氧化鈰觸媒的硫化前處理及氨氣選擇性脫硝還原反應

Ceria Catalyst: Sulfurization Pretreatment and Selective Catalytic Reduction of Nitric Oxide with Ammonia

陳柏瑋(Po-Wei Chen)

指導教授 ： 游文岳

國立臺灣大學/工學院/化學工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103062

摘要

工廠與汽車所排放的氮氧化物(Nitrogen oxide, NOx)造成非常嚴重的環境汙染，如酸雨、光化學霧霾以及PM2.5，選擇性觸媒還原(selectivity catalytic reduction, SCR)反應通常被應用在脫除氮氧化物，V2O5-WO3/TiO2商業觸媒長久以來已經被廣泛的應用在SCR反應，然而五氧化二釩被世界衛生組織視為人類潛在的致癌物質，而被分類為group 2B，因此尋找可以替代V2O5-WO3/TiO2觸媒已成為一項急迫的工作。文獻報導指出CeO2由於具有強還原性質與氧儲存能力，因此成為具潛力的替代觸媒，最近研究指出在以在無氧條件(anaerobic)下以SO2硫化處理CeO2可以大幅提升反應活性。
本研究中，我們在無氧條件下以500 ppm SO2/Ar氣體硫化處理CeO2觸媒，改變硫化處理溫度與時間的變因: 在200、300、400與500 oC硫化處理30分鐘，以及在300 oC硫化處理5、15、30與120分鐘，並測試硫化處理後CeO2觸媒於選擇性脫硝還原(SCR)反應的活性，分析硫化處理溫度與時間的變因對硫化物在觸媒表面上形成的影響。我們發現硫化處理溫度200 – 500 oC改善觸媒活性的效果相同；在不同硫化處理時間下，以硫化處理30分鐘，觸媒活性即達到最佳表現，可能原因為CeO2表面的chelating sulfate達到飽和。CeO2主要為路易士酸性位點，而硫化處理後CeO2則為路易士與布羅忍斯特酸性位點，布羅忍斯特酸性位點使NH3吸附量大幅提升。在NH3升溫IR實驗中，我們確認表面sulfate為布羅忍斯特酸性位點，而sulfate並不影響原有的路易士酸性位點存在。CeO2上的NH3容易進行H-abstraction而形成N2O與NO等副產物，而硫化處理後CeO2則抑制NH3進行H-abstraction。由於NO吸附位點被sulfate占據，因此硫化處理後CeO2的NO吸附量大幅下降。在in-situ IR實驗中，我們可以得知CeO2上imide species容易與氣相NO反應，而nitrate卻不易與氣相NH3反應；硫化處理後的CeO2上之NH3與NH4+容易與氣相NO進行反應，然而硫化處理後的CeO2的NO吸附量較少，因此我們推測兩者主要皆以Eley-Rideal機制進行反應。

關鍵字

氧化鈰觸媒 ； 氮氧化物 ； 脫硝技術 ； 選擇性觸媒還原反應 ； 酸性位點 ； 硫化處理 ； 原位擴散反射傅立葉轉換紅外線光譜

並列摘要

Nitrogen oxide, NOx emitted by the factory and automobile has caused serious environmental issues, such as acid rain, photochemical smog, and PM2.5. Selectivity catalytic reduction (SCR) has been widely used as one of efficient way to remove NOx, and V2O5-WO3/TiO2 catalyst has long been a commercial catalyst which is adopted as SCR catalyst. Nevertheless, vanadium pentaoxide is recognized possible human carcinogens (group 2B) by world health organization, thus finding a substitute for V2O5-WO3/TiO2 catalyst has become an urging work. Some studies have reported CeO2 catalyst as a potential substitute for V2O5-WO3/TiO2 catalyst due to strong redox properties and oxygen storage abilities. Recently, some study reported that CeO2 pretreated with SO2 gas in anaerobic condition (without oxygen) can greatly improve catalyst activity.
In this work, commercial CeO2 catalyst pretreated in anaerobic condition with 500 ppm SO2/Ar gas flow for various time (5, 15, 30 and 120 min) at 300 oC, and at various temperature (200, 300, 400 and 500 oC) for 30 min were prepared for SCR reaction. We found that CeO2 sulfated at 200 – 500 oC for 30 min have almost the same improvement on SCR activity. With increasing sulfation time at 300 oC, the sulfated CeO2 has reach the maximum SCR activity for 30 min sulfation time, since chelating sulfate formed on CeO2 was saturated. CeO2 are mainly Lewis acid site. After sulfation treatment, the Brønsted acid site, which is considered to be sulfate, formed on CeO2, thus more NH3 adsorption amount for sulfated CeO2. Also sulfate formed on CeO2 wouldn’t affect Lewis acid site. NH3 would go through H-abstraction reaction and further produce NO and N2O side product on CeO2. After sulfation treatment, H-abstraction reaction of NH3 has been restrained on CeO2. Since NO adsorption sites were cover by sulfate, NO adsorption amount of sulfated CeO2 declined. In in-situ IR experiment, imide group on CeO2 easily react with gaseous NO, while nitrate doesn’t react with gaseous NH3. NH3 and NH4+ on sulfated CeO2 easily react with gaseous NO. However, the NO adsorption amount on sulfated CeO2 is quiet low. We may confirm that both CeO2 and sulfated CeO2 follow Eley-Rideal mechanism on SCR reaction.

並列關鍵字

CeO2 catalyst ； NOx removal ； selective catalytic reduction ； sulfurization treatment ； acid site ； in-situ IR

參考文獻

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