探討電解質對二氧化鈦奈米顆粒於紫外光照下光降解酚之效應

Abstract

二氧化鈦奈米顆粒(titanium dioxide nanoparticle)為典型且具有發展潛力的光催化奈米材料之一。因此許多研究致力於改質TiO2之型態與結構來提升光催化效率。Phenol是化學工業的基本原料，然而其具有生物毒性與致癌性，因此酚在環境中的流變與宿命受到長久的關注。於含有鹽類的環境下，二氧化鈦奈米顆粒會發生聚集與沉降現象，進而減少比表面積與表面活性位置，然而其光降解phenol之速率不但沒有降低，甚至隨著鹽類濃度上升而升高。本研究的目的是為瞭解含有鹽類下TiO2的光降解機制，並提出可能使之促進的原因。 於水溶液中以TiO2作為光催化劑降解phenol，當添加的NaBr濃度上升，phenol的降解速率也隨之上升。但當添加10到1000 mM的NaCl時，降解速率輕微地下降，而NaCl濃度達2000 mM時則使降解速率略微提升。當添加50 mM的NaI時，phenol的降解速率為無添加NaI時的15倍，然而當NaI濃度上升時，降解速率隨之下降。在含有500 mM的NaBr時，酸性的水溶液比中性或鹼性環境具有更高的降解速率。當溶液中[Br-]濃度固定在100或500 mM時，phenol在pH為3時有較高的降解速率。 為探究phenol以TiO2為光催化劑在含有NaBr下的光降解機制，在水溶液中加入不同種類及濃度的清除劑(scavengers)來分別去除特定自由基。當添加1.0 M的methanol以去除水中的HO•時，光降解phenol的反應被部分抑制；而為了去除TiO2表面之光致電洞(photogenerated hole)，添加10 mM的Na2C2O4或H2C2O4，分別可使水溶液之pH調至約接近中性的6.5與酸性的2.0，其可幾乎完全抑制phenol的降解反應。由此可知，二氧化鈦與NaBr的水溶液系統中，HO•及電洞是phenol光降解的關鍵。 為偵測系統中產生之RHS，進而推測phenol在含NaBr水溶液中的降解機制，本研究進行了數個實驗。第一點：在含有TiO2與NaBr或NaI的水溶液中照射波長254 nm之紫外光，能以紫外光可見光光譜分析，分別於波長268 nm得到產生之Br3-特徵峰，並在288及353 nm得I3-特徵峰，此兩物種之產生與控制組相比有明顯的差異，顯示TiO2能使Br-與I-在紫外光光照下產生RHS，最後轉化成Br3-與I3-，而反應中產生的RHS可能為促進phenol光降解反應的原因。第二點：在含有ATiO2與3 M之NaBr酸性環境水溶液中連續照射UV光，利用間斷曝氣收集產生之溴素並以IC定量，其中收集瓶中之Br-濃度比未添加ATiO2的控制組高出1.4倍，顯示ATiO2有促進Br-氧化生成Br•或Br2之效應。第三點：在含有CTiO2奈米顆粒與NaCl或NaBr的水溶液中照射UV光，並以DPD法隨時間測定產生之氯素或溴素量，其濃度隨NaCl或NaBr添加量與UV光照時間增加而上升，顯示CTiO2具有與Cl-或Br-反應產生氯素或溴素之能力。第四點：以波長254 nm之激發光照射含有ATiO2與NaBr的水溶液，並以螢光光譜儀偵測其可見光波段的光譜，在TiO2的放光波長(350-500 nm)下可發現添加NaBr組別的放光強度比未添加的控制組弱，顯示可能為電洞氧化Br-而減緩電子-電洞對復合的速率。最後一點：以ATiO2於含有500 mM 的NaBr水溶液中光降解phenol產生之副產物，透過液相層析串聯質譜儀分析得到數種含溴酚類化合物，顯示源自於光催化反應的RHS具有與phenol反應及加速其降解之能力。

Titanium dioxide nanoparticle (TiO2 NP) is one of the most typical and promising nanomaterials especially in photocatalysis. Phenol is harmful ecotoxin which could result in carcinogenicity so phenol in the environment has been concerned. When there are salts in the solution, ATiO2 NPs would aggregate and sediment which results in the decrease of surface area and active site on the surface. However, the photodegradation rate of phenol using ATiO2 with salts increased instead of decreasing, and the rate increased with increasing of salt concentrations. This study aims to understand the mechanism of phenol photodegradation using ATiO2 in the solution containing salts. The photodegradation of phenol increased with increasing NaBr concentration. The rate slightly decreased with 10-1000 mM of NaCl, but increased with 2000 mM NaCl. A 15-fold increase of the degradation rate with 50 mM NaI was compared with the rate with 0 mM NaI. However, the degradation rate decreased with increasing NaI concentration. Phenol had a higher degradation rate under acidic conditions with 500 mM NaBr compared to neutral and basic conditions. When the Br- concentration was set at 100 or 500 mM, phenol had a higher degradation rate at pH 3. In order to investigate the mechanism of phenol photodegradation with NaBr, different concentrations of scavengers were added into the aqueous solution to eliminate radicals. The photodegradation of phenol was partially inhibited with methanol. In the case of photogenerated hole, the photodegradation of phenol with Na2C2O4 at pH around 6.5 and H2C2O4 at pH around 2.0 was almost inhibited, which indicated that HO• and hole play important roles in phenol degradation in TiO2 system. In order to detect reactive halogen species (RHS) produced from the photoreaction system of TiO2 and estimate the mechanism of phenol photodegradation in the solution with NaBr or NaI, several experiments were conducted. First, the Br3- was detected at 268 nm and the I3- was detected at 288 and 353 nm by UV/Vis spectrophotometer after the ATiO2 solutions with NaBr or NaI were irradiated at the wavelength of 254 nm. The amount of Br3- and I3- were more than that in the control tests. The result indicated that with ATiO2 and UV irradiation, Br- and I- could be transformed to RHS then produced Br3- and I3-. And the RHS in the reaction may lead to acceleration of phenol photo-degradation. Second, the system containing ATiO2 and 3 M NaBr at pH 3 was continuously irradiated with UV light and intermittently purged. Bromine produced from the system was collected and analyzed by IC. There was a 1.4-fold increase in the concentration of bromide ion compared to the control test, which represented that ATiO2 could promote oxidation of Br- and generate Br• or Br2. Third, the CTiO2 NP suspensions with NaCl or NaBr were irradiated under UV and conducted chlorine or bromine analysis using DPD method. The concentration of chlorine or bromine increased with increasing NaCl or NaBr concentration and UV irradiation time. The results indicated that CTiO2 possessed the capability of reacting with chloride or bromide and generating chlorine and bromine. Fourth, the photoluminescence of the solution with ATiO2, NaBr and UV irradiation showed a lower intensity of emission light than ATiO2 without NaBr. The result indicated that the hole might oxidize Br- ion and caused the reduction of electron-hole pairs recombination rate. Finally, the LC-MS analysis showed the information of brominated phenol from the solution with ATiO2 and NaBr after UV irradiation. RHS originated from photocatalytic reaction of halide ions on TiO2 NPs were capable of reacting with phenol and accelerating rate of phenol degradation.