| Diesel engine is widely used by virtue of its favoxurable power performance,economy and durability. However, the pollutants in the exhaust gas, especially the emissions of particulate matter (PM) and nitrogen oxides (NOx), are threat to human health and to the environment. It is difficult to simultaneous remove both just rely on internal purification technology. In order to meet the increasingly stringent emission regulations,it is of great research significance to further explore the diesel engine aftertreatment technology to reduce PM and NOx simultaneously. The cerium-based catalyst has a unique structure and exhibits a good catalytic activity for reducing PM and NOx. In this paper, we first studied the crystal structure of CeO2 and Ce1-xMnxO2 by density functional theory (DFT) and revealed the catalytic properties of CeO2 andCe1-xMnxO2 catalysts from the molecular structure level. Then CeO2 catalyst was prepared by precipitation method. The effects of catalyst particle size on its physical and chemical properties and catalytic properties were compared by XRD, SEM, BET,H2-TPR, catalytic activity evaluation system and thermal dynamic analysis. The catalytic performance for soot oxidation and NOx reduction were investigated. On this basis, CeO2 catalyst was doped with Mn by sol-gel method. The effects of calcination temperature and Mn doping concentration on the activity of the catalyst were analyzed. Finally, CeO2 and Ce0.5Mn0.5O2 with the best activity were used as theresearch objects, coated on the carrier and encapsulated, and then the engine bench test with catalytic aftertreatment was carried out. In the actual diesel engine operation,the effect of catalyst on PM and NOx in the exhaust gas was also studied. The specific research works are as follows:(1) The crystal structures of Ce02 and Ce1-xMnxO2 were studied by DFT. Theoxygen vacancies on CeO2( 110) surface and the adsorption of NO on clean and with oxygen vacancies CeO2(110) surface were calculated. The results showed that adsorption energy of NO on clean CeO2 (110) surface was lower, which was onlyphysical adsorption. When there was oxygen vacancy on CeO2(110) surface, the adsorption energy was increased obviously, which was only chemical adsorption. The N-O bond length was elongated after adsorption, which can promote the NO dissociation.(2) The effects of different Mn doping concentration on the crystal structure,oxygen vacancy formation energy and electronic structure of CeO2 were studied. The results showed that the formation energy of oxygen vacancy can be greatly reduced after Mn doping CeO2, which indicated that Mn doping can enhance its oxygen-releasing ability, thus enhancing its redox capacity. When Mn concentration was 50%, the oxygen vacancy formation energy can reach a minimum value of 0.31eV, and Mn entered the CeO2 lattice to form solid solution with high structure stability.Mn doping made a large number of redundant free electrons deposited on the bottom of conduction band, increased the possibility of CeO2 price changes after doping Mn,and also can provide more empty space to accept incident electrons,resulting in reducing capacity decreased, thus showing a larger oxygen storage capacity.(3) Ce02 catalyst was prepared by precipitation method. The CeO2 microstructure and catalytic activity for soot oxidation and NOx reduction were investigated by XRD, SEM, BET, H2-TPR, catalytic activity evaluation system and thermal dynamic analysis. The results showed that the crystal structure of prepared CeO2 samples were still cubic fluorite structure, and the appearances of particles were relatively uniform, which were mostly spherical or near spherical particles with clear boundaries and better dispersion. The particle sizes of CeO2 samples were 7, 12, 20 nm, which were significantly lower than particle size (100 nm) of commercial CeO2 catalyst. Moreover, compared with commercial CeI2, specific surface area of the prepared CeO2 was relatively large. The ignition temperature and the peak temperature of the soot particles with CeO2 catalyst were greatly reduced, and the activation energy of the particles was greatly reduced and the pre-exponential factor was increased. With the increase of temperature, the NO conversion increased firstly and then decreased. The NO conversion rate was all higher than 60% at 3 50?450?,which proved that the prepared CeO2 has a wide temperature window.(4) Ce1-xMnxO2 (x=0.03, 0.125, 0.25, 0.5) catalysts were prepared by sol-gel method. The effects of Mn doping concentration and catalyst calcination temperature on soot and NOx catalytic activity were investigated. And the thermal dynamic analysis method was used to calculate the thermogravimetric results of diesel particle oxidation. The results showed that the weightlessness curves of diesel particle shifted to the low temperature region with the increase of Mn doping concentration at the same calcination temperature, which indicated that the particle ignition temperature and peak temperature decreased gradually. This showed that the oxidation activity of Ce1-xMnxO2 catalyst was improved after adding Mn and reached the optimum when the Mn concentration was 50%. When the Mn doping concentration was the same, the catalytic oxidation of soot was best under the calcination temperature of 450?.Therefore, the activity of Ce0.5Mn0.5O2 catalyst prepared at 450? was the best, and the corresponding soot oxidation ignition temperature and peak temperature were 158? and 381?, respectively. N2 conversion rate increased with the increase of Mn concentration. When the Mn concentration was 50%, the N2 conversion reached a maximum of 83% at 375?. Compared with the pure CeO2, the activation energy of soot oxidation reaction was decreased and the pre-exponential factor was significantly increased, which indicates that the substitution of Mn significantly increased the number of active sites on soot and catalyst surface,thus enhanced the the catalytic activity for soot oxidation reaction.(5) The catalytic reaction mechanism of Ce1-xMnxO2 catalyst was revealed by means of XRD, BET, H2-TPR, XPS, Raman and UV-vis characterization from the microstructure level. The results showed that the cubic fluorite of CeO2 crystal structure did not change after being doped with Mn, the structure was still stable. The partial substitution of Mn inhibited the growth of catalyst grains and increased the catalyst specific surface area. Mn-doped CeO2 increased the lattice defects and enriched the oxygen species, which was favorable for adsorption, migration and transformation of oxygen. And lattice oxygen increased with the increase of Mn doping concentration, while lattice oxygen had stronger adsorption and activation on NO. Mn doping was also beneficial for electrons from valence band excitation to conduction band, prompting more electronic-hole pair, which led to higher catalytic performance.(6) Ce1-xMnxO2 catalyst was coated on DPF carrier and encapsulated, and then the catalytic aftertreatment engine bench test was set up. In the actual diesel engine operation, the effect of PM and NOx in the exhaust gas was studied and an off-line regeneration test of DPF was carried out by a regeneration test system. The results showed that Ce1-xMnxO2 catalyst had a remarkable catalytic effect on HC,CO, PM and NO, emissions in the ESC test cycle. The weighted oxidation efficiencies of CeO2 and Ceo.5Mn0.5O2 for HC specific emissions were 38.1% and 49.2%, respectively. The weighted oxidation efficiencies for CO specific emissions were 44.6% and 53.8%,respectively. The weighted oxidation efficiencies for PM specific emissions were 29.1% and 38.6%, respectively. The catalytic conversion efficiencies for NOx were 16.6% and 28.7%, respectively. At the speed of 2230 r/min and full load, the oxidation efficiencies of Ce0.5Mn0.5O2 and CeO2 were 55.1% and 37.6%, respectively.CO2 volume fraction of DPF-CeO2and DPF-Ce0.5Mn0.5O2 showed a trend of decline after increasing first and then gradually flattened through offline regeneration experiment. CO2 volume fraction of DPF-Ce0.5Mn0.5O2 changed greatly and back pressure dropped fastly, which indicates it had an excellent regeneration effect. |
PDF Full Text Request