Design Of Light-storing Assisted Photocatalytic Composite For NO Removal In Tunnel And Study On Its Synergy Mechanism

As an important form of underground space,highway tunnel plays an active role in shortening the running distance,optimizing the traffic path and improving the transportation capacity.However,due to its closed space structure,the exhaust pollutants from past vehicles are easy to stay in the tunnel and difficult to be effectively discharged,which makes the air pollution problem more prominent.In serious cases,it may even affect the health of drivers and passengers and the driving safety.At present,the air pollution in tunnels is mainly reduced by adding ventilation shafts,jet fans and air purification devices.These not only increase the complexity of tunnel structure and the difficulty of construction,but also bring inconvenience to the maintenance and replacement of facilities,consume huge energy and investment costs,which cannot meet the requirements of energy conservation and environmental protection.This paper was supported by the National Natural Science Foundation of China,"Design and synergistic mechanism of g-C₃N₄/long afterglow photocatalytic composites for NOx degradation in tunnel".Based on the technology of photocatalytic purification of pollutants and the energy storage-luminescence characteristics of long afterglow materials,a porous Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺was designed as a functional carrier to support graphitic carbon nitride（g-C₃N₄）and achieved a light-storing assisted photocatalytic composite for continuous catalytic removal of pollutants under illumination and dark.For the first time,a series of indicators such as cumulative pollutant degradation concentration（S_C）were defined to accurately describe the comprehensive pollutant removal effect in light and dark state.The interaction mechanism between g-C₃N₄and Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺was revealed by means of microscopic characterizations and molecular simulation calculations,which discussed the reasons for the change of macro properties from the atomic and molecular levels.In addition,the fluorinated acrylate emulsion with low surface energy was synthesized.Using this emulsion as the binder and the light-storing assisted photocatalytic composites as the functional fillers to prepare an energy-storing photocatalytic paint with good hydrophobicity and oil repellency.A self-cleaning experiment was designed to study the influence of oil pollution on the wetting behavior of the coating surface with time under illumination.And the durability of the coating under UV aging and photocatalysis was also investigated.Finally,the energy-storing photocatalytic paint was applied in the tunnel test section to evaluate the practical purification effect of nitric oxide（NO）pollutant.The researches of this paper aimed to seek an energy conservation and environmental protection way to purify the air quality in the tunnel,meanwhile,improve the functional durability and cleaning convenience of the photocatalytic paint to promote the application of new environmental protection technology.The specific research contents are as follows:（1）The Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺（recorded as SMSO:（Eu,Dy））blue long afterglow phosphors with conventional block morphology were prepared by a sol-gel method.The optimum preparation conditions of SMSO:（Eu,Dy）were determined by single-factor experiments and orthogonal designs,which used the crystal purity and the initial brightness of afterglow as the measurement indexes.In order to further enhance the brightness of afterglow and facilitate the loading of photocatalysts,SiO₂aerogels were introduced instead of some silicon sources to prepare the SMSO:（Eu,Dy）with porous morphology.The distribution characteristics of pores were analyzed by the mercury injection experiment.The effects of aerogels dosage on the luminescence intensity and afterglow properties of porous SMSO:（Eu,Dy）were also investigated.（2）The CN/SMSO:（Eu,Dy）light-storing assisted photocatalytic composite was prepared by pyrolysis polymerization to make g-C₃N₄load on porous SMSO:（Eu,Dy）,and its feasibility of continuous removing nitric oxide（NO）was explored.Taking the self-defined cumulative pollutant degradation concentration（S_C）as the evaluation index of catalytic performance,which quantitatively described the comprehensive removal effect of NO under illumination and dark,and the rationality of the index was verified.The coupled mode of single components and the change characteristics of the performance before and after combination were explored from the aspects of bonding,morphology,optical properties,afterglow properties and photoelectrochemical properties.Furthermore,the mechanism of NO removal by the CN/SMSO:（Eu,Dy）composite was explained.（3）In order to deeply understand the reasons of the change of macro properties from atomic level,the bulk and surface structures of g-C₃N₄and SMSO:（Eu,Dy）and the CN/SMSO:（Eu,Dy）interface structure were modeled and calculated by the CASTEP module in Materials Studio.The SMSO model,SMSO:（Eu）single-doping model and SMSO:（Eu,Dy）co-doping model were constructed by gradual optimization.The electronic properties of three models were analyzed and compared to provide evidences for the afterglow mechanism of SMSO:（Eu,Dy）from atomic level.Then the appropriate surface structures of g-C₃N₄and SMSO:（Eu,Dy）were cleaved to construct the CN/SMSO:（Eu,Dy）interface structure,and the rationality of this model was judged by the stability of interface structure.Finally,the transition difficulty and transfer paths of electrons and holes in the interface structure were discussed from the aspects of electronic density of states,band structure,differential charge density,layout analysis and work function.The possible synergy mechanism between g-C₃N₄and SMSO:（Eu,Dy）was revealed.（4）In order to improve the durability of photocatalytic materials in application,the low surface energy fluorinated acrylate emulsion with good hydrophobicity and oil repellency was synthesized.Using this as binder and CN/SMSO:（Eu,Dy）composites as functional fillers to prepare an energy-storing photocatalytic paint with other fillers and auxiliaries.The optimum content of CN/SMSO:（Eu,Dy）composite in paint was determined according to the NO removal effect and the surface wettability of the coating.The conventional properties were evaluated referred to the specifications and standards of exterior latex paint.In addition,a self-cleaning test was designed with glycerol as the pollutant to study the influence of oil pollution on the wetting behavior of the coating surface under catalysis.The durability of the coating under UV aging and photocatalysis was investigated from the aspects of film composition,micro morphology and appearance grade.Finally,the self-made energy-storing photocatalytic paint was applied to the side wall of the tunnel in Chongqing Communications Technology Research&Design Institute Co.Lto.The statistical analysis results showed that about 12.7%of NO was effectively removed during illumination and the coating still played a role in continuously purifying NO after turning off the light source.