以光電反應器搭配改質二氧化鈦降解甲醛之研究

Abstract

本國於2012年起實施室內空氣品質管理法，並陸續公告「應符合室內空氣品質管理法之第一批公告場所」及「應符合室內空氣品質管理法之第二批公告場所」，其中甲醛在16類公告場所中皆被列為管制項目。而常見的甲醛室內環境來源廣泛，包括合板、油漆、隔音材料、壁紙、地毯、室內燃燒行為及香菸，都有可能釋放甲醛。甲醛被國際癌症研究署(International Agency for Research on Cancer, IARC)分類為致癌物質，長期暴露可能造成鼻咽癌、白血病、血液淋巴腫瘤等疾病。而針對室內甲醛處理方式，仍以吸附材為主，然吸附材使用受限於吸附容量之限制，並有再脫附之可能，因此希望開發可長期使用的光觸媒降解方式。 本研究使用實驗室研發之蜂巢狀光電反應器，披覆Ag/AgBr/TiO2改質觸媒及P25- TiO2進行甲醛降解試驗。而實驗結果顯示，在停留時間3秒、進流濃度1 ppm之條件下，Ag/AgBr/TiO2觸媒於照射紫外光或可見光之條件下，可達到92.8~96.8%之甲醛轉化率，出流濃度可符合本國室內空氣品質標準，而P25-TiO2對應之轉化率僅為74.41%；停留時間3秒、進流濃度2 ppm，Ag/AgBr/TiO2之甲醛轉化率介於90.3~93.2%、P25-TiO2之轉化率為79.3%；停留時間1秒、進流濃度1 ppm，Ag/AgBr/TiO2轉化率轉化率於不同光源約為87.5%及91.3%、P25-TiO2轉化率為38.9%；停留時間1秒、進流濃度 2 ppm，Ag/AgBr/TiO2處理轉化率約為79.9%、P25-TiO2轉化率為58.2%。 實驗結果顯示，本實驗所製備之Ag/AgBr/TiO2於實驗條件之濃度區間及停留時間，有優於P25-TiO2之處理效果。整體之甲醛轉化率皆會隨停留時間縮短而降低，但隨濃度提升，P25-TiO2之轉化率提高，而Ag/AgBr/TiO2之轉化率則隨濃度提升而些微降低，可能與較高濃度甲醛能有效提升對P25-TiO2表面擴散速度有關，即P25-TiO2可能較適用於高濃度、高停留時間之降解方式。而光電催化於本實驗並未出現顯著影響，主要原因推測包括停留時間過短、甲醛進流濃度過低，使表面擴散成為速率決定步驟，進而使光電催化延長電洞維持時間之效果無法有效影響反應速率。 單位能源所分解的甲醛，以可見光搭配Ag/AgBr/TiO2處理6 LPM(停留時間1秒)、2 ppm甲醛氣體之46.01 mg /kWh為最優，其次為紫外光搭配Ag/AgBr/TiO2處理同條件甲醛氣體之28.77 mg /kWh次之，其中能量效率差異源自於紫外光燈管耗能較高。以降解甲醛達室內空氣品質為目的，使用可見光做為激發光源為最經濟之選擇，且Ag/AgBr/TiO2效果仍是遠大於未改質之P25-TiO2。

Indoor Air Quality Act has been announced by EPA since 2011 in R.O.C. Following administrative ordinances implemented in 2014 and 2017 requested that formaldehyde concentration of indoor air in 16 categories of public place should be monitored and controlled. Formaldehyde is listed as a carcinogenic chemical by IARC(International Agency for Research on Cancer) and IRIS (Integrated Risk Information System) of U.S. EPA, and long term exposure to formaldehyde could result in higher rick to Nasopharyngeal carcinoma, leukemia, Hemolymph tumor, etc. Most common indoor formaldehyde source include plywood, paint, acoustic insulating material and carpet made of material with formaldehyde, and indoor combustion such as cooking and smoking would emit different levels of formaldehyde. However, the most of the available commercial indoor air cleaners are equipped with adsorbent to control indoor VOCs including formaldehyde, which has operating restriction on adsorption capacity and possibility of desorption. Therefore, the focus of this study is to develop an indoor VOCs photocatalysis technique with stable characteristic. This study uses 2 different photocatalysts including modified Ag/AgBr/TiO2 composites and commercial P25-TiO2 coating on a honeycomb photoelectrocatalytic reactor to degrade formaldehyde in continuous air flow. The results show while operating with 3 sec retention time (RT) and 1 ppm inflow concentration (Cin), Ag/AgBr/TiO2 irradiated by UV and visible light have 92.8~96.8% conversion rate of formaldehyde, and P25-TiO2 has only 74.41%; and RT=3 sec, Cin=2 ppm, conversion rate corresponding to Ag/AgBr/TiO2 and P25-TiO2 are 90.3~93.2% and 79.3%. If the RT decrease to 1 sec, conversion rates of formaldehyde in 1 ppm with Ag/AgBr/TiO2 decreased to 87.5% ~91.3%, and 38.9% for P25-TiO2; in 2 ppm, conversion rates become 79.9% for Ag/AgBr/TiO2 and 58.2% for P25-TiO2. These results indicate that Ag/AgBr/TiO2 has better photocatalytic oxidation efficiency in dealing with formaldehyde in setting conditions of this study. Overall, the conversion rate would decrease with the RT in 2 catalysts, but conversion rate of P25-TiO2 would raise with higher inflow concentration. It could relate to higher surface diffusing rate in higher concentration, which means that P25-TiO2 is suitable for higher concentration and longer RT. However, photoelectrocatalysis didn’t show obvious change in conversion rate with photocatalysis. The major reasons possibly including short RT and low concentration in this study which causes surface diffusing rate to be rate-determining step and means that longer lifetime of excited electron-hole pair in photocatalyst can’t improve the efficiency of photocatalysis. Moreover, Ag/AgBr/TiO2 excited by visible light in this study has the highest energy efficiency to oxidate formaldehyde with 1 second RT and 2 ppm concentration (46.01 mg /kWh), the following combination is Ag/AgBr/TiO2 excited by UV lamp in the same conditions (28.77 mg /kWh). The deviation of energy efficiency is derived from higher energy consumed by UV lamp. It is most economical way to use visible light as energy source for Ag/AgBr/TiO2 to photocatalysis the indoor formaldehyde for the purpose of fitting IAQ.