反應器與電子電漿共振於光觸媒之分析與應用

Characterization and Application of Reactor and Surface Plasmon Resonance on Photocatalysis

張志毅(Chih-Yi Chang)

指導教授 ： 吳乃立

國立臺灣大學/工學院/化學工程學研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.00373

摘要

直接利用太陽能進行光觸媒廢水處理為相當具有潛力之綠色製程。光觸媒反應之優勢除可直接採用再生能源外，其觸媒反應之特性，可減低製程中環境之二次污染。然而，現今偏低的反應轉化效能為實現光觸媒被實際應用所需要克服的關鍵之一。本論文以提高光觸媒反應程序效能為目標，由反應器設計與可見光利用之觀點著手進行研究。

第一，吾人設計一旋轉盤反應器（Rotating disk reactor），並測試與評價其應用於光觸媒廢水染料污染物之降解能力。在此光觸媒廢水處理製程中，二氧化鈦光觸媒奈米粉體均勻塗佈於旋轉盤表面，而染料（甲基橙）水溶液於轉盤旋轉與紫外光照射下，由轉盤中心注入並由轉盤徑向流出。結合流體力學與動力學模型，反應器之基本特性參數（滯留時間與液膜膜厚）與操作變數（試液體積流量與旋轉盤轉速）之間的關係可被緊密建立。實驗結果顯示，反應器旋轉盤上之液膜膜厚受試液體積流量與旋轉盤轉速交互控制。而於一關鍵膜厚厚度之下，膜厚的減低可使觸媒反應於光反應限制區進行操作，而不受質傳反應限制。即使於光反應限制區，總體反應速率仍顯著受制於液膜膜厚。這是由於入射光會於到達二氧化鈦觸媒表面進行反應之前，被染料溶液吸收而衰減。於最佳之實驗條件下，甲基橙染料整體轉化率可超過50%，而僅需數秒之反應器滯留時間。旋轉盤反應器之設計模型方程式已被推導，並顯現相當具有前景之製程規模放大潛力。

第二，銀奈米粒子於可見光照射下具有之表面電子電漿共振現象（Surface plasmon resonance），其應用於二氧化鈦光觸媒之潛力已被探討。研究重點強調於分別量化評估蕭基障壁（Schottky-barrier）與表面電子電漿共振所貢獻之光觸媒活性增益，並進一步深入了解特定光子激發能量與銀奈米結構對於表面電子電漿共振現象引發之光觸媒活性增益的交互關係。本研究製備兩種形貌之複合光觸媒，包含銀奈米粒子/二氧化鈦奈米粒子（Ag-NP/TiO2-NP）與銀奈米環/二氧化鈦奈米管（Ag-NR/TiO2-NT）。除探討不同的銀奈米結構所產生之表面電子電漿共振模式外，並觀察兩種複合光觸媒對於甲基橙染料降解之能力。紫外光源與四種不同之可見單色光源分別使用於激發二氧化鈦之電子電洞對以及銀奈米粒子之表面電子電漿共振。藉由可見/紫外光的不同組合照射實驗，蕭基障壁與表面電子電漿共振所貢獻之光觸媒活性增益可被清楚區分。實驗結果顯示，兩種不同形貌之觸媒皆顯現表面電子電漿共振產生之光觸媒活性增益，而二氧化鈦與銀奈米粒子之形貌與光觸媒活性有複雜之關係，暗示激發之二氧化鈦電子電洞對與銀奈米粒子內之表面電子電漿共振有強烈之交互作用。本研究最特殊之貢獻在於首次證實，可見光源之光子能量與複合光觸媒之表面電子電漿共振活性增益有其特定選擇性：表面電子電漿共振貢獻之活性增益程度，隨著可見光光子入射頻率遠離表面電子電漿共振頻率而降低，而與共振頻率之絕對值無關。Ag-NR/TiO2-NT光觸媒與Ag-NP/TiO2-NP光觸媒相比，表現出較佳之表面電子電漿共振增益活性。

最後，吾人進一步致力於觸媒材料與反應器的設計，以期提升表面電子電漿共振對於二氧化鈦光觸媒降解反應的活性增益。對於前者，吾人研發一創新之”奈米球模板製程”，製作大範圍面積之單層奈米金顆粒，並使用旋轉塗佈法旋鍍二氧化鈦光觸媒，成功合成出金奈米粒子嵌入式二氧化鈦薄膜光觸媒材料。奈米金顆粒於二氧化鈦薄膜內之嵌入量與大小可藉由金靶材之濺鍍時間精準控制。對於後者，一搭載雙光源之”微反應器”，被設計於量測此特殊結構的金二氧化鈦觸媒材料對於染料降解反應之活性增益現象。活性實驗結果發現，嵌入金奈米粒子之二氧化鈦薄膜光觸媒於不同組合之光源照射下，清楚顯現蕭基障壁與表面電子電漿共振產生之光觸媒活性增益。

關鍵字

光觸媒 ； 旋轉盤反應器 ； 表面電子電漿共振 ； 銀/二氧化鈦 ； 金/二氧化鈦 ； 廢水處理

並列摘要

Wastewater treatment based on solar energy-effected photocatalytic reaction is a green process that utilizes renewable energy resources and minimizes secondary pollution. Low conversion efficiency is one of the key issues to overcome for realizing its practical application. This study aims at significantly raising the process efficiency from the viewpoints of photocatalytic reactor and visible light utilization, respectively.

First, a rotating disk reactor (RDR) has been evaluated for the application of photocatalytic decomposition of dye pollutants in water. In this process, photocatalyst (TiO2) particles are immobilized onto a disk, and dye (methyl orange)-containing solution is allowed to flow in radial direction along the surface of the disk, which is rotating and illuminated with UV light. The correlations between the fundamental characteristics of the reactor, including residence time and film thickness, and its operating variables, including volumetric flow rate and disk rotating speed, have been established by the combination of fluid dynamic and kinetic models. The results indicate that the reactor can be operating beyond mass-transfer limitation by reducing the liquid film thickness, which is a complex function of both flow rate and disk rotating speed, below certain critical value. Even under such a condition, the overall reaction rate remains strongly affected by the liquid film thickness due to the intensity attenuation of incidence light through the liquid film before reaching the TiO2 surface. With selected operation conditions, conversions greater than 50% have been achieved within only a few seconds of residence time. A reactor design equation has been derived, indicating promising scale-up potential of the process.

Second, a fascinating surface plasmon resonance (SPR) phenomenon of Ag nanostructures within visible light wavelength region has been investigated for the potential application to TiO2 photocatalysis, and emphasis has been placed on quantifying the Schottky-barrier effect (SB-effect) and the SPR effect and on looking for energy-specificity and morphology-dependence of the SPR effect. Two types of Ag/TiO¬2 composite photocatalysts, including Ag nanoparticle-on-TiO2 nanoparticle (Ag-NP/TiO2-NP) and Ag nanoring-on-TiO2 nanotube (Ag-NR/TiO2-NT), that generate different SPR modes, have been evaluated for photocatalytic bleaching of methyl orange (MO). Irradiation from ultraviolet light (UVL) and visible-light (VL) emitting diodes (LEDs) of four different wavelengths are applied to excite excitons in TiO2 and surface plasmon of Ag, respectively. Under various combinations of the UVL and VL LED, the SB- and SPR-effects due to the Ag nanostructures have been clearly distinguished. The photocatalytic activity exhibits complex dependence on the morphology of TiO2 and Ag, suggesting strong interaction between the excited electron-hole pairs and SPR effect. In particular, SPR-enabled enhancement in photocatalytic activity has been confirmed for both catalysts, and the frequency-specificity of such enhancement has for the first time been demonstrated: the extent of the enhancement decreases with increasing departure of the VL photon energy from the resonance peak energy, irrespective of the absolute value of the resonance energy. Ag-NR/TiO2-NT nanostructure in general exhibits greater SPR-enhanced enhancement than Ag-NP/TiO2-NP one.

Finally, we further delicate effort to material and reactor design for the purpose of enhancing SPR effects of noble metal on TiO2 photocatalysis. We successfully synthesize a large-scale Au NPs-embedded TiO2 thin film photocatalyst with applying a series of fabricating process involving a novel “nanospheres template method” followed by spin-coating of TiO2 thin film. The Au NPs concentration and diameter in the TiO2 matrix can be easily controlled by the sputtering time of Au target. A customized “micro-reactor” with dual-light sources is designed to evaluate the photocatalytic activities and the activity data showed clearly the SB- and SPR-enabled activity enhancement in TiO2 photocatalysis in the presence of embedded Au NPs.

並列關鍵字

Photocatalysis ； Rotating disk reactor ； Surface plasmon resonance ； Ag/TiO2 ； Au/TiO2 ； Wastewater treatment

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