利用理論計算方法探討O2及C2H4在Au38及Cu38奈米團簇上的吸附及反應機制

Adsorption and reaction mechanisms of O2 and C2H4 on Au38 and Cu38 nanoclusters: A computational study

10.6840/CYCU.2015.00198

李偵綺

銅奈米團簇 ； 乙烯的環氧化反應 ； 吸附反應 ； DFT計算法 ； 金奈米團簇 ； copper nanocluster ； adsorption ； gold nanocluster ； DFT calculations ； Ethylene epoxidation

中原大學化學系學位論文

2015年

碩士

陳欣聰

繁體中文

我們利用週期性之密度泛函理論 (Density functional theory, DFT) 的方法來探討乙烯及氧氣在Au38和Cu38奈米團簇上的吸附及反應機制。對C2H4/M38、O2/M38以及C2H4-O2/M38進行吸附結構預測 (M=Au, Cu) 。計算的結果發現吸附的位置有正上方 (top, T) 、橋接 (bridge, B) 、三角形面中心 (hexagonal, h) 、四面形中心 (hollow, H) ，在Au38奈米團簇上，C2H4和O2最穩定的吸附能分別為 -0.66和 -0.99 eV，Cu38奈米團簇為 -0.51和 -2.19 eV。乙烯在Au38和Cu38奈米團簇上的環氧化反應機制的位能曲面圖是使用nudged elastic band (NEB) 方法建構。氧化反應的進行是以Langmuir-Hinshelwood (LH) 機制產生環氧乙烷 (ethylene oxide, EO) 以及乙醛 (acetaldehyde, AA) 。整體反應分成兩個部分： (1) C2H4 + O2 + M38→環氧乙烷 (ethylene oxide, EO) + O/M38，此反應M為Au會放熱約 2.20 ~ 2.40 eV，若為Cu則會放熱約 2.20 ~ 3.56 eV； (2) C2H4 + O2 + M38 → 乙醛 (acetaldehyde, AA) + O/M38，反應中M為Au放熱約為3.03 ~ 3.08 eV，為Cu時，放熱約3.08 ~ 4.31 eV。最後，我們也計算了局部態密度分析 (local density of state, LDOS) 了解金或者銅奈米團簇和被吸附物之間的高催化性質。

We have investigated the adsorption and reaction mechanisms of C2H4 and O2 catalyzed by a Au38 and Cu38 nanoclusters based on periodic density-functional theory (DFT) calculations. The configurations of the adsorption of C2H4/M38, and O2/M38, as well as the coadsorption of C2H4-O2/M38 (M=Au, Cu) were predicted. We considered four different positions: top (T), bridge (B), hexagonal (h), and hollow (H) sites. The calculation results show that C2H4 and O2 are preferably adsorbed at top (T) and bridge (B) with adsorption energies of -0.66, and -0.99 eV, respectively for Au38 nanoclusters, and -0.51 and -2.19 eV for Cu38 nanoclusters. The detailed reaction mechanisms for ethylene epoxidation on Au38 and Cu38 nanoclusters has been illustrated using the nudged elastic band (NEB) method. The oxidation process takes place via the Langmuir-Hinshelwood (LH) mechanism to generate ethylene oxide andacetaldehyde. The overall reaction of C2H4 + O2 + M38 → ethylene oxide + O/M38, exothermic by 2.20 ~ 2.40 eV or 3.20 ~ 3.56 eV for M is Au or Cu respectively, while those are 3.03 ~ 3.08 eV or 3.83 ~ 4.31 eV for the production of acetaldehyde and O/M38.The nature of interaction between adsorbate and gold or copper nanoclusters has been analyzed by the detailed electronic local density of states (LDOS) to understand high catalytic activity of the gold or copper nanoclusters.