Study On Catalytic Performance Of LaMnO₃ Under Different Acid Treatment Conditions

Behind the rapid development of China’s economy,there are also many thorny problems,especially the increasing number of motor vehicles.CO in motor vehicle exhaust not only causes environmental impact,but also threatens human health.Therefore,it is a hot research topic to study a kind of catalyst which can purify CO.At present,perovskite catalysts have outstanding catalytic activity for CO oxidation.In this paper,perovskite type catalyst LaMnO₃was prepared by sol-gel method,and acid treatment was carried out to compare the catalytic activity of LaMnO₃to CO before and after acid treatment.The principle of the reaction is that CO is activated on the surface of the catalyst through adsorption,and then reacts with adsorbed oxygen to form CO₂,ad,and finally gaseous CO₂.XRD（powder X-ray diffraction）:the information about the periodic pore structure of the sample and the crystal structure of the material were determined,and the specific surface area and pore structure were measured by N₂adsorption desorption.The catalytic activity for CO was determined by temperature programmed oxidation.Temperature programmed reduction was used to test the reducibility of the catalyst,temperature programmed desorption was used to test the content of adsorbed oxygen,scanning electron microscope was used to observe the surface morphology,and X-ray photoelectron spectroscopy was used to analyze the surface elements and chemical states.The experimental results show that the catalytic activity of the catalyst for CO is obviously improved after acid treatment.After acid treatment,the perovskite structure of the catalyst is still maintained,and the manganese element in LaMnO₃forms Mn³⁺and Mn⁴⁺.In the activity test of temperature programmed oxidation of CO,the acid treated samples can start to transform at a lower temperature,and the initial conversion temperature of CO decreases by nearly 120℃.The results of N₂adsorption and desorption showed that the specific surface area was93.4 m²/g after treated with nitric acid for 24 h.The specific surface area of the catalyst reaches the maximum value of 127.8 m²/g after acid treatment for 15 min.The reduction temperature and desorption temperature of adsorbed oxygen decrease with time.The conversion of CO was 100%.The initial conversion temperature and complete conversion temperature reached the lowest point at 83℃and 167℃respectively after acid treatment for 24 h.By changing the amount of dilute nitric acid used in acid treatment,it was found that the catalytic performance of the catalyst treated with the amount of acid treatment increased from 100 ml>200 ml>50 ml.The catalyst treated with 3 mol/L nitric acid will cause excessive corrosion,resulting in the reduction of specific surface area to 75.6 m²/g and particle size to 5.56 nm,which makes the agglomeration of the catalyst more serious and reduces the chance of contact with Co.therefore,the catalytic activity of the catalyst treated with 0.6 mol/L nitric acid is not as good as that of the catalyst treated with 0.6 mol/L nitric acid.It was concluded that the best catalytic performance was obtained when LaMnO₃was treated with 100 ml 0.6mol/L HNO₃for 24 h.