Preparation Method Of Pt-based Nanocatalyst And Its Catalytic Oxidation Performance Of VOCs

Volatile organic compounds（VOCs）produced by various industrial processes and automobile exhausts are key species leading to the photochemical formation of ground ozone（O₃）,secondary organic aerosols（SOA）,which are also important precursors of PM2.5.They display a great negative impact on human health and environmental pollution.Among the available technologies for eliminating VOCs,catalytic oxidation is an effective method.In the catalytic oxidation of VOCs,supported noble metals are considered the most effective catalysts,especially Pt-based catalysts,which show remarkable low-temperature catalytic activity and high selectivity to CO₂and H₂O in the oxidation of VOCs.However,how to improve the utilization of Pt has becomes an issue that needs to be solved due to the high price of Pt,.In this paper,several methods are used to prepare Pt nanoparticles（NPs）and supported Pt catalysts,in order to improve the catalytic oxidation performance for VOCs and stability of Pt catalysts.In this thesis,three methods were used to prepare Pt nanoparticles which were organometallic liquid phase decomposition method,metal salt liquid phase hydrogen reduction method and metal salt triethylamine liquid phase reduction method.Among them,the latter two methods were firstly proposed in this paper.Then the bimetallic core-shell nanoparticles were further prepared by the seed growth method.Based on this,a series of 0.1wt%Pt/Al₂O₃and Pt@Au/Al₂O₃catalysts were prepared by a combined method.Toluene was used as a VOCs model molecule to evaluate the catalytic oxidation performance of the prepared catalysts,and the structure of the catalyst was systematically characterized.The internal relationship between the nanoparticles prepared by different methods and the catalytic oxidation performance of toluene was studied in this paper.The particle size and dispersion of Pt NPs prepared by organometallic liquid phase decomposition method were closely related to the preparation conditions.With the increase of temperature,reduction time and hydrogen pressure,the organic metals were gradually decomposed.The particle size of Pt NPs increased with the increase of Pt concentration and reduction time,but the degree of dispersion was reversed.Under better preparation conditions（temperature was 40℃,pressure was 4 MPa,reduction time was 3 h,Pt concentration was 0.3 mg/m L）,the particle size of Pt NPs was small（average 2.4 nm）and the dispersion was higher.The oxidation activity was better,and the reaction temperature at 95%toluene conversion（T95）was 235℃.The particle size of Pt NPs prepared by metal salt liquid phase hydrogen reduction method increased with the increase of Pt concentration,reduction time and temperature,while the degree of dispersion increased with pressure increased.Therefore,the particle size of Pt NPs was relatively smaller（average 3.2 nm）and the dispersion was better under the suitable preparation conditions（temperature was 10℃,pressure was 4 MPa,reduction time was 3 h,Pt concentration was 0.3 mg/m L）,leading to better catalytic performance with lower reaction T95（240℃）.The particle size of Pt NPs prepared by metal salt triethylamine liquid phase reduction method increased with the increase of Pt concentration,reduction time and temperature,while decreased with the increase of the amount of triethylamine.However,with the increase of the amount of triethylamine,due to the bridging effect,agglomeration of triethylamine occurred,so the degree of dispersion tends to decrease.When the temperature was 100℃,the amount of triethylamine（molar ratio）was 1:10,the reduction time was 1 h,and the Pt concentration was 0.3 mg/m L,Pt NPs showed smaller particle size（average 2.5 nm）and higher dispersion,causing the activity of toluene oxidation significantly improved with the T95 reaching 235℃.In summary,the catalytic activity of Pt/Al₂O₃catalysts depended on the balance between the particle size and dispersion of Pt NPs.Among the above three methods,the particle size of Pt NPs prepared by organometallic liquid phase decomposition method was smaller and the degree of dispersion was better.Pt NPs prepared by metal salt liquid phase hydrogen reduction method had better dispersion but its particle size was larger.The Pt NPs prepared by metal salt triethylamine liquid phase reduction method had small particle size and high degree of dispersion.Therefore,the Pt NPs obtained by the organometallic liquid phase decomposition method and the metal salt triethylamine liquid phase reduction method exhibit a better balance between the particle size and the degree of dispersion,leading to better catalytic activity for the oxidation of toluene.Pt@Au/Al₂O₃core-shell catalyst with controllable shell thickness was prepared by seed growth method using Pt₂（dba）₃and HAu Cl₄·4H₂O as precursors through adjusting the ratio of the two precursors.Combining with the activity evaluation and characterization results,it could be found that the Pt₁@Au₁/Al₂O₃catalyst had better catalytic performance,which may be due to the specific Au shell layer effectively enhancing the interaction between Pt and Au.This enhanced the electron transfer from Pt to Au,thereby reducing the binding energy and increasing the amount of active oxygen on the surface of Au,which further improved the catalytic activity of the catalyst.Meanwhile,the presence of the Au shell effectively reduced the oxidation of Pt,thereby producing more active sites Pt⁰and Au⁰that promoted the catalytic oxidation of toluene.The research on the preparation of Pt₁@Au₁/Al₂O₃catalyst by the seed growth method provided an important theoretical basis for the application of core-shell nano-catalysts in the catalytic combustion of VOCs.Simultaneously,this method also has great development potential.