Study On The Generation Mechanism Of Typical Pollutants In Industrial Environment By In-situ Synchrotron Radiation Photoionization Mass Spectromtry

Air pollution has become one of the limiting factors that restrict industrial produc-tion,e.g.metallurgy,and economy in the 21st century and even on.Therefore,the re-searches on the formation mechanism,emission reduction and prevention mechanism of air pollutants have been in the frontier.Polychlorinated dibenzo-p-dioxins/dibenzo-furans(PCDD/Fs)and NO_x produced in industrial metallurgy and waste incineration are important inducementsleading to serious air pollution.Having the formation mech-anism of PCDD/Fs been studied for decades,the key intermediates are yet to be exper-imentally detected.In this paper,the basic theoretical study on the formation mecha-nism of chlorinated organic pollutants such as PCDD/Fs was carried out by SR-PIMS,which can be used to detect free radicals and active intermediates in situ.Meanwhile,the mechanism of N₂O generation and the effect of pressure on NH₃-selective catalytic reduction(NH₃-SCR)denitration over Fe-Mn catalyst were discussed.1.Study on pyrolysis of chlorobenzene and the mechanism of formation of PCNs and PCBsPyrolysis of chlorobenzene at two different pressures(101 k Pa and 4 k Pa)was studied in the temperature range of 873?1337 K.The pyrolysis products including in-termediates and chlorinated aromatics were detected and quantified via SR-PIMS.Based on the experimental results,the decomposition pathways of chlorobenzene were discussed,as well as the generation and consumption pathways of the main products.Benzene(C₆H₆)is the main product of chlorobenzene pyrolysis.Chlorobiphenyl(C₁₂H₉Cl),dichlorobiphenyl(C₁₂H₈Cl₂),and chlorotriphenylene(C₁₈H₁₁Cl)predomi-nated in trace chlorinated aromatic products.Chlorobenzene decomposed initially to form two radicals[chlorophenyl(·C₆H₄Cl)and phenyl(·C₆H₅)]and an important inter-mediate o-benzyne(o-C₆H₄).The propagation processes of chlorinated aromatics,in-cluding polychlorinated naphthalenes(PCNs)and polychlorinated biphenyls(PCBs),were mainly triggered by chlorobenzene,chlorophenyl,and benzene via the even-num-bered-carbon growth mechanism.Besides,the small-molecule products such as acety-lene(C₂H₂),1,3,5-hexatriyne(C₆H₂),and diacetylene(C₄H₂)were formed via decom-position of o-benzyne.The formation of chlorine-containing organics such as chlorin-ated aromatic hydrocarbons was prohibited at low pressure,compared with that at at-mospheric pressure.This finding provides us a clue that the conversion of chloroben-zene to chlorinated aromatic hydrocarbons can be controlled by adjusting the pressure,to further achieve the goal of pollutant prevention and control.2.Study on pyrolysis of 2-chlorophenol and the mechanism of PCDD/Fs and PCNs formationThe thermal decomposition of 2-chlorophenol was studied in the temperature range of 623?1373 K with a flow tube reactor at two different pressures(101 k Pa and4 k Pa).The nascent pyrolysis products were identified in situ in a supersonic molecular beam using SR-PIMS.The photoionization efficiency(PIE)spectra were used to iden-tify more than 42 pyrolysis products,including short-lived radicals,unstable interme-diates,isomers,stable oxygen-containing compounds,and hydrocarbons.Radicals[HCCCHCCH(·C₅H₃),cyclopentadienyl(·C₅H₅),and chlorocyclopentadienyl radical(·C₅H₄Cl)]and intermediates[chlorocyclopentadiene(C₅H₅Cl)and o-benzyne],were directly observed for the first time during the 2-chlorophenol pyrolysis process.In the meanwhile,the potential energy surface(PES)profile,rate constants,and branching ratios of five hypothetical initial pyrolytic species[2-hydroxylphenyl radical(·C₆H₅O),chlorophenoxy radical(C₆H₄Cl O·),chlorocyclopentadiene,fulvenone ketene(C₆H₄O),and 4-chloro-dehydrobenzol(C₆H₃Cl)]have been calculated by the CBS-QB3 and RRKM method during 2-chlorophenol initial pyrolysis.The calculation results show that fulvenone ketene and chlorocyclopentadiene are the main products of 2-chlorophe-nol initial decomposition,as observed in the experiment.In addition,the dissociation pathways of 2-chlorophenol and the five formation pathways of detected PCDD/Fs at high temperature are discussed,as well as the formation pathways of PCNs.The reac-tion of phenyl radical(·C₆H₅)and chlorophenoxyl radical(C₆H₄Cl O·)leads to the for-mation of PCDD/Fs.PCNs are yielded from the recombination of chlorocyclopentadi-enyl.3.The study of effects of pressure on the formation pathways of N₂O over Fe-Mn catalystIn order to study the formation pathway of N₂O in NH₃-SCR of NO_x process and the variation of N₂O selectivity affected by pressure,in situ SR-PIMS and flow tube reactor were used to detect the gaseous species in the NH₃-SCR reaction over3Mn10Fe/Ni catalyst,and the mass spectra of each component were obtained.The var-iations of N₂O selectivity,NO_x conversion and NH₃ conversion were analyzed under different working conditions,temperature and pressure.The results show that the for-mation of N₂O mainly comes from non-selective catalytic reaction(NSCR)and ad-sorbed NH₃ oxidation(NSNO).Thereinto,NSCR plays a dominant role in N₂O for-mation in the temperature range of 373?523 K.The contribution of the two formation pathways in the temperature range of 523?673 K is equivalent,and NSNO is the main source in the temperature range of 673?773 K.In addition,low pressure reduced the denitration activity of the catalyst at low temperature,but promoted the formation of N₂O via NSNO reaction at high temperature.The experimental results provide guidance for the further optimization of NO_x control conditions.