機車觸媒轉化器劣化現象之研究

Abstract

機車引擎排放廢氣為台灣主要空氣污染來源之一，其空氣污染物排放貢獻約為CO：19%、HC：11%及NOx：3%。我國第三期空氣污染物排放標準規定87年起所有新出廠的機車都要加裝觸媒轉化器，以降低機車引擎排放污染量。然而使用過程中高溫的操作環境與引擎排放廢氣中所含不純物將造成觸媒轉化效率隨著使用時間的增長而衰減，致使排氣污染情形惡化。因此本研究之主要內容為了解機車觸媒轉化器於使用過程中可能遭受之劣化機制以及觸媒轉化器表面特性與處理效能之變化，並藉由實驗室劣化模擬程序以探討操作溫度、時間及引擎排放廢氣中所含不純物對觸媒轉化器特性之影響。 本研究針對不同車齡及里程數機車觸媒轉化器特性分析結果顯示，熱劣化與毒化為機車觸媒轉化器之主要劣化機制，且劣化現象隨著車齡及里程數的增加而更加顯著。熱劣化及毒化機制造成使用過觸媒孔洞損失，且孔隙尺寸往大孔徑移動，觸媒比表面積減少，並影響觸媒之效能。 熱劣化模擬試驗中探討操作溫度與劣化模擬時間對觸媒特性之影響，其中操作溫度越高，熱劣化現象越顯著。觸媒比表面積損失情形有隨操作溫度上升而增加之趨勢：

Motorcycle engine exhaust is one of the major sources of air pollutants in Taiwan. Their pollutant emissions were evaluated to contribute about 3%, 11% and 13% to NOx, HC and CO emissions respectively. Catalytic converters have been enforced to be assembled in exhaust pipes of motorcycles produced later than 1998, to reduce pollutant emissions. Nevertheless, catalytic converters were reported as becoming deactivated, which was attributed to operation conditions of high temperature and impurities. Consequently, motorcycles equipped with those deactivated catalytic converters cause severe impact on the environment. In this study, catalytic converters used for different ages and mileages were characterized by forms of analytical techniques to investigate the deactivation phenomena. The laboratory aging tests were performed to simulate the real operation conditions of motorcycle catalytic converters and determine the effects of operation temperature, operation time, poison’s type and poison’s concentration on catalytic properties. According to the analytical results of catalysts used for different ages and mileages, thermal deactivation and poisoning were described as the main deactivation mechanisms. The aging induced phase transitions, loss in specific surface areas and pore volume, the growth in the pore size and the deterioration of catalytic performances were observed. The accumulation of the used time stands for the increase in the deactivation potential. Thermal deactivation tests were performed to determine the effects of operation temperature and operation time on catalytic properties. The thermal deactivation phenomena became more significant with the increase in the operation temperature. The loss in specific surface area exhibited an increasing trend as: