以CuOx/ MnxCe1-xO2觸媒去除揮發性有機污染物-甲醛之研究; Catalytic oxidation of formaldehyde over CuOx/ MnxCe1-xO2 catalysts

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Title:	以CuOx/ MnxCe1-xO2觸媒去除揮發性有機污染物-甲醛之研究;Catalytic oxidation of formaldehyde over CuOx/ MnxCe1-xO2 catalysts
Authors:	李家維;Li,Jia-wei
Contributors:	環境工程研究所
Keywords:	甲醛;揮發性有機物;室內污染物;觸媒焚化;臭氧氧化催化;formaldehyde;volatile organic compounds;indoor air pollutant;thermal catalytic oxidation;ozone catalytic oxidation
Date:	2013-08-23
Issue Date:	2013-10-08 15:36:03 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究致力於開發低溫、便宜、高效率的氣相揮發性有機物甲醛去除技術，包括觸媒焚化(thermal catalytic oxidation)與臭氧氧化催化(ozone catalytic oxidation)兩種控制技術，分別探討反應溫度、擔體改質、臭氧含量、水氧含量及空間流速對甲醛去除效率的影響。第一部分利用觸媒焚化法，藉由改質MnxCe1?xO2擔體觸媒，由XRD結果顯示Mn的引入量為x=0.3、0.5時，大多呈現氧化鈰立方螢石型結構晶格，顯示離子半徑較小之錳金屬已滲入螢石型結構晶格中形成結晶度良好的固溶相，其中以Mn0.5Ce0.5O2有最佳活性表現T100= (270~275?C)，為降低操作溫度也利用含浸法方式，將5%CuOx含浸至MnxCe1-xO2擔體上，結果顯示溫度下降40?C並有最低之T100= (230~235?C)，比起商業擔體5%CuOx/γ- Al2O3之T100必需達305?C，溫度下降達75~80?C，顯示本研究開發之觸媒可在中等溫度操作。第二部分利用臭氧氧化催化法，文獻指出錳系觸媒對於分解氣流中的臭氧，具有較佳的分解效率，由自行製備p型的Mn2O3觸媒於常溫下操作分別有92.1%去除效率，將兩種p型金屬氧化物CeO2、 Mn2O3製備成Mn0.5Ce0.5O2共氧化物觸媒，去除效率則明顯提升至99.2%，因此Mn0.5Ce0.5O2為OZCO法選用之觸媒。利用OZCO法可於[O3]/[HCHO]=3、溫度(T)=25?C時即有83.3%之去除效率，即達到觸媒焚化法(thermal catalytic oxidation)在185?C時的效率，顯示OZCO法(ozone catalytic oxidation)可大幅降低觸媒焚化法所需之反應溫度。未添加臭氧情況下，270?C時甲醛的去除速率為0.222 μmol HCHO/g-cat./s，加入203 ppm 臭氧使甲醛的去除速率大幅提升至1.49 μmol HCHO /g-cat./s，比在沒有臭氧的狀況下高出6.71倍，顯示結合臭氧可有效提高甲醛氧化的能力。80小時觸媒耐久性測試結果顯示，Mn0.5Ce0.5O2觸媒可維持高的甲醛去除率，並且失活情況不明顯。綜觀而言，臭氧氧化催化法使用綠色、便宜的錳系觸媒材料，搭配適量的臭氧添加即能在低溫條件下、有效氧化氣相甲醛，就工業製程廢氣及室內空氣品質控制而言具實場應用潛力。 The aim of this study is to develop a low-temperature, cost-effective, and energy-saving technology for the destruction of volatile organic compounds (VOCs) such as formaldehyde. The effectiveness of thermal catalytic oxidation (TCO) and ozone catalytic oxidation (OZCO) in removing formaldehyde from gas streams will be studied. Catalytic reactors are constructed to investigate the effects of operating temperature, O3 concentrations, water vapor, and gas hour space velocity on formaldehyde removal efficiency. In the first part of this study, thermal catalytic oxidation of formaldehyde was investigated using MnxCe1?xO2 catalysts with different Mn/Ce ratios. At x = 0.3 and 0.5, most of manganese was fixed in the fluorite structure of CeO2 to form a solid solution. The best performance of the catalytic activity was found with x=0.5, which has a minimum T100= (270 ~ 275?C). T100 of HCHO oxidation over Mn0.5Ce0.5O2 (270~275?C) exceeded that over 5%CuOx/ Mn0.5Ce0.5O2 by about 40 ?C. On the other hand, T100 of HCHO oxidation over 5%CuOx/γ- Al2O3 was 305?C. Even though only MnxCe1?xO2 catalysts have high oxidation ability toward HCHO, experimental results indicate that the modified catalysts can be operated at moderate temperatures. In the second part of this study, complete oxidation of formaldehyde was investigated with OZCO. Ozone decomposition efficiencies of the p-type oxide Mn2O3 is 92.1%. Apparently, the modified catalyst has the highest ozone decomposition efficiency of 99.2%. Therefore, catalytic oxidation of gaseous formaldehyde with ozone over Mn0.5Ce0.5O2 was experimentally carried out with a packed bed reactor to investigate the feasibility of the low-temperature decomposition process. HCHO removal efficiency achieved is 83.3% at 25?C and [O3]/[HCHO] =3, the same removal efficiency is achieved when TCO is operated at 185?C. In addition, the removal rate of formaldehyde is 0.222 μmol HCHO/g-cat./s for TCO at 270?C, while it is significantly raised to 1.49 μmol HCHO/g-cat./s with the OZCO process. Addition of O3 greatly enhanced the catalytic activity of the Mn0.5Ce0.5O2 catalyst toward the oxidation of formaldehyde. With the gas hour space velocity of 10000 hr-1, [O3]/[HCHO] =3 and temperature of 25?C, high formaldehyde removal efficiency (81.8%) is maintained during the durability test of 80 hours, showing its long-term stability. Overall, this OZCO process applies eco-friendly, cost-effective manganese oxide catalysts to remove HCHO. Adding appropriate amount of ozone results in the effective removal of formaldehyde at room temperature. It can be employed directly for industrial emissions and indoor air quality control.
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