| The diesel exhaust belongs to the excess of oxygen and low temperature, which makes it difficult to reduce NOx to N2 by catalyst. It is a great significance to control the emission of harmful NOx gases. NOx Storage and Reduction(NSR) catalysts have yielded the most promising results to reduce NOx emission of light duy diesel engine due to high NOx conversion, no additional reducing agent and small size, etc. Large amount of NO and small amout of NO2 composed of Diesel NOx emissions. NO2 is much more easily to be stored than NO. Therefore, improving the NO2/NOx ratio in diesel engine exhaust will favour NOx storage. The study found that NO can be oxidized to NO2 by Diesel Oxidation Catalyst(DOC), which contributed to NOx storage process of NSR catalysts. Therefore, DOC combined NSR technology was put forward, and it may be a great help to reduce NOx emission from diesel engine.In this paper, a series of DOC Mn-Ce/γ-Al2O3 were prepared by citric acid gel method, and a series of NSR catalysts Ce-Ba/γ-Al2O3 and Pt-Ce-Ba/γ-Al2O3 were prepared by the incipient wetness method. These prepared catalysts were characterized using comprehensive detection technology. Then, performance of the DOC on NO oxidation as well as NOx storage capacity and NOx removal of NSR catalysts was evaluated by test-bed. NOx removal of DOC combined NSR technology system was evaluated. Finally, the oxidation capacity of CO, HC, and variation of NO2/NOx ratio of DOC as well as the NOx conversion of NSR catalyst were researched through bench test. Experiment of DOC combined NSR technology system for NOx removal from Diesel Engine were carry out to evaluate the performance of the system. The main work and conclusions are as follows:(1) A series of NSR catalysts Ce-Ba/γ-Al2O3 and Pt-Ce-Ba/γ-Al2O3 were prepared by the incipient wetness method, and a series of DOC Mn-Ce/γ-Al2O3 were prepared by citric acid gel method. These prepared catalysts were characterized using X-ray diffraction(XRD), Scanning Electron Microscope(SEM)/Energy Dispersive Spectroscopy(EDS), Transmission Electron Microscope(TEM), and X-ray Photoelectron Spectroscopy(XPS). The characterization results showed that Ce elments of 6Mn10Ce/γ-Al2O3 and 8Mn10Ce/γ-Al2O3 mainly exist as the form of large amount of Ce4+ and small amount of Ce3+. Mn3+ and Mn4+ coexist in Mn elments of DOC, and Mn3+/( Mn3++ Mn4+) ratio was about 1/3. The size in active components of NSR catalyst 10Ce15Ba/γ-Al2O3 has good uniformity. Active component was highly dispersed in the surface of γ-Al2O3 support. Ce elements of NSR catalysts mainly exist in Ce4+ and a small amount of Ce3+, and Ba elements are in Ba O form. Due to the interaction between Pt and Ce, the active substance in a spherical shape of the 10Ce15Ba/γ-Al2O3 catalyst changed into a long rod, and polygons. Pt0 and Pt2+ coexist in platinum of Pt10Ce15Ba/γ-Al2O3 catalyst.(2) Experiments were performed in the test-bed to research the NO conversion and NO conversion rate by DOC at different temperatures. At the same time, the influence of operating parameters(temperature, reduction time) on the NSR catalysts performance was evaluated. Then, DOC combined NSR technology were tested in the test bed to evaluate the NOx conversion. The resuls revealed that 6Mn10Ce/-Al2O3 catalyst among all DOC has the best NO oxidation ablity, and the maximum NO conversion was up to 83% at 300℃. NSR catalyst Pt10Ce15Ba/γ-Al2O3 has low NOx storage capacity at low temperature. As the temperature rises the NOx storage capacity of Pt10Ce15Ba/γ-Al2O3 was significantly improved. The NSR catalyst Pt10Ce15Ba/γ-Al2O3 has good NOx removal capability. And the maxmun NOx storage capability of the NSR catalysts was 554 μmol?g-1 at 400℃.The highest NOx conversion and maximum NOx adsorption conversion of the NSR catalyst were 58% and 74.8%, respetively. Under the treatment of DOC coordination, NOx storage speed and NOx desorption rate of NSR catalyst in DOC combined NSR technology system significantly increased. Due to strong oxidation ability of DOC, resulting in the reducing agent of NSR catalyst in DOC combined NSR technology system significantly declined, and causing the amount of the released NOx reduced to N2 decreased and the residual amount of NOx stored in the catalyst was much more. Therefore, the NOx conversion and the NOx absorption conversion of NSR catalyst in DOC combined NSR technology system significantly dropped.(3) The experiment on DOC combined NSR technology for NOx, CO and HC emissions removal of Diesel Engine under European steady state cycle(ESC) test cycle were researched through the bench test. The oxidation ability of NO, CO and HC by DOC as well as the variation of NOx emissions under the treatment of NSR catalyst was evaluated. The results of Bench test displayed that DOC has preferab NO conversion ability as well as CO and HC oxidization ability. Three highest conversion of them were 36%, 53% and 62%, respectively. Moreover, NO2/NOx ratio was effectively improved by DOC. NSR catalyst has a good NOx removal efficiency. Under the condition of light engine load, NOx conversion of NSR catalysts is higher, reaching to 36%. With the increase of the engine load and engine speed, NOx conversion gradually decreased. Under the condition of full engine load, the NOx conversion is only 16%. Compared with NSR catalyst, DOC combined NSR technology system could significantly improve the value of CO and HC conversion, and promote the NOx storage speed of NSR catalyst in the system. However, due to oxidation of the DOC, the amount of CO and HC which act as the reducing agent of NSR catalysts significantly decreased, and leading to lower NOx conversion of the system. |
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