Study On The Catalytic Oxidation Performance And Mechanism Of Rare Earth Metal Catalysts

In recent years,with the rapid increase of the earth’s population,the degre e of industrialization has gradually deepened,and more and more harmful gase s have been emitted due to human life and production.The problem of air pol lution cannot be ignored.Because of the flow and diffusion effects of gases,t he problem of air pollution is not a simple local problem,but a global and co mplex problem,which has caused widespread concern in countries around the world.Among the many gaseous pollutants,VOCs（volatile organic compounds）are one of the main sources.Most of these organic substances have a certain degree of toxicity,and are accompanied by irritating odors such as malodor,an d some substances also have hidden dangers of causing cancer.These volatile organic waste gases not only cause serious pollution to the environment,but al so endanger human health.Therefore,they must be purified and treated before they can be discharged.At present,there are many commonly used pollutant control technologies,among which the catalytic combustion method is admired by most scholars.Th is method uses the catalytic effect of the catalyst to reduce the difficulty of th e oxidation reaction of pollutants and accelerate the reaction process.The core lies in the development of excellent catalysts.Among different types of catalys ts,perovskite catalysts composed of rare earth elements have certain research a nd development potential.Based on La Mn O3 perovskite,the catalyst is modifie d by ion doping,acid etching and urea modification.The results obtained are as follows:（1）La Mn O₃ synthesized by traditional sol-gel method in La_1-xCe_xMn O₃ ser ies catalyst has complete crystal phase structure.According to XRD,it can be found that when the substitution amount of Ce ions reaches 0.2,La_0.8Ce_0.2Mn O₃ catalyst has Ce O₂ is generated.With the increase of the substitution amount,the perovskite phase gradually decreased and the Ce O₂ phase gradually increase d.The SEM morphology of the catalyst changed from cotton floc to dense fla ke.Among La_1-xCe_xMn O₃ series catalysts,La_0.8Ce_0.2Mn O₃ catalyst has the best catalytic activity for toluene oxidation.The temperature at which the sample ca talyzes toluene to 50%conversion is 245°C,and the temperature at 90%conve rsion is 265°C.Characterization and analysis A small amount of Ce ions instea d of La ions can increase the specific surface area of the catalyst,increase the surface Mn⁴⁺ions and oxygen content,and thus improve the ability to catalyz e toluene.（2）Among La_0.8Ce_0.2Mn O₃ catalysts treated with different concentrations of nitric acid,when the nitric acid concentration reached 10M,the Mn O₂/La_0.8Ce_0.2Mn O₃ catalyst showed the best catalytic oxidation activity.The temperature a t which the sample catalyzes toluene to 50%conversion is 237°C,and the tem perature at 90%conversion is 255°C.Excessive nitric acid concentration will d estroy the crystal structure of perovskite.Characterization shows that the specifi c surface area of the catalyst is increased and the reducibility is enhanced after a certain concentration of nitric acid is etched.La ions are partially removed,and more Mn O₂ is exposed on the catalyst surface,which increases the surface Mn⁴⁺ion and oxygen content,which is conducive to the improvement of cata lytic activity.（3）The catalytic activity of the catalysts in the samples with different ure a additions is ordered from high to low（stoichiometric ratio urea:citric acid）i s 2:1>1:1>4:1>0.5:1>0:1.Compared with the La Mn O₃ catalyst modified without urea,the four catalysts with added urea show the improvement of catalytic ac tivity regardless of the proportion of urea added,which is mainly due to the i ncrease in specific surface area.（Stoichiometric ratio of urea:citric acid）The s ample with 2:1 has the best catalytic activity.The temperature corresponding t o 50%conversion of toluene is 216°C,and the temperature corresponding to 90%conversion is 230°C.Associated with an increase in specific surface area a nd enhanced low-temperature reducibility.