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  • 學位論文

以過渡金屬氧化物觸媒與臭氧分解氯苯及戴奧辛之研究

Catalytic Oxidation of Chlorobenzene and PCDD/Fs with Ozone over Transition Metal Oxides

指導教授 : 張木彬
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摘要


本研究之目的是開發低溫(<200 °C)、高效、便宜的氣相氯苯及戴奧辛分解技術,採用結合臭氧與過渡金屬氧化物觸媒(如:氧化鐵或氧化錳)方式(簡稱臭氧觸媒氧化法),於60-200 °C進行氣相氯苯及戴奧辛氧化分解之測試,分別探討反應溫度、臭氧添加濃度、空間速度、含水率等操作因子對轉化率的影響,再者,對於觸媒反應的動力學探討、觸媒長效性及產物分析等也將進行瞭解。 首先,以戴奧辛前驅物(氯苯)作為測試對象,實驗結果顯示:氯苯於200 °C、20% O2時,氧化鐵觸媒對氯苯的轉化率非常低(<5%),然而,添加1,200 ppm臭氧於氧化鐵觸媒,只需150 °C、極短的滯留時間(300,000 h-1),即有90%以上的氯苯轉化率;一氧化碳及二氧化碳是唯一可偵測到的氣相含碳反應產物,反應生成的一氧化碳、二氧化碳濃度與碳回收率皆隨臭氧添加量的增加而遞增。長效測試結果顯示:氧化鐵觸媒對氯苯的轉化率能保持96小時以上的穩定性,然而,氧化錳的轉化活性卻呈現些微下降(3%),其原因可能是反應生成的中間產物沉積於觸媒表面;其次,氧化錳觸媒會與氯離子生成MnCl2導致觸媒失活。使用後的氧化錳觸媒可於20%氧氣、400 °C下進行加熱再生。動力實驗結果顯示:以Langmuir-Hinshelwood 模式能合理解釋臭氧觸媒分解氯苯的反應行為,臭氧於觸媒表面分解生成的原子氧物質,是低溫氧化氯苯之重要氧化劑。於20%O2時,氧化鐵分解氯苯的活化能為43 kJ mol-1,添加O3後,反應的活化能明顯降至17 kJ mol-1。顯示添加臭氧不僅能有效降低觸媒分解氯苯的反應溫度,還能降低反應之活化能。 第二部份是探討臭氧觸媒氧化法對氣相戴奧辛的去除,結果顯示:添加100 ppm臭氧於氧化鐵觸媒,180 °C時、戴奧辛的分解率可達90%以上。再者,針對17種不同的戴奧辛同分異構物進行分析,在不含臭氧、120 °C時,各種異構物的分解率皆低於20%,顯示大部份只吸附於觸媒表面,當添加臭氧後,於180 °C,各種異構物的分解率皆高於80%。最後,適量的水氣有助於分解氣相戴奧辛,於150 °C、5%含水量時、戴奧辛分解率達90%,其原因可能是含水氣時,會生成高反應性的OH自由基,另外,也可移除觸媒表面的氯離子。 綜觀來說,臭氧觸媒氧化法使用的是綠色、便宜的環保觸媒材料,配合少量臭氧的添加即能低溫、高效分解氣相氯苯及戴奧辛,於實廠應用,可直接安裝於集塵機或洗滌塔之出口。

並列摘要


This dissertation describes a quest to develop a low-temperature (<200 °C), cost-effective, and energy-saving technology for the destruction of chlorobenzene (CB) and dioxins [polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs)]. The catalytic oxidation of gaseous CB and PCDD/Fs with ozone [O3 catalytic oxidation (OZCO)] over transition metal oxides (iron oxide and manganese oxide) was investigated at temperatures of 60–210 °C, while monitoring the effects of operating temperature, O3 concentration, space velocity (SV), and water vapor content on conversion. Moreover, the catalyst stability and the kinetics of the conversion processes were also studied. CB was tested initially as a model dioxin compound to evaluate the oxidation behavior. Substituting O3 for O2 during the course of catalytic oxidation of CB decreased both the operating temperature and the activation energy. In the absence of O3, the iron oxide and manganese oxide catalysts were almost inactive toward CB oxidation at temperatures up to 200 °C in 20% of O2. In contrast, high conversions of CB (ca. 90%) were obtained in the presence of 1,200 ppm of O3 at a relatively low temperature (150 °C) and a short gas residence time (GHSV = 300,000 h–1). CO and CO2 were the only carbon-containing products detected in the effluent gas stream; their concentrations and the carbon recovery all increased upon increasing the concentration of O3 from 0 to 1,200 ppm. The iron oxide catalyst provided a stable conversion during testing for more than 96 h. In contrast, a 3% reduction of the CB conversion occurred over manganese oxide and O3 because (i) small amounts of partially oxidized byproducts (e.g., carboxylic acids) deposited on the surface of the catalyst and (ii) deactivating MnCl2 species were formed during the oxidation process. Complete regeneration of the manganese oxide catalyst was possible at temperatures higher than 400 °C in 20% of O2. The Langmuir–Hinshelwood model adequately described the kinetic behavior for the oxidation of CB over iron oxide and O3, suggesting that the atomic oxygen species that formed on the surfaces of the catalyst upon the decomposition of O3 played important roles in the catalytic oxidation of CB. In the second part of this study, total oxidation of gaseous PCDD/Fs was investigated using the OZCO process. The addition of O3 greatly enhanced the catalytic activity of the iron oxide catalyst toward the oxidation of gaseous PCDD/Fs. At 180 °C, the destruction efficiencies of gaseous PCDD/Fs over iron oxide and 100 ppm O3 exceeded 90%. At a relatively low temperature of 120 °C and in the absence of O3, the destruction efficiencies of all PCDD/F congeners were less than 20% and decreased upon increasing the degree of chlorination of the dioxin congener. At 180 °C in the presence of O3, however, the destruction efficiencies were greater than 80% for all of the PCDD/F congeners over iron oxide. Moreover, in the presence of 5% water vapor, the destruction efficiencies of PCDD/Fs were greater than 90%, even at a relatively low operating temperature of 150 °C. Thus, the presence of an appropriate amount of water vapor enhanced the catalytic activity for the decomposition of gas-phase PCDD/Fs, presumably because (i) highly reactive OH radicals were formed to oxidize PCDD/Fs and (ii) water vapor facilitated the removal of Cl– ions from the catalyst surfaces. Overall, this OZCO method—using eco-friendly, cost-effective iron oxide and manganese oxide catalysts and small amounts of O3—is a novel, feasible, and economical approach for the removal of low-concentrations of CB and dioxins from gas streams. It can be employed directly to treat the flue gas at the outlet of a baghouse or scrubber in field application.

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