Develop Novel And High-Efficiency Ru-containing Catalysts For Toluene Combustion:The Effect Of Ru Dispersion And Distribution On The Reaction Performance

With the rapid development of national economy,air pollution problems caused by VOCs emission are becoming more and more severe.Catalytic combustion of VOCs into CO₂ and H₂O is one of the feasible ways to control their pollutions.The development of efficient and inexpensive catalysts is the core issue to improve the effectiveness of catalytic combustion technology.In this paper,from the perspective of cost control,a relatively cheaper noble metal Ru is used as the active component to prepare catalysts for toluene combustion.（1）A₂B₂O₇compounds have high thermal stability and excellent oxygen mobility,which can meet the structure and active site requirements of VOCs combustion catalysts.Ru distribution in/on Ru-Pr₂Sn₂O₇ pyrochlore catalysts,the structure-activity relationship and its eventual effect on catalytic performance have been systematically studied.It mainly includes the following two parts:Part I:Ru surface and bulk-modified Pr₂Sn₂O₇ pyrochlore catalysts were prepared with impregnation and hydrothermal methods for toluene combustion.XRD,TEM,XPS and in-situ DRIFTS results have shown that for the Ru/Pr₂Sn₂O₇catalysts prepared by the impregnation method,Ru species distributes on the surface of Pr₂Sn₂O₇ as both metallic Ru⁰ and Ru O₂.In contrast,for the Ru-Pr₂Sn₂O₇ catalysts prepared by the hydrothermal method,Ru species is incorporated into the crystal matrix of the pyrochlore structure in the form of Ru⁴⁺.O₂-TPD,EPR and Toluene-TPD results have testified that with the co-existence of both metallic Ru⁰ and Ru O₂ on the surface of Pr₂Sn₂O₇,more abundant active-surface oxygen sites can be formed.At the same time,surface Ru⁰ sites are favorable for the adsorption and activation of both toluene and gaseous O₂ molecules.In-situ DRIFTS results have proven that toluene can be oxidized into active benzoyl oxide,benzyl alcohol and benzoic acid intermediates.Simultaneously,gas phase O₂ can be activated quickly on the catalyst surface,and regrate the surface oxygen vacancies.The reaction follows a Mars-van-Krevelen mechanism.In conclusion,Ru surface modification on Pr₂Sn₂O₇pyrochlore is a better way than the bulk modification,which can improve more significantly the performance of the prepared catalysts.PartⅡ:Based on monolayer dispersion theory,a series of Ru/Pr₂Sn₂O₇catalysts with different Ru/Ru O₂ loadings were prepared by impregnation method for toluene combustion.Through XRD and XPS extrapolation methods,it is quantified that the dispersion capacity of Ru O₂ is 1.83 mmol 100 m^-2 Pr₂Sn₂O₇ surface,which equals to 3.2wt.%.When the Ru loading is below the dispersion capacity,Ru/Ru O₂species disperses highly on the surface of Pr₂Sn₂O₇.In contrast,when the Ru loading is above the dispersion capacity,Ru O₂ micro-crystallites appears.It has been revealed that when the Ru loading is near the dispersion capacity,the largest amount of metallic Ru⁰ sites can be generated,which is beneficial to the dissociative adsorption of gas phase O₂and toluene molecules,thus obtaining a catalyst with the optimal performance.Whereas,the Ru O₂ micro-crystallites formed above the dispersion capacity have negative influence on the inherent activity of the prepared catalysts.（2）An Si-Al zeolite has usually strong adsorption capability for VOCs pollutants,which can enrich VOCs firstly,and then undergo further catalytic combustion,thereby improving the combustion efficiency of low-concentration VOCs.Thus,in this part of work,the valence state distribution of Ru,Ru dispersion,the interaction between Ru and ZSM-5 support,the surface-active oxygen sites,the toluene adsorption ability and their impact on the reaction performance have been systematically investigated.The main results are summarized here:With an impregnation method,a series of Ru/ZSM-5 catalysts were prepared by supporting Ru/Ru O₂（2 wt.%Ru）on HZSM-5 supports with different Si/Al ratios for toluene combustion.The reaction results have shown that the activity of the Ru/ZSM-5 catalysts can be enhanced with the increasing of Si/Al ratios.TEM,XPS and in-situ DRIFTS results have demonstrated that the active Ru⁰ amount/ratio improves significantly with the Si/Al ratio increment.O₂-TPD,EPR,Toluene-TPD and in-situ DRIFTS results have indicated that the metallic Ru⁰ sites are of advantageous for the adsorption and activation of toluene and gas phase O₂ molecules,hence generating active benzoic acid intermediates and oxygen sites(O₂^-/O₂^2-).Nevertheless,the surface lattice O^2-was not involved into the reaction evidently.At the same time,TGA-DSC results have testified that the hydrophobicity of the catalysts improves with the Si/Al ratio increment,which can impede H₂O molecule adsorption,and restrict its competition of surface-active sites with the reactant molecules effectively.In-situ DRIFTS reaction results have revealed that over Ru/HZSM-5,the reaction follows a Langmuir-Hinshelwood mechanism.In brief,the results in this part have concluded that the mixed interaction of higher hydrophobicity,more abundant surface Ru⁰ and active oxygen sites is the major reason accounting for the remarkably enhanced activity of a Ru/ZSM-5 catalyst having a higher Si/Al ratio.