Pore-Modified Effect Over Toluene Catalytic Combustion Performance Of Zeolite Foam Supported Pt Catalysts

Volatile Organic Compounds?VOCs?,as precursors of PM_2.5 and ozone,have caused urgent environmental problems of fine particles and ozone pollution in China in recent years.Over the past decades,developing techniques such as adsorption,catalytic combustion,thermal incineration,biological processes,plasma catalytic oxidation,and photocatalytic degradation,etc.have attracted great attention to remove VOCs.Among them,catalytic combustion is a well-known technology of high catalytic performance,economic feasibility,high selectivity,low operation cost,and non-organic by-products for its approximately completely catalytic oxidation of VOCs into CO₂ and H₂O at low temperatures.The development of a catalyst with low temperature activity and strong stability is the key to enhancing the core competitiveness of the technology.Herein,to investigate the pore effect on toluene catalytic oxidation activity,novel supports for Pt nanoparticles—ZSM-5 foam?ZF?fabricated using polyurethane foam?PUF?templates and pore-modified ZSM-5 foam?ZF-D?treated by acid etching,comparing with conventional ZSM-5 and pore-modified ZSM-5?ZSM-5-D?,were successfully synthesized.Pt nanoparticles were loaded on series ZSM-5 supports by the impregnation method.In order to compare the effects of different supports on the catalytic oxidation performance of toluene,and to understand the pore characteristics of zeolite foam and the difference between its pore structure and traditional ZSM-5,Pt/ZF and Pt/ZSM-5 was firstly prepared by impregnation method in this study.Through XRD,SEM,TEM,XPS,ICP,BET,IGA,and other characterization methods,it is found that ZF contains the unique hierarchical porous structure?micropore-mesoporous-macropore?,while ZSM-5 mainly contains microporous structure.The unique hierarchical porous structure of the ZF is conducive to the dispersion of Pt particles,the reduction of Pt⁰ species,the reduction of active oxygen species,and the improvement of the adsorption and diffusion capacity of toluene,which makes the significantly enhancement of catalytic oxidation activity of toluene.Pore-modified ZSM-5 foam?ZF-D?treated by acid etching,comparing with conventional ZSM-5 and pore-modified ZSM-5?ZSM-5-D?,were successfully synthesized.Pt nanoparticles were loaded on series ZSM-5 supports by the impregnation method.Pt/ZF-D has much better toluene catalytic performance than Pt/ZF.According to various characterization results,it is confirmed that the pore-modification of ZF via acid treatment play a significant role in improving the catalytic performance of toluene oxidation.The mesopores volume of ZF-D significantly increased due to acid etching,which enlarged toluene adsorption capacity and led to a better Pt distribution since some Pt nanoparticles were immobilized into some mesopores.Specifically,the microporous distribution was centered in the range of 0.7–0.8 nm close to the molecular diameter of toluene?ca.0.67 nm?,which was key to the increasing toluene diffusion rate due to pore levitation effect of catalysts and accessibility of metal.Furthermore,the reducibility of Pt nanoparticles was improved on Pt/ZF-D,which enhanced the activity of toluene catalytic oxidation.According to the confinement effect,this unique microporous structure which only focused on 0.7-0.8 nm provided almost the most favorable channel conditions for the diffusion and adsorption of toluene.Catalysts with ZF-D supports was synthesized with different Pt loadings.According to the results of toluene activity test,it is not that the higher the Pt loading,the better the catalytic oxidation activity of toluene.The results show that too much Pt loading will cause the agglomeration of Pt particles,thereby weakening the metal-support interactions,reducing the proportion of Pt⁰,reducing the catalytically active components,and inhibiting the improvement of catalytic activity.The optimal Pt loading was determined to be 1 wt%.Load Pt with the same loadings on a series of ZFx-D?x=25,50,100,200 Si/Al?.By adjusting the ratio of Si/Al,the porous structure of ZF-D is further modified.The results show that the adjustment of Si/Al can not only adjust the mesoporous structure of zeolite,but also the microporous structure of zeolite.According to the characterizations,the greater the Si/Al,both the mesoporous and microporous structure of zeolites are adjusted to a smaller pore size structure.After adjusting the Si/Al,among all the catalysts,the Pt/ZF_x-D with a Si/Al of 25 exhibit the most excellent catalytic toluene oxidation performance.