| 摘要: | 台灣有超過1000萬輛的機車，因此機車所排放的廢氣已變成一嚴重的環保問題。為了降低機車的廢氣排放，由於機車對於空燃比控制較不精準，操作條件也不穩定，往往由於劇烈操作或間斷性迴油造成大量未燃燒油氣排入排器管中，而在觸媒表面反應，由於大量放熱所產生的高溫有可能達到1100 oC以上，會對觸媒轉化器造成極大傷害。因此，提昇觸媒的高溫熱穩定性與低溫起燃特性，是未來機車觸媒轉化器研究發展的重點。 本論文首先利用微濕含浸法將熱安定添加劑加入洗覆層的主要成份γ-A12O3中，經1100 ℃，4小時處理後，由 BET比表面積與XRD繞射圖譜結果發現添加1~5 wt.%的La與5~10 wt.%的Ba對於γ-A12O3晶相的熱穩定性效果最佳。且添加熱穩定劑的Pt/γ- Al2O3 與 Pd/γ- Al2O3觸媒具有低起燃溫度及較佳的CO、C3H6轉化效率，而Pd/γ- Al2O3觸媒對C3H6的轉化率與熱穩定性較Pt/γ- Al2O3觸媒為佳。 其次針對不同製備方法(溶膠-凝膠法、共沉澱法、物理混合及含浸法)合成CeO2-Al2O3擔體，探討如何增進γ- Al2O3中添加劑二氧化鈰的熱穩定性。結果顯示以溶膠凝膠法合成之CeO2-Al2O3擔體，較其他合成方法合成之CeO2-Al2O3擔體，具有較高的表面積，其二氧化鈰的結晶顆粒最小，負載於其上的Pd分散度也最佳。由TPR結果顯示以溶膠凝膠法合成之Pd/CeO2-Al2O3觸媒有較低溫的還原峰，且於缺氧反應條件下，以溶膠凝膠法製備之觸媒對一氧化碳及碳氫化合物的氧化催化活性最好。但Pd/CeO2-Al2O3觸媒經950℃，3小時的高溫處理後，由於擔體燒結與貴重金屬分散變差，觸媒的活性變差。為了改善觸媒的熱劣化情形，以溶膠凝膠法合成的Pd/CeO2-BaO-Al2O3觸媒，其對一氧化碳及碳氫化合物的催化反應效果最好。 本文又嘗試以酸及鹼含浸處理氧化鋁及氧化鋯作觸媒擔體，再製成Pt及Pd觸媒，以瞭解酸鹼效應對於觸媒催化反應的影響。結果發現將活性金屬支撐於含浸NaOH的鹼性擔體觸媒上，對於當量點的CO及丙烯氧化反應有助於起燃溫度的降低，而含浸H2SO4的酸性擔體觸媒對CO及丙烯氧化反應則有抑制反應的效應。另外，將活性金屬支撐於含浸H2SO4的酸性擔體觸媒上，對C3H8氧化反應可使觸媒的起燃溫度降低，而含浸NaOH的鹼性擔體觸媒則有抑制C3H8氧化反應的效應。 由於鈣鈦礦型氧化物(Perovskite oxides，ABO3)結構穩定且耐熱性佳，因此本文探討La1-xSrxCo1-yMnyO3與含有Pd的 LaCo0.9-xPdxMn0.1O3鈣鈦礦型氧化物。結果顯示La1-xSrxCo1-yMnyO3觸媒對CO與C3H6的氧化反應較LaCoO3、LaMnO3觸媒為佳。EXAFS的分析結果發現，分別以鍶及錳取代部分之鑭及鈷的LaxSr1-xCoyMn1-yO3 氧化物，可以增加鈷周圍之氧空缺，提昇鈣鈦礦型氧化物對於CO及丙烯的氧化活性。且將La、Co、Mn、Pd鹽依比例同時含浸於γ- Al2O3中，組成為LaCo0.84Pd0.06Mn0.1O3 (LaCo0.84Pd0.06Mn0.1O3/Al2O3)，以700℃煅燒2小時。LaCo0.84Pd0.06Mn0.1O3/Al2O3觸媒較Pd/CeO2-Al2O3 及 Pt-Rh/CeO2-Al2O3觸媒反應性及熱穩定性佳。而由ECE-R47法規測試的結果顯示LaCo0.84Pd0.06Mn0.1O3/Al2O3觸媒對CO與HC的轉化率與商用的Pt-Rh/CeO2-Al2O3機車觸媒相當。 最後以台灣50 cc二行程與125 cc四行程機車為例，說明為了因應未來日益嚴格機車排氣法規，未來機車排氣污染控制系統必須包含二次空氣導入及雙觸媒轉化器。 There are more than ten millions motorcycles in Taiwan. Exhaust emissions from motorcycles became a serious environmental issue. A common characteristic of motorcycles is operating in oxygen deficient conditions. For two-stroke motorcycles, short-circuiting loss and incomplete combustion result in high hydrocarbon (HC) concentration in the exhaust emission. On the other hand, the temperature of the catalyst may exceed 1100 ℃ when CO and HC are oxidized. In order to comply with the more stringent emission regulation in the future, this research will focus on the improvement of the thermal stability and light-off temperature of the catalyst. There are five parts in the thesis. In the first part, additives added by impregnation of γ- alumina support for improving the thermal stability is reported. BET surface area and XRD results reveal thatγ-Al2O3 exhibits improved thermal stability by adding 1-5 wt % La or 5-10 wt% Ba after aging at 1100 ℃ for 4 hours. The additives can also reduce the light-off temperature and enhance the conversion of CO and C3H6 for Pt/γ-Al2O3 and Pd/γ-Al2O3. Under oxygen deficient condition, the conversion of CO over Pt/γ-Al2O3 is higher than that of Pd/γ- Al2O3. While the conversion of C3H6 is higher over Pd/γ-Al2O3, and better thermal stability is reported for Pd/γ- Al2O3. Improvements of thermal stability of cerium oxide additives in alumina (CeO2-Al2O3) prepared by different methods (impregnation, sol-gel, co-precipitation and physical mixing) are investigated in the second part. The results show that CeO2-Al2O3 prepared by sol-gel method exhibits the largest surface areas, highest Pd dispersion and smallest CeO2 crystalline size than other methods. TPR results also show that the reduction of Pd/CeO2-Al2O3 prepared by sol-gel method shift to lower temperature. In addition, Pd/ CeO2-Al2O3 prepared by sol-gel method exhibits higher activity for CO and C3H6 oxidation. However, the activity of Pd/ CeO2-Al2O3 was inevitably deactivated due to thermal aging at 900 ℃ for 3 hours. Improvement of thermal stability can be achieved by using CeO2-BaO- Al2O3 prepared by sol-gel method as the support for Pd. The catalyst also exhibits better catalytic activity for CO and HC oxidation. The effects of acid and base treatment of alumina and zirconia support on the light-off temperature of supported Pt and Pd are investigated in the third part. Under stoichiometric conditions, the activities of CO and C3H6 oxidation over the catalysts on supports impregnated with NaOH solution are promoted. In contrast, the activities are suppressed for catalysts on supports impregnated with H2SO4 solution. On the other hand, the activities for C3H8 oxidation are promoted over catalysts on supports impregnated with H2SO4, and are suppressed over catalysts on supports impregnated with NaOH. In the fourth part the activities and thermal stability of perovskite, La1-xSrxCo1-yMnyO3, and Pd containing perovskite LaCo0.9-xPdxMn0.1O3 are reported. La1-xSrxCo1-yMnyO3 exhibits higher activity than LaCoO3 and LaMnO3 for CO and HC oxidation. EXAFS results reveal that oxygen vacancy around Co ion in LaCoO3 and LaMnO3 increased by Sr, Mn, Co partially substitutes La, Co, and Mn, respectively. The activity is related to oxygen vacancies. The activity of propylene oxidation was furthering promoted when LaCo0.9Mn0.1O3 containing a small amount of Pd. The best activity is observed for the composition of LaCo0.84Pd0.06Mn0.1O3. La, Co, Mn, Pd salts solution impregnated into alumina simultaneously and calcined at 700 ℃for 2 hours with the composition LaCo0.84Pd0.06Mn0.1O3 (LaCo0.84Pd0.06Mn0.1O3/Al2O3) exhibits higher activity and thermal stability than Pd/CeO2-Al2O3 and Pt-Rh/ CeO2-Al2O3. The test results of ECE-R47 driving cycle also reveals that HC and CO conversions on LaCo0.84Pd0.06Mn0.1O3/Al2O3 is close to those on Pt-Rh/CeO2-Al2O3, which is a typical composition of the commercial catalyst for motorcycles. In the last part, potential emission control strategies are proposed for motorcycles. For 50 cc two-stroke motorcycles and 125 cc four-stroke motorcycles in Taiwan, the emission control system should include secondary air input and multi-type catalysts in order to comply with the stringent emission regulation in the future. |
