機車使用觸媒熱劣化現象之探討

Abstract

機車為我國廣為應用之交通工具，然而以往移動源研究對象以汽車為多，機車相關研究十分有限。以往探討三元觸媒轉化器劣化機制 之研究，亦多侷限以汽車為對象。有鑑於此，本研究利用配製模擬廢氣及實驗室加速劣化程序，檢討操作溫度、操作時間及廢氣含氧量對三元觸媒轉化器之影響。本研究利用排氣分析儀配製模擬廢氣通入觸媒中並加以高溫進行劣化程序，再將經劣化後之觸媒樣品，進行特性分析及相關檢討。 觸媒因高溫操作導致熱劣化之機制主要分為載體燒結、活性金屬 燒結及具有活性之結晶因高溫轉變成不具活性。由比較面積分析儀、X光粉末繞射儀、掃瞄式電子顯微鏡—能量分散光譜儀分析結果顯示：在機車操作溫度範圍內，主要熱劣化機制為載體燒結，不論在何種劣化條件下，觸媒之比表面積及孔徑分佈隨劣化溫度提高而往低比表面積及大孔徑分佈方向成長。 在不同劣化條件劣化之觸媒，其起燃溫度曲線呈現不同特性的改變，經5 小時劣化之觸媒活性測試結果與其他條件劣化之觸媒呈趨勢不規則之變化。經當量點及氧化性劣化之觸媒活性測試結果相似，而經還原性劣化之觸媒，活性指標T75 優於其餘二者。溫度與廢氣含氧量對觸媒熱劣化影響較劣化時間顯著。在本研究設定之劣化條件範圍內，本研究建構之模式可合理推估觸媒經長時間操作後之比表面積變化情形。

Motorcycles constitute important parts of air pollution emission in Taiwan. However, only limited studies were focused on the deactivation mechanisms of three-way catalyst converters used in motorcycles. Furthermore, the performance of deactivated catalyst converters on motorcycles has not been extensively studied. In this study, thermal aging of the catalyst is simulated in laboratory scale. Based on results of several surface characterization techniques, the effects of aging test exhaust, temperature, and time were examined. Thermal deactivation mechanism is a combination of several aging phenomena, including sintering of active phase, the collapse in surface area and the thermal deactivation of solid phase. According to the measurement results of specific surface area, XRPD, SEM-EDS, the main mechanism of thermal deactivation is deduced as the collapse in surface area. For all thermal aging operating modes studied, the temperature increase reduced the specific surface area and increased the pore size. For all thermal aging operating modes studied, light-off curves of aged catalysts were different. The activity test results of 5 hours aged catalysts were differ from others. The results of activity test of during stochiometric and oxidative aging were similar, and the activity indicator, T75 of during reductive aging catalysts, was lower than the others. The aging temperature and composition of test exhaust are more effective than aging time. The constructed model has good performance on predictions within the thermal aging operating range studied.