SnO₂-supported RuO₂ And Co₃O₄ For Toluene Combustion:Study The Monolayer Dispersion Behavior Of Metal Oxides And Threshold Effect

In the industry development process,the emission of volatile organic compounds（VOCs）becomes increasing seriously,which will not only pollute the environment,but also harm human health.Therefore,it is necessary to find efficient,affordable and sustainable ways to deal with VOCs emissions.At present,VOCs treatment technologies mainly include recycling and degradation technologies,among which catalytic combustion method is one of the efficient and simple treatment technologies.Under the action of catalysts,VOCs can be converted into H₂O and CO₂ by deep oxidation.In current research,both precious metals and non-precious metals have received extensive attention.Precious metals have the advantages of high activity and low dosage,but due to high price,scarce resources and other factors,they are limited in the catalytic applications.Non-precious metals have attracted wide attentions in the research of catalysts because of their low price and high catalytic activity.Therefore,in this thesis for the catalytic combustion of VOCs,the selection of low-cost industrialized CVD-SnO₂ material as a support,on which precious metals or non-precious metals were loaded according to the monolayer dispersion theory to design catalysts with high efficiency and good stability.Through a series of characterization techniques such as XRD,Raman,N₂ adsorption and desorption,TEM,Mapping,H₂-TPR,O₂-TPD,XPS,ICP,NH₃-TPD,and Toluene-TPD,the influence of the physical chemical properties of the catalysts on the catalytic performance was investigated.The main results are summarized here:In the first part,a series of precious metal catalysts with different RuO₂contents on CVD-SnO₂ supports were prepared by a precipitation-deposition method for the deep oxidation of toluene,and the monolayer dispersion phenomenon was investigated.The monolayer dispersion threshold of RuO₂ on SnO₂ was quantified by an XRD extrapolation method,which is 0.94 mmol 100m^-2 SnO₂ surface,being equivalent to3.5 wt%of RuO₂ loading.Raman,TEM,HRTEM,STEM-Mapping and XPS results have testified further that the monolayer dispersion of RuO₂ on SnO₂ does be present,and the surface accumulation and crystallization of RuO₂ occurs when the loading exceeds the threshold value,and the activity increase becomes mild.The monolayer dispersion threshold was further confirmed by an XPS extrapolation method,which is about 3.5 wt%and consistent with XRD results.The results of H₂-TPR,O₂-TPD,NH₃-TPD and Toluene-TPD tests show that RuO₂ could interact with the surface of SnO₂ to form interfacial Ru-O-Sn bonds,which makes the redox performance of the catalyst become better,and produces more surface oxygen vacancies and acidic sites.Thus,the catalytic performance of the catalysts is improved,as well as their stability,water and sulfur resistance.In the second part,a series of non-noble metal catalysts with different Co₃O₄contents on CVD-SnO₂ supports were prepared by an impregnation method for the deep oxidation of toluene,and the monolayer dispersion was also investigated.It was calculated by an XRD extrapolation method.The monolayer dispersion threshold of Co₃O₄ on SnO₂ is 0.48 mmol 100m^-2 SnO₂ surface,which is equivalent to 3.4 wt%.In terms of reactivity,with the increase of Co₃O₄ loading,though it gradually improves,the threshold effect is not edvident.After reaching the threshold capacity,the activity did not increase significantly.TEM,HRTEM and STEM-Mapping results show that Co₃O₄ is highly dispersed on SnO₂ when the Co₃O₄loading is lower than the monolayer dispersion amount.When the Co₃O₄content reaches monolayer dispersion capacity,Co₃O₄ begins to partially accumulate and crystallize on SnO₂,which is consistent with XRD results.The results of H₂-TPR,O₂-TPD,NH₃-TPD and Toluene-TPD showed that after loading Co₃O₄ on SnO₂,the low temperature redox performance of the catalyst can be improved,and generate more surface oxygen vacancies and acidic sites,thus improving the catalytic performance of the catalysts.The catalysts showed also improved stability,water and sulfur resistance.