Catalytic Removal Of Soot And NOx Over Co- Or Mn-based Hydrotalcite-derived Complex Oxides Catalysts

Soot and NOx are main pollutants in the emission of diesel engines. How to decrease the soot combustion temperature and increase the NOx reduction efficiency is important for the elimination of these pollutants. Highly effective oxidation catalysts can remarkably decrease the activation energy of soot combustion reaction, and the catalysts with reductive active sites can increase the efficiency of NOx reduction. In this dessertation, Co- and Mn-based hydrotalcite-derived catalysts, as well as the K-promoted ones, are selected. The composition of catalysts, the loading of K and the calcination temperature are optimized. Moreover, the stored NOx species were investigated carefully by in situ DRIFTS in order to reveal the reaction mechanisms.Firstly, hydrotalcite-based CoMgAlO mixed oxides were synthesized by pH-constant coprecipitation and then impregnated with K. All the catalysts display good catalytic activity in the range of 200⁴00 oC. When the weight loading of K reaches 4.5 %, prominent enhancement effect was observed. The addition of K increases the amount of active Co sites, leading to the high soot oxidation activity. Higher NOx trapping efficiency is also obtained over K-promoted catalysts. As dopant K mounts up, N-related species vary from chelating bidentate nitrates to monodentate nitrates and ionic nitrates gradually. The main storage phase is K2O.The loading of K is fixed at 4.5 wt.%. Further researches were carried out on the influence of calcination temperature. High catalytic performance could be maintained on K-promoted catalysts. The strong interaction between K and Co induces the formation of the active oxygen species in a new K-Co-O phase, which readily reacts with soot. Continuous transferring of oxygen from gaseous phase ensures the high soot combustion activity. The NOx reduction activity can also be improved. The catalyst calcined at 600 oC shows the highest NOx removal efficiency (³2%), attributing to the strong interaction between K and Co. Therefore, NO molecules can be easily oxidized to NO2 by the referred active oxygen species and then stored as nitrates on K species, which can be reduced by soot.Moreover, the performance of hydrotalcite-derived MnMgAlO catalysts was evaluated. The influence of the Mn content on the activity of the catalysts is carefully studied. Mn1.5Mg1.5AlO and Mn1.0Mg2.0AlO show the best catalytic performance for soot combustion. As dopant Mn increases, major Mn-related species vary from MnAl₂O₄ and Mg₂MnO₄ to Mn₃O₄ and Mn₂O₃. The highly reducible Mn⁴⁺ in Mg₂MnO₄ is most active for soot combustion. The Mn_1.0Mg_2.0AlO exhibited the highest activity for NOx removal reaction. The existence of Mn⁴⁺ in Mg₂MnO₄ can enhance the formation of ionic nitrates; the Mn²⁺ in MnAl₂O₄ is proposed to be the active site for NOx reduction. The coexistence of appropriate amounts of Mn²⁺ and Mn⁴⁺ is beneficial to the whole reaction.Finally, Mn_1.5Mg_1.5AlO catalyst promoted by K was investigated and the effect of K was also studied. For soot combustion, when the dopant K was lower than 10 wt.%, the average soot oxidation rate is lowered. The spillover mechanism is proposed. While when the content of K is higher than 10 wt.%, the characteristic temperatures of soot combustion are decreased. NOx-aided gas-phase mechanism was proposed. As dopant K increases, the NOx reduction percentage reaches the maximum value at the point of 7.5 wt.% of K. The introduction of proper content of K can facilitate the formation of monodentate nitrates, which show high reactivity with soot, leading to the higher removal efficiencies. When the K content is lower than 10 wt. %, the strong interaction between K and Mn species in K₂Mn₄O₈ phase stabilizes the K species, making the catalysts possess higher thermal stability, in the view of application, these catalysts are more suitable for practical use.