| 摘要: | 目前全球暖化逐漸加劇，車輛數量卻不斷上升，車輛污染排放亦持續增加特別是機車數量的成長，因此各國對機車污染的排放限制是越來越嚴格，再加上能源的危機，使得政府與車廠對車輛油耗量降低受到相當重視，然目前讓油耗降低方法除改用替代能源外，最快速的就是將空燃比提高，利用較少的油量達到相同的動力，但傳統車輛污染控制的觸媒轉化器在高空燃比時，NOX有著相當低的轉化效率，故在法規要求且較低轉化效率的雙重壓力下，提升觸媒在高空燃比時的NOX轉化率是刻不容緩的，為此本研究利用NSR (NOX storaged Reduction)觸媒，在高空燃比的情況之下，探討該觸媒的活性，並將其實際應用於成車上，以評估其應用之可行性。 本研究利用Ba添加至Pt/Al2O3中形成Pt/Ba/Al2O3，與添加至Pd/Al2O3中形成Ba/Pd/Al2O3，視添加Ba之後觸媒為NSR觸媒，並進行活性與物性測試，由結果可知添加Ba之後對觸媒的物性影響相當少，且並不會造成明顯或大量表面積下降，僅約1%，且由XRD得知由於添加量較少且鍛燒溫度情況下晶相並無法明顯表現，但由SEM/EDS結果得知Ba確實存在於觸媒之中，另由金屬分散度實驗得知，在Pt觸媒製備過程中先將Ba與Al2O3合成後再添加貴金的方式，可提升其分散度，且Ba本身並無吸附任何CO，然由於Pd觸媒是待Pd/Al2O3合成完成後，再將Ba含浸上觸媒，故分散度測試結果中添加Ba時並無法有效提升。 NSR活性的測試中可以發現添加Ba之後無論是Pt/Ba/Al2O3或Ba/Pd/ Al2O3皆有提升NSR轉化率的效益，且Pt/Ba/Al2O3隨溫度上升，轉化率下降，Ba/Pd/ Al2O3則隨溫度上升，而維持穩定。最終成車測試的活性可以發現Pt觸媒與Pd觸媒對CO與HC的轉化率是相似的，且添加Ba之後CO與HC的轉化效率皆提升約12~13％，主要原因應為起燃點較低，污染物轉化提前的效果，然在NOX轉化效率的部分，雖然兩觸媒皆有提升轉化率效益，但Pt/Ba/Al2O3觸媒在本研究上顯然是優於Ba/Pd/Al2O3，差異約6%，主要原因應為本研究觸媒在成車測試時排氣溫度維持在350℃，對於Ba/Pd/Al2O3在NSR活性測試實驗中顯示需維持較高溫度才能穩定轉化率，因此在成車測試的最後一個cycle Ba/Pd/Al2O3與Pd/Al2O3才發生轉化率差異，且在金屬分散度的實驗亦發現Pd的分散度較低，因此造成吸脫附時並無法最有效率的進行轉化，故若未來應用於車輛上時需與車輛性能及特性進行搭配，才可達到最佳的轉化效率。 最後本研究已證實NSR觸媒在運用上確實能夠提升NOX的污染削減，但須配合電子噴射供油系統以於加速時變化空燃比達到NSR觸媒的效用。 In order to alleviate global warming, governments and vehicle makers endeavor to suppress carbon dioxide emissions and clean up mobile exhaust gases at the same time. Therefore, many countries limited the vehicle emissions by more serious emission standard, especially for motorcycles. Although motorcycles have adopted new EMS (engine management system) from automobiles, better fuel economy and less emission are still needed. The gasoline fuel lean-burn engine is one solution to enhance the fuel efficiency of vehicles. However, the A/F of the lean-burn engine is usually controlled above 18-22, it is difficult to reduce the NOX emission with the conventional catalyst of vehicle at this high A/F. To address the conflict of high fuel efficiency and low NOX emissions, a possible solution involves the use of a NOX storage reduction (NSR) catalyst in combination with mixed-lean engine operation. In this study, Pt/Al2O3 and Pd/Al2O3 are used as the conventional catalyst, while Pt/Ba/Al2O3 and Ba/Pd/Al2O3 are used as NSR catalyst after adding Ba. Experimental results indicate the BET surface areas of NSR catalyst were slightly smaller that of the conventional catalyst. The dispersion of Pt/Al2O3 and Pt/Ba/Al2O3 were obviously better than that of Pd/Al2O3 and Ba/Pd/Al2O3, indicating the dispersion of Pt/Al2O3 was increased by adding Ba. However Ba/Pd/Al2O3 has a different trend. It may be due to different preparation methods. Pt/Ba/Al2O3 impregnated Ba and Al2O3 in first step and caused the support (Al2O3) surface variation, but the Ba/Pd/Al2O3 impregnated the Pd and Al2O3 ahead. Hence, the dispersion degree was not effectively increased by adding Ba. The activity of NSR was improved by adding Ba, and the NOX conversion achieved with NSR catalyst (Pt/Ba/Al2O3) decreased with increasing temperature, However, the activity of Ba/Pd/Al2O3 increased with increasing temperature, the light-off temperatures of conventional catalyst were higher than NSR catalyst. Conversions of HC and NOX achieved with Pt/Ba/Al2O3 and Ba/Pd/Al2O3 were higher than that of Pt/Al2O3 and Pd/Al2O3, respectively, with A/F below 14.5, and the conversions of CO were similar in different A/F. In the real application to the vehicle, the CO and HC conversions achieved with NSR catalyst Pt/Ba/Al2O3 and Ba/Pd/Al2O3 increased about 12~13% compared with conventional catalyst. The reason should be the lower light-off temperature to make the pollutant convert earlier. The NOX conversion of Pt/Ba/Al2O3 catalyst was superior to Pt/Al2O3 by 12%, but the NOX conversion of Ba/Pd/Al2O3 just increased by 4.5% compared with Pd/Al2O3. This research proved that the NSR catalyst could improve the NOX conversion in lean-burn engine of motorcycle, but it is still needed to switch A/F to rich region at right time to improve the performances. |
