為了開發高性能的儲氣材料，了解氣體與吸附材料間之相互作用是必須的。本理論
計算研究深究探討了 MOF-74-Ni 對於甲烷儲存能力的吸附機制。在此我們使用凡
德瓦校正之密度泛函理論（ DFT）並在 MOF-74-Ni 的單孔中吸附了 15 個甲烷分子。
在 MOF-74-Ni中不同吸附位點（包括 iSBU（ NiO5）， L（苯接頭）和 P（孔中心）
位點）的逐步計算中，吸附能在-0.3794eV 至-0.1982eV 之間變化。同時我們也觀察
到吸附能量呈現線性趨勢。計算表明，隨著系統的甲烷數達到完全飽和，甲烷在這
些位置上的吸附強度差異開始減小。接著我們通過使用不同的電子分析探討當增加
甲烷負載量時，甲烷分子與 MOF-74-Ni 吸附材料之間的電子結構變化。其中，
partial crystal orbital Hamiltonian population（ PCOHP）分析預測 fermi-level 周圍的骨
架狀態本質上是反鍵合的，暗示即使在電子損失後骨架也將保持穩定。此外基於
DDEC6 的淨電荷分析表明甲烷在 iSBU 和 L 位置上都獲得電子。 Electron density
difference（ EDD）圖顯示甲烷和 L位點之間的電子增加，並且 partial density of state
（ PDOS）分析顯示了甲烷和 iSBU 位置上的氧之間的密度重疊。這些結果分別揭
示了甲烷和 L 以及 iSBU 位置之間的 C-H…pi 和 C-H…O 相互作用。此外部分
DDEC6 結果也顯示甲烷在 iSBU 位置會失去電子， 說明甲烷和 iSBU 位置之間存有
些許 agostic 相互作用。 這些甲烷的增益和失去電子將構築一個電子轉移循環，這
也導致了甲烷吸附的協同效應。這種協同作用可以促進吸附在 MOF-74-Ni 上的甲
烷數量增加。 在此研究的第二部分中， 我們在 MOF-74-Ni 中參入銥金屬。 我們觀
察到銥金屬的聚集比銥金屬在 MOF-74-Ni 中分散略微有利， 其能量差為 0.3184 eV。
相對於純 MOF-74-Ni 上的一個甲烷的吸附能，在成簇的 Ir / MOF-74-Ni 上觀察到甲
烷吸附能會增加，而在分散的 Ir / MOF-74-Ni 上會觀察到其減小。 此研究提供了有
趣的結果，可用於未來的多孔材料開發和催化研究。 此外了解氣體分子與吸附材料
間的相互作用亦將有助於多孔材料性能的改進。

To develop a high-performance gas storage material, an understanding of host-guest and
host-host interactions is essential. This theoretical study highlights the key mechanism
behind the methane storage capacity of MOF-74-Ni. We performed stepwise Van der Waals
corrected density functional theory (DFT) calculations and were able to adsorb fifteen CH4 molecules in a single pore of a unit cell. We identified three adsorption sites, iSBU (NiO5), L (benzene linker), and the P (pore center) site. The adsorption energy at these sites varied between -0.3794 eV to -0.2489 eV with a linear trend of adsorption energy. The calculation showed as the system reaches complete saturation the difference in adsorptive strength of those sites starts to diminish. Variation in adsorbate-adsorbent electronic properties as a function of increased CH4 loading was monitored by using different electronic analysis. Partial crystal orbital Hamiltonian population (PCOHP) analysis predicted that states of framework around fermi level are antibonding in nature, hinting that the framework will remain stable even after electron loss. DDEC6 based net charge analysis demonstrated an interplay of charge gain/loss by CH4 molecules as system approaches to complete saturation. This charge gain/loss makes an electron transfer cycle which results in the cooperative interaction between host-guest and host-host. This cooperative effects can facilitate a high methane storage capacity by MOF-74-Ni. Electron density difference (EDD) maps showed the electrons accumulation between CH4 and framework sites which increases as the CH4 loading increases. Partial density of state (PDOS) analysis displayed the small overlap of host-guest states. The results also indicated the C-H…pi type of interaction between CH4 and L site. In the second part of this study, iridium was introduced in MOF-74-Ni. Clustering of iridium was observed to be slightly favorable by 0.3 eV than the dispersion in the framework. Relative to one CH4 adsorption energy on pure MOF-74-Ni, an increase in adsorption energy was observed on the clustered as well as dispersed Ir/MOF-74-Ni, where dispersed Ir/MOF-74-Ni showing the highest CH4 adsorption energy i.e. -0.4237 eV. This study provides interesting results which can be useful for future porous material development and catalysis studies. Understanding of host-guest interactions will be beneficial in finding improvements in the performance of porous material.

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