以紫外光/臭氧程序增進光觸媒對室內生物源揮發性有機物去除效率之研究

Enhancement Effect of UV/O3 on the Effectiveness of Photocatalytic Oxidation for Removing Indoor Biogenic Volatile Organic Compounds

黃國豪(Guo-Hao Huang)

指導教授 ： 李慧梅

國立臺灣大學/工學院/環境工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.00712

摘要

揮發性有機物(Volatile organic compounds, VOCs)一直是室內環境中的主要汙染物，且被認為是引起病態大樓症候群的原因。其中，像是木質傢俱及裝潢塗料或是清潔劑及香水等消費型產物甚至是使用精油芳香療法來試圖達到提神或改善室內空氣的作用時，其所產生的檸檬烯等生物源揮發性有機物會有不同程度的逸散。長時間的暴露加上檸檬烯與氧化物質反應產生次級氣膠的能力很強，所以本實驗選擇檸檬烯當作目標汙染物。
而揮發性有機物的處理方法很多，其中，由於光催化反應在室溫下有處理效率高、較少的能源消耗及氧化完全的優點，且較無操作成本過高的問題，所以成為近來發展最廣的室內空氣清淨技術。加上有文獻指出臭氧對光催化反應有正面的效應，且臭氧本身與檸檬烯的反應性很強，所以實驗添加臭氧。但由於臭氧在室內環境中有其規範濃度值，所以本實驗探討不同環境條件下添加臭氧後，光催化增進效率與尾氣中臭氧之去除效率間的關係。
影響光催化效率的因子有很多，但本實驗僅探討反應物的濃度、相對濕度、氣體流率及臭氧濃度的影響，而將其他條件固定，像是溫度控制在 25±1℃、紫外光波長為 254 nm、固定紫外光強度及選用奈米級之 Degussa P25 TiO2為實驗觸媒。
實驗結果顯示，在氣體流率大於 1600 ml/min時，氣相質傳效應可以忽略，且此時光催化反應動力模式符合雙分子之 Langmuir-Hinshelwood model。而光催化反應中的轉化率及二氧化碳產率都隨著檸檬烯濃度的增加而遞減，但反應之氧化速率則相反，其中臭氧去除效率從 45.01％提升到 53.77％，且反應後之臭氧濃度值多為接近或低於 50 ppb。而由反應動力模式回歸模擬可得在相對濕度20％、50％、80％檸檬烯之氫氧根自由基反應常數(k_OH)分別為 15.6、19.7、16.0 μ-mole/m2-s，彙整文獻中之 toluene、 p-xylene、 m-xylene、 mesitylene物種之氫氧根自由基反應常數，可知光觸媒反應之速率常數(k)與氫氧根自由基反應常數無論在相對濕度為何，皆是呈現一個線性的正相關。而由反應動力模式回歸模擬也可得不同濕度下檸檬烯與水分子之 Langmuir吸附常數(K_1 、K_2)，彙整上列文獻中之物種可得其 Langmuir吸附常數的倒數與亨利定律常數(K_H)無論相對溼度為何皆是呈現一個線性的正相關。
在光催化反應中，隨著相對濕度增加，反應轉化率及二氧化碳產率皆在相對濕度約 40％前後分別呈現提升及下降的趨勢，中間產物的殘餘量則呈現持續下降，但在相對濕度約 40％後呈現趨緩的現象。然而添加臭氧可使得觸媒在相對濕度提升至 30％時，就可以有效的減少中間產物殘餘量，延長觸媒的使用年限，且可以在高相對濕度下，將中間產物殘餘量控制在 10~20%間，使礦化更完全。
在光觸媒反應中，臭氧濃度的增加會增加反應的轉化率及二氧化碳產率，但卻會造成中間產物的殘餘量增加。由實驗結果可得檸檬烯之臭氧增進效應指標為 4.15×〖10〗^(-4) μ-mole-m-2-s-1/ppb-O3，結合文獻中上述物種更可知，化合物之氫氧根自由基反應速率常數與臭氧之增進效應指標可能呈現正相關的趨勢。
在光催化反應中，當臭氧濃度增加時，反應之臭氧去除效率是呈現上升的趨勢，其中，有無檸檬烯之臭氧去除效率分別從 50.13％提升到 89.93％及從 40.23％提升到 77.29％，發現檸檬烯確實會與臭氧反應，且當臭氧濃度增加時，光催化的反應速率也會提高，所以，推測光催化反應速率與臭氧的去除效率有關。

關鍵字

光觸媒 ； 臭氧 ； 揮發性有機物 ； 檸檬烯

並列摘要

Volatile organic compounds(VOCs) is one of the major indoor air pollutants, and is considered as an cause for sick building syndrome. Using wood furniture , paint, cleanser and perfume even for freshening and improving indoor air quality by Aromatherapy will produce biogenic volatile organic compounds like limonene. Because of long time exposure and possible production of secondary organic aerosols from reaction between limonene and oxidant so in this essay I choose limonene as a target pollutant in this study.
Among the indoor air-cleaning technology for controlling VOCs, photocatalytic oxidation (PCO) reaction is widely used in recent years because of the advantages such as higher cleaning efficiency, the lower energy consumption, and the fully oxidation and lower operation cost. Some researches have demonstrated that ozone has positive effects on effectiveness of PCO, and also ozone has high oxidation with limonene. The objective of this research was to investigate the enhancement effect of ozone on VOCs removal efficiency and the ozone removal efficiency.
There are many factors influencing PCO efficiency. In this study, the effects of limonene concentration, reactive humidity, gas flow rate and the ozone concentration on the removal of limonene were investigated. Other factors were fixed like controlling temperature on 25±1℃, using UV light source (wavelength is 254 nm), constant UV intensity, and choosing Degussa P25 TiO2 for the catalyst in this study.
The effect of gas-phase mass transfer was negligible when gas flow rate was higher than 1600 ml/min. And the PCO kinetics fitted Langmuir-Hinshelwood model for bimolecular competitive adsorption form. The VOCs conversion and CO2 evolution decreased with increasing limonene concentrations. However, the VOCs oxidation rate has opposite effect. In this experiment, removal efficiency of ozone ranged from 45.01% to 53.77%. After the reaction, concentration of ozone are close or below to 50 ppb.
The VOCs-hydroxyl radical rate constants (k_OH) of limonene is 15.6, 19.7, 16.0 μ-mole/m2-s respectively for humidity 20%, 50%, 80%. PCO rate constants of toluene, p-xylene, m-xylene, mesitylene, and limonene were proportional to k_OH no matter what humidity is. Getting together with Langmuir adsorption constants of the above-mentioned VOCs and water. A linear positive relationship was found between reciprocal of Langmuir adsorption constants and Henry’s Law constants no matter what reactive humidity is.
Increasing reactive humidity showed a dual effect on VOCs conversion, CO2 evolution, and the residual intermediate. The reactive humidity turning point is 40％. The experiment showed that adding ozone can reduce it significantly when the humidity move up to 30％. And it showed that in high humidity, adding ozone can control the residual intermediate on 10~20%, and extending the catalyst effectiveness.
In PCO reaction, the VOCs conversion and CO2 evolution increased with increasing concentration of ozone. But it will enhance the residual intermediate relatively. The slopes of plot of VOCs oxidation rates & ozone concentration were defined as enhancement indices of ozone. The experiment result showed enhancement indices of ozone on limonene is 4.15×〖10〗^(-4) μ-mole-m-2-s-1/ppb-O3. Getting together with above-mentioned species, enhancement indices of ozone were proportional to k_OH.
In PCO reaction, removing efficiency of ozone and VOCs oxidation increased with increasing the ozone removal efficiency. The ozone removal efficiency in the presence and absence of limonene ranged from 50.13% to 89.93% and from 40.23% to 77.29%. So this study found that VOCs oxidation is related to removing efficiency of ozone.

並列關鍵字

photocatalysis ； ozone ； VOCs ； limonene

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