以臭氧與電漿輔助觸媒氧化去除氣相萘之研究

Abstract

本研究以臭氧與電漿輔助觸媒氧化破壞去除氣相有機污染物。研究選定的目標物種為在多環芳香族 (polycyclic aromatic hydrocarbons, PAHs) 中化學結構最簡單、揮發性最高、且為氣相PAH污染物最具有代表性的萘 (naphthalene, Nap)。破壞去除方法包括臭氧觸媒氧化程序 (ozone-catalytic oxidation process, OZCO)、大氣電漿噴射束 (atmospheric-pressure plasma jet, APPJ) 及觸媒大氣電漿噴射束 (catalytic-APPJ, CAPPJ) 等之新穎技術。 OZCO程序分解Nap實驗主要系統參數為反應溫度 (T)、空間流速 (space velocities, SV)及O3進流濃度 (inlet concentrations of ozone, CO3,in)。實驗結果顯示在相同Nap轉化率 (conversion, XNap) 下，反應所需溫度隨著CO3,in增加而降低。此外，XNap及其礦化率 (mineralization extent, MNap) 與CO3,in的增加呈現線性關係。比較XNap到達50%之反應所需溫度 (T50) 結果，CO3,in = 1,750 ppmv及SV = 100,000 1/h時，T50為460 K，低於相同條件無臭氧添加時之T50 (480 K)。反應動力模式考慮CNap,in及CO3,in之二階反應方程式，以描述Nap在Pt/γ-Al2O3觸媒與O3之反應行為。其活化能 (E) 及頻率因子 (Af) 為68.3 kJ/mol及5.36×1012 L2/(mol g-cat. s)。與CO3,in為零時之觸媒氧化程序活化能 (150.0 kJ/mol) 相比，E約降低了2.2倍。CO3,in = 0時，其Af為3.26×1017 1/s。對實際工程應用，降低反應所需溫度，即降低在高溫燃燒所需之操作成本。另外，MNap的增加表示觸媒的完全氧化能力增加，因而積碳量亦減少。故OZCO方法可應用於實際工業界之廢氣焚化系統，且其反應動力參數能提供工程設計使用。 探討APPJ及CAPPJ程序分解Nap/n-butanol (Nap/n-b)，n-butanol及vinyl chloride (VC) 時，其實驗主要系統參數為輸入功率 (input power, PWI)、電漿能量密度 (plasma energy density, Ē)、污染物初始濃度 (C0或Cin)、工作氣體 (air與air/Ar) 及觸媒之SV。分析APPJ尾氣之光譜圖顯示主要活性氧物種為氧原子及介穩態 (metastable) 氧分子 (230-330 nm)。不同工作氣體之光譜圖顯示Ar離子會與污染物Nap/n-b激烈碰撞，在600-800 nm產生可見光波峰，因此電漿系統溫度 (plateau temperature, TP) 也比單獨air為工作氣體時高。另外，TP隨著PWI增加而增加，表示電漿能量從加速電子轉移到氣體介質，促使電漿氣體TP提高。當觸媒量增加 (即SV減低或接觸時間增長)，其TP降低。此現象表示觸媒吸附許多活性物質及帶電粒子在其表面上，造成氣相上的能量轉移減少。 APPJ實驗結果顯示TP受到PWI (或Ē) 之影響，PWI越高TP越高，目標污染物轉化率亦提高。結合Pt/γ-Al2O3觸媒之CAPPJ系統能有效提升其對污染物之分解效率及反應速率。在相同PWI下，觸媒量增加 (即SV減低或接觸時間增長)，則污染物轉化率提升。在air/Ar的Nap/n-b系統，當PWI = 250 W及無觸媒時，其XNap及Xn-b分別為30%及19%。而相同PWI及SV = 26,400 1/h時，其轉化率Xn-b及XNap分別高達96%及98%以上；在SV = 17,400 1/h，XNap及Xn-b可到達99%以上。比較單成份n-butanol系統，其Xn-b與雙成份Nap/n-b之Xn-b無明顯差異。此外，污染物初始濃度較高，其轉化率較低。例如，在air系統中，當PWI = 250 W而無觸媒時，CVC,in = 200 ppmv及450 ppmv之XVC分別為14.0%及5.4%。在PWI = 250 W且SV = 17,400 1/h時，CVC,in = 200 ppmv及450 ppmv時之XVC分別為49%及39%。 本研究亦提出單獨APPJ電漿程序及結合觸媒之CAPPJ操作程序分解Nap、n-butanol及VC之動力模式，以建立C/C0與各系統參數間之關係。模擬結果與實驗結果具有良好符合度。污染物初始濃度在APPJ電漿系統之反應動力階數為–1.5；於CAPPJ之反應物初始濃度及觸媒量之反應動力階數分別為–0.5及1。所提出之動力模式能有效描述反應行為，並提供電漿設備之操作設計使用。

This study investigated ozone- and plasma-assisted catalytic oxidation for the decomposition of gaseous organic compound in gas phase. Naphthalene (Nap), the most volatile, simple structured and abundant polycyclic aromatic hydrocarbons (PAHs) observed in the atmosphere, is taken as a target compound. The processes examined include the ozone catalytic oxidation (OZCO) and radio frequency-powered atmospheric-pressure plasma jet (APPJ) oxidation and APPJ with Pt/γ-Al2O3 catalysts, noted as catalytic APPJ (CAPPJ) oxidation. The OZCO experiments were carried out at various constant reaction temperatures (T), space velocities (SV) and inlet concentrations of ozone (CO3,in) for the decomposition of Nap. The results indicate that the required T for the effective decomposition of Nap decreases with the increase in CO3,in at the same conversion level of Nap (XNap). Further, the values of XNap and mineralization extent of Nap (MNap) increase linearly with the increase of CO3,in. Regarding the T at XNap = 50% (T50), there is about 20 K reduction at SV = 100,000 1/h for the case of OZCO process with CO3,in of 1,750 ppmv (T50 = 460 K) compared to the process without ozone (T50 = 480 K). Further, the power law can be applied to describe the data of OZCO process by using the second order expression with respect to ozone and Nap concentration. The observed activation energy (E) and frequency factor (Af) are 68.3 kJ/mol and 5.36×1012 L2/(mol g-cat. s), respectively. For the sole catalytic oxidation process (CATO), the reaction is first order respect to Nap with E and Af of 150.0 kJ/mol and 3.26×1017 1/s. A significant reduction of E for the OZCO compared to CATO reflects the enhancement role of ozone in conjunction with catalyst. The APPJ experiments were carried out at various input power (PWI), plasma energy density (Ē), initial concentration of pollutant (C0 or Cin), working gas (air and air/Ar) and SV of the catalyst for the decomposition of Nap/n-butanol (Nap/n-b), n-butanol and vinyl chloride (VC). The emission spectra of APPJ discharge show that the oxygen atom and metastable oxygen molecule (230-330 nm) are possibly the dominated active species in the jet effluent. The visible glow (600-800 nm) results from the interaction of discharged Ar molecule and Nap/n-b. It turns out that the value of plateau temperature (TP) of reactor in the presence of Ar is higher than that in air. Further TP increases with PWI because of more energy transfer from the heated electrons to gas molecules. However, TP decreases in the presence of the catalysts. It suggests that the role of the catalyst acts as a conducting medium with low electrical resistance and high surface area, absorbing charged as well as active species for the catalytic decomposition. The effectiveness of plasma-assisted catalysis is evident as indicated by the increase of conversion and rate constant. At PWI = 250 W without catalyst, the values of XNap and Xn-b are 30% and 19%, respectively. In the presence of Pt/Al2O3 catalyst with SV = 17,400 1/h, XNap and Xn-b can reach as high as 99%. Note that the Xn-b in Nap/n-b is close to XN-b in n-butanol system. Therefore, the interaction of Nap and n-butanol is minor in APPJ and CAPPJ. The conversion decreases with increasing initial concentration. For example, at PWI = 250 W without catalyst, the values of XVC are 14% and 5.4% for CVC,in = 200 and 450 ppmv, respectively. At PWI = 250 W with SV = 17,400 1/h, XVC for CVC,in = 200 and 450 ppmv are 49% and 39%, respectively. The kinetic models were proposed to describe the relationships of C/C0 (or =C/Cin) with the major parameters for the plasma and plasma-assisted catalytic decomposition of Nap, n-butanol and VC, showing good agreement with the experimental data. For the plasma system of APPJ the reaction order is –1.5 with respect to Cin, while –0.5 and 1 with respect to Cin and catalyst amount, respectively, for the plasma-catalysis system of CAPPJ. The information obtained is useful for the rational design and operation for the destruction of gaseous pollutants via the OZCO, APPJ and CAPPJ processes.