利用衝擊波管研究高溫下亞甲基結合反應及其與乙烯反應之動力學

Abstract

我們利用活塞型衝擊波管‐原子共振吸收光譜(ARAS)技術研究了在1750 - 2000 K溫度區間之C2H4熱解、CH2自由基之自身結合反應及CH2與C2H4反應之動力學，利用動力學軟體ChemKin模擬適解由實驗所測得之氫原子濃度變化圖譜，進而求得反應途徑分支比以及反應速率常數。10 ppm、20 ppm C2H4熱解結果顯示氫原子之產生非主要途徑，分支比為ψH1(C2H2 + 2H)：0.04±0.01。CH2自身結合反應在高溫狀態下會先形成激發態的C2H4，其相對位能高於基態之C2H4 ,我們預估相同分子以不同能態為起始點，在高溫下裂解會得到不同的反應途徑分支比。我們利用CH2I2(3 ppm、5 ppm)作為CH2的來源，其ψH2為0.14±0.01，與Jasper團隊利用理論計算(VRT-TST)方法所提出的結果(ψH2 = 0.8)有很大差異；但與Bauerle團隊所報導的(ψH2 = 0.14)相符。此外我們還進行0.5 ppm CH2I2 + 10 ppm C2H4 在2005 K下反應，並與C2H4/Ar熱解結果做對照，得到1CH2在高溫狀態下與C2H4及C2H2之反應動力學資料。約為K. L. Gannon團隊在低溫範圍(300-800 K)之反應速率常數值之1/3：k4-10a (1CH2 + C2H4=>aC3H5 + H) = 1.66 × 10-11 (cm3 molecule-1 s-1)；k4-12 (1CH2 + C2H2=>C3H3 + H) = 5.00 × 10-11 (cm3 molecule-1 s-1)。

A diaphramless shock tube coupled with atomic resonance absorption spectrophotometry (ARAS) was employed to study the kinetics of the dissociation of C2H4, and the reactions of CH2 with CH2 and C2H4 between 1750 and 2000 K. Kinetics software Chemkin was used to fit the experiment data to obtain the reaction rate constants and branching ratios. 10 ppm、20 ppm C2H4 in Ar were applied in its thermal decomposition study, and we found that the hydrogen atom producing channel is minor with a branching ratioψH1(C2H2 + 2H) = 0.04±0.01. Recombination of CH2 at high temperature will produce excited C2H4. We anticipate that the decompositions of these molecules with high internal energy would proceed through different production channel from the ground state C2H4. We used CH2I2(3ppm、5ppm) as the source of CH2, and obtained the hydrogen atom producing channel ratioψH2(C2H2 + 2H) = 0.14±0.01, which is consistent with that reported by Bauerle et al. (ψH2 = 0.14); but disagrees with a theoretical result by Jasper et al. (ψH2 = 0.80). Furthermore, we also conducted the experiment of 0.5 ppm CH2I2 with 10 ppm C2H4 at 2005 K and compared the results to the back-to-back study of C2H4/Ar under the same experimental conditions. We predicted that 1CH2 would participate in the mechanism of this reaction. Our study yielded rate constants for CH2 + C2H4 and CH2 + C2H2：k4-10a (1CH2 + C2H4  aC3H5 + H) = 1.66 × 10-11 (cm3 molecule-1 s-1)；k4-12 (1CH2 + C2H2 C3H3 + H) = 5.00 × 10-11 (cm3 molecule-1 s-1), which are ~1/3 of the values reported by Gannon et al. at low temperatures (300-800 K).