Catalytic Oxidation Of Toluene Over Platinum Supported On Ceria Catalysts:Performance And Reaction Mechanism

Volatile organic compounds?VOCs?are known as the important precursors of ozone and photochemical smog,having detrimental effects on air quality and human health.Catalytic oxidation is generally regarded as one of the most effective routes for eliminating VOCs emissions with lots of distinguished advantages,such as high destructive efficiency and no generation of organic by-products.The key issue of such a technology is the development of high performance catalytic materials.The supported noble metals?NMs?,especially Pt-based materials,are considered as the most commonly promising catalysts for VOCs catalytic combustion.As an important rare-earth metal oxide,ceria?CeO₂?exhibits a nice ability of oxygen releasing and storage.In addition,as an example of‘‘active support”for noble metals,CeO₂ could not only bind NMs particles strongly through surface oxygen vacancies but also involve in redox process and create new active sites for reaction.Therefore,this paper selected catalytic oxidation of toluene over Pt/CeO₂ catalysts as the research system.A series of catalysts with various structures were prepared,and their relationships between structure and catalytic performance were studied.Moreover,the reaction mechanism of toluene catalytic oxidation over Pt/CeO₂ had been investigated.The main research contents and results are as follows:1.CeO₂ material with rod,particle,and cube shape were prepared by hydrothermal method and then employed to immobilize Pt nanoparticles with tunable size by adsorption method.The Pt/CeO₂-r sample exhibited the best catalytic activity due to the best reducibility and highest surface oxygen vacancies concentration.Two types of TOFs based on Pt dispersion and surface oxygen vacancies concentration were calculated,and the results suggested that the oxygen vacancies on ceria surface controlled the reaction rate.In addition,the variation of surface oxygen vacancies concentration were investigated by in-situ UV Raman,demonstrating that the surface oxygen vacancies played a role in replenishing active oxygen species from gaseous phase and accelerating oxygen cycle for the toluene oxidation.2.A series of size-controllable Pt nanoparticles had been fabricated through a glycol reduction method and then supported on CeO₂ nanorod by adsorption.The XPS,H₂-TPR,Raman and OSC characterization results showed that Pt/CeO₂-1.8 sample exhibited the best catalytic performance due to the balance of both Pt dispersion and oxygen vacancy concentration of ceria.Three types of TOFs values based on Pt dispersion,oxygen vacancy concentration and both of them were calculated respectively,and the result demonstrated that the reaction rate was controlled by both of the exposed Pt atom and oxygen vacancies.In addition,the Pt/CeO₂-1.8 catalyst exhibited excellent stability and resistance to toluene concentration and could be completely negligible for 5 vol%water vapor at 155 ^oC.3.A series of Ce_0.8Ln_0.2O₂ nanorods?Ln:La,Pr,Nd,Sm and Gd?were prepared by hydrothermal method and then employed to Pt nanoparticles via wet impregnation method.XRD,XPS,H₂-TPR and Raman results showed that Pt/CeGdO₂ catalyst possessed the highest content of Ce³⁺and Pt⁰,and had the best reduction performance and surface oxygen adsorption capacity,exhibiting the best performance for toluene catalytic oxidation.While for Pr,because of larger ion radius and+4 state,its doping had a negative effect on the activity.Therefore,in the case of toluene catalytic oxidation over Pt/CeO₂ catalysts,it was beneficial to promote the catalytic activity by doping aliovalent element with a small ion radius.4.Pt/CeO₂ and Pt/Al₂O₃ catalysts were successfully prepared via wet impregnation method.A comparative study of the two catalysts on Pt electronic environment,toluene adsorption-desorption and intermediate products/by-products had been carried out by H₂-TPR,Toluene-TPD,CO-FTIR and GC-MS.And the results showed that the Pt nanoparticle over Pt/CeO₂ catalyst presented higher electron concentration,resulting in better performance for toluene activation,and CeO₂ support could supply more reactive oxygen species and increase the catalytic activity for toluene oxidation.In addition,the main reaction process of toluene on two catalysts was basically the same:toluene?benzoic aldehyde?benzoic acid?long-chain acids and aldehyde?small molecules of acid and aldehydes?carbon dioxide and water.