二氧化鈦氣凝膠在光催化分解水產氫之應用

Application of TiO2 Aerogels in Photocatalytic Water Splitting for Hydrogen Production

林佳潔(Chia-Chieh Lin)

指導教授 ： 呂世源

國立清華大學/工學院/化學工程學系/碩士(2010年)

摘要

光解水產氫反應是一種異相反應，而反應關鍵在於觸媒與反應物的表面反應，因此研究大多傾向於製備高比表面積觸媒使反應接觸面積增加。本研究首次嘗試以二氧化鈦氣凝膠作為水分解產氫反應的光觸媒。利用溶膠-凝膠法製備濕凝膠，經過二氧化碳超臨界流體萃取乾燥後，取得二氧化鈦氣凝膠。由於氣凝膠擁有以下特性：1. 高比表面積可提高表面反應機會；2. 完整連續性三維立體網狀結構，可使光生電荷載子易於表面移動；3. 中孔洞結構有利於反應物以及產物進出。本研究比較氣凝膠與水熱法製備之二氧化鈦奈米晶粒以及商用P25二氧化鈦之間的產氫效率。利用XRD、BET、TEM及DLS檢測觸媒晶相、比表面積、孔洞體積、表面結構以及分散於水中聚集時團簇大小，藉以解釋實驗結果。

研究系統為甲醇水溶液，其中甲醇為系統中的犧牲劑，於400 W 高壓汞燈照射下進行反應，平均光照功率密度為0.24 mW/cm2。反應溫度為20℃時，二氧化鈦氣凝膠產氫效率為6.4 μmol/g h，相較於另外兩種二氧化鈦光觸媒，P25(0.9 μmol/g h)及水熱法合成之奈米晶粒(1.9 μmol/g h)產氫速率高於2~6倍。另外，電化學交流阻抗光譜(electrochemical impedance spectroscopy，EIS)圖中顯示二氧化鈦氣凝膠有效分離光生電子電洞對以及在固液界面上發生快速電荷轉移。改變反應溫度時產氫速率隨溫度上升而增加，氣凝膠在50℃時產氫效率可達45 μmol/g h。根據阿瑞尼氏(Arrhenius)方程式，簡化水分解反應為0級反應，求得氣凝膠反應活化能為50.9 kJ/mole。利用多元醇還原法負載助觸媒鉑，其中負載量為還原時前驅物溶液中鉑與二氧化鈦氣凝膠重量比例為0.5 %時，產氫速率提昇約50倍。負載鉑之氣凝膠反應活化能為13.2 kJ/mole；可知助觸媒鉑提供另一種的反應途徑，有效降低活化能。

另一研究重點為比較不同負載助觸媒鉑的方式：多元醇還原法及含浸-鍛燒-還原法。助觸媒分散性會決定反應活性位的多寡進而影響反應速率，研究顯示含浸-鍛燒-還原法產氫速率可達到288.7 μmol/g h，此時助觸媒負載量為含浸步驟中，前驅物溶液的鉑與二氧化鈦氣凝膠重量比例為0.05 %，與多元醇還原法負載0.5 wt.%的鉑產氫速率相近。此負載量的差異是由於含浸-鍛燒-還原法所製備的鉑顆粒較小且較為分散。

關鍵字

水分解 ； 光催化 ； 二氧化鈦 ； 氣凝膠 ； 氫氣

並列摘要

The key point in water splitting as a heterogeneous reaction is the surface reaction between the catalyst and reactant. In order to enhance the reaction at the interface, there have been many efforts made on increasing the surface area of the catalysts. We first used TiO2 aerogels prepared with a sol-gel process and subsequent supercritical fluid drying, as the photocatalyst in water splitting. The high surface area and 3-D connected pore structure of large porosity are the advantages of aerogels for serving as the photocatalysts. We compared the hydrogen production efficiencies of aerogels, commercial P25, and TiO2 nanoparticles prepared with a hydrothermal process. The properties of these catalysts were characterized with XRD, BET, TEM, and DLS.

The photocatalytic water splitting reaction was carried out in a methanol solution under a 400 W Hg light source for 8 hours. The methanol served as the sacrificial agent. The hydrogen evolution rate of the TiO2 aerogels was 6.4 μmol/g h, which was six times and twice of those achieved by P25 (0.9 μmol/g h) and TiO2 nanoparticle (1.9 μmol/g h), respectively. The results of EIS (Electrochemical Impedance Spectroscopy) Nyquist plot indicated that a more effective separation of photogenerated electron-hole pairs, and faster interfacial charge transfer to the reactant occurred in the TiO2 aerogel system. The activation energy of the reaction in the aerogel system was determined with the Arrhenius equation to be 50.9 kJ/mole, assuming a zeroth order kinetics for the photocatalytic reaction. We raised the hydrogen production rate from 6.4 μmol/g h to 287 μmol/g h by incorporating Pt nanoparticles as an assistant catalyst, by a polyol process based on 0.5 wt% Pt. The activation energy determined for the Pt loaded TiO2 arogel was 13.2 kJ/mole, much reduced from the plain TiO2 arogel case. The result showed that the assistant catalyst indeed provides another reaction path to lower the activation energy.

Another part of this research focused on the ways of loading Pt – polyol process and immersion-calcination-reduction process. We found that the immersion-calcination-reduction process achieved the same level of hydrogen evolution rate by using a much lower concentration of starting Pt, 0.05 wt. % Pt in this case. The result may be due to better dispersion and smaller particle size of the Pt nanoparticle realized in the immersion-calcination-reduction process.

並列關鍵字

water splitting ； photocatalysis ； TiO2 ； aerogel ； hydrogen

參考文獻

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