Preparation Of Pd-based Nano-catalyst And Its Catalytic Performance In Toluene Oxidation

Volatile organic compounds（VOCs）are important precursors of secondary pollutants such as PM_2.5and O₃,which could cause atmospheric environmental problems such as haze and photochemical smog.The VOCs emission situation in China is severe and is an urgent air pollution problem that needs to be solved.Among the numerous kinds of VOCs,toluene is a typical one that has a molecular structure consisting of a benzene ring and an alkyl side chain,and that is stable and hard to degrade in terms of its chemical properties.Catalytic oxidation could completely oxidize toluene to CO₂and H₂O,and has the advantages of low operating temperature and no secondary pollution.The key to catalytic oxidation is to develop catalysts with low temperature and high efficiency,high conversion rate and good stability.Pd-based catalysts have good activity and stability in catalytic oxidation of benzene compounds.This thesis focuses on the types and structures of catalyst supports,and the dispersion and chemical states of active sites Pd nanoparticles,and investigated the effects of different properties of supports and Pd nanoparticles dispersion on the catalytic activity of Pd-based catalysts for toluene oxidation.By combining XRD,BET,TEM,SEM,FTIR,in-situ XPS,O₂-TPD,H₂-TPR,NH₃-TPD,ICP-OES and other characterization methods,the structure-activity relationship of different Pd catalysts in toluene catalytic oxidation reaction was established.The main research contents and conclusions were as follows.（1）Pd/TiO₂,Pd/ZrO₂,Pd/CeO₂and Pd/Al₂O₃catalysts with a loading of 0.5 wt%were prepared by isovolumetric impregnation method.The effects of different properties of supports and metal-support interactions on the catalytic performance of toluene oxidation were investigated.Pd/ZrO₂catalyst had the highest catalytic activity for toluene oxidation,achieving 90%conversion of toluene at 177℃,and had the lowest apparent activation energy（199.7 k J/mol）.The catalytic activity was not positively correlated with the specific surface area.The high activity of Pd/ZrO₂catalyst might be due to the small particle size of Pd,the strong acidity of the catalyst surface,and the strong reducibility of Pd²⁺on the catalyst surface,which could expose more active sites.（2）The effects of different crystalline ZrO₂supports on the catalytic performance of toluene oxidation were further investigated.Monoclinic,tetragonal and mixed monoclinic-tetragonal ZrO₂supports were prepared,and Pd/mixed ZrO₂,Pd/tetragonal ZrO₂and Pd/monoclinic ZrO₂catalysts with a loading of 0.5 wt%were prepared by isovolumetric impregnation method.The three catalysts showed significant differences in catalytic activity,among which Pd/monoclinic ZrO₂had the highest catalytic activity and the lowest apparent activation energy（146.3 k J/mol）in the reaction.Pd/monoclinic ZrO₂catalyst had better low-temperature reducibility and stronger surface acidity,and the surface Pd⁰showed an electron-rich state,which was conducive to the activation of reactants and intermediates,and thus improved the catalytic activity of the catalyst.（3）Pd/NH₂-KIT-6 catalyst was prepared by immobilization method using the coordination complexation of amino groups on the surface of catalyst support,and the effect of Pd nanoparticles dispersion on the catalytic performance of toluene oxidation was investigated.The introduction of-NH₂groups on KIT-6 support resulted in the formation of highly dispersed nano-Pd active centers on the catalyst,and the electron-rich Pd⁰activated the electrophilic O₂molecules to generate more active oxygen,promoting the conversion of toluene.The catalytic activity test showed that compared with the catalyst prepared by impregnation method,the immobilized Pd catalyst had higher catalytic activity,and 0.7 wt%Pd/NH₂-KIT-6 catalyst could achieve complete conversion of toluene at 165℃.The apparent activation energy（97.3 k J/mol）of 0.5%Pd/GR catalyzed oxidation of toluene by immobilized catalyst was lower than that of 0.5%Pd/IM（111.4k J/mol）by impregnated catalyst.The stability test showed that 0.5 wt%Pd/NH₂-KIT-6catalyst had good stability,and there was no obvious deactivation in 900 min of reaction.