Research On The Catalytic Performance Of Pd/K Co-supported Catalyst In Removal Of Diesel Exhaust

In recent years,due to the frequent occurrence of atmospheric pollution such as PM2.5 and severe haze,the atmospheric environmental issues have attracted more and more attention.Many heavy industries and developed cities,especially in the Beijing-Tianjin-Hebei region,have experienced many serious hazy weather and the air quality is getting worse.This kind of weather is harmful to human health and can easily cause many respiratory diseases such as bronchitis and asthma.The occurrence of severe atmospheric pollution,in addition to the fact that China's coal-based energy structure has resulted in a large number of industrial coal boilers as the primary pollution source of coal-fired flue gas emissions,is also associated with the considerable amount of vehicle and the ever-growing number of vehicle in our country.Among them,the proportion of diesel vehicles exhaust to the contribution of pollutant emissions about NOx and particulate matter?PM?in motor vehicles is large.Effectively controlling the emission of NOx and particulate matter from diesel vehicle exhaust is a key measure to purify vehicle exhaust pollution.In this paper,the effects and mechanisms of Pd,K co-doped perovskite-type and Mg-Al hydrotalcite composite oxide catalysts in the removal of four pollutants,such as CO,HC,NOx and soot in diesel vehicle exhaust were studied.The catalysts of different K-doped La_1-xK_xMnO₃?x=0,0.1,0.2,0.3,0.4,0.5?type and xK/MgAlO?x=0,3,5,7,9,x is the mass Percentage?series mixed oxides were synthesized by sol-gel method and precipitation method and characterized.The La_0.5K_0.5MnO₃ catalyst with the best K doping amount x=0.5 and the 7K/MgAl O catalyst with x=7 were selected.Then the different amounts of noble metals of Pd doping catalysts were analyzed,and the best catalysis efficiency of Pd doped La_0.5K_0.5Mn_0.97Pd_0.03O₃ perovskite and 1Pd/7K Mg-Al hydrotalcite-based composite oxide catalysts were optimized.The microstructure,properties and the reaction mechanism with the contaminated gas of Pd/K co-doped perovskite-type catalysts and Mg-Al hydrotalcite-compounded oxide catalysts were studied through a series of characterization test methods such as XRD,SEM,FT-IR,HRTEM,XPS,H₂-TPR.The kinetic parameters of the catalytic reaction were calculated.A bench test was performed on the selected Pd/K co-doped composite oxide catalyst.The main conclusions and findings of this paper are as follows:?1?The La_1-xK_xMn O₃?x=0,0.1,0.2,0.3,0.4,0.5?catalysts were obtained by doping different K amounts into LaMnO3 type perovskites.From the results of mixture gas experiment,it was found that when the amount of K doping?x?was 0.5,the conversion rate of CO and NO reached the maximum,and the conversion rate of CO was 47%at 300°C,and the rate of NO reacheed 95%at 500°C.The soot ignition temperature of La_0.5K_0.5MnO₃ catalyst was the lowest,reaching 267°C,which was65°C lower than that without doping K.By analyzing the characterization of the catalyst before and after doping K,it was found that the doping of K improves the dispersity of the catalyst to a certain extent.The doping of K made the Mn³⁺at the B site of the perovskite into a valence state of Mn⁴⁺,which helped to increase the oxygen vacancies,thereby effectively improving the storage performance of the catalyst for NOx.?2?Based on the selection results of the best K doping amount,different amount of noble metal Pd doping was performed.It was found that the Pd doped catalyst significantly improved the conversion rate of CO,C₃H₆ and NO.When the conversion rate of CO reacheed 50%,the reaction temperature of the catalyst without doping Pd was 331?,while the temperature of La_0.5K_0.5Mn_0.97Pd_0.03O₃ catalyst?doping amount of Pd=0.03?was only 145?,which significantly improved the low temperature conversion efficiency of the catalyst for CO.Through the XRD tests of different Pd-doped catalysts,it was found that the peaks of the perovskite diffraction peaks shifted to small angles in the Pd-doped catalyst,which proved that Pd was successfully doped into the catalyst structure and existed in the form of Pd³⁺and Pd⁴⁺.The results of SEM test showed that the doping of Pd further improved the dispersion and anti-sintering ability of the catalyst,effectively increased the contact area between the catalyst and the contaminated gas,and thus improved catalytic activity.?3?The structures of LaMnO₃,La_0.5K_0.5MnO₃ and La_0.5K_0.5Mn_0.97Pd_0.03O₃catalysts were characterized and the redox properties were tested.It was found that the vibration peak area at 848cm^-1 in La_0.5K_0.5MnO₃ attributable to carbonate was significantly higher than that of LaMnO₃ catalyst without doping K.The Pd,K co-doped catalyst La_0.5K_0.5Mn_0.97Pd_0.03O₃ also showed characteristic peaks of K₂CO₃ at1736cm^-1 and 1458cm^-1.The formation of carbonate helped the catalyst to react with NOx to form nitrates,and then increased the NO storage and conversion rate.The peak area of the O_I at the high binding energy of the P and K co-doped catalysts was significantly increased,and the O_II of the low binding energy was decreased,indicating that the doping of Pd increased the defects on the surface of the catalyst,reduced the lattice oxygen,and increased oxygen fluidity,which helped the transfer of electrons and improved the catalytic activity.The redox performance of the H₂-TPR shows that the reduction peak of the catalyst La_0.5K_0.5MnO₃ after doping with K shifted ahead of the reduction peak of LaMnO₃.After doping Pd,the reduction peak of the catalyst La_0.5K_0.5Mn_0.97Pd_0.03O₃ was further moved forward,and the reduction peak area was increased.It was proved that the doping of Pd enhanced the redox performance of the catalyst.?4?The magnesium-aluminum hydrotalcite-based composite oxides with different K doping amounts xK/MgAlO?x=1,3,5,7,9,x was the mass percentage?were prepared by co-precipitation method using Mg-Al hydrotalcite as a precursor.Through the results of gas mixture experiments,it was found that when the doping amount of K was 7,the conversion rate of CO reached 61%at 300°C,which was 13%higher than that of MgAlO.There was an optimum amount of K doping.When x K was less than 7%,the surface of MgAlO is weaker,While the K content x was more than 7%,too much K covered the active sites on the catalyst surface.As a result,the catalytic activity of the catalyst was difficult to be enhanced.The characterization results show that the doping of K effectively increases the dispersion of the catalyst.After doping K,the single-tooth CO₃^2-characteristic peak in the hydrotalcite disappeared,and a double-toothed CO₃^2-characteristic peak appeared,indicating K doping produced a new alkaline site of hydrotalcite.In addition,K reacted with OH^-on the weak alkaline site of magnesium aluminum hydrotalcite,replacing protons on OH^-,forming Mg?Al?-O-K.Besides,surface CO₃^2-coordinated with K⁺also appeared.?5?The 7K/MgAlO catalyst with the best K doping amount was selected through characterization and mixed gas test.Based on this,the doping of different amounts of noble metal Pd was performed.Through mixture gas test and catalyst structure characterization,it was found that the Pd-doped catalyst yPd/7K/MgAlO had higher CO conversion at lower temperature than the undoped Pd catalyst 7K/MgAlO,indicating that the doping of Pd increased the combustion activity of CO at low temperature.When the conversion rate of CO and C₃H₆ reached 50%,the reaction temperature of 1.0Pd/7K/MgAlO catalyst was 250°C and 244°C respectively,which increased 31°C and 24°C over 7K/MgAlO.The Pd-doped catalyst yPd/7K/MgAlO showed a crystalline structure,and the lamellar structure formed by the1.0Pd/7K/MgAlO catalyst was the most fluffy and orderly.It indicates that there is an interaction between Pd and K,forming Pd-O-K species and enhancing the dispersion of K,thus effectively inhibiting the occurrence of catalyst binding to a certain extent.?6?The structure characterization and redox properties of three catalysts MgAlO,7K/MgAlO and 1.0Pd/K/MgAlO were tested.The analytical results showed that K doping reacted with OH^-instead of protons on OH^-to form Mg?Al?-OK or Mg-OK species,and Pd doping did not change the formed Mg?Al?-OK or Mg-OK.The splitting peak spacing of CO₃^2-doped with noble metal Pd was less than that of7K/MgAlO,which showed that when Pd and K were co-doped,there was an interaction between Pd and K,K covered part of Pd and reduced the degree of Pd dispersion.XPS characterization results showed that the relationship between the O_III content of the three catalysts was MgAlO<7K/MgAlO<1.0Pd/K/MgAl O,indicating that the doping of K and Pd increased the surface defects of the catalyst and reduced the lattice oxygen content,which increased the mobility of oxygen,facilitated electron transfer and improved catalytic activity.The results of H₂-TPR showed that there were no hydrogen reduction peaks in the catalysts MgAlO and 7K/MgAl O,indicating that the two catalysts were not easily reduced by H₂ and its redox performance was poor.It may be due to the fact that the state of K,Mg,Al metal ions in the catalyst was relatively stable and the valence state was not easily changed.The Pd-doped catalyst1Pd/7K/MgAlO showed a reduction peak at 71°C,indicating that the doping of Pd improved the redox performance of the catalyst effectively.?7?The NOx storage performance of the preferred Pd/K co-doped perovskite-type catalyst La_0.5K_0.5Mn_0.97Pd_0.03O₃ and Mg-Al hydrotalcite-based composite oxides catalyst 1.0Pd/7K/MgAl O were verified.The results showed that after stopping the O₂supply,the CO concentration of the Pd/K co-doped catalyst did not rapidly increase back to the initial concentration,but increased very slowly,and it was always lower than the initial concentration of CO.In the anaerobic environment,the production of CO₂ did not decrease rapidly,but remained at a higher concentration and then slowly decreased.It showed that after stopping the oxygen supply,the NOx stored in the catalyst was decomposed and decomposed,and then the redox reaction would occur with reducing gas CO and soot particles?PM?,generating CO₂,N₂ and H₂O.Therefore,the harmful gas was purified.?8?Through comparative analysis of the structural properties and reaction pathways of the catalysts of La_0.5K_0.5MnO₃ and 7K/MgAlO,we have clarified the reason of K increases the catalytic activity of soot combustion.The reason for K-doped perovskite-type La_0.5K_0.5MnO₃ catalyst enhances soot catalytic activity is that K promotes the generation of"oxygen vacancy",while K-doped Mg-Al hydrotalcite-based 7K/MgAlO catalyst is that K promotes the occurrence of"oxygen overflow".?9?Based on the results of the previous study,the best Pd,K co-doped perovskite catalyst La_0.5K_0.5Mn_0.97Pd_0.03O₃ and Mg-Al hydrotalcite-based composite oxide catalyst 1.0Pd/7K/MgAlO were selected.Then the two kinds of catalysts were coated on cordierite honeycomb ceramics pretreated with hydrochloric acid by using the catalyst powder slurry coating method.After packaging of the stainless steel,two kinds of monolithic ceramic honeycomb catalysts DPNR1 and DPNR2 were prepared,and the bench test was carried out.Through the analysis of the bench results,it was found that the DPNR2 encapsulated on the Mg-Al hydrotalcite-based composite oxide catalyst has a high CO removal efficiency,and the conversion rate was 62%,but the removal effect of NOx was not good,which was only about 9%.However,through the test results of the platform,we found that it was necessary to optimize the coating method of the catalyst and improve the accurate control of the engine working conditions.Besides,it was beneficial to the storage and reduction of the catalyst a working environment with alternating poor and rich oxygen was crested.