以第一原理計算探討銠金合金表面對甲醇反應的效果及反應路徑的影響

Abstract

本文運用密度泛函理論計算在RhAu合金催化表面，甲醇氧化反應的反應機構。RhAu是在將Rh原子層建構在Au(100)表面，其原子半徑差距會使Rh層在表面扭曲，低配位的Rh原子將形成類似團簇(cluster)的結構。相較於其在Rh(100)純金屬表面上的結果，會有更佳的反應活性。因此將從扭曲表面形成物理結構上的差別，以及Rh與輔層的Au接觸產生電子層面的影響，兩方面討論RhAu 表面能有效進行MOR的原因。RhAu表面的Rh電子分布更集中，強化其吸附能力並降低了O-H斷鍵的反應能障，影響了較重要的兩項反應途徑(CH3OHCH3OCH2O)。 此外，我們利用類似構型的Rh2Au36、Rh2Au38、Rh36Au2、Rh38Au2團簇，分析電子層面對於反應能力的影響。其中Rh2Au38有更多的輔層Au原子並表現出較RhAu表面更強的反應能力。DOS(density of state)的結果顯示RhAu表面的Rh會有較高密度的電子形成鍵結，強化吸附能力並帶來更佳的反應活性。計算結果與實驗觀察到，少量Rh添加於Au表面，甲醇反應活性將會急遽增強，而增強幅度將隨Rh比例上升而趨帄緩。 最後我們Rh(100)表面吸附O及OH，討論甲醇在氧的輔助下的斷鍵能力，並與乾淨Rh(100)的結果比較。與氧相較，羥基在表面的輔助斷鍵的表現較弱。First principles calculations have been used to investigate the mechanism of methanol reaction on bimetallic RhAu catalysts. We initially compare the reactions on RhAu and Rh(100) surfaces. RhAu surface was built by adding a Rh layer on Au(100) to simulate the Rh deposition on Au substrate in the experiment; the Rh layer shrunk forming the low-coordinate surface Rh atoms. The higher active of methanol reaction on RhAu surface, thus, corresponds to both the physical effect from the shrunk Rh layer and the electronic effect caused by the subsurface Au atoms. Surface Rh atoms of RhAu surface has more localized charge distribution due to the low coordination and accumulate more charge form subsurface Au atoms that can strengthen the methanol adsorption and lower the dissociation barrier for initial O-H bond cleavage, the two key steps that control the activity of methanol reaction. Furthermore, we examined RhAu clusters of Rh2Au36, Rh2Au38, Rh36Au2 and Rh38Au2 with similar shapes to analyze electronic effect contributed to reaction activity. Rh2Au38 with more substrate Au elements shows the highest activity, better than that on RhAu surface. The detailed electronic structures of those RhAu bimetals have also been examined by the density of state analysis. The computational results have been confirmed from the experimental observation that the activity increases dramatically as small amount of Rh deposited on Au substrate; the activity becomes saturated as the deposited Rh increases. Finally, we introduce surface oxygen and hydroxyl neighboring to the methanol and its fragment to investigate the oxidation assisted methanol reaction on Rh(100). Surface hydroxyl moderately enhanced the MOR while surface oxygen shows small assistance.