| Diesel engines are more popular due to the relatively high thermal efficiency andfuel economy than gasoline engines. Diesel engine emissions have led to seriousenvironmental problems especially their carbon particles content. The small size ofdiesel soot particles(≤2μm) may be linked to a number of health problems by itsability to penetrate the body through the respiratory system.The non-catalytic ignition temperature of soot generally exceeds 550℃, while thetemperature of typical exhausts is 400℃or below in light duty applications. Thecombination of traps and oxidation catalysts appears to be the most plausibleafter-treatment technique to eliminate soot particles. However, the temperature ofdiesel exhaust is in the range of 175-400℃. To lower the soot ignition temperature,one solution is using a catalyst-based DPF. The catalysts used in a catalytic DPFshould display good activity in the temperature range of diesel exhaust, and goodstability and durability under practical working conditions. Many attempts have beenmade to develop the useful catalysts that promote soot oxidation, whereas there is noeffective catalyst like three-way catalysts that can be applied in practice.In this paper, V2O5-KCl and NH4VO3-KNO3 catalysts have been prepared byimpregnation and coating method, V-ⅡA, V-Ln, Co-ⅡA, Sm-ⅡA, and Ce-Zrcatalysts have been produced by grinding method, V-ⅡA-K, Ln-K, V-Ln-K,Co-ⅡA-K, Sin-ⅡA-K, and Ce-Zr-K catalysts have been prepared by coating method.In addition, Ce0.5Zr0.5O2 have been prepared by co-precipitation, sol-gel andpolymer-network gel methods. XRD and TG-DSC have been used to characterize the catalysts, and their catalytic activities have been evaluated by a temperatureprogrammed reaction system (TPR). The surface morphologies of catalysts and sootparticles have been observed by SEM. The morphologies of Ce-Zr catalysts powderprepared by various methods have been observed by TEM, and the BET has beenmeasured too. The relation between the structure and catalytic activity has beenanalyzed. Otherwise, the effect of contact mode between catalysts and soot on catalyticactivity has been studied, and the catalytic mechanism has been explored.The catalytic activity of NH4VO3-KNO3 catalysts prepared by coating method isbetter than those prepared by impregnation method. As the catalysts coated on thesubstrate can form accidented surface and increase the contact area between thecatalyst and the soot. However, the catalyst prepared by impregnation can enter intosubstrate and the surface is relatively smooth, which lead to poor contact between thesoot and the catalyst.The catalytic activity of KNO3 prepared by coating method is higher than that ofKCl. On the one hand, the melting KNO3 can enhance the contact between the catalystand the soot as the melting point of KNO3 is 330℃; on the other hand, the probablemechanism is that one in which the nitrate is reduced to nitrite by the reaction withcarbon and that the oxygen or the nitrogen oxide oxidize the nitrite again to nitrate.The soot onset ignition temperature is 362℃by the synergistic catalysis of KNO3with KVO3 for NH4VO3-KNO3 catalyst with a molar ratio of 1:1, which is lower thanthat of single KNO3 or KVO3. The catalytic activity of NH4VO3-KNO3 catalysts islowered with the increase of KNO3 content. One reason is that the higher KHCO3content hinders the contact between the catalyst and the soot; the other is that thesynergistic catalysis is lowered with the decrease of KVO3.The soot onset ignition temperature is relatively high, and soot oxidation rate isslow for V-K catalysts. In addition, the V-K catalysts are unstable due to the lowermelting point of KNO3. The V-K catalysts added alkaline earth metal oxide or nitratecould improve the stability of catalysts, and increase the KNO3 content avoiding thedeterioration of catalytic activity led by the loss of partial KNO3. The catalytic activityof CaO/MgO is worse than that of Sr(NO3)2/Ba(NO-3)2 which are oxidant. The catalyticmechanism of Sr(NO3)2/Ba(NO3)2 is: M(NO3)2+C=M(NO2)2+CO2 (1) 2M(NO2)2+C=2MN2O3+CO2 (2) 2MN2O3+O2=2M(NO2)2 (3) M(NO2)2+O2=M(NO3)2 (Mis SrorBa) (4)The soot onset ignition temperature of-Ⅴ-ⅡA-K catalyst with a molar ratio of 1:1:6 is the lowest among the prepared V-ⅡA-K catalysts.The catalytic activity is improved by adding KNO3 to rare earth metal oxide, Thecrystal lattice defect formed by partial K+ entering into the rare earth metal oxide canpromote the soot combustion. The soot onset ignition temperature order of Ln-Kcatalysts is as follows: Ce-K(330℃C=Pr-K(330℃)＜La-K(350℃)=Y-K(350℃)＜Sm-K(365℃). The soot oxidation rate comply to the sequence: Sm-K＞Ce-K=La-K＞Y-K＞Pr-K。The catalytic activity of V-Ce/Pr-K catalyst with a molar ratio of 1:0.25:2 isalmost the same as that of Ce/Pr-K catalyst with a molar ratio of 1:2. The catalyticactivity of V-La/Y-K catalyst with a molar ratio of 1:0.5:2 is higher slightly than that ofLa/Y-K catalyst with a molar ratio of 1:2. The catalytic activity of V-Sm-K catalyst ishigher obviously than that of Sm-K catalyst with a molar ratio of 1:2.The effect of contact mode between the catalyst and the soot has an importanteffect on the catalytic activity of Co-ⅡA-K catalysts, especially on the soot oxidationrate. The soot can react with crystal lattice O only in tight contact condition made bygrinding, while the soot oxidation rate becomes slow in tight contact condition madeby melting KNO3. Although it can improve the contact condition between the soot andthe catalyst, the melting KNO3 can encumber the synergistic catalysis of severalcrystalline phases. The catalytic activity of Co-ⅡA-K catalyst with a molar ratio of0.5:0.25:2 is the best, and the soot onset ignition temperature is almost same whetherin tight contact or in loose contact condition, while the soot oxidation rate is morerapidly in tight contact condition than that in loose contact condition. The catalyticmechanism of Co3O4 is: 2Co3O4+C=6CoO+CO2 (5) 2CoO+C=2Co+CO2 (6) 2Co+O2=2CoO (7) 6CoO+O2=2Co3O4 (8)The catalytic activity of Sin-ⅡA-K catalyst is good. The soot oxidation rate isquickened by adding MgO/CaO/Sr(NO3)2/Ba(NO3)2, and the cold boot performance isimproved by adding Sm2O3. KNO3 can enhance the contact between catalyst and soot,and act as an oxidant. The soot onset ignition temperature is 318, 323,332, aad 325℃when the weight ratio ofKNO3-Mg, KNO3-Ca, KNO3-Sr and KNO3-Ba is 8.41, 5.78,7.57, and 7.36 respectively for Sin-ⅡA-K catalysts. In addition, the soot oxidation rateof the Sin-ⅡA-K catalysts is more rapidly than that of V-Sm-K catalysts.The catalytic activity of Ce-Zr catalysts is higher than Zr0.5Ce0.5O2 prepared byco-precipitation, sol-gel and polymer-network gel methods, wfiich shows that the synergistic catalysis of ZrO2-CeO2 is better thafi that of Zro.sCeo.502. The catalyticactivity of all the Ce-Zr catalysts is higher in tight contact mode than that in loosecontact mode. The soot onset ignition temperature of Ce-Zr catalyst prepared bygrinding method in tight contact condition.is lower than the catalysts containing KNO3...
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