銀奈米粒子負載於沸石應用於甲醛吸附與降解

Abstract

由於甲醛是常見的室內揮發性有機物，其主要來源為居家房屋內的裝修建材，人體若長時間低濃度下接觸到甲醛，會對人體產生危害，輕則會造成皮膚或呼吸道感染，嚴重則會有致癌的風險。目前移除甲醛的方法中，觸媒氧化法是最受矚目的，因為此方法可以將甲醛直接氧化轉換成無毒的二氧化碳和水，然而能在室溫下催化此反應的的都是貴金屬觸媒，如: Pt, Pd, Au；因其價格昂貴，科學界與產業界都希望能找到取代性的觸媒材料，以達到降低成本的目的，並且能實際應用於產業中。 本論文是以含浸法將銀奈米粒子負載於不同離子交換的Y型沸石與鈉離子型的X型沸石上，利用X光粉末繞射(XRD)、恆溫氮氣吸脫附、程式控溫還原(H2-TPR)、誘導偶極電漿質譜(ICP-MS)、掃描式電子顯微鏡(SEM)、高解析穿透式電子顯微鏡(HR-TEM)、X光吸收光譜(XAS)、固態核磁共振光譜(NMR)、熱重分析(TGA)、傅立葉轉換式紅外線光譜(FTIR)來鑑定觸媒材料的性質。甲醛催化反應測試是利用固定床流體系統，在甲醛濃度為320 ppm的情況下觀察觸媒的催化效果，再依據行政院環境保護署所頒布的｢排放管道中甲醛標準鑑測方法-4-胺基-3-聠基-硫醇基-1,2,4三唑比色法｣ (NIEA A724.72B)檢測方法來偵測反應前後甲醛濃度的改變，並且探討不同離子交換之沸石、煆燒步驟、還原步驟以及不同矽鋁比之Faujasite沸石等變因對於催化結果所造成的影響。從固態核磁共振光譜圖可以發現MnY的沸石在離子交換的過程結構受到破壞，因此Ag/MnY是三種離子交換沸石中甲醛轉換率最低的，此外經過還原步驟後的銀觸媒其表面上的銀奈米顆粒相較只經過煆燒步驟的銀觸媒，從高解析穿透式電子顯微鏡圖中可以發現還原步驟讓銀顆粒變大，因此其催化效果較差。在本論文所探討的Y型沸石中，6Ag/NaY-c的催化效果最好，在反應溫度為100 ℃反應24小時後，有100%的甲醛轉換率，當反應溫度降為80 ℃時，仍有70%左右的甲醛轉換率，雖然在溼度為50%的條件下，其甲醛轉換率下降幅度比6Ag/HY-c大，但其催化效果仍然較好。相較之下，在反應溫度為80 ℃，6Ag/NaX-c催化效果較6Ag/NaY-c好，反應8小時後仍有80% 的轉換率。HCHO-TPD的結果顯示，NaX對於甲醛的吸附能力較NaY好，推測因為NaX的矽鋁比較NaY低，所含的鈉離子較NaY多，而鈉離子會幫助具有極性的甲醛分子吸附，增加與銀粒子作用的甲醛表面濃度，因此在本論文中發現使用矽鋁比值低的X型沸石作為銀載體其催化效果較矽鋁比值高的Y型沸石佳。

Formaldehyde (HCHO) is one of the most common indoor pollutants, coming from building and decorative materials. Long-term exposure to ppm level of HCHO can cause serious health problems. Catalytic oxidation of HCHO into harmless CO2 and H2O is the most promising technique to remove HCHO. Noble metals (e.g., Pt, Pd and Au) are efficient catalysts in oxidation of HCHO at ambient temperature. However, the high cost is the drawback. In this work, silver was impregnated on zeolite Y exchanged with different ions and NaX. Catalysts were characterized by XRD, BET, H2-TPR, SEM, HR-TEM, 27Al-NMR, ICP-MS, TGA, FTIR and XAS. Catalytic activity test was performed in a fixed-bed system under a flow of 320 ppm HCHO, and the HCHO analysis was based on A724.72B method established by the National Institute of Environmental Analysis (NIEA), Taiwan. The factors which might affect the catalytic activities were examined, including the cations on Y-zeolite, calcination temperature, reduction in hydrogen and different Si/Al ratio of Faujasite zeolte. MnY zeolite has its zeolitic framework destroyed after ion-exchange experiment, and Ag/MnY shows lower catalytic activity compared to other silver catalysts. Besides, the reduced silver catalysts have lower catalytic activities than those only through calcination, due to that the size of silver nanoparticles becomes larger after reduction process. In this study, 6Ag/NaY-c shows highest catalytic activity. It can achieve 100% conversion at 100 ℃and around 70% conversion at 80 ℃. Although the conversion over 6Ag/NaY-c reduced more dramatically than 6Ag/HY-c when relative humidity raising from 0% to 50%. 6Ag/NaY-c still has higher catalytic activity than 6Ag/HY-c. Comparing NaX with NaY, 6Ag/NaX-c shows higher catalytic activity than 6Ag/NaY-c. It can achieve around 80% conversion at 80 ℃. The HCHO-TPD results showed that the adsorption capacity for formaldehyde of NaX is much higher than NaY. It is attributed to that the Si/Al ratio in NaX is lower than that of NaY and higher Na+ ions content helps the adsorption of polar formaldehyde molecules. Therefore, NaX has higher surface formaldehyde concentration to react with Ag particles. In this study, NaX is a more suitable support of silver for formaldehyde degradation.