The Preparation Of Cerium-based Catalyst And Its Research On Toluene Catalytic Performance And Mechanism

In recent decades,with the rapid development of modern industry in China,volatile organic solvents are widely used in five fields:chemical industry,petrochemical industry,packaging and printing,industrial painting and oil storage and transportation,etc.However,volatile organic pollutants（VOCs）produced in the production process are considered to be important precursors for the formation of O₃and PM_2.5,which have certain negative effects on the ecological environment and human health,and have attracted more and more attention.Therefore,the national"Fourteen Five"plan outline have replaced the original total index of sulfur dioxide（SO₂）into VOCs,and VOCs are taken as one of the assessment indicators of urban air quality.This means that the governance of VOCs will become the focus of work in the future.Cerium dioxide（CeO₂）,as a common rare earth element oxide,has a good ability to store and release oxygen,and the oxygen vacancy generated on its surface can provide active sites for the catalytic reaction.In this paper,CeO₂was selected as the basis of the study,toluene gas was taken as the target pollutant,and the appearance morphology of the catalytic material（bulk structure,nanoparticle structure and three-dimensional ordered porous structure）was controlled by the hard template method.The effects of template,doping elements（Co and Cu）,doping ratio and calcination process on the structure and performance of the catalyst were studied.The effect of cerium-based catalyst on the catalytic performance of toluene under different reaction conditions（O₂,H₂O,SO₂,NO_x,toluene concentration,etc.）was investigated,the adsorption and oxidation behavior of toluene in the removal process was revealed,and the reaction mechanism of toluene in the catalytic oxidation process on cerium-based catalyst was clarified.The main conclusions obtained through experimental analysis and research are as follows:（1）CeO₂ with different morphologies were prepared by the template method,The relationship between the structure and performance of the catalysts was explored,and the path of toluene catalytic reaction was clarified.The experimental study found that CeO₂-NP exhibited excellent catalytic performance for the oxidation of toluene(T₅₀=180℃,T₉₀=199℃).XPS analysis showed that temperature increase was beneficial to the conversion of O_αto O_βon CeO₂-NP catalyst,which indicated that both O_αand O_βwere involved in the catalytic oxidation of toluene and played a positive role in promoting the oxidation of toluene.CeO₂-NP catalyst showed good stability and reusability in the catalytic oxidation process of toluene,which provided potential possibilities for the effective elimination of toluene in industrial waste gas in actual production.The results of In situ DRIFTS and DFT simulation showed that the adsorption and catalytic oxidation of toluene on the surface of CeO₂-NP catalyst was easier when toluene approached the catalyst with an aromatic ring parallel to the（111）crystal plane,and the ring-opening process of aromatic ring was the key rate control step of toluene catalytic oxidation.（In Chapter 3）（2）In order to solve the problem of catalyst poisoning and deactivation caused by sulfur component in complex flue gas,CuCeO_x-NP catalyst was prepared by in situ Cu-doped modification of CeO₂on the basis of the research in the previous chapter,and applied to the catalytic oxidation of toluene in the presence of SO₂.The CuCeO_x-1-NP-400 with the best catalytic activity for toluene removal(T₅₀=212℃ and T₉₀=226℃)was selected by adjusting the molar ratio of Cu/Ce,calcination atmosphere,and calcination temperature.Experiments with different SO₂concentrations showed that CuCeO_x-1-NP catalyst has excellent stability and recycling for the catalytic oxidation of toluene in the presence of high sulfur.XPS and In situ DRIFTS experimental analysis showed that the catalytic reaction of toluene on CuCeO_x-1-NP catalyst can be oxidized to produce benzyl alcohol→benzaldehyde→benzoic acid→maleic anhydride and other intermediate products under the action of active oxygen species.Among them,the conversion from benzoic acid to maleic anhydride is a key rate control step in the catalytic reaction of toluene.（In Chapter 4）（3）In order to explore the synergistic effect between cobalt and cerium and further improve the stability of cerium-based catalysts,on the basic of the research in Chapter 3,Co was used to modify CeO₂by in-situ doping.The experimental results showed that appropriate Co/Ce molar ratio doping can increase the specific surface area and the number of oxygen vacancies on the surface of the catalyst,and enhance the migration ability of oxygen species.In addition,the synergistic effect between cobalt and cerium significantly promoted the removal of toluene and greatly improved the low-temperature reducibility of CoCeO_x-NP catalyst.The CoCeO_x-2-NP showed the best catalytic oxidation performance(T₅₀=209℃,T₉₀=218℃)and good water resistance.Meanwhile,the CoCeO_x-2-NP catalyst showed good stability and recycling performance in the catalytic oxidation of toluene.Raman characterization results showed that Co was successfully embedded into the lattice of CeO₂,which promoted the formation of more lattice defects and oxygen vacancies.XPS results further showed that O_αand O_βwere involved in the reaction and played an important role in promoting toluene catalytic oxidation.（In Chapter 5）（4）The previous studies show that cerium-based catalysis is feasible for the separate removal of toluene and NO.On this basis,CoCeO_x-3DOM catalyst with three-dimensional ordered macroporous structure with different Co/Ce molar ratios was successfully prepared through ordered PMMA-O template,and successfully applied in the experimental study of the simultaneous removal of toluene and NO_x.The three-dimensional ordered macroporous structure effectively reduces the mass transfer resistance and improves the mass transfer efficiency during the catalytic reaction.Due to the partial replacement of Ce by Co cation,lattice defects and oxygen vacancies were generated in cerium dioxide lattice,which enhanced the catalytic oxidation of toluene and NO_xcatalytic reduction of CoCeO_x-3DOM catalyst.Among them,CoCeO_x-1-3DOM catalyst showed good catalytic activity(T₉₀=216℃,E_NO=76.74%)and excellent stability and recycling performance in the simultaneous removal of toluene and NO_x.The results of in situ DRIFTS showed that benzoic acid was the main intermediate product in the catalytic oxidation of toluene.The Lewis acid sites and Br?nsted acid sites on CoCeO_x-3DOM catalyst contribute to the adsorption of NH₃,thus promoting the catalytic reduction of NO_x,while the existence of oxygen vacancies promotes the catalytic oxidation of toluene.Therefore,CoCeO_x-3DOM catalyst has become a promising catalytic material for the simultaneous removal of toluene and NO_x.（In Chapter 6）...