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研究生： 鄧亦棋
Deng, Yi-Qi
論文名稱： 探討鐵基催化劑的活性位點及載體效應於產氨反應之影響
Mechanistic Understanding of Active Sites and Support Effects in Fe-based Catalysts for NH3 Production
指導教授： 陳馨怡
Chen, Hsin-Yi Tiffany
口試委員: 郭錦龍
Kuo, Chin-Lung
蔡明剛
Tsai, Ming-Kang
楊家銘
Yang, Chia-Min
高橋開人
Takahashi, Kaito
學位類別： 碩士
Master
系所名稱： 原子科學院 - 工程與系統科學系
Department of Engineering and System Science
論文出版年： 2023
畢業學年度： 111
語文別： 英文
論文頁數： 80
中文關鍵詞： 氨合成 、鐵觸媒 、活性位點 、載體效應 、密度泛函理論
外文關鍵詞： NH3 Synthesis, Fe catalysts, Active Sites, Support Effect, Density Functional Theory
相關次數： 點閱：53  下載：0 
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    • 工業界哈伯製氨法所使用的鐵觸媒除了容易取得且價格便宜外，與其產氨效率相比可接受的催化活性，使得其在現今產氨工業仍具優勢。過往的研究結果已發現Fe(111)比起其他表面具有最高的產氨效率，因為其具有C7活性位點。然而尚未有詳細的研究提出C7活性位點促進產氨反應的機制，因此本研究將利用密度泛函理論 (density functional theory, DFT) 研究擁有C7活性位點之Fe(111)上的產氨反應，並與沒有C7活性位點的Fe(110)進行比較。根據本項工作，態密度與Bader電荷分析表明，側向(side-on)吸附的N2得到了更多的π*電子反向回饋(π* back donation) (1.49 |e|)，而在Fe(110)上則是1.18 |e|，這也導致了側向吸附的N2在Fe(111)上鍵長為1.332 Å，在Fe(110)上為1.261 Å。此外，從能量分析結果證實，Fe(111)表面上N2解離的活化能為0.44 eV，遠遠低於Fe(110)上的1.12 eV。以上分析皆說明了N2在Fe(111)上更加地被活化近年來，單原子觸媒及單團簇觸媒引起了廣泛關注，因其提升觸媒利用率及明確的活性中心，故被視為溫和條件下合成具有高效能和低碳足跡的綠氨之潛在觸媒。例如在先前的研究中，搭載在MgO(111)上的釕單原子及團簇在N2活化及氨合成反應表現出更加的性能。因此，我們進一步研究了負載在MgO(111)上的鐵基觸媒對於N2活化與產氨之效能。在所構建的Fe1/MgO(111)系統中，N2為端向(end-on)吸附，其鍵長為1.114 Å，並沒有被活化的跡象。此外，解離的2N在Fe1/MgO(111)上的吸附並不穩定(1.93 eV)，推論解離機制在該系統上不易發生。因此，我們考慮了締合機制(Associative mechanism)，其為被吸附的N2分子直接氫化。然而，由於MgO(111)具有強極性，因此模擬H2在Fe1/MgO(111)上的活化後，發現解離的氫原子很容易溢流到載體表面的氧原子上，此氫溢流抑制了吸附於觸媒上的N2分子進行氫化。計算結果指出，只有一個氫原子可以與吸附的N2鍵結，而進一步增加氫原子將會導致氫原子溢流到MgO(111)表面，這說明締合機制在Fe1/MgO(111)系統上也不易發生。本研究為Fe(111)上的產氨途徑提供了新的見解，闡明了由C7活性位點所促進的低反應能障之N2活化機制。對Fe1/MgO(111)的研究結果則表明，鐵單原子催化劑不適合用於產氨反應。以上計算結果及機制了解，於鐵觸媒產氨反應，說明C7活性位點為活化氮氣之關鍵，希冀以上說明對於開發新一代鐵基觸媒以實現更高效的氨生產能有所助益。

    In addition to their reactivity, iron (Fe) catalysts are accessible and inexpensive, which qualifies them as promising materials for industrial thermal catalytic ammonia production. In previous studies, it has been found that, due to the presence of C7 active sites, Fe(111) has the highest ammonia production efficiency than other surfaces. However, the detailed mechanism explaining how the C7 sites facilitate improved ammonia production remains poorly understood, limiting the development of modern Fe catalysts for ammonia production. In this study, density functional theory (DFT) is used to understand the effect of the special C7 active sites by comparing the complete ammonia production reaction on a Fe(111) with the C7 active sites to that on a Fe(110) surface without a C7 site. A distinct mechanism for N2 dissociation, commonly the rate-determining step, is observed on the two studied Fe surfaces. The density of states and Bader charge analyses show that a side-on adsorbed N2 receives more π* back donation on Fe(111) (1.49 |e|), as compared to that on Fe(110) (1.18 |e|). This leads to a more activated side-on N2 bond length of 1.332 Å on Fe(111) as compared to 1.261 Å on Fe(110). Thus, the activation energy for the subsequent N2 dissociation of 0.44 eV on the surface of Fe(111) is much lower than the 1.12 eV on Fe(110).For instance, a previous study on Ru single atoms and clusters supported on the surface of MgO(111) showed enhanced performance for ammonia synthesis and N2 activation. In addition to the pure metal (Fe) catalysts, we have also investigated the N2 activation mechanism and ammonia production on models of single Fe atom or Fe clusters supported on MgO(111). On the constructed Fe1/MgO(111) models, N2 adsorbs in an end-on configuration, whose bond with a length of 1.114 Å, is not significantly activated. Furthermore, the adsorption of dissociated 2N on Fe1/MgO(111) is not stable (1.93 eV), indicating that a dissociative mechanism is not favored on the system. Therefore, the associative mechanism, characterized by direct hydrogenation of adsorbed N2 molecules, was considered. However, simulation of H2 activation on Fe1/MgO(111), easily leads to spillover of dissociated H atom to surface O atoms of the support because of its strong polarity. Such H spillover inhibits the hydrogenation of the adsorbed N2 molecule. In our calculations, only one H atom could bind to the adsorbed N2 while further addition of H atoms leads to spillover to the MgO(111) surface, indicating that the associative mechanism is also not favored on the Fe1/MgO(111) system. The present study provides novel insights into the ammonia production pathway on Fe(111) where the mechanism of a low-barrier N2 activation facilitated by the C7 sites are successfully elucidated. The results for Fe1/MgO(111) suggest that a single-atom catalyst is not suitable for ammonia-producing reactions. These interesting findings will be useful for the development of Fe-based catalysts for more efficient ammonia production.

    摘要 ii Abstract iv Acknowledgement vi List of Tables xii List of Figures xiii List of Figure Appendix xxi 1. Introduction 1 1.1 Review of Industrial Methods for Ammonia Synthesis 1 1.2 Current State of Ammonia Synthesis and Challenges 3 1.3 Objectives and Experiments Design 5 2. Literature Review 8 2.1 Evolution of Ammonia Synthesis Issues 8 2.2 Method for Synthesizing Ammonia and the Importance of Iron Catalysts 11 2.3 DFT Study for N2 Activation Mechanism 13 2.4 Method for Promoting Ammonia Synthesis 14 2.5 The Discovery of Fe C7 Site and Its Importance for N2 Activation 17 3. Methodology 21 3.1 Density Functional Theory, DFT 21 3.1.1 Born-Oppenheimer Approximation 23 3.1.2 Hohenberg-Kohn Theorem 24 3.1.3 Kohn-Sham Formulation 25 3.1.4 Exchange-Correlation Functional 26 3.1.5 Pseudopotential 27 3.1.6 Dispersion Corrections 29 3.2 Computational Details 30 3.2.1 Surface Optimization 30 3.2.2 Surface Sites 31 3.2.3 Energy 32 3.2.4 Transition State Search 33 3.2.5 Bader Charge Analysis 36 4. Results and Discussions 37 4.1 Fe(110) and Fe(111) 38 4.1.1 Adsorption Energy Analysis 38 4.1.2 N2 Dissociation 39 4.1.3 Charge Analysis of N2 dissociation 43 4.1.4 N* Hydrogenation 46 4.2 H2 activation 48 4.2.1 H2 activation on MgO(111) 48 4.2.2 H2 activation on Fe1/MgO(111) 50 4.3 Fe1/MgO(111) 52 4.3.1 The H spillover on Fe1/MgO(111) & Ru1/MgO(111) 52 4.3.2 Adsorption Energy Analysis 54 4.3.3 N2 Hydrogenation 56 4.4 Fe9/MgO(111) 60 4.4.1 Adsorption Energy Analysis 62 4.4.2 Dissociative & Associative Mechanism 64 5. Conclusions 66 Reference 68 Appendix 74 Appendix I: N2 hydrogenation on Fe(110), (111) 74 Appendix II: N2, H2 on MgO (100), (110), (111) 75 Appendix III: N2, H2 on Ru1/MgO (100), (110), (111) 77 The Training at EMMG 79 1. Conference Attendance 79 2. Awards 79 3. Paper to be Submitted 80

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