SnO₂-based Catalytic Materials For Toluene Deep Oxidation

With the development of industry,environmental pollution caused by volatile organic compounds(VOCs)is becoming gradually serious.Catalytic oxidation is one of the effective means to eliminate its pollution.The development of non-noble metal and low noble metal catalysts with high activity and stability are the focuses of current research.In this work,a series of highly active and stable SnO2-based catalytic materials were designed and prepared for toluene deep oxidation,and their surface active centers and key factors affecting toluene oxidation were investigated by combining the characterization methods.The main conclusions are listed here:First,to elucidate the fomation of the surface oxygen sites on SnO2,and the relationship between the the active sites and the toluene oxidation peformance,a series of SnO2 samples have been fabricated by using microwave/ultrasound and KCC-1 hard template.By adjusting the preparation methods,SnO2 with varied surface and texture properties were obtained,which display different intrinsic activity for toluene deep oxidation,following the sequence of SnO2-Ultra-OH>SnO2-Stir-OH>SnO2-Mic>SnO2-Pre.Raman,H2-TPR,O2-TPD and XPS results show that the presence of oxygen vacancy on the surface of catalyst is conducive to the formation of more abundant surface active oxygen species.For the first time,it is discerned clearly with In Situ DRIFTS,XPS and EPR techniques that both superoxide O2-and peroxide O22-anions are present on SnO2 surface as active oxygen species.NH3-TPD and Toluene-TPD show that the acid sites on the catalyst surface can effectively improve the adsorption and activation process of Toluene molecules.Generally,those more active SnO2-Ultra-OH samples possess richer surface active O2-,O22-anions and acidic sites.The concerted interaction between surface O2-,O22-anions and acidic sites is concluded to be the predominant reason determining the toluene total oxidation activity over SnO2.Second,metal oxides with different properties supported on CVD-SnO2 have been prepared as catalysts by the simple impregnation method for toluene deep oxidation.The toluene deep oxidation activity of the catalysts follows the sequence of 1%Mn/SnO2>1%Co/SnO2>1%Cr/SnO2>1/%Bi/SnO2>1%In/SnO2>SnO2.In-situ DRIFTS,EPR,XPS and H2-TPR results testified the amount of surface peroxide and superoxide species on the SnO2 catalyst have been enhanced by the addition of metal oxides with redox feature.NH3-TPD,Toluene-TPD and in-situ DRIFTS have demonstrated that the surface acidic sties and the toluene adsorption sites of the modified SnO2 catalysts are all increased.Moreover,both the active oxygen species content and the toluene adsorption sites on the catalyst surface play important roles in toluene deep oxidation.1%Mn/SnO2 catalyst possesses more surface peroxide and superoxide species,the highest toluene adsorption amount and low-temperature reducibility,thus having the optimal toluene deep oxidation activity in all the catalysts.In addition,this catalyst possesses excellent stability,water vapor and sulfur tolerance.Third,SnO2 with two different morphologies,nano-rod(SnO2-R)and nano-particle(SnO2-P),have been prepared by precipitation and hydrothermal method,and used as a support for Pd to prepare low noble metal content catalysts for toluene deep oxidation reaction.The test results show that the activity of Pd/SnO2-R catalysts is significantly higher than that of Pd/SnO2-P catalysts with the same Pd content,and their catalytic behaviors are significantly different.Pd/SnO2-R catalysts mainly follow the L-H mechanism,while Pd/SnO2-P catalysts mainly follow the MVK mechanism.XRD,SEM and H2-TPR resultshave demonstrated that compared with SnO2 nanoparticles,SnO2 nanorods have a smaller specific surface area and very high crystallinity,with fewer oxygen species on the surface.Due to the different morphologies and properties of the two SnO2 supports,the valence state and distribution of the loaded Pd are different.HRTEM,XPS and ICP results have shown that both Pd0 and Pd2+ are present and highly enriched on Pd/SnO2-R surfaces.In contrast,on Pd/SnO2-P catalysts,Pd species mainly exist as Pd2+.The presence of high content Pd0 species on Pd/SnO2-R surface is favorable for the adsorption and activation of oxygen and toluene molecules,which effectively promotes toluene oxidation.The valence distribution of Pd species on the surface of SnO2 support with different morphologies is the key factor affecting the toluene deep oxidation activity.