以改質二氧化鈦光電催化處理室內甲苯之研究

Abstract

揮發性有機物(VOCs)為室內主要空氣污染物，與室內空氣品質(IAQ)有關，且被認為是引起病態大樓症候群(SBS)的原因之一。其中室內裝潢材料、室外汽機車、消費型產品、吸菸等，會產生不同程度的甲苯逸散，造成人體健康危害。 目前室內VOCs處理技術以光催化(Photocatalytic Oxidation，PCO)搭配紫外光為主，但有礦化率不足的問題，光催化反應產生的中間產物可能毒性更高，反而造成對室內人員的健康危害。另外，紫外光應用於室內環境，仍有安全疑慮。 本研究於光反應室中進行光電催化(Photoelectrocatalysis，PEC)反應，反應室內部放置披覆光觸媒之蜂巢狀金屬載體，能使電流均勻流經整個反應器，確保光電催化反應的進行。實驗使用的紫外光及可見光燈管波長分別為254nm及420nm，光觸媒選用Degussa P25 TiO2商業光觸媒及改質Ag/AgBr/TiO2光觸媒，溫度控制在25±1℃，濕度控制在30%。實驗之影響因子包含甲苯進流濃度、氣體流率、光波長及電壓。 本實驗參考文獻使用高反應效率比例(139.2%)配製Ag/AgBr/TiO2，與一般P25二氧化鈦進行光催化及光電催化室內甲苯比較。由實驗可知，以紫外光照射改質Ag/AgBr/TiO2，在流速0.5LPM及甲苯濃度0.2ppm下，光電催化轉化率可達95.9%；單照可見光的轉化率最高為59.9%，外加電壓後轉化率大幅提升至97.4%。而一般TiO2在相同條件下最高轉化率為97.7%。實驗結果發現進流甲苯濃度越高，轉化率越大，推測原因為光觸媒表面活性位址有限，當甲苯濃度較高時，光觸媒表面活性位址不足導致反應不完全，降低光催化效率。 利用GC-MS測量光電催化反應的產物及礦化率，發現光催化降解甲苯主要副產物為苯甲醛及苯甲酸，苯甲酸在光觸媒表面的累積是造成觸媒失活的主要原因。另外，部分苯甲醛上的氫被甲基取代反應形成苯乙酮。以紫外光照射TiO2，礦化率為85%至89%，產生較多副產物。使用改質Ag/AgBr/TiO2光觸媒，不論是照紫外光或可見光，處理甲苯的礦化率皆可以維持在95%以上，證實Ag/AgBr/TiO2改質光觸媒有助於提升處理甲苯的礦化率。 利用改質及未改質光觸媒進行5次光電催化循環實驗，TiO2照紫外光2小時後，觸媒表面有變黃漸漸失活的現象，再利用性不佳。而使用Ag/AgBr/TiO2在紫外光及可見光照射下，5次循環之後仍維持80%以上的甲苯轉化率，穩定性及再現性佳。實驗結果證明可見光光電催化搭配改質光觸媒處理室內VOCs具有未來之發展潛力。 由能源效益結果可知，Ag/AgBr/TiO2搭配可見光的能源效益(Ee)最高，兩個濃度下Ee值分別為0.2714mg kW-1 h-1及2.5891mg kW-1 h-1，而以紫外光為光源之兩種觸媒的能源效益非常接近。評估甲苯轉化率及能源效益，改質Ag/AgBr/TiO2搭配可見光光電催化處理室內VOCs，具有未來發展之潛力。

Volatile organic compounds (VOCs) are the major indoor air pollutant which are associated with indoor air quality (IAQ). It is considered as a cause of sick building syndrome (SBS). Source of VOCs indoor including upholstery materials, outdoor motor vehicle, consumer products and smoking, etc., will produce different levels of toluene emission, causing human health hazards. Currently, the technology of indoor VOCs treatment is based on photocatalytic oxidation (PCO) with UV light, but still have the problem of insufficient mineralization rate. The intermediate products produced by photocatalytic reaction may be more toxic, causing health hazards to indoor personnel instead. In addition, there are still have safety doubt for UV light apply to the indoor environment. In this study, toluene was degraded by photoelectrocatalytic (PEC) reaction in the photoreaction chamber, and a honeycomb metal monolith was placed inside, which can make the current flow through the whole reactor uniformly to ensure the process of photoelectrocatalytic reaction. The UV and visible light were controlled at 254nm and 420nm, and the photocatalyst was choosed the Degussa P25 TiO2 and modified Ag/AgBr/TiO2. The temperature was controlled at 25±1℃ and the humidity was maintained 30%. The impact factors of the experiment including toluene concentration, gas flow rate, light wavelength and voltage. In this study, Ag/AgBr/TiO2 was prepared with high reaction efficiency ratio (139.2%), and compared photocatalytic and photoelectrocatalytic activity with the P25 titanium dioxide. When Ag/AgBr/TiO2 was irradiated with ultraviolet light, the photoelectrocatalytic conversion of toluene was 95.9% at 0.5LPM flow rate and 0.2ppm toluene concentration. The photoelectrocatalytic conversion was significant increase from 59.9% to 97.4% after added voltage under visble light irradiation. And the highest conversion is 97.7% under the same conditions with using TiO2. The experimental results show that the higher the concentration of toluene is, the higher the conversion rate is. The reason is that active site of the photocatalyst surface is insufficient, causing the incomplete reaction , and reducing the photocatalytic efficiency. The products and mineralization rate of photoelectrocatalytic reaction were measured by GC-MS. It was found that the main byproduct of photocatalytic degradation of toluene was benzaldehyde and benzoic acid. The accumulation of benzoic acid on the surface of photocatalyst was the main reason of photocatalyst deactivation. In addition, part of the benzaldehyde form acetophenone by substitution reaction. When TiO2 was irradiated with ultraviolet light, the mineralization was 85% to 89%, resulting in more by-products. Mineralization can be maintained at more than 95% when using Ag/AgBr/TiO2 as photocatalyst, improving that it was helpful to increase the mineralization of toluene by modified photocatalyst. In the five times circulaing experiments, the TiO2 with poor reusability which was gradually deactivation after irradiated with ultraviolet light for 2 hours. It showed good stability and reproducibility that maintain at more than 80% conversion after 5 cycles when using Ag/AgBr/TiO2 as photocatalyst. The results of experiment showed the potential of processing indoor VOCs for future development by photoelectrocatalysis with modified photocatalyst and visible light. According to the result of energy efficiency, the energy efficiency of Ag/AgBr/TiO2 with visible light is the highest. The value are 0.2714mg kW-1 h-1 and 2.5891mg kW-1 h-1 respectively in two different concentration. And the energy efficiency are very close between two kinds of catalysts with UV light. Finally, evaluating the toluene conversion and energy efficiency, Ag/AgBr/TiO2 with visble light photoelectrocatalytic treatment of indoor VOCs has thepotential for future development.