The Preparation Of Novel SnO₂ Catalytic Materials For Toluene Total Oxidation:Study On The Structure-reaction Relationship

Catalytic VOCs total oxidation is important for both industrial application and fundamental research.It is currently a hot research topic to prepare low cost and high-performance catalysts for this reaction.In this thesis,the preparation and properties of novel SnO₂ catalysts with high activity and good stability have been studied for toluene total oxidation.The physicochemical properties of the prepared catalysts have been characterized by means of N2 adsorption-desorption,XRD,H2-TPR,SEM,XPS,TEM,ICP,NH3-TPD,and Toluene-TPD.The structure-reactivity relationship of the catalysts has been investigated and elucidated.The main results are summarized here:First,SnO₂ catalysts with different surface areas have been fabricated by using hydrothermal method and evaluated for toluene total oxidation.Among all the catalysts,SnO₂-217,a catalyst with the hig hest surface area shows the best activity,which is over 430 times of Sn O 2-2 catalyst.XRD,TEM and N2-sorption results testify that with the increasing of the surface area,the crystallinity of the catalysts becomes lower,the grain sizes and pore diameters beomce smaller,but the pore volumes becomes larger.As a result,the amount of the surface loosely bond oxygen species improves evidently,as testified by H2-T PR and XPS results,which is favorable for the oxidation of the adsorbed toluene species.In addition,Toluene-T PD and NH3-T PD results substantiate that the surface acidity of the catalysts can also be significantly strengthened due to the increase of the surface area,which favors the adsorption and activation of the toluene molecules.It is believed that these two are the major reasons accounting for the high activity of the Sn O 2 catalysts.After supporting 1 wt.% Pd,toluene t otal oxidation follows Mars-van Krevelen mechanism on SnO₂ nano-particle supports,but it follows Langmuir-Hinshelwood mechanism on SnO₂ nano-rod support.HRT EM and ST EM-mapping results demonstrate that Pd is mainly present as metallic Pd0 uniformly on SnO₂ nano-rod support but present uniformly as Pd2+ on those SnO₂ nano-particle supports.Second,SnO₂ nano-rods doped by In,Cr and Al cations have been fabricated by using hydrothermal method and evaluated for toluene total oxidation.SEM results demonstrate that the initial nano-rods aggregated into three-dimensional microspheres for all the samples.XRD testifies that all the SnO₂ nano-rod catalysts are well crystallized with low surface areas and big crystallite sizes.It is revealed by HRTEM images that the pure SnO₂ and Sn99Al1 nano-rods preferentially exposed?101?facets,while the Sn99In1 and Sn99Cr1 nano-rods exposed?110?facets,which demonstrated that the doping with a small amount of the secondary cations into the crystal matrix can significantly influence the crystallization process and change the preferentially exposed facets of SnO₂ nano-rods.As testified by XPS and O2-TPD,the introduction of Al³⁺ cations into the crystal matrix of SnO₂ nano-rods leads to the formation of more mobile oxygen species.In addition,NH3-TPD results substantiate that the surface acidity of some catalysts can also be significantly enhanced,which is favorable for the adsorption and activation of the toluene molecules.In conclusion,these are regarded as the two predominant factors accounting for the improved activity of the doped SnO₂ nano-rod catalysts for toluene total oxidation.Due to the optimized concerted action of the two factors,Sn99Al1-rod,a catalyst doped by Al cations shows the highest activity among all the catalysts.