利用第一原理計算探討金屬氧化物表面對於小分子氣體之催化反應

Catalytic Reaction of Gas Molecules Adsorption on the Metal Oxide Surfaces: A First-Principles Study

盧禹寰(Yu-Huan Lu)

指導教授 ： 陳欣聰

中原大學/理學院/化學研究所/博士(2016年)

https://doi.org/10.6840/cycu201600369

摘要

第一部分：NO2 分子吸附在M-dope CeO2(111)表面上之反應探討(M = Mn, Fe)我們使用自旋極化週期性密度泛函理論探討NO2吸附於M-doped CeO2(111) (M= Mn, Fe)表面上的反應機制，我們將Mn或 Fe原子置換到CeO2(111)表面用來修飾純CeO2表面的活性、並探討氧缺陷的形成和分子吸附行為與其相關反應機制，在CeO2(111)相關計算中，我們以密度泛函理論之並加入Hubbard U term (DFT+U)做計算。結果發現與未摻雜的表面相比NO2吸附在M摻雜氧化鈰表面上吸附力較強，並經由Bader charge以及振動頻率計算分析發現NO2會在表面上形成類硝酸鹽之結構；反之，NO2吸附在氧缺陷之未摻雜的表面上則較M摻雜氧化鈰表面上吸附力較強，經由Bader charge以及振動頻率計算分析也發現NO2會於氧空缺表面上形成類亞硝酸鹽之結構。
第二部分：H2O 分子吸附在M-dope CeO2(111) 表面上之反應探討(M = Mn, Fe)我們使用自旋極化週期性密度泛函理論並加入Hubbard U term (DFT+U)做計算，探討H2O吸附於M-doped CeO2(111) (M= Mn, Fe)表面上的反應機制，我們將Mn或 Fe原子置換到CeO2(111)表面用來修飾純CeO2表面的活性、並探討氧缺陷的形成和分子吸附行為與其相關反應機制，從計算結果中發現， H2O吸附在M參雜之氧化鈰表面上更為牢固，若吸附在氧缺陷之表面上，則在未摻雜的表面上之吸附能較為穩定，在結構上H2O在未參雜的表面上並無法尋找到為解離前的吸附結構。由吸附能來看也可以發現氧缺陷可以增強H2O與表面間的作用力，整體吸附能之趨勢也和氧缺陷之形成能有關：Ce0.875Mn0.125O2 < Ce0.875Fe0.125O2 < CeO2。
第三部分：H2O 分子吸附在α-Al2O3(0001) 表面上之反應探討
我們使用週期性密度泛函理論來研究H2O吸附於α-Al2O3(0001)表面之吸附與解離反應，我們分別計算兩種不同的表面進行H2O的催化反應，分別為Al, O-terminated以及Al-terminated的α-Al2O3(0001)表面，所使用的表面為2 × 2的supercell，結果顯示Al-terminated α-Al2O3(0001)主要有兩種裂解路徑，1. H2O解離為OH + H並經由提供一定的能量讓整體反應趨向產生H2氣體，總反應放熱2.37 eV；2.將H2O完全解離為2H + O的形式，並放熱4.18 ~ 4.22 eV。Al, O-terminated的α-Al2O3(0001)表面由於 的s軌域與Al原子的p軌域也有著明顯混成情況形成強作用力，讓H2O吸附於Al, O-terminated α-Al2O3(0001)表面僅能形成H + OH，總反應放熱約1.64 eV。
第四部分：H2O 分子吸附在γ-Al2O3(110) 表面上之反應探討我們使用週期性密度泛函理論來研究H2O分子吸附於γ-Al2O3(110)表面之吸附與解離反應，我們計算了H2O、OH、O以及H分別在表面上最穩定的吸附位置，分別為Al(I)-top、Al(III)-bridge、Al(I, II)-bridge以及Al(III)-bridge，並利用這些吸附位置去推斷可能的反應路徑；除了H2O的解離反應我們也做了H2的生成預測並繪製於位能曲面圖中，並成功尋找到H2O在γ-Al2O3(110)表面上的最佳路徑，總反應僅需兩步驟即可得到H2為最終產物，透過分子動力學模擬我們也驗證了H2在γ-Al2O3(110)表面逐步變化的過程，另外，我們也計算Bader charge去分析吸附物與表面之間的作用力。

關鍵字

第一原理計算 ； 二氧化鈰 ； 氧化鋁 ； 二氧化氮吸附 ； 水吸附 ； 產氫 ； 金屬置換

並列摘要

1st part: The interaction of NO2 on M-doped CeO2(111) (M = Mn, Fe) surfaces investigated with theoretical calculations
We use spin-polarized Density functional theory with on-site columbic repulsion via a Hubbard U term (DFT + U) to study NO2 adsorption on the M-doped CeO2(111) (M = Mn, Fe) surfaces. The dopant metal is used to increase the surface activity due to the formation of oxygen vacancy for NO2 absorption and reduction reaction. Our calculations show NO2 adsorbs more strongly on the stoichiometric M-doped CeO2 surfaces compared with the undoped surface and forming nitrate-like species (NO3–). On the contrary, the adsorption of NO2 is less stable at the oxygen vacancy site on the defective M-doped CeO2 surfaces and forming nitrite-like species (NO2–). The vibrational frequency calculations as well as the Bader charge analysis are carried out to characterize the adsorbed species.
2nd part: The interaction of H2O on M-doped CeO2(111) (M = Mn, Fe) surfaces investigated with theoretical calculations
We use spin-polarized Density functional theory with on-site columbic repulsion via a Hubbard U term (DFT + U) to study H2O adsorption on the M-doped CeO2(111) (M = Mn, Fe) surfaces. The results show that H2O adsorbs more strongly on the stoichiometric M-doped CeO2 surfaces compared with the undoped surface. On the defective surfaces, there exist both molecular adsorption and dissociation adsorption modes on the defective M-doped surfaces. For the undoped defective surface just dissociation adsorption mode was found, the adsorption energy of H2O is less stable at the defective M-doped surfaces. It is obvious that oxygen vacancies can enhance the interaction of water with the surfaces. We also notice that reaction energies depend on the oxygen vacancy formation energies, for which Ce0.875Mn0.125O2 < Ce0.875Fe0.125O2 < CeO2.
3rd part: The interaction of H2O on α-Al2O3(0001) surfaces investigated with theoretical calculations
We applied periodic Density functional theory to investigate the adsorption and dissociation of H2O on α-Al2O3(0001) surfaces. We designed two surfaces (Al-terminated and Al, O-terminated) with 2 × 2 supercell slab models to investigate the coverage-dependent hydroxylation of the surface. Our results show there has two pathway of H2O dissociation on Al-terminated α-Al2O3(0001) surface, (i) H2O dissociation on Al-terminated α-Al2O3(0001) surface and form H2 generation with energy suppling. (ii) H2O can fully dehydrogenation on Al-terminated α-Al2O3(0001) surface forming 2H + O as the product, the reaction process with an overall exothermicity of 4.18 – 4.22 eV. Because of the s orbital of has an overlap with the p orbital of the top-layer Al atom forming strong interaction, resulting in H2O molecular adsorption on Al, O-termitated α-Al2O3(0001) surface and dissociation to OH + H, and the reaction process with an exothermicity of 1.64 eV.
4th part: The interaction of H2O on γ-Al2O3(110) surface investigated with theoretical calculations
We applied periodic Density functional theory to investigate the adsorption and dissociation of H2O on γ-Al2O3(110) surface. The results show that H2O, OH, O and H are preferably adsorbed at Al(I)-top, Al(III)-bridge, Al(I, II)-bridge and Al(III)-bridge sites. In the following study, we predict the reaction pathway of H2O dissociation and H2 generation on γ-Al2O3(110) surface and map out the potential energy profiles. We find that H2 generation on the surface occurs via two processes, H2O dehydrogenation and direct H2 generation with an owerall exothermicity of 2.24 eV. The mechanism of H2 generation process was also demonstrated using molecular dynamics simulation. The interaction between the adsorbate and the surface during the reaction also analysis by the local density of states and Bader charge calculation.

並列關鍵字

First-Principles study ； Ceria oxide ； Aluminium oxide ； NO2 adsorption ； H2O adsorption ； Hydrogen generation ； Metal doped

參考文獻

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