| Diesel exhaust particulate matter is one of the main sources of fine particulate matter(PM2.5) in the atmosphere, and its formation mechanism and control technology have become research focuses. The present study offers an opportunity to systematically understand the effects of typical diesel after-treatment devices(DOC-diesel oxidation catalyst,POC-particle oxidation catalyst) on particle size distribution, microscopic physic-chemical properties, oxidation activity of the diesel particles, and the relations among them under various operating conditions. The results have important theoretical significance to understand the nature as well as formation and evolution mechanism of particulate matter, and potential practical value to guide the purification technology applications.In order to investigate the impacts of typical after-treatment devices on particles' oxidation properties, this dissertation studied the transformation of the following influence factors of particle oxidation activity: the percentage composition of SOF(soluble organic fractions) in diesel particles, the particle size distribution, the types and the corresponding relative amount of particles' various surface functional groups, and the order degree of particles' micro structure. The transient operating conditions of engine were substituted by various steady working conditions to simplify the investigation. Firstly, the samples were collected under certain steady condition at the three sample positions located before DOC, after DOC, and after POC, respectively. Then the sample characterizations were performed on devices or instruments such as Soxhlet's extractor, electronic microbalance, engine exhaust particulates size spectrometer, Fourier transformation infrared spectrometer and micro Raman spectroscopy to study the transformation of micro physic-chemical properties of diesel particles. Finally, the thermo-gravimetric analysis was applied to analysis the oxidation properties of diesel particles.The major achievements obtained through the above experiments and analyses are listed as follows. From the macroscopic aspect, it can be seen that part of soluble organic fractions(SOF)on particles can be removed by DOC; and the effect of DOC on the total particle number concentration and particle size distribution can be ignored under most of the operating conditions. Then the POC further reduced the amount of SOF under small load or/and low speed; while under large load or/and high speed conditions, the formation of nuclear particles increased, resulting in the increment of SOF content after the POC. From the aspect of microscopic physic-chemical properties, the DOC reduced the relative amount of methyl and methylene(compared with oxygen-containing functional groups) and sharply diminished the peak area ratio of C=O structures and C-O structures; although the order degree of particles didn't experience a large change in the DOC, the amount of disordered graphene crystals increased. The particles collected after the POC exhibited the increase of the ratio of methyl/methylene and were far more disordered with much larger amount of organic molecules, functional groups and amorphous carbon, as well as the disordered grapheme crystals. The results from thermo-gravimetric analysis indicated that under small load or/and low speed, the ignition temperature and activation energy showed a significant decrease after the influence of the DOC and POC. However, the numerical values of these two parameters firstly increased with the effect of the DOC then decreased after the POC under large load or/and high speed. The activation energy of particles collected after POC was larger than that of original diesel particles under these large load or/and high speed operating conditions. |
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