The investigation on photoelectrocatalysis of sulfadiazine by carbon doped TiO2

Abstract

[[abstract]]本研究應用碳摻雜改質之二氧化鈦/氧化銦錫(TiO2/ITO)複合光觸媒薄膜電極，在近紫外光與可見光藍光下，進行水中磺胺嘧啶之光催化(photocatalytic)與光電催化(photoelectrocatalytic)分解研究，磺胺嘧啶為一常見的抗生素用藥，同時也是新興污染物(emerging contaminates)。本研究除了進行污染物之分解實驗外，同時也藉由UV-Vis、SEM、XRD等精密儀器來檢測光觸媒能隙、薄膜表面形貌、金屬氧化物晶相等物化特性，探討光觸媒物化特性與污染物分解速率之關連性；此外，研究中也針對光觸媒電極表面氫氧自由基生成濃度進行檢測，並應用循環伏安法量測定觸媒電極之光生電流(photocurrent)與平帶電位( flat-band potential)，藉以探討光觸媒光電特性與其觸媒活性之關係；最後，本研究也進行分解產物之鑑定與分析，藉以確認磺胺嘧啶的光催化與光電催化分解路徑，以及污染物被礦化分解的程度。 本研究系列實驗係在批次式光電催化反應器中進行，以製備之各類雜複合光觸媒薄膜為工作電極、白金絲為相對電極，以主要波長為365 nm之近紫外光及470 nm LED之可見光源，為反應所需之光源，測試光觸媒種類(TiO2-ITO、TiO2-Ni-CuO-ITO、TiO2-ITO/C)、外加電壓大小(0-1 0 V)、水溶液pH值(pH=3、7、11)等實驗參數，對磺胺嘧啶之光電催化分解效能影響。本研究針對光觸媒之製備方式，則採用RF磁控濺鍍(sputtering)技術。 研究結果發現：在光電催化反應中，光觸媒於製程中摻雜碳之光觸媒薄膜電極，可藉由在外加電動勢的協助，加速污染物的分解，而且反應速率普遍隨外加電壓的增加而增加。相較於單純光催化反應，整體反應速率最大可提高48倍，一般情形也有數倍反應速率的增幅；此外，碳摻雜光觸媒薄膜電極，確實具有紅移效果，相關污染物可在可見光光源下，進行分解反應；另外，就水溶液酸鹼條件之影響方面，於酸性(pH= 3)條件下，磺胺嘧啶有最快的反應速率，其最大的一階分解速率常數k= 2 405hr-1(BC120)，中性則是次之。雖然在鹼性環境中，觸媒表面可產生較多的氫氧自由基，但污染物在鹼性環境中的分解速率反而較慢，顯示反應物與觸媒之吸附關係，仍可能影響整體反應速率之重要因素；其次，就本研究所製備的四種光觸媒薄膜電極而言，以摻雜較多碳之BC120試片，其催化分解污染物的反應速率最快。經比對此觸媒的平帶電位、光生電流強度、以及觸媒上氫氧自由基之生成濃度等結果，印證此觸媒應具有較高的催化活性，也顯示適當的碳摻雜，確實有利於增加觸媒的活性，也有利於可見光的吸收。 本研究成功地呈現在施以適當的外加電壓下，可加速二氧化鈦光催化分解反應的進行，不僅可增加數倍至數十倍的污染物分解速率，同時也可以增進污染物分解的礦化程度。本研究利用GC high mass鑑定污染物完全礦化分解前，可能衍生的中間產物，包括：苯胺、2 - 氨基吡啶、4 -（2 -氨基嘧啶-1（2氫）- 基）苯胺，並藉以研擬其分解反應路徑，也發現污染物之光催化與光電催化分解反應路徑，略有差異。[[abstract]]This study aimed to investigate both photocatalysis (PC) and photoelectrocatalysis (PEC) of sulfadiazine under illumination of near-UV (l= 365 nm) or LED blue light (l= 470 nm) by using TiO2/ITO composite thin-films as photocatalysts prepared with RF magnetron sputtering processes Sulfadiazine (SZD) a widely using antibiotic but concerned as an emerging contaminant was tested as a target compound Three TiO2/ITO composite thin-film photocatalysts including bare TiO2 carbon doped TiO2 and Ni-CuO co-doped TiO2 composited with ITO substrates were tested in the study Other than testing decomposition rate of SDZ both physical and chemical properties of the photocatalyst including absorption spectra surface morphology and crystalinty were also measured by UV-Vis、SEM and XRD for identifying their basic properties This study also measured both hydroxyl radical formation concentrations on photocatalyst surfaces and flat-band potential of the prepared photocatalysts for correlating their oxidation capabilities with SDZ degradation rates Several reacting variables including solution pH levels (pH= 3 7 11) applied external bias (0~1 0 V) types of photocatalysts and light sources (l= 365 nm and l= 470 nm) were tested in the study All of experiments were conducted in a bench-scale photoelectrocatalytic reactor The reaction intermediates/products and mineralization ratio of SDZ were identified as well for discussing SDZ degradation The experimental results indicate SDZ degradation rate was enhanced with increasing applied bias Higher SDZ degradation rate was achieved by PEC reaction than by simple PC reaction In comparison of simple PC reaction the PEC reaction rate could be increased as high as 48 folds In addition it was observed that the carbon-doped TiO2 had a higher degradation rate for SDZ than the simple TiO2 sample As a matter of fact more hydroxyl radicals were yielded by the carbon-doped TiO2 The carbon-doped TiO2 has more “negative” flat-band potential as well The TiO2 doped with carbon can absorb more visible light which results in fast decomposition of SDZ while using LED blue light as a light source For the effect of the solution pH on SDZ degradation rate SDZ has higher degradation rate in acid environments It should be due to more SDZ adsorbed by the photocatalysts in acidic conditions showing the typical heterogeneous reaction characteristics of photocatalytic reactions This study successfully demonstrates the enhanced photocatalytic degradation rate of typical antibiotic SDZ by providing an external bias to the reaction system It is also interesting to find that he provided biases not only can increase SDZ reaction rate but also can promote the mineralization of SDZ This study also found three reaction intermediates before complete mineralization of SDZ They may result from the reactions of hydroxylating heteroaromatic ring reacting with amine groups to form acetyl and formyl groups and reacting with aniline to produce 2-aminopyridine and 4-(2-iminopryrimidine-1(2H)-yl)anilin