Performance And Mechanism Of Catalytic Oxidation Of Toluene And NO By Supported Perovskite

Vehicle exhaust emissions are one of the main sources of volatile organic compounds（VOCs）and nitrogen oxides（NOx）in the atmosphere.Effective control of vehicle exhaust emissions,especially diesel vehicle exhaust emissions,is an important way to reduce near-surface ozone（O₃）,haze and other air pollution.Diesel exhaust is usually purified by a combined post-treatment system,that is,catalytic oxidizer（DOC）+particulate filter（DPF）+selective catalytic reduction（SCR）technology.DOC device can catalyze the oxidation of VOCs in tail gas to CO₂and H₂O,and convert part of NO to NO₂,which is conducive to the subsequent DPF regeneration and SCR denitration reaction,and is the core element in the post-treatment system.At present,DOC catalyst mainly uses precious metals as the active component,which has high cost,easy sintering and limited application.The development of efficient non-precious metals DOC catalyst has become a hot spot in the field of environmental catalysis.Perovskite oxides are suitable candidate materials for non-noble metal DOC catalysts because of their high oxidation activity,low cost and good thermal stability.However,there are still some problems such as high activation temperature and narrow activity window.Therefore,it is of great practical significance to carry out modification research on perovskite-type oxides to improve the performance of toluene and NO oxidation and broaden the activity temperature window.A series of different supported LaCoO₃/MO₂catalysts（M=Zr,Ti,Ce）were prepared by citric acid sol-gel method.The physicochemical properties of the supported perovskite catalysts were characterized by BET,XRD,SEM-EDS,H₂-TPR,XPS,etc.The catalytic oxidation of toluene and NO by LaCoO₃/CeO₂was investigated.The catalytic oxidation mechanism of toluene and NO by LaCoO₃/CeO₂was proposed based on the influence of space velocity,oxygen content and in-situ infrared spectroscopy.On this basis,the A-site Ce doping of LaCoO₃/CeO₂and the optimization of the preparation method of the support were studied,in order to further improve the catalytic activity,broaden the activity window,and provide A basis for the development of wide temperature and high efficiency perovskite DOC catalyst.The main research results are as follows:（1）Compared with non-supported perovskite catalyst,supported perovskite catalyst LaCoO₃/MO₂（M=Zr,Ti,Ce）has a larger specific surface area,can provide more effective adsorption sites,and has more active lattice oxygen,thus showing better oxidation performance.The oxidation activity of LaCoO₃/CeO₂was the best.The T90 of toluene was 245℃,and the conversion rate of NO reached 68%at 300℃.This was mainly due to the interaction of electron transfer between Co and Ce ions in LaCoO₃/CeO₂,which promoted the redox cycle and was conducive to the catalytic oxidation of toluene and NO.（2）Space speed has a certain effect on the catalytic oxidation of toluene and NO by LaCoO₃/CeO₂.With the increase of space speed,toluene conversion rate,CO₂production rate and NO conversion rate all show a decreasing trend,and the effect on NO oxidation is greater than that on LaCoO₃/CeO₂oxidation,indicating that NO oxidation may be a control step in synchronous oxidation.Under the action of LaCoO₃/CeO₂catalyst,NO and toluene also react in the absence of oxygen,and the conversion of NO and toluene is not affected by oxygen content.On this basis,further in-situ diffuse reflection characterization analysis showed that the oxidation mechanism of NO was Langmuir-Hinshelwood and that of toluene was Mars-van Krevelen on LaCoO₃/CeO₂catalyst.The effect of NO on toluene oxidation is mainly realized by competitive chemisorption sites and changing the reaction process of intermediate products,while the effect of toluene on NO oxidation is mainly competitive adsorption of reactive oxygen species.（3）The low temperature oxidation activity of La_1-XCe_xCoO₃/CeO₂catalyst was improved by Ce doping at A site.The main mechanism may be the formation of more oxygen vacancies due to lattice defects.La_0.8Ce_0.2CoO₃/CeO₂catalyst supported by CeO₂carrier was prepared by template method,and the interaction between the active component and the carrier was enhanced,thus the oxidation activity was significantly improved.Finally,the water resistance of the modified catalyst was tested,and the experiment showed that the template method La_0.8Ce_0.2CoO₃/CeO₂had better water resistance.