以理論計算的方式研究乙炔在金屬Fe(111)和W(111)以及Fe-W(111)雙金屬表面的氫化反應

Abstract

本篇藉由密度泛函理論來研究乙炔分子在催化表面進行選擇性氫化反應，並且能夠順利使乙炔分子100%由氫化的方式轉換成乙烯。在我們的計算結果顯示出，利用Fe原子取代W(111)表面最上面兩層W原子形成的Fe(1,2)@W(111)表面有最好氫化選擇性效果。在乙炔氫化選擇性的反應中主要會有三種路徑發生：(1) 乙炔直接氫化至乙烯且可以順利於表面上脫附；(2) 乙炔過度氫化至乙烷；(3) 乙炔氫化至乙烯後進行C-C斷鍵形成兩個CH¬2吸附在表面。我們也針對其他雙金屬催化表面進行相同的反應(Fe(111)、W(111)以及利用不同層數所組合而成的Fe-W(111)雙金屬(在W(111)上以一層Fe原子取代形成Fe(1)@W(111)；在Fe(111)上以一層W原子取代形成W(1)@Fe(111)以及在Fe(111)上以兩層W原子取代形成W(1,2)@Fe(111))。在Fe(1,2)@W(111)表面上形成乙烯所需誇越的活化能只要0.84eV；而C¬2H4進行氫化形成C2H5所需經過的能障為2.43eV，若進行乙烯的C-C斷鍵反應則需要跨越2.27eV的活化能。以上兩種反應途徑皆較形成乙烯後直接於表面脫附(0.42eV)的能量來得高。因此在Fe(12)@W(111)表面能夠有效提升C2H2氫化的反應活性以及達到將乙烯脫附的的效果。

The selectivity of ethylene formation from the hydrogenation of acetylene could be tuned to 100% in our catalytic surface design, Fe(1,2)@W(111), in which the first two layers of W(111) surface is replaced by the Fe atoms. There are three possible reaction pathways in the hydrogenation of acetylene carried out on the catalytic metal surfaces: (1) solely formation of ethylene then desorbed from the surface; (2) complete hydrogenation to ethyl radical then ethane; (3) decompose to two methylene fragments. We introduced severalmonometallic and bimetallic surfaces (W(111), Fe(1,2)@W(111), Fe(111), W(1,2)@Fe(111), Fe(1)@W(111), and W(1)@Fe(111); where Fe(1)@W(111) represents the top layer of tungsten (111) surface replaced by the iron atoms, while W(1)@Fe(111) denotes the tungsten atoms replacing the first layer iron (111) surface ) to systematically tune the selectivity of ethylene formation via acetylene hydrogenation by employing DFT (density functional theory) calculations. On Fe(1,2)@W(111) surface, the barrier of ethylene formation is only 0.84 eV, the smallest among those bimetallic surfaces, and the barrier of further hydrogenation to C2H5 is 2.43 eV, while the alternative pathway of C-C bond scission is 2.27eV; these two latter barriers are much higher than C2H4 desorption energy (0.42eV). Therefore, the ethylene molecule could be the sole and final product to be desorbed from the catalytic tuned Fe(1,2)@W(111) surface.