- 學位論文
以金奈米棒-介孔洞二氧化矽核殼粒子作為乙二醇氧化成乙醇醛之催化劑
Mesoporous silica-coated gold nanorod as a catalyst for oxidation of ethyleneglycol to glycolaldehyde
摘要
1987年日本科學家 Masatake Haruta 首次以金奈米粒子鑲嵌於過渡金屬氧化物上催化一氧化碳氧化反應，證實金的尺寸縮小至 5 奈米等級時具備催化活性。Haruta認為催化反應發生於金奈米粒子與其吸附之過渡金屬氧化物界面上，催化效果以粒徑為 2 至 5 奈米之金奈米球最佳。本研究將介孔洞二氧化矽包覆金奈米棒（AuNRs@SiO2）加入EG中，金棒的直徑在13 ~ 24 nm間發現其具備催化乙二醇（ethylene glycol，EG）氧化成乙醇醛（glycolaldehyde，GA）之能力，使我們進一步探討其催化效率與催化原因。 乙醇醛為製備銀奈米立方體之還原劑，然其結構極不穩定，無法市售取得，可經由EG加熱至150 ℃以上氧化而成。考量 EG 加熱不容易控制，我們檢驗金奈米棒對此反應的催化能力。我們首先選擇尺寸為 70 × 20 nm2 ( 長寬比為 3.5 ) 的金奈米棒，為了防止金奈米棒在反應過程中聚集，利用介孔洞二氧化矽（mesoporous silica）作為保護殼層與反應通道，使小分子可進到殼層中進行反應。檢測產物GA之方法改良自2,4-二硝基苯肼（2,4-dinitrophenylhydrazine）分光光度法。我們以甲苯取代高毒性之萃取溶劑苯，並在萃取過程中全程冰浴降低顯色產物分解速率，使其檢測結果更加精確。我們發現金奈米棒-介孔洞二氧化矽核殼粒子具備催化 EG 氧化成 GA 之能力，可將反應溫度由 150˚C 降至最低 60˚C 。且初步得出介孔洞二氧化矽包覆金奈米棒做催化劑可重複使用共 4 次，回收次數為 3 次。 我們探討不同尺寸之金奈米棒（長 44 ~ 70 nm、粒徑 14 ~ 23 nm）對催化之影響。經過原子吸收光譜儀決定金奈米棒濃度，對金奈米棒催化能力做濃度正規化之後，發現催化效果與金奈米棒之表面積有正相關。推測此催化反應與金奈米棒之表面吸附有關。最後我們使用 HCl 並通 O2下酸蝕介孔洞二氧化矽殼層包覆之金奈米棒。酸蝕後金奈米棒與二氧化矽殼層會分離。將其加入 EG 反應中，發現在反應溫度100˚C、反應時間 10 分鐘下，卻不具備催化能力。因此我們認為催化反應是發生在金奈米棒與二氧化矽殼層的接觸面上，是金奈米棒與二氧化矽殼層的協同效應而產生催化能力。此外我們利用吸收光譜檢測產物GA的量。取 EG 與在150˚C 下加熱 1 小時的 EG 進行檢測。產物 GA 比反應物 EG 的濃度相對低 300000 倍。
並列摘要
In 1987, Japan scientist M. Haruta was the first person to use gold nanoparticles embedded in transition metal oxide to catalyze the oxidation of carbon monoxide. Haruta pointed out that the catalytic ability was related to the interaction between gold nanoparticles and metallic oxides. Gold nanoparticles could be a catalyst when the sizes of the particles are around 2 nm to 5 nm, according to the literature. Here, we examined the catalytical ability of mesoporous silica-coated gold nanorod (AuNR@SiO2) with diameter from 13 nm to 24 nm and found they could catalyze the oxidation of ethylene glycol (EG) to glycolaldehyde (GA). GA is the reducing agent for the wet chemical synthesis of silver nanocubes. However, GA is unstable and not commercially available. We obtained it only by heating the EG up to 150 ˚C, but the yield was hard to control. We examined if AuNR@SiO2 could be a catalyst for this reaction. We first used AuNRs with dimensions of 70 × 20 nm2 (aspect ratio of 3.5). Then, we coated the surface of AuNRs with a mesoporous silica shell to protect them from aggregation and destruction. The mesoporous pores in the silica shell serve as sieves and channels, allowing only small reagents pass through. We improved the method of 2,4-DNPH spectrophotometry for the detection of GA. We replaced extraction solvent of benzene with toluene and decreased the deterioration rate of products by using ice bath. We found that AuNR@SiO2 can decrease the reaction temperature from 150°C to as low as 60°C. We examined the recycling times of AuNR@SiO2. The preliminary results indicated that AuNR@SiO2 could be used for four times. So, the number of recycling are three. We further investigated the catalytic ability of AuNR@SiO2 with different sizes of AuNRs（length = 44 ~ 70 nm, diameter = 14 ~ 23 nm）. We found that catalytic ability of AuNR@SiO2 was mainly related to the adsorption of reactants to the whole surface of AuNRs, not particularly sensitive to the ends of the nanorods. Finally, we etched AuNRs in AuNR@SiO2 by using HCl and bubbling with O2. Etching made AuNRs shorter and forming a hollow space between AuNR and the silica shell. Then we found they have no catalytic ability. So, we thought that the catalytic ability of AuNR@SiO2 may be attributed to the synergic effect of the AuNRs with the silica shell in contact from the above experimental results. By using absorption spectra, we also found the amount of GA in EG that is heated in 150˚C for 1 hour is about 1/300000 of EG.