Controllable Preparation of CeO2-based Catalysts and Study on Their Mechanism of Toluene Catalytic Degradation

Author:Feng Zhen Tao

Supervisor:ye dai qi


Degree Year:2019





In recent years,China’s air pollution control has achieved remarkable results,and atmosphere quality has been significantly improved.The concentration of PM2.5 in the air has dropped obviously.But the ozone problems have become more and more seriously with the continuous increase of volatile organic compounds(VOCs).Therefore,reducing and controlling the emission of atmospheric pollutants is the key to solve the above problems.Among all the technologies,catalytic oxidation has become the most important technologies for air pollutant emission control because of its advantages of cost-effectiveness and less by-products.In this paper,the CeO2-based catalysts widely used in catalytic oxidation technology were selected as the research object.The toluene was used as the target pollutant.By controlling the hydrothermal reaction conditions,CeO2 catalysts with different morphologies were prepared,and the structural properties were further investigated.Catalytic degradation of toluene by different morphological CeO2 catalysts;three-dimensional hierarchical CeO2 catalysts with different compositions were synthesized by adjusting the preparation method.The differences in physicochemical properties between different structures of CeO2 nanospheres were investigated.The key factors of catalytic degradation of toluene by CeO2 catalyst were as follows.The CeO2 supported Pt catalyst was prepared and its surface was modified by aniline treatment.The effect of modification on the catalyst was investigated.The degradation mechanism of aniline modified CeO2 catalyst on toluene was revealed.The in-situ method was used to study the true reaction process of toluene degradation on CeO2 catalyst,and the reaction mechanism of toluene catalytic degradation under different CeO2 catalysts was elucidated.The main conclusions are as follows:(1)We successfully synthesized CeO2 catalysts with different morphologies(rod,cubic and sphere,named as CeO2-R,CeO2-C and CeO2-S,respectively)by hydrothermal method..Among them,CeO2-S catalyst has the largest specific surface area(130.2 m2 g-1),highest oxygen vacancy content and the most outstanding redox capacity.Thus exhibits the most outstanding and excellent toluene catalytic oxidation performance,and excellent redox properties.More importantly,the CeO2-S catalyst exhibits excellent long-term reaction stability,reusability,and water resistance,which may be attributed to the unique hollow spherical structure of the CeO2-S catalyst,and the relative large specific surface area and richer surface oxygen vacancies.(2)The three CeO2 nanosphere catalysts prepared by hydrothermal method have unique three-dimensional hierarchical structure,which are respectively formed by self-assembly of nanoparticles,nanorods and nanospheres.The HS catalyst has the highest specific surface area,the highest Ce3+content and the most excellent redox performance,while also having the most surface oxygen vacancies and bulk oxygen vacancy content.Thus exhibited the highest toluene catalytic oxidation activity compared with the PS and RS samples,and highest TOF value.The HS catalyst also exhibits excellent long-term stability and water resistance.(3)A variety of in-situ methods were used to study the degradation of toluene by different morphological CeO2 catalysts.It was found that toluene was adsorbed parallelly on the surface of CeO2-R and CeO2-S catalysts viaπbond,and this adsorption method was produced.The strong force can activate the benzene ring in toluene,which is favorable for the ring-opening decomposition of toluene.On the surface of the CeO2-C catalyst,toluene is vertically adsorbed on the surface in the form ofσbond.The O2 molecules in the air adsorbed on the surface of CeO2-R and CeO2-S catalysts to formη1-superoxide species O-2ads andη2-peroxy species O22-ads.The O-2ads species is very important for the deep activation of toluene.While the O22-species favors the"re-oxidation"process of the CeO2 catalyst.By analyzing the intermediates on the catalyst surface,we can conclude that there are two mechanisms for the catalytic degradation of toluene on the surface of CeO2 catalyst:some of the toluene adsorbed on the surface of the catalyst in parallel is directly oxidized by O-2ads species,undergoing ring-opening degradation to form CO2 and H2O.The another part of toluene is coated on the surface of the catalyst,followed by the process of reactive oxygen species passing through benzyl alcohol-benzaldehyde-benzoic acid and finally completely decomposed to form CO2 and H2O.(4)Furthermore,the prepared CeO2 was subjected to Pt loading by impregnation method,and the loading process was modulated by aniline pretreatment.The results showed that aniline modification treatment can increase the dispersion of noble metal on the surface of the Pt/CeO2catalyst,and the N doping can also affect the electronic structure of the Pt atom in contact therewith.After the aniline modification,the strong interaction between the metal supports is stronger,so that the number of Pt atoms and surface oxygen vacancies near the interface increases,and the activity becomes stronger.The toluene catalytic oxidation on the Pt/CeO2catalysts follows the"double active sites mechanism",and toluene catalytic activity is the result of a combination of Pt particles and surface oxygen vacancies.The 0.9Pt/CeO2-A catalyst exhibited the highest toluene catalytic activity with a T90 of 149°C,which can be attributed to the aniline modification treatment to give it a higher Pt dispersion,higher oxygen vacancy concentration and higher activity.The unit active site conversion frequency.After the aniline modification treatment,the stability and water resistance of the supported Pt CeO2 catalyst have been significantly improved.