氮氧化物在鉑/氧化鋇/氧化鋁觸媒上的吸脫附行為及反應機制

Abstract

氮氧化物(NOx)對人體健康與環境皆具有危害，因此將其還原為無害的氮氣以降低排放，已成為當前迫切且必要的課題。氮氧化物儲存及還原(NOx storage/reduction, NSR)是經常使用在稀薄燃燒引擎的技術，稀薄燃燒引擎雖然提高了燃油效率，但造成了大量的氮氧化物排放，而NSR技術就是在氮氧化物豐富的稀燃階段進行氮氧化物的儲存，並在富燃階段將儲存的氮氧化物還原，而氮氧化物的儲存及升溫釋放對於NSR來說是很重要的步驟。過去對氮氧化物的吸脫附行為的研究，多集中於低溫吸附後再升溫脫附，因此本研究希望利用程序升溫脫附(temperature-programmed desorption, TPD)研究在高溫下的吸脫附行為，以符合多數處理氮氧化物的溫度環境，並透過原位擴散反射式紅外光光譜儀(in-situ DRIFTS)來了解在高溫時氮氧化物的儲存機制。 本研究中選用氧化鋁(Al2O3)為擔體，以含浸法(impregnation method)擔載1%的鉑(Pt)及20%的鋇(Ba)製備1Pt/20BaO/Al2O3、20BaO/Al2O3、1Pt/Al2O3及Al2O3等觸媒，探討一氧化氮、一氧化氮及氧氣和二氧化氮等氣體在觸媒上的吸脫附行為及儲存機制。NO-TPD結果顯示，NO氣體可於350 oC下在Al2O3表面及Ba金屬上形成表面硝酸鹽，其中，在Al2O3表面上的硝酸鹽脫附行為以NO2與O2共脫附的形式進行，而Ba金屬表面的硝酸鹽則是單獨以NO₂形式脫附。此外，NO亦可與Ba生成體相硝酸鹽Ba(NO3)2，其脫附產物為NO與O2，且脫附溫度較高，顯示其具有較佳熱穩定性。NO/O2-TPD的結果顯示O2的加入有助於硝酸鹽吸附量提升，但不顯著改變脫附行為。NO2-TPD的結果說明，NO2進氣則能大幅增加儲存量，並促進體相硝酸鹽生成，尤以含Pt與Ba之觸媒最為顯著。低溫50 °C的環境下，NOx吸附量雖有提高，但會使熱穩定性較差之表面硝酸鹽生成比例大幅增加。 本研究以原位紅外光光譜探討儲存機制，結果顯示NO可快速與Al2O3表面形成bidentate nitrate，並於無O2下於Ba中生成ionic nitrite，並進一步形成ionic nitrate。Al2O3表面的bidentate nitrate隨著吸附時間的增加，也會遷移至Ba表面，Pt則可以幫助NO的氧化與NO2解離，兩者共同增加NOx儲存效率。

Nitrogen oxides (NOx) pose serious threats to human health and the environment. Therefore, reducing NOx emissions by converting them into harmless nitrogen gas is essential. NOx storage and reduction (NSR) is a key technology used in lean-burn engines, which, while improving fuel efficiency, emit higher levels of NOx. NSR stores NOx during the lean phase and reduces it during the rich phase. In this study, high-temperature NOx adsorption–desorption behavior was investigated using temperature-programmed desorption (TPD) to better reflect practical engine conditions. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was also employed to clarify storage mechanisms. In this study, alumina (Al2O3) was used as the catalyst support, and catalysts were prepared by the impregnation method: 1Pt/20BaO/Al2O3, 20BaO/Al2O3, 1Pt/Al2O3, and Al2O3. The adsorption–desorption behavior and storage mechanisms of various gases , including NO, NO/O2, and NO2, were investigated. NO-TPD results showed that NO could form surface nitrates on both Al₂O₃ and Ba sites at 350 °C. The desorption products were NO2 and O2 from Al2O3, and only NO2 from Ba. Additionally, NO reacted with Ba to form bulk nitrate (Ba(NO3) 2), which desorbed as NO and O2 at higher temperatures, indicating greater thermal stability. NO/O2-TPD results revealed that the addition of O2 increased the total storage capacity without significantly altering desorption behavior. NO2-TPD results showed that NO2 greatly enhanced the formation of bulk nitrates, especially on catalysts containing both Pt and Ba. Although storage at 50 °C increased the adsorbed amount of NOx, it mainly led to the formation of thermally unstable surface nitrates. The storage mechanisms were further analyzed using in situ DRIFTS. The results demonstrated that NO rapidly formed bidentate nitrates on the Al2O3 surface and could also generate ionic nitrite on Ba even in the absence of O2, which subsequently transformed into ionic nitrate. Over time, bidentate nitrates migrated from Al2O3 to Ba surfaces. Pt facilitated both NO oxidation and NO2 dissociation, synergistically enhancing NOx storage efficiency alongside Ba.