於熱催化產氨製程，釕金屬為其一有潛力高效能催化劑，氮氣活化為其中之關鍵。本研究利用密度泛函理論（density functional theory, DFT）探討釕金屬(Ru)參雜於23種過渡金屬(TM)表面，和吸附在過渡金屬表面，藉此改變催化劑表面的結構與電子結構性質，討論其對氮氣活化機制的影響。本研究首先研究兩種單原子釕催化劑模型(參雜及吸附Ru1/TM)的穩定度，接著計算釕金屬參雜過渡金屬表面時的電子結構，研究釕金屬和過渡金屬表面之間的電子轉移。因參雜Ru1/TM的模型(單原子合金SAA)較穩定，在進行氮氣吸附時，只考慮此模型，計算結果說明，無論是end-on氮氣吸附或是side-on 氮氣吸附較穩定，氮氣活化皆以dissociative mechanism 為主，故接下來之機制探討只著眼於氮分子解離活化能之比較。計算結果顯示單原子合金模型(Ru1/TM)之氮分子解離活化能與其純金屬模型(TM)相近，再者，文獻指出釕金屬B5 活性位點為活化氮氣參鍵之關鍵，故我們設計了類B5 活性位點的釕金屬團簇於過渡金屬表面模型(Ru8/TM)，結果發現氮氣的解離活化能於Ru8/TM模型的氮氣分解活化能(0.46 – 0.79 eV) 接近純釕金屬B5模型(0.71 eV)，指出此較經濟之Ru8/TM催化劑設計，有潛力媲美釕金屬具B5 活性位點之氮活化效能。我們未來將會繼續探討Ru8/TM模型解離後氮原子的氫化反應至氨生成表現，以確認此新催化劑設計於產氨反應之效能，以提供實驗組催化劑設計方針。

New-generation Ruthenium (Ru)-based catalyst is considered to be a potential catalyst that can react at milder conditions than the traditional Haber-Bosch process in ammonia production. However, Ru is a scarce and expensive material. To address this limitation, we herein use density functional theory to investigate Ru Single Atom Alloy (SAA) models composed of a few atomically dispersed Ru doped in or adsorbed on the bulk structure of 23 different transition metals (a total of 46 SAA models), as potential novel and economical alternative to commercial Ru-based catalysts. Specifically, we elucidated the mechanism and energetics of N2 activation, which is the rate-determining step in ammonia production on Ru SAA models compared to pure Ru catalyst. We calculated the adsorption energy of molecular N2 intermediates on the 46 SAA models. We then calculated the activation energy of N2 dissociation and N2 hydrogenation, which are the respective rate-limiting steps of dissociative and associative ammonia synthesis mechanisms. We found that the dissociative mechanism has a lower activation energy than the associative mechanism. We further aimed to understand how B5 sites and clustering could affect the dissociation of N2. We, therefore, constructed four types of slab models: (i) terrace of pure host transition metals, (ii) Ru SAA model in host metal, (iii) B5 step sites of host transition metals, and (iv) Ru cluster on the host metal. Herein, five transition metals were considered as a host including Fe, Co, Ni, Mo, and Cr. The difference in the N2 dissociation activation energy between the corresponding terrace and SAA models, which both lack B5 active site only varies in the 0.01-0.22 eV range. However, if we compare terrace models with corresponding B5 step site models the N2 activation energy is lower on B5 step sites about 0.50-1.06 eV. This confirms that B5 step sites can facilitate N2 activation by lowering the activation energy even on metals other than Ru. As for Ru cluster on the transition metal model (Ru8/TM), the N2 dissociation activation energy is in the 0.82-2.00 eV range for Ru8/TM. Specifically, Ru8/Ni has N2 dissociation activation energy of about 0.82 eV, which is comparable with that on the pure Ru B5 step site (0.72 eV). This indicates Ru8/Ni which will be cheaper than pure Ru can maintain a high N2 dissociation activity and has the potential to become a cost-effective catalysts. In the future, the complete hydrogenation of dissociated N atoms to form ammonia on Ru8/TM will also be studied. This study provides a novel catalyst design paradigm for cost-effective ammonia production based on Ru catalysts.

摘要 ii
Abstract iii
Acknowledgement v
Table of Contents vii
List of Tables x
List of Figures xii
1. Introduction - 1 -
1.1 Brief Introduction of Ammonia Synthesis - 1 -
1.2 Challenges of Ammonia Synthesis - 4 -
1.3 Ru-based Catalyst for Ammonia Synthesis - 6 -
1.4 DFT Modeling and Analyses - 7 -
2. Literature Review - 9 -
2.1 Experimental Studies on Single Atom Catalysts (SAC) - 9 -
2.2 Utilization of Single Atom Alloy - 11 -
2.3 Mechanism for N2 Activation - 13 -
2.4 DFT Studies on Single Atom Catalysts - 15 -
2.5 d-Orbital and π Back-Donation Effect Nitrogen Activation - 17 -
3. Methodology - 21 -
3.1 Density Functional Theory, DFT - 21 -
3.1.1 Schrodinger Equation - 22 -
3.1.2 Kohn and Hohenberg Theorem - 23 -
3.1.3 Kohn-Sham Equation - 25 -
3.1.4 Exchange-Correlation Functional - 27 -
3.1.5 Pseudopotential - 28 -
3.2 Computational Details - 29 -
3.2.1 Model Construction - 29 -
3.2.2 Energetic Analyses - 32 -
3.2.3 Transition States Search - 33 -
4. Results and Discussion - 35 -
4.1 Fundamental Properties of Single Atom Alloy Model - 35 -
4.1.1 Binding Energy between Ru and Transition Metal Host in Adsorbed Model - 35 -
4.1.2 Formation Energy of Doped Model and Adsorbed Model - 37 -
4.1.3 Bader Charge Analyses of Doped Model and Adsorbed Model - 40 -
4.1.4 Partial Density of States Analysis (PDOS) - 41 -
4.2 Nitrogen Adsorption on Single Atom Alloy Model - 42 -
4.2.1 Adsorption Energy of N2 - 44 -
4.3 N2 Dissociation Mechanism on SAA - 46 -
4.4 Ru (0001) Terrace - 48 -
4.5 N2 Dissociation on Terrace and SAA Model - 50 -
4.5.1 Reaction Coordinate of N2 Dissociation on Terrace and SAA Model - 51 -
4.5.2 Electronic Structure of SAA and Slab Model - 54 -
4.6 N2 Dissociation on B5 Steps Sites and Ru8/TM Model - 57 -
5. Conclusions - 60 -
6. Future work - 62 -
References - 63 -
The Training at EMMG - 67 -

1. Zhang, Y., et al., Understanding and Modifying the Scaling Relations for Ammonia Synthesis on Dilute Metal Alloys: From Single-Atom Alloys to Dimer Alloys. ACS Catalysis, 2022. 12(15): p. 9201-9212.
2. Liu, H., Ammonia synthesis catalyst 100 years: Practice, enlightenment and challenge. Chinese journal of catalysis, 2014. 35(10): p. 1619-1640.
3. Erisman, J.W., et al., How a century of ammonia synthesis changed the world. Nature Geoscience, 2008. 1(10): p. 636-639.
4. Smith, C., A.K. Hill, and L. Torrente-Murciano, Current and future role of Haber–Bosch ammonia in a carbon-free energy landscape. Energy & Environmental Science, 2020. 13(2): p. 331-344.
5. Medford, A.J., et al., From the Sabatier principle to a predictive theory of transition-metal heterogeneous catalysis. Journal of Catalysis, 2015. 328: p. 36-42.
6. Saadatjou, N., A. Jafari, and S. Sahebdelfar, Ruthenium nanocatalysts for ammonia synthesis: a review. Chemical Engineering Communications, 2015. 202(4): p. 420-448.
7. Zhou, Y., et al., A highly stable and active mesoporous ruthenium catalyst for ammonia synthesis prepared by a RuCl3/SiO2-templated approach. Chinese Journal of Catalysis, 2019. 40(1): p. 114-123.
8. Li, L., et al., Operando spectroscopic and isotopic-label-directed observation of LaN-promoted Ru/ZrH2 catalyst for ammonia synthesis via associative and chemical looping route. Journal of catalysis, 2020. 389: p. 218-228.
9. Wu, Y., et al., Enhanced ammonia synthesis performance of ceria-supported Ru catalysts via introduction of titanium. Chemical Communications, 2020. 56(7): p. 1141-1144.
10. Li, L., et al., Size sensitivity of supported Ru catalysts for ammonia synthesis: From nanoparticles to subnanometric clusters and atomic clusters. Chem, 2022. 8(3): p. 749-768.
11. Tang, Y., et al., Metal‐Dependent Support Effects of Oxyhydride‐Supported Ru, Fe, Co Catalysts for Ammonia Synthesis. Advanced Energy Materials, 2018. 8(36): p. 1801772.
12. Kitano, M., et al., Low-temperature synthesis of perovskite oxynitride-hydrides as ammonia synthesis catalysts. Journal of the American Chemical Society, 2019. 141(51): p. 20344-20353.
13. Fang, H., et al., Challenges and Opportunities of Ru-Based Catalysts toward the Synthesis and Utilization of Ammonia. ACS Catalysis, 2022. 12(7): p. 3938-3954.
14. Hannagan, R.T., et al., Single-atom alloy catalysis. Chemical Reviews, 2020. 120(21): p. 12044-12088.
15. Kresse, G. and J. Hafner, Ab initio molecular dynamics for liquid metals. Physical Review B, 1993. 47(1): p. 558-561.
16. Nomoev, A.V., et al., Structure and mechanism of the formation of core-shell nanoparticles obtained through a one-step gas-phase synthesis by electron beam evaporation. Beilstein J Nanotechnol, 2015. 6: p. 874-80.
17. Yang, X.-F., et al., Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis. Accounts of Chemical Research, 2013. 46(8): p. 1740-1748.
18. Lin, J., et al., Remarkable Performance of Ir1/FeOx Single-Atom Catalyst in Water Gas Shift Reaction. Journal of the American Chemical Society, 2013. 135(41): p. 15314-15317.
19. Qiao, B., et al., Single-atom catalysis of CO oxidation using Pt1/FeOx. Nature Chemistry, 2011. 3(8): p. 634-641.
20. Zhang, Y., et al., Insight into the critical role of strong interaction between Ru and Co in RuCo single-atom alloy structure for significant enhancement of ammonia synthesis performance. Journal of Catalysis, 2022. 410: p. 256-265.
21. Darby, M.T., et al., Lonely atoms with special gifts: breaking linear scaling relationships in heterogeneous catalysis with single-atom alloys. The Journal of Physical Chemistry Letters, 2018. 9(18): p. 5636-5646.
22. Thirumalai, H. and J.R. Kitchin, Investigating the reactivity of single atom alloys using density functional theory. Topics in Catalysis, 2018. 61(5): p. 462-474.
23. Greiner, M.T., et al., Free-atom-like d states in single-atom alloy catalysts. Nature chemistry, 2018. 10(10): p. 1008-1015.
24. Yan, H., et al., Single-atom catalysts and their applications in organic chemistry. Journal of Materials Chemistry A, 2018. 6(19): p. 8793-8814.
25. Cheng, N., et al., Single-atom catalysts: from design to application. Electrochemical Energy Reviews, 2019. 2(4): p. 539-573.
26. Mao, J., et al., Single atom alloy: An emerging atomic site material for catalytic applications. Nano Today, 2020. 34: p. 100917.
27. Zhao, G.-C., Y.-Q. Qiu, and C.-G. Liu, A systematic theoretical study of hydrogen activation, spillover and desorption in single-atom alloys. Applied Catalysis A: General, 2021. 610: p. 117948.
28. Shen, T., et al., Recent Advances of Single-Atom-Alloy for Energy Electrocatalysis. Advanced Energy Materials, 2022. 12(39): p. 2201823.
29. Zhang, T., et al., Ru alloying with La or Y for ammonia synthesis via integrated dissociative and associative mechanism with superior operational stability. Chemical Engineering Science, 2022. 252: p. 117255.
30. Kyriakou, G., et al., Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations. Science, 2012. 335(6073): p. 1209-1212.
31. Liu, J., et al., Tackling CO poisoning with single-atom alloy catalysts. Journal of the American Chemical Society, 2016. 138(20): p. 6396-6399.
32. Darby, M.T., et al., Elucidating the stability and reactivity of surface intermediates on single-atom alloy catalysts. ACS Catalysis, 2018. 8(6): p. 5038-5050.
33. Giannakakis, G., M. Flytzani-Stephanopoulos, and E.C.H. Sykes, Single-atom alloys as a reductionist approach to the rational design of heterogeneous catalysts. Accounts of chemical research, 2018. 52(1): p. 237-247.
34. Boudart, M., Kinetics and Mechanism of Ammonia Synthesis. Catalysis Reviews, 1981. 23(1-2): p. 1-15.
35. Li, L., et al., Size sensitivity of supported Ru catalysts for ammonia synthesis: From nanoparticles to subnanometric clusters and atomic clusters. Chem, 2022. 8(3): p. 749-768.
36. Nørskov, J.K., Theory nof chemisorption and heterogeneous catalysis. Physica B+C, 1984. 127(1): p. 193-202.
37. Gutterød, E.S., An experimental and computational study of nitrogen activation on promoted ruthenium catalysts. 2016.
38. Dai, T., et al., “Sabatier principle” of d electron number for describing the nitrogen reduction reaction performance of single-atom alloy catalysts. Journal of Materials Chemistry A, 2022. 10(32): p. 16900-16907.
39. Nilsson, A., et al., The electronic structure effect in heterogeneous catalysis. Catalysis Letters, 2005. 100(3): p. 111-114.
40. Jónsson, H., G. Mills, and K.W. Jacobsen, Nudged elastic band method for finding minimum energy paths of transitions, in Classical and quantum dynamics in condensed phase simulations. 1998, World Scientific. p. 385-404.
41. Henkelman, G., B.P. Uberuaga, and H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths. The Journal of chemical physics, 2000. 113(22): p. 9901-9904.
42. Reyes, Y.I.A., et al., Mechanistic understanding of N 2 activation: a comparison of unsupported and supported Ru catalysts. Faraday Discussions, 2023.
43. Eric., Density Functional Theory Understanding of Nitrogen Activation Mechanisms by Ru Catalysts. Thesis, 2022.
44. Dahl, S., et al., Role of steps in N 2 activation on Ru (0001). Physical Review Letters, 1999. 83(9): p. 1814.
45. Mortensen, J.J., et al., Density functional calculations of N2Adsorption and dissociation on a Ru (0001) surface. Journal of Catalysis, 1997. 169(1): p. 85-92.
46. Dronskowski, R. and P.E. Blöchl, Crystal orbital Hamilton populations (COHP): energy-resolved visualization of chemical bonding in solids based on density-functional calculations. The Journal of Physical Chemistry, 1993. 97(33): p. 8617-8624.