液相非熱電漿技術備制奈米級N-doped TiO2降解偶氮染料並結合陶瓷膜分離/回收光觸媒之研究

Preparation of N-doped TiO2 Nanocatalysts by LPNTP Technique for Decolorization of Azo Dye with the Catalyst Separation/Recovering System using Ceramic Membrane

鄭旭惠(Hsu-Hui Cheng)

指導教授 ： 陳孝行

國立臺北科技大學/工程學院/工程科技研究所/博士(2011年)

摘要

商業級之二氧化鈦 (Degussa P-25) 及氯化銨 (NH4Cl) 粉末混和後，放置於液相非熱電漿反應系統中，藉以備製出淡黃色之含氮二氧化鈦光觸媒 (TiOxNy);並於可見光下應用偶氮染料 (Acid Orange 7) 進行光催化降解試驗。液相非熱電漿技術之優點即可於常溫常壓下，快速備製出含氮二氧化鈦光觸媒，其原理即應用高壓放電予瞬間產生一強烈之電場與伴隨之UV光，促使TiO2受到低於臨界波長的光線照射後，可激發價帶 (valance band) 電子躍遷至傳導帶 (conduction band) 而產生電子-電洞對，更進而將TiO2中之O原子順利與N原子進行置換，以降低光觸媒之能隙，備製出含氮二氧化鈦光觸媒。除可同步改善光觸媒於可見光源下之光催化效益，同時亦發展一新的連續式光催化反應槽；亦更進一步利用UF陶瓷薄膜 (孔徑為10 nm) 系統回收TiOxNy之光觸媒顆粒，藉由反覆清洗方式再次循環使用。不但符合綠色科技及能源之趨勢，亦達到資源「零浪費」的目標。
其研究結果顯示備製TiOxNy光觸媒藉由液相非熱電漿系統於常溫常壓下僅需40分鐘之備制時間，且最佳備制條件為DP-25與NH4Cl氯化銨之比例為0.6：3.6 (g/ ml)；此外,依據ESCA、EDS及UV/Visible分析結果得知：樣品於電漿製備40分鐘即能分析出NH4+之N原子摻雜於二氧化鈦表面上，並藉由UV/Visible觀察到含氮二氧化鈦之吸收光譜皆位移至可見光區域 (＞400nm)，能隙從原本3.2eV（DP-25）降低至2.82eV (TiOxNy)，亦由XRD分析中發現，常溫常壓放電過程 (13.5W、40min) 並不會改變二氧化鈦晶相，仍舊維持原來Anatase晶相。製備出之TiOxNy於可見光下進行偶氮染料光催化降解試驗，經10小時之批次試驗後，氮掺雜比為TiO2：NH4Cl = 1：6之含氮光觸媒，其反應速率常數為0.1519 hr-1；已明顯優於DP-25之反應速率常數 0.045 hr-1之光催化活性，其TiOxNy光催化反應速率為DP-25的3.4倍。
在連續式可見光光催化系統中，水力停留時間控制在10小時，經反應16小時後，隨著曝氣含氧量增加去除效果亦增加，當系統內曝氣含氧量達40% 時去除效果達62%。此外，隨著本研究光觸媒添加量增加反應效果亦增加，於光觸媒添加量為0.5 g/L時，反應去除率達96%，其pH = 2時反應去除效果為最佳。因達本研究設訂之光催化效果目標，故未在持續添加其光觸媒之量，使致系統內未發生所謂之遮蔽現象(screening effect)。在陶瓷薄膜回收部分，反應在60分鐘之前，光觸媒TiOxNy顆粒回收率已達64%以上，在80分鐘後，光觸媒TiOxNy顆粒回收率超過83 %以上，當系統操作超過90分鐘以上，則回收率可達99.5%以上，且光觸媒於陶瓷膜分離程序中，光觸媒TiOxNy顆粒可被完全分離濃縮。回收後之TiOxNy顆粒經光催化再利用後，仍可反覆處理11次之偶氮染料廢水，但經回收多次後之光觸媒，因表面被染料分子慢慢佔據，進而產生所謂之觸媒劣化現象(catalyst deactivation)，直接影響觸媒的重複使用性。若於系統加入曝氣條件，則能延緩光觸媒毒化現象發生，提高光觸媒回收再利用效率，亦讓光觸媒使用壽命可再次被延長。

關鍵字

液相非熱電漿 ； 含氮二氧化鈦 ； 可見光 ； 偶氮染料 ； 陶瓷薄膜

並列摘要

Yellow-colored N-doped TiO2 (TiOxNy) powders, synthesized by the liquid-phase non-thermal plasma (LPNTP) technique, were produced from a mixed aqueous solution, containing commercial titanium dioxide (Degussa P-25) and an N-precursor (ammonium chloride, NH4Cl), and utilized for photodegradation of C. I. Acid Orange 7 (AO7) under visible light. The benefit of such technology is mainly on rapid production of visible light photocatalyst N-doped TiO2 under room temperature and pressure. A high voltage applied to the electrodes was to generate a strong and transient electric field, and the discharge emit ultraviolet light of sub-critical wavelengths was used to illuminate TiO2 producing electron-hole pairs by exciting electrons from the valance band to the conduction band. These promote the replacement of O atoms with N atoms to narrow the band gap of photocatalyst. To enhance the photocatalytic efficiency of TiOxNy under visible light, this study also developed a new continuous-flow photoreactor by further combining a UF ceramic membrane system to recover the TiOxNy particles in the suspension system. The recovered TiOxNy particles were subjected to repetitive cleaning and reuse. This not only complies with the practices of green technology and energy, but also achieves the aim of "zero waste".

The TiOxNy photocatalysts obtained through the LPNTP process were characterized with UV-Vis spectrophotometer, XRD, ESCA, TEM, and EDS, respectively. The UV-Vis spectrum of N-doped TiO2 showed that the absorption band was shifted to 439 nm and the band gap was reduced to 2.82 eV. The structure analysis of XRD spectra showed that the peak positions and the crystal structure were changed by plasma-treating at 14.1 W for 40 min. In the batch photocatalytic system, ninety percent of azo dyes were degraded under visible light irradiation for 10 hours in the optimum condition of TiOxNy-1:6. Furthermore, a screening effect of excess particles was observed when the dosage of TiOxNy-1:6 was over 0.2 g/L, and an optimum dosage of TiOxNy-1:6 at 0.15 g/L was determined.

Additionally, the optimal conditions for preparation of the photocatalyst as TiOxNy-1:6 was chosen to perform the photocatalytic activity and durability test in a continuous-flow photocatalysis/membrane separation system (PMSS). The experimental results showed that the optimal dosage of TiOxNy was 0.5 g/L, and the AO7 degradation efficiency was effectively improved by increasing oxygen concentration from 0% to 40% or decreasing the initial AO7 concentration from 15 to 5 mg/L. The degradation efficiency of AO7 increases as pH decreases, exhibiting a maximum efficiency at pH 2. For membrane separation/recover system, the recovery efficiency reached 99.5% after the ultrafiltration had been carried out for 90 min, and the result indicated that the photocatalyst was able to be separated/recovered completely. For this reason, the photocatalyst was recovered completely. The durability of the photocatalyst was evaluated by reusing the photocatalyst 11 times and the recycled catalyst was capable of repeating 5 runs without significant decrease in treatment efficiency. In addition, the durability of TiOxNy catalysts with aeration, which decreases catalyst deactivation, was longer than that without aeration.

並列關鍵字

Liquid-phase non-thermal plasma ； N-dope TiO2 ； Visible light ； Azo dyes ； Ceramic membranes

參考文獻

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