial Bi4Ti3O12.The formation of Bi and O dual defects optimized the band structure and endowed the optimal Bi@Bi4Ti3O12 with near-infrared light photocatalytic activity,as well as promoted the generation of active oxygen species.The formation of elemental Bi and dual defects converted part of the absorbed light energy into thermal energy accelerated the desorption of surface adsorbed H2O molecules,improving the utilization of active sites,and inhibiting the conversion of bi-NO3-to m-NO3-,thereby enhancing the stability of the modified Bi4Ti3O12 and reducing the emission of NO2.In summary,by modifying bismuth based composite oxides with surface defects and plasma Bi,the light response range was widened,the photo-generated carrier dynamics process was optimized,the surface photocatalytic reaction process was improved,and the efficiencies of photocatalytic conversion of NO were gradually improved.This thesis has guiding significance for the development of photocatalytic NO conversion materials for practical environmental applications.