利用蜂巢狀光觸媒處理甲醛之研究

Abstract

揮發性有機物(Volatile organic compounds, 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。不同室內揮發性有機物來源有所差異，其中室內木質家俱、消費型產品，抽菸及烹飪等，會產生不同程度之甲醛逸散。甲醛對人體健康影響甚重，IARC將其列為第一類致癌物質，在台灣2012年11月公布之室內空氣品質管理法，對室內甲醛濃度進行規範(0.08 ppm)，故本實驗選擇甲醛當作目標污染物。 本研究於光反應器中進行光催化反應，光反應器內部放置蜂巢狀載體，藉由蜂巢狀載體增加反應面積、觸媒批覆量，並搭配光纖作為導光通道，確保紫外光可以深入光反應器內激發光觸媒。實驗之紫外光燈管為8 W，波長為254 nm、光觸媒選用Degussa P25 TiO2商業光觸媒、溫度控制在25±1℃。實驗之影響因子包含HCHO進流濃度、相對濕度、氣體流率。 實驗將氣體流率控制在1400 ml/min，避免氣相質傳效應影響光催化反應速率。在相對濕度(30%、40%、70%)，HCHO進流濃度由0.80 ppm至2.00 ppm時，光觸媒轉化效率隨濃度增加而上升；而在相對濕度(50%、60%)，HCHO進流濃度由0.80 ppm至2.00 ppm時，光觸媒轉化效率一開始呈上升趨勢，但於1.40 ppm時轉化率下降，之後HCHO濃度增加，轉化率上升。光觸媒反應速率在相對濕度固定的情況下，隨HCHO進流濃度增加，光觸媒反應速率呈線性增加。 在甲醛進流濃度0.80 ppm至2.00 ppm時，不論在何種相對濕度(30%、40%、50%、60%、70%)，光觸媒轉化效率皆可以維持在90%至95%，較佳相對濕度條件為40至50%。光觸媒反應中，相對濕度具有生成氫氧自由基及與反應物產生競爭吸附的作用，由實驗可知，相對濕度30%時，提高相對濕度有助於轉化率上升；相對濕度50%時，提高相對濕度會造成轉化率下降。 利用光催化反應雙分子競爭動力模式(Langmuir-Hinshelwood model)來進行模擬，可得在相對濕度30%下，HCHO與水分子之Langmuir吸附常數為0.03075與0.7432 ppm-1，HCHO之反應速率常數為0.04793 μmole/m2-s；相對濕度50%下，HCHO與水分子之Langmuir吸附常數為0.03221與0.7066 ppm-1，HCHO之反應速率常數為0.04503 μmole/m2-s；相對濕度70%下，HCHO與水分子之Langmuir吸附常數為0.5247與0.8923 ppm-1，HCHO之反應速率常數為0.00457 μmole/m2-s。

Volatile organic compounds (VOCs) are the major indoor air pollutants, which are significantly related to indoor air quality (IAQ), and it is considered as a cause for sick building syndrome (SBS). Different indoor volatile organic compounds come from different sources. Sources of formaldehyde indoor include wooden furniture, consumer products, smoking and cooking, etc., and each of them will cause different levels of formaldehyde emission. Formaldehyde may cause health problems and the international agency for research on cancer (IARC) classified it as a human carcinogen. The Indoor Air Quality Management Act of Taiwan, which restricted indoor formaldehyde concentration to 0.08 ppm, so formaldehyde was chosen as the aim pollutant in this study. In this study, formaldehyde was degraded by photocatalytic oxidation (PCO) in photocatalytic reactor, and placed a honeycomb monolith inside it, which can increase the reaction area and the amount of coated photocatalysts. The honeycomb monolith needs to be operated with optical fiber for UV light to excite photocatalysts. The UV light source was controlled at 254 nm and 8 W, controlling temperature at 25±1℃, and choosing Degussa P25 TiO2 for the catalyst in this study. The key factors of the formaldehyde removal efficiency including formaldehyde concentration, relative humidity and gas flow rate. The gas flow rate was controlled at 1400 ml/min, which avoided the gas phase mass transfer affecting photocatalytic reaction rate. Controlling HCHO inlet concentration from 0.80 ppm to 2.00 ppm at relative humidity of 30%, 40%, 70%, respectively, the photocatalytic conversion efficiency increased with the increasing inlet concentration. However, in the same inlet concentration gradient of HCHO, controlled the relative humidity of 50% and 60%, respectively, the photocatalytic conversion efficiency started an upward trend, but the efficiency decreased at 1.40 ppm. Then, the photocatalytic conversion efficiency increased with the increasing concentration. The photocatalytic reaction rate increases linearly with the increasing concentration at constant relative humidity. The conversion efficiency was maintained between 90% and 95% regardless of the relative humidity which was from 30% to 70% at inlet concentration of HCHO from 0.80 ppm to 2.00 ppm. Relative humidity may affect the production of hydroxyl radicals and the competitive adsorption in photocatalytic reaction. In this study, increasing relative humidity had an enhancement effect on conversion at humidity 30%; though the relative humidity 50% had an inhibition effect on conversion. The PCO kinetics fitted Langmuir-Hinshelwood model for biomoecular competitive adsorption form. The Langmuir adsorption constants of HCHO and water at relative humidity 30% is 0.03075 and 0.7432 ppm-1. The reaction rate constants of HCHO at relative humidity 30% is 0.04793 μmole/m2-s；The Langmuir adsorption constants of HCHO and water at relative humidity 50% is 0.03221 and 0.7066 ppm-1. The reaction rate constants of HCHO at relative humidity 30% is 0.04503 μmole/m2-s；The Langmuir adsorption constants of HCHO and water at relative humidity 70% is 0.5247 and 0.8923 ppm-1. The reaction rate constants of HCHO at relative humidity 30% is 0.00457 μmole/m2-s.