NOx ( x=1, 2) 吸附與分解反應在M(111) (M=Cu, Ir, CuIr) 表面之理論計算研究

Abstract

第一部分 : NOx ( x = 1, 2)在Cu(111)表面之吸附與分解反應 我們使用週期性密度泛函理論來研究NOx ( x= 1, 2)在Cu(111)表面之吸附與分解反應，計算結果顯示NO2在表面上最穩定的吸附結構為μ-O,O´-nitrito，以兩個O原子接在表面Cu原子上，而NO2要進行分解時，會轉換成μ-N,O-nitrito結構，以N原子與一端O原子接在Cu原子上。NO2逐步分解反應第一步活化能為1.05 eV，第二步為2.08 eV，最後在表面上形成N(a) + 2O(a)。另外，我們也計算了三組NO分解的模型，分別為NO / Cu(111)、O + NO / Cu(111)以及N + NO / Cu(111)，探討NO在三種環境中的分解能障。結果發現，有O原子共吸附時，NO的5σ軌域面積是三組中最大的，而有N原子共吸附時的5σ面積最小，代表NO在O-pre-adsorbed的環境下要行斷鍵反應最不易。計算三組NO斷鍵活化能：O + NO(2.08 eV)＞NO(1.88 eV)＞N + NO(1.28 eV)，與先前計算吸附後NO的5σ軌域面積大小呈線性關係。

第二部分 : NO在Cu(111)、Ir(111)、Ir@Cu(111)、Cu@Ir(111)表面的吸附與分解反應 我們使用週期性密度泛函理論來研究NO在單金屬Cu(111)與Ir(111)以及雙金屬Ir@Cu(111)、Cu@Ir(111)表面之吸附與分解反應，其中雙金屬表面又分不同比例(在M(111)表層分別取代1、5、9顆之M´)的金屬取代。計算結果發現，NO在Ir(111)純金屬表面的吸附與分解皆較Cu(111)容易。比較雙金屬Ir@Cu(111)系列，吸附的部分以1Ir@Cu(111)表面可得到最大的NO吸附能(-2.56 eV)，而分解的部分則是在5Ir@Cu(111)表面有最低的活化能(0.76 eV)。另外，比較Cu@Ir(111)系列，吸附的部分以5Cu@Ir(111)表面可得到最大的NO吸附能(-2.72 eV)，而分解的部分同樣在5 Cu@Ir(111)表面有最低的活化能(1.26 eV)。不論是Ir@Cu(111)或Cu@Ir(111)系列，在NO吸附的選擇上，皆是偏好在Ir原子位置上，而NO斷鍵部分也發現在雙金屬表面上大部分有低於純金屬表面的活化能，除了1Ir@Cu(111)表面外。1st part: Adsorption and dissociation of NOx (x = 1, 2) molecules on the Cu(111) surface. Spin-polarized density functional theory calculation is employed to study the adsorption and dissociation of NO2 molecule on Cu(111) surface. It is shown that the most favorable adsorption structure is the NO2 (T,T-O,O'-nitrito) configuration and has an adsorption energy of -1.49eV. The barriers for step-wise NO2 dissociation reaction, NO2(g)  N(a) + 2O(a), are 1.05 (for O-NO bond activation), and 2.08 eV (for N-O bond activation), respectively, and the entire process is 0.6 eV exothermic. The energies of single NO dissociation and its dissociation in the presence of N atom or O atom on the surface are also calculated. The results indicate that in the presence of O atom on Cu(111) surface would raise the NO dissociation barrier, whereas in the presence of N atom decrease it. The interaction nature between adsorbates and substrate is also analyzed by the local density of states (LDOS) analysis.

2nd part: NO Adsorption and dissociation on monometallic Cu(111), Ir(111), and bimetallic Ir@Cu(111) and Cu@Ir(111) surfaces. Spin-polarized density functional theory calculation is employed to study NO adsorption and dissociation on monometallic Cu(111), Ir(111), and bimetallic Ir@Cu(111) and Cu@Ir(111) surfaces. The NO adsorption energy is bigger and the dissociation barrier is lower on the Ir(111) surface than that on the Cu(111) counterparts. In the Ir@Cu(111) series we find that the most stable NO adsorption is on the Ir atop site with the adsorption energy being -2.56 eV. Besides, we calculate the NO dissociation barrier on these surfaces, and the lowest energy barrier exists on the 5Ir@Cu(111), only 0.76 eV. In the Ir@Cu(111) series NO adsorption site will be on the Ir atom instead of Cu atoms. Furthermore, in the Cu@Ir(111) series the NO adsorption site will still be favor on the Ir atoms. The most stable NO adsorption occurs on the Ir atop site of 5Cu@Ir(111) surface with theadsorption energy being -2.72 eV. Moreover, the calculated results show that the lowest NO dissociation barrier exists on the 5Cu@Ir(111) of 1.26 eV. The interaction nature between adsorbates and substrate is also studied with the local density of states (LDOS) analysis.