Study And Application Of Dual-comb Spectroscopy In NO₂ Dimerization Reaction

Nitrogen dioxide（NO₂）is one of the major air pollutants that seriously endangers human health and ecological systems.Meanwhile,it also serves as a high-value and multifunctional reagent in organic synthesis and chemical production,and has important application in rocket propellants.However,due to the dimerization reaction,NO₂ always exists in equilibrium with its dimer,dinitrogen tetroxide（N₂O₄）.Therefore,the study of NO₂ dimerization can not only qualitatively understand the existing forms of nitrogen oxides in the atmosphere and dimerization equilibrium system,but also contribute to quantitatively analyzing the equilibrium mixture of NO₂and N₂O₄,which is important for the study of atmospheric material transformation,environmental monitoring and management,and industrial production.Dual-comb spectroscopy（DCS）,compared to other spectral measurement techniques,is an advanced spectral measurement tool with the advantages of broad bandwidth,high precision,and a simple system.Thus,it’s widely used in many fields such as greenhouse gas monitoring,combustion diagnosis,and absolute distance measurement.In this paper,the measurement of N₂O₄ absorption spectra at different temperatures in the mid-infrared band,and parallel measurement and analysis of reactants and products in the NO₂ dimerization reaction at different temperatures are achieved with DCS.The results promote the potential of the DCS for equilibrium reaction monitoring and measurement analysis.The details and innovations are summarized as follows:1.The mid-infrared dual-comb spectrometer is obtained.Generation of mid-infrared optical frequency comb（OFC）in periodically polarized lithium niobate crystal is achieved by the optical modulation process of a 1030 nm high-power ytterbium-doped fiber laser and a 3374 nm narrow linewidth continuous-wave laser.Based on the two above OFCs using the same continuous-wave laser,a mid-infrared dual-comb spectrometer with endogenous interdependence and comb-tooth-resolved is obtained.The spectral resolution is 108 MHz,the measuring speed is 600 Hz,and the spectral coverage ranging is from 3170 nm to 3540 nm covering the absorption bands of both NO₂ and N₂O₄,providing a tool basis for NO₂ dimer monitoring.2.The parallel high-precision absorption spectrum measurements of NO₂ and N₂O₄ based on DCS are performed.The results of NO₂ are in good agreement with that from the high-resolution transmission molecular absorption database（HITRAN）database,which provides the accuracy of the molecular spectrum measurement by DCS.While the commonly used spectral databases have not yet included the spectral parameters of N₂O₄,the absorption spectra of N₂O₄ at different temperatures are obtained experimentally and the absorption line strength are calculated,which provides a reference basis for further measurement and analysis of N₂O₄.3.The study of the NO₂ dimerization reaction monitoring in different temperatures is performed.Based on the mid-infrared dual-comb spectrometer with broad bandwidth,simultaneous online monitoring and analysis of the molecules concentrations of monomer NO₂ and its dimer N₂O₄ in the NO₂ dimerization reaction at temperatures between 296 K and 423 K are achieved.When the temperature is from296 K to 343 K,the standard deviation between the measured concentration and the theoretical concentration calculated based on the theoretical equilibrium constant is1.37%.Based On this,the relationship between the degree of reaction proceeding and temperature is analyzed.When the temperature is above 353 K,the degree of dimerization is low and NO₂ is the main gas in the equilibrium mixture.