發光二極體結合改質光觸媒處理室內揮發性有機污染物之研究

Treatment of Volatile Organic Compound from Indoor Pollution Source Using Modified Photocatalyst with LED

李佳香(Chia-Hsiang Lee)

指導教授 ： 張慶源

國立臺灣大學/工學院/環境工程學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.00323

摘要

本研究內容分為光催化反應之改質觸媒合成與室內有機污染物之光催化分解反應（目標污染物則選用甲醛）兩部分。第一部分之改質光催化觸媒合成係以初溼含浸法（impregnation method）添加銀離子製備Ag/TiO2。希望藉由將金屬銀批覆於TiO2表面以期減緩其電子-電洞再結合，提升其氧化效能。此外並利用化學沉澱法（precipitation method）自行合成鈦系半導體觸媒，改變其能隙（energy band gap）使其適用之光源波長能接近於近紫外光與可見光。
第二部分進而探討以發光二極體（light emitting diode, LED）為光源，取代傳統燈管之光催化反應處理室內揮發性有機污染物甲醛。主要探討光催化之各項反應操作參數（如不同光源、不同觸媒及不同觸媒載體等）對甲醛光催化分解之影響。反應系統採用不同之載體，使觸媒能更均勻散佈於系統中增加反應面積，提升反應速率，以提高光催化反應之分解效率（decomposition efficiency）及礦化效率（mineralization efficiency）。並進一步討論不同初始濃度對甲醛分解之影響，以建立甲醛光催化分解之反應動力式。
實驗結果顯示，各種改質觸媒中以TiNH400顆粒較為細緻，粒徑約在30-40 nm之間，且表面積也相對的較高；但團聚情形相當嚴重，團聚顆粒明顯較Ag/TiO2為大，分子團之大小亦不均一，皆可達數十至數百微米。TiNH400可利用之最長吸收光譜波長明顯位移至約400-440 nm處，故其可適用於短波長之可見光與近可見光光源。其餘改質之光觸媒，如TiO2-N、Ag/TiO2-N、TiON400、Ag/TiO2等，其可利用之最長吸收光譜波長皆與TiO2相同，為360-380 nm左右。
此外，光催化分解甲醛之速率會隨著系統濕度增加而略微上升。在光源為紫外光時，使用Ag/TiO2觸媒不論在分解效率及礦化效率方面均比TiO2或無披覆觸媒有顯著提升。而在甲醛初始濃度500 ppmv時，利用Ag/TiO2觸媒批覆於玻璃片上在365 nm 之UV燈照射下且1 h反應時間內，甲醛分解效率達82%，而使用254 nm之UV燈同樣也是反應時間1 h即可達80%之降解率，當使用UVLED時，在反應時間為1 h及7 h之降解率為58%及65%。而以玻璃棒載體批覆Ag/TiO2為反應條件時，在使用365 nm之UV燈，控制100%的相對濕度，其甲醛最終分解效率達96%，而UVLED燈之最終分解效率亦可達95%，且礦化效率亦明顯地增加，顯示以UVLED取代傳統UV燈管是一可行的方法。本實驗亦將藉由Langmuir-Hinshelwood 方程式模擬紫外光光催化甲醛系統中反應速率與濃度的關係，並求得有機物甲醛之反應速率常數與吸附平衡常數。
實驗結果顯示甲醛最終分解效率隨光源波長變短、能量增加而有所提升。因此紫外光系統分解效果皆比可見光系統好，最終分解效率幾乎可達100%；使用改質觸媒TiNH400且以可見光LED做為光源時，其分解效率較低，但仍有約50-60%之最終降解率，顯見改質光觸媒的仍具相當成效，代表著摻雜氮之改質觸媒在未來使用上更適合於自然光下使用，可應用在500-600 nm為主的一般陽光照射環境下，且從能量效益觀點來看，以LED為光源之能量效益均比傳統燈管高。因此，本研究可提供未來開發設計新穎LED及光觸媒之室內空氣污染物處理器之參考，極具商業應用價值。

關鍵字

揮發性有機物 ； 甲醛 ； 發光二極體 ； 室內污染物 ； 光觸媒改質 ； 可見光光催化 ； 二氧化鈦。

並列摘要

This study investigated the surface modification of photocatalyst and photo-decomposition of volatile organic compounds (VOCs) from indoor pollution source. Formaldehyde, which has the highest concentration among VOCs in the indoor air pollutants, was taken as a model compound. The photocatalyst of Ag/TiO2 was made by the impregnation method. Ti-semiconductor photocatalyst was prepared by precitation method, and expected to change the energy band gap of visible-light-active photocatalyst making it suitable for the wavelengthes of visible light and near UV light.
Regarding the light sources, this study explored the feasibility of the application of the light emitting diode (LED) instead of the traditional light lamp to treat the indoor air pollutant of formaldehyde. The operation parameters, which affect the formaldhyde decomposition in photo-decomposition reaction, were examined. These parameters induded different light sources, photocatalysts and catalytic supports. The modified composed photocatalyst was coated on the glass plates or sticks uniformly. The catalytic effects enhancing the decomposition (ηD) and mineralization (ηM) efficiencies of formaldehyde were studied. Furthermore, the photocatalytic decomposition of formaldehyde at various initial concentrations was elucidated according to the Langmuir-Hinshelwood model.
The results showed that TiNH400 had the finest size of about 30-40 nm and the largest surface area among the modificated photocatalysts examined in this study. In opposition to Ag/TiO2, TiNH400 aggregated more easily to form larger particles with size up to hundreds of nm. The absorption spectra of the TiNH400 indicated a stronger absorption than those of other modificated photocatalysts. Also its absorption spectra were shifted to a lower energy region of about 400-440 nm, which revealed the ability of adsorption of the visible light and near UV light.
The photo-reaction with Ag/TiO2 had better ηD and ηM than those with TiO2 and with UV lamp alone (without catalyst). Under the initial concentration of 500 ppmv formaldehyde, the ηD of formaldehyde reached 82% at 1 h with 365 nm UV lamp (16 W) and 0.05 g Ag/TiO2 coated on glass plates. However, the ηD of formaldehyde became 80% at 1 h with 254 nm UV lamp (16 W) and 0.05 g Ag/TiO2 coated on glass plates. The lamp emitting 365 nm UV was better than that emitting 254 nm UV in this study. Applying the 383 nm UVLED light (Δλ = 18 nm) with 800 mW (20 mW of one LED and 40 totally) and 0.05 g Ag/TiO2 coated on glass plates gave 58% and 65% of ηD at 1 and 7 h, respectively. Using the glass sticks with 0.334 g catalyst instead of the glass plates with 0.05 g catalyst enhanced the ηD to 96% and 95% with 365 nm UV lamp of 16 W and 383 nm UVLED of 800 mW (20 mW of one LED and 40 totally) at 7 h, respectively. The ηM also increased apparently for the case using the glass sticks coated with catalyst. Regarding the humidity effects, the results showed that the ηD increased slightly as relative humidity increased. Using UVLED light as light source in this study can enhance the safety and energy usage efficiency for the application of photocatalytic technology. Thus, this study showed the feasible and potential use of UVLED.
The results also showed that the finally ηD of formaldehyde increased with the higher light energy and shorter wavelength of light sources. Therefore, the ηD in the UV light illumination system was better than that in the visible light illumination system. However, the visible light illumination system still had ηD of 50-60% via LEDs (BLED and WLED). The results of photocatalytic activity of N-doped TiO2 indicated that the photocatalytic activity of the N-doped TiO2 was higher than that of the commercial TiO2 photocatalyst Degussa P25 for the decomposition of formaldhyde under visible light irradiation. It seemed that nitrogen atoms in doped TiO2 polycrystalline powder were responsible for the significant enhancement of the photoactivity of N-doped TiO2 under visible light irradiation. Therefore, the energy effectiveness of LED was higher than those of the traditional lamps. Considering all such advantages of LED and photocatalysts, the potentially high market value for indoor air pollutant cleaners applying the said technology can be anticipated in the near future.

並列關鍵字

Volatile organic compounds (VOCs) ； formaldehyde ； light emitting diode (LED) ； Ag/TiO2 ； indoor air pollutant ； N-doped TiO2.

參考文獻

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