利用衝擊波管研究氫原子與丙烷、異丁烷 及正丁烷的高溫反應動力學

Abstract

我們利用活塞式衝擊波管-原子共振吸收光譜(ARAS)技術來研究氫原子和丙烷、正丁烷及異丁烷在溫度1000-1150 K之反應動力學，經扣除由丙烷、正丁烷及異丁烷在此溫度自行吸收之背景訊號後，再適解氫原子濃度隨時間之變化來得到各反應之分支反應速率常數:氫原子擷取丙烷及異丁烷上之一級碳上氫原子及正丁烷上之二級碳上氫原子。對於我們所得到的H + (C3-C4)烷類的支反應速率實驗結果歸納如下: (1) H + C3H8 → H2 + C3H7 : 〖"lnk" 〗_"4-6" ("T" )" = -(21.81±1.25)-(4.90±1.36)×" 〖10〗^3/T cm3molecule-1s-1 (2) H + i-C4H10 → H2 + i-C4H9 : 〖"lnk" 〗_"4-9" ("T" )" = -(18.74±1.74)-(8.02±1.88)×" 〖10〗^3/T cm3molecule-1s-1 (3) H + n-C4H10 → H2 + s-C4H9 : 〖"lnk" 〗_"4-13" ("T" )" = -(24.14±1.74)-(1.86±0.48)×" 〖10〗^3/T cm3molecule-1s-1 本實驗為此溫度區間首次之直接量測研究，所得結果與一些理論計算團隊利用量子化學計算過渡狀態及位能面後所得之速率常數值相比較。對於H原子抓取一級碳上氫原子(C3H8、i-C4H10)的實驗結果與理論計算的結果是相當符合的;而對於二級碳上氫原子(n-C4H10)的實驗結果就比理論計算的結果要小了超過30％。我們認為有可能是一級碳上氫的遮蔽造成二級碳上氫被抓取的反應速率比起理論值還要低些，且此結果亦有可能會對總反應速率造成程度上的影響。

A diaphragmless shock tube coupled with atomic resonance absorption spectrophotometry (ARAS) was employed to measure the site specific rate of H + C3H8, H + i-C4H10 and H + n-C4H10 between temperature 1000 and 1150 K. C2H5I was used as the source of H atom and the net temporal profile of H atoms in the C2H5I + alkane mixture is derived by simply subtracting the absorbance of alkane itself from C2H5I + alkane mixture at the same conditions. The site specific rate of H + (C3-C4) alkanes are summarized as , lnk4-6(T) = -(21.81±1.25)-(4.90±1.36)×〖10〗^3/T cm3molecule-1s-1 for the reaction H + C3H8 → H2 + C3H7 , lnk4-9(T) = -(18.74±1.74)-(8.02±1.88)×〖10〗^3/T cm3molecule-1s-1 for the reaction H + i-C4H10 → H2 + i-C4H9 , lnk4-13(T) = -(24.14±1.74)-(1.86±0.48)×〖10〗^3/T cm3molecule-1s-1 for the reaction H + n-C4H10 → H2 + s-C4H9, respectively. The present experimental results are compared to the previous theoretical calculations based on the quantum chemical calculation of the potential energy surface and transition state theory. The theoretical calculations in the literatures on the reaction rates of the attacking to the primary site for H + C3H8 and H + i-C4H10 are consistent with the present experiment but that for the attacking to the secondary site of n-C4H10 is about 30％ larger than this measurement. The hindrance by the C-H bonds of the primary sites may attribute to the reduction for the secondary site of n-C4H10 and the results will probably make some influence to the total rates of H + alkanes.