| In recent years,with the attention and continuous promotion of the prevention and control of environmental pollution in China,the control of air pollution begins to change from the control of conventional pollutants(NOx,CO,particulate matter,SO2)to the coordinated control of conventional and unconventional pollutan ts(PM2.5,VOCs,O3,Hg.and other heavy metals,toxic substances,etc.).Among them,volatile organic compounds(VOCs)have become an important problem to be solved urgently in air pollution control due to their late start and governance difficulties.Catalytic oxidation technology can remove VOCs efficiently under low temperature and mild operating conditions,which is considered to be the most promising industrial VOCs purification method in the future.Perovskite-type oxides have many advantages such as low cost,high stability,excellent catalytic performance,strong adjustability and high thermal stability,so they have become the hot materials in VOCs catalytic oxidation catalyst research.However,there are several key problems for perovskite-type catalyst to be solved in order to realize industrial application urgently.First,the perovskite-type oxide has limited low-temperature catalytic activity and low ignition temperature,which did not reach the practical application standard at present.Second,perovskite is easily poisoned and deactivated in sulfurcontaining VOCs gas,which seriously affects the catalytic performance.Third,the research on the monolithic molding of perovskite-type catalyst lags behind,and further optimization is needed to give full play to their catalytic activity.In order to solve the above problems,in this paper,the LaCoO3 was selected as a typical perovskite oxide template,and the toluene was selected as the representative VOCs pollutant,then we conducts a series of basic studies on the low-temperature catalytic activity,stability,and sulfur resistance of the catalyst from the perspectives of the catalyst preparation,doping modification regulation,element synergistic enhancement,nano coating development,testing and experimental verification.The physicochemical properties of the catalyst,intermediate process of the catalytic reaction,element synergistic enhancement effect,catalytic reaction mechanism and sulfur resistance mechanism were deeply explored by various characterization methods including in-situ testing.The main work and results are as follows:The method of introducing specific dopants(Fe,Cr,Cu)into the B site of LaCoO3 perovskite can not only improve the low-temperature catalytic activity,but also weaken the negative effect of SO2 poisoning to a certain extent.Among them,the LaCoO.6Fe0.4O3 perovskite catalyst shows the best low-temperature activity,stability and sulfur resistance.The doped perovskite oxide can well maintain the typical ABO3 type structure,and its oxidation state at the B site,reducibility and oxygen vacancy content will be improved to varying degrees,thus improving the catalytic activity.In addition,it was also found that there was some variability in the increase and mechanism of sulfur resistance caused by different dopants.SO2 poisoning will form some sulfite or sulfate species,which will reduce the catalytic activity of perovskite.Different dopants will cause certain differences in reducibility,lattice stability,sulfur adsorption and sulfate content,eventually leading to the distinction of the sulfur resistance.Among them,the high stability of perovskite lattice caused by Fe doping and its insensitivity to sulfur species are the key factors for its optimal sulfur resistance;Co-Cr synergism caused by Cr doping and enhanced reduction performance also formed good sulfur resistance;Cu doping did not improve the sulfur resistance significantly,but the improvement of reduction performance caused by doping could partially counteract the activity loss caused by poisoning.The catalytic activity and sulfur resistance of LaCoO3 perovskite oxides can be improved simultaneously by modifying them with appropriate proportion of A/B site dopants(Ce,Fe).Among all the synthesized doped samples with single/dual element,La0.6Ce0.4Co0.6Fe0.4O3 sample has the best low-temperature activity,stability and sulfur resistance.Compared with the effect of single doping,Ce doping has more advantages in improving catalytic activity,while Fe doping is more beneficial for sulfur resistance.The co-doping of two elements has a good synergistic effect and achieves the best catalytic performance.Ce doping will generate more oxygen vacancies and form CeO2 components with high oxygen storage capacity,which will promote the migration of lattice oxygen and the low-temperature reducibility,thereby enhancing the catalytic activity;Fe doping can change the oxygen vacancy content and the oxidation state of B site metal,thus improving the catalytic activity;Although the codoping of Ce and Fe will reduce the crystallinity of the perovskite components and make the sample composition more complex,the strong the strong synergism of co-doping elements further enhances the low-temperature reducibility and the overall catalytic activity of the catalyst.In addition,the CeO2 generated by Ce doping can limit the poisoning of the main active phase,and Fe dopants results in highly stable perovskite lattices that are insensitive to SO2.The advantages of the individual dopants were preserved,and the sulfates formed in the Ce-Fe solid solution were more mobile and decomposable after heating,which further enhanced the sulfur tolerance of the catalyst.Various types of LaCoO3/washcoat/cordierite monolithic catalysts were prepared by a facile two-step procedure which employed the nano oxides(CeO2,SiO2,γ-Al2O3,ZrO2,TiO2)as coating materials,the monolithic catalysts can not only give full play to the low-temperature catalytic activity of active components,but also show a high catalytic stability.Although the washcoat material has certain catalytic oxidation capacity,the interaction between the washcoat and the active component is the key factor determining the performance of the monolithic catalyst.Therefore,the catalytic properties of the monolithic catalysts vary notably with the species of nanowashcoat.Among the catalysts studied,the LaCoO3/CeO2/cordierite and the LaCoO3/ZrO2/cordierite monolithic catalysts showed better low-temperature activity and stability for toluene oxidation.Compared with other washcoat,CeO2 and ZrO2 nano washcoat helps to form a more uniform distribution of active components,more favorable porous surface structure,higher ratio of surface adsorbed oxygen,stronger low-temperature reduction and better adhesion on the monolithic catalyst,which greatly promote the catalytic reaction.The above research results are expected to optimize the actual composition,life cycle and industrial development for perovskite-type.catalysts,and also provide important theoretical basis,scientific basis and new ideas for the practical industrial application of catalytic oxidation of VOCs. |
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