Design And Preparation Of High-Performance Pt-CeO₂-TiO₂/ZSM-5 Catalysts For VOCs Oxidation

The design and preparation of catalysts with high performance is the key factor for the efficient oxidation degradation of volatile organic compounds（VOCs）.Metal oxide catalysts,especially the CeO₂-based oxides,such as the CeO₂-TiO₂ mixed oxides possess excellent catalytic activity and stability for chlorinated VOCs（CVOCs）oxidation.However,these catalysts show the inferior catalytic oxidation activity of non-chlorinated VOCs,which are often coexisted with CVOCs in the actual industrial emissions.Due to the above drawbacks,their wide application is limited to a large extent.Although the noble metal（Pt,Pd,etc.）supported catalysts have been extensively used in the catalytic oxidation of non-chlorinated VOCs owing to the advantages of the satisfactory low-temperature oxidation activity,stability and water resistance.Nevertheless,these catalysts are easy to suffer from deactivation when the CVOCs exist in the reaction system due to the formation of oxychloride species.A large number of toxic polychlorinated byproducts also would generate though the chlorination reaction.Therefore,it is of great significance for the practical applications to improve the adaptability for various VOCs oxidation,the catalytic oxidation activity at low temperature,deep oxidation ability as well as the chlorine resistance of catalysts in the CVOCs oxidation process.In this present dissertation,the strong interaction between Pt and CeO₂-TiO₂ mixed oxides in the Pt/CeO₂-TiO₂ catalysts is enhanced by the adjustment of the Ce/Ti molar ratio and Pt loading methods,which accelerates the mobility of oxygen species and further results in the improved adaptation and stability of the catalysts for various VOCs oxidation.On this basis,the Pt particle size effect of Pt/CeO₂-TiO₂ catalysts on VOCs oxidation is also investigated.The Pt-CeO₂-TiO₂/ZSM-5 catalysts with high-performance for VOCs oxidation are designed and prepared,and the physicochemical properties of the catalysts are analyzed by the X-ray diffraction（XRD）,the N₂adsorption-desorption,the high resolution transmission electron microscopy（HRTEM）,the CO in-situ diffuse reflectance infrared Fourier transform spectroscopy（CO in-situ DRIFTS）,the X-ray photoelectron spectroscopy（XPS）,the temperature-programmed reduction by H₂（H₂-TPR）and the temperature-programmed desorption of NH₃（NH₃-TPD）characterizations.Moreover,combined with the in-situ techniques,such as the temperature-programmed desorption and surface reaction of VOCs（VOCs TPD/TPSR）and VOCs adsorption and reaction in-situ diffuse reflectance infrared Fourier transform spectroscopy（VOCs in-situ DRIFTS）,the structure-activity relationship between catalyst structure and redox properties,catalytic performance of various VOCs oxidation as well as the effects of the synergy of each component in the Pt-CeO₂-TiO₂/ZSM-5 catalysts on catalytic performance and mechanisms of VOCs oxidation are clarified.The main conclusions are as follows:（1）The Pt/CeO₂-TiO₂ catalysts were prepared by the electron free deposition method and the effect of Ce/Ti molar ratios on the catalytic performance of chlorinated VOC（1,2-dichloroethane）and non-chlorinated VOC（n-hexane）oxidation was investigated.The results reveal that,for the Ti-rich CeO₂-TiO₂ mixed oxides（Ce/Ti molar ratio<1）,the CeO₂ and TiO_x nanoparticles are highly dispersed into each other.The strong interaction between them evidently improves the acidity properties of the CeO₂-TiO₂ mixed oxides,which promotes the adsorption and dichlorination of DCE.On the other hand,the Ce-rich CeO₂-TiO₂ mixed oxides（Ce/Ti molar ratio>1）are more conducive to the dispersion of Pt species,among them,Pt/Ce Ti-2/1-EFD catalyst with the Ce/Ti molar ratio of 2/1 exhibits the best Pt dispersion that Pt species mainly exist in the form of Pt single atoms.The stronger interaction between Pt single atoms and CeO₂-TiO₂ mixed oxides significantly promotes the formation of the more Pt²⁺species,absorbed oxygen species and the improved low-temperature redox properties,which is beneficial to the catalytic oxidation of n-hexane.（2）The CeO₂-TiO₂ mixed oxides with the Ce/Ti mass ratio of 1/4 were selected as the support and the effect of Pt loading methods on the catalytic activity of Pt/CeO₂-TiO₂ catalysts for 1,2-dichloroethane and benzene oxidation was investigated.For the Pt/Ce Ti-1/4-EFD catalyst prepared by the electron free deposition method,the Pt species are highly dispersed in the form of single atoms and nanoclusters on the surface of the catalyst,resulting in the highest catalytic activity for benzene and DCE oxidation.For the Pt/Ce Ti-1/4-ER（11）and Pt/Ce Ti-1/4-HR catalysts prepared by the ethylene glycol reduction and hydrazine hydrate reduction methods,most of the Pt species are Pt⁰ and Pt²⁺species.A good balance between Pt dispersion and Pt⁰/Pt²⁺proportion accelerates their catalytic activity for benzene oxidation.As for the Pt/Ce Ti-1/4-IM catalyst prepared by the incipient-wetness impregnation method,although most Pt species are dispersed as the large Pt nanoparticles and Pt nanoclusters,a large amount of Pt⁴⁺species inhabit the catalytic oxidation activity of benzene.The study of the Pt particle size effect of catalysts on the benzene and DCE catalytic oxidation also further confirmed that there is a stronger interaction between smaller Pt nanoparticles and CeO₂-TiO₂ mixed oxides,which is beneficial to the formation of more Pt²⁺and active oxygen species,and further promotes the catalytic oxidation of benzene.While the Pt/CeO₂-TiO₂ catalysts with larger Pt nanoparticles（>2.95 nm）possess more acid sites（especially for the strong acid sites）,which is in favor of the adsorption and dichlorination of DCE,and results in the better catalytic activity of the catalysts for DCE oxidation.（3）The Pt-CeO₂-TiO₂/ZSM-5 catalysts with high performance for VOCs oxidation were designed and prepared.It can be found that the Pt species are mainly located on the surface of CeO₂-TiO₂ mixed oxides rather than ZSM-5 zeolites in the Pt-CeO₂-TiO₂/ZSM-5 catalysts.Both Pt and CeO₂-TiO₂ dispersion can be evidently promoted by introducing the appropriate amount of CeO₂-TiO₂ mixed oxides,which obviously enhances the Pt-support interaction and further improves the catalytic activity of catalysts for various VOCs oxidation at low temperature.For example,the Pt-Ce Ti-1/4/ZSM-5（1/10）catalyst with the CeO₂-TiO₂/ZSM-5 mass ratio of 1/10shows the excellent low-temperature oxidation activity and adaptability,and the T_90%values of various VOCs oxidation as follows:1,2-dichloroethane（245 ^oC）,benzene（145 ^oC）,ethyl acetate（176 ^oC）,acetonitrile（240 ^oC）and n-hexane（170 ^oC）.The synergy of the components in the Pt-CeO₂-TiO₂/ZSM-5 catalyst for the catalytic oxidation of various VOCs is clarified,which provides guiding significance for developing novel VOCs oxidation catalysts.For the DCE oxidation,the multifunctional reactive sites on the surface of the catalysts are required.The synergistic effect of abundant acid sites（especially for the strong acid sites）and strong oxidizing sites is beneficial to catalytic oxidation of CVOCs at lower temperature.The abundant acid sites（especially for the strong acid sites）are conducive to the adsorption and dichlorination of DCE molecules,while the enhanced Pt dispersion and redox properties of the catalysts are in favor of the deep oxidation of intermediates and byproducts produced in the DCE oxidation process.For the catalytic oxidation of hydrocarbon VOCs（such as benzene）,the surface acidity properties of ZSM-5 zeolites have little effect on the oxidation activity of catalysts,and the dispersion of Pt species plays a key role.As for the VOCs containing oxygen and nitrogen（such as ethyl acetate and acetonitrile）oxidation,the surface acidity properties of catalysts play a more important role,and the good matching and synergy between the abundant acid sites and oxidation sites would significantly improve the low-temperature oxidation activity of the catalysts for these VOCs oxidation.（4）The adsorption-desorption and the catalytic oxidation mechanisms of VOCs over the Pt-CeO₂-TiO₂/ZSM-5 catalysts were discussed in depth.The results indicate that the Pt/CeO₂-TiO₂ catalyst exhibits inferior adsorption capacity for VOCs,while the adsorption capacity of the catalysts containing ZSM-5 zeolites is obviously improved due to the abundant VOCs adsorption sites with different strength and the hierarchical pore structure of ZSM-5 zeolites.The introduction of a small amount of CeO₂-TiO₂mixed oxides would cover some strong adsorption sites on the surface of zeolites,which is beneficial to the desorption of VOCs molecules.The catalytic activity of Pt-CeO₂-TiO₂/ZSM-5 catalysts for VOCs oxidation at low temperature can be evidently promoted by improving their adsorption-desorption capacity for VOCs,because the VOCs desorption at low temperature is also accompanied with the oxidation process.The conversion of phenol and vinyl chloride intermediates to benzoquinone and acetic acid are the key steps of the benzene and DCE oxidation,while the hydrolysis reaction on the strong acid sites of catalysts is key step of ethyl acetate and acetonitrile oxidation.Thus,the improvement of oxidation capacity and surface acidity properties of catalysts is the main factor to promote the catalytic activity of catalysts for VOCs oxidation.Moreover,the carbon deposition is the main factor affecting the stability of Pt-CeO₂-TiO₂/ZSM-5 catalysts for DCE oxidation.The activity and mobility of oxygen species can be significantly accelerated by introducing a small amount of CeO₂-TiO₂ mixed oxides,which would reduce the carbon deposition and improve the stability of the catalysts.