Study On The Co-and Mn-based Oxides Catalysts Used For Catalytic Purification Of The Exhausts From Diesel Vehicles

Soot and NOx are the main pollutants in diesel emission. Catalytic purificationof diesel exhaust is one of the most efficient methods for the removal of thesepollutants. The key factor of catalytic technique is the exploration of highly effectivecatalysts which can decrease the soot combustion temperature and increase the NOxreduction efficiency. In this dessertation, Co-based hydrotalcite-derived catalysts andMn-based oxides catalysts are selected and applied to soot-NOx removal and sootcombustion, respectively. The doping amount of rare earth element, the preparationmethods of MnOx and the calcination temperature of catalysts are optimized.Moreover, according to the characterization results, the potential C-NO-O2reactionpathways were proposed for Co-based hydrotalcite-derived catalyst; the active oxygentransfer pathway was investigated for Mn-based oxide catalysts.Firstly, Ce-substituted hydrotalcite-based Co2.5Mg0.5Al1-x%Cex%O mixed oxideswere synthesized by pH-constant coprecipitation and applied for PM-NOx removal.The results of activity measurements indicate that the optimal Ce-substitution amountis8%, showing the best PM-NOx removal efficiency. The addition of Ce increases thespecific surface area, activates the surface lattice oxygen and enhances the reductivityof the catalysts. The results of in-situ DRIFTS revealed that NO was oxidized andstored as nitrates over the catalyst surface, promting the soot combustion.La element was also introduced into the layer of Co2.5Mg0.5Al1.0O to partlyreplace Al. It is found that La-substituted catalysts can lower the activation energy ofsoot oxidation, and improve NOx-soot reduction efficiency. The characterizationresults display that La can reduce the size of Co3O4crystallites, improve the redoxproperties and enhance the mobility of oxygen species over the catalysts, thuspromoting NO oxidation, storage NOx and the simultaneous NOx-soot removal.Moreover, by comparision studies, it is reavealed that the stored nitrates can directlyreact with soot, confirming the nitrate pathway for soot combustion is available.Manganese oxides with different valence were synthesized by differentpreparation methods and applied for soot combustion. It is found that the singlecrystalline Mn2O3prepared by precipitation of manganese acetate and ammoniumcarbonate displays the highest performance for soot oxidation. Compared with otherMn-related samples, Mn-Ac-NH4exhibits much better reducibility and uniformsponge-like nanostructure favorable to soot-catalyst contact. Meanwhile, Mn-Ac-NH4 possesses the most active oxygen species which can fastly transfer to soot surface,resulting in the best soot oxidation activity.By use of the optimal prepation method, different content of Ce was dopped intopure Mn2O3and applied for soot combustion. There are three kinds of Mn speciesover the catalysts; the reducibility of the catalysts is improved due to the interactionbetween Mn-Ce; the content of weakly chemisorbed surface active oxygen species isincreased and the mobility of oxygen species is also enhanced because of theformation of MnCeOx solid solution. Meanwhile, since the oxygen species can easilytransfer from catalyst to soot, the presence of NOx did not show obvious enhancementfor soot oxidation. The oxygen spillover mechanism is proposed for soot combustionover MnCeOx catalysts.Finally, a certain amount of La was introduced into the optimal MnCeOx catalyst.It is indicated that even calcined at high temperature, the MnCeLaOx catalysts stillshow high activity for soot oxidation. Although the high calcination temperature canresult in the decomposition of the MnCeOx solid solution, making the migration offormation of Mn2O3, the interaction of Mn-Ce is still present, enhancing the transferof oxygen species; meanwhile, the presence of structural promoter La can stabilize theactive component, as a result, the high catalytic activity is well maintained.