利用第一原理探討材料表面上二氧化碳電催化還原之特性 I.含氮碳管中類有機金屬結構對於二氧化碳的催化特性及碳管雜化後性質 II.應力效應對材料表面之電子結構及其對二氧化碳電催化還原反應特性之影響

Abstract

我們使用DFT理論計算探討了二氧化碳還原反應在過度金屬螯合於氮參雜碳管(TM-4N2v-CNT)的特性。為了要模擬平面四方體均相催化劑，在這個模型當中包括了四個氮原子取代及兩個空缺位置，並在其空缺位螯合上Fe, Ru, Os, Co, Rh, Ir, Ni, Pt及Cu。接著使用此結構沿著二氧化碳還原路徑尋找可能存在的中間體。在本篇所有研究的金屬，皆偏向進行水還原(Hydrogen evolution reaction)，且只有第八族元素可與CO產生強鍵結，並生成後續還原產物，而其他金屬則偏向生成HCOOH。而我們也以ligand field theory解釋這個中心金屬對於CO鍵結強弱的差異。透過增加電壓去穩定CO中間體，可在中心金屬為Ru及Os時產生甲烷，在中心金屬為Fe時產生甲醇。並且當增加碳管曲率時可減少電催化所需之電壓。然而反應中的主要產物主要還是由中心金屬種類決定。

我們透過DFT理論計算展示了應力效應如何影響銅催化電極對二氧化碳還原反應(CO2RR)的催化選擇性，我們討論了在二氧化碳電催化路徑上幾個關鍵的中間體，例如(_^*)H、(_^*)COOH、(_^*)CO、(_^*)CHO及(_^*)OCCOH，透過在考慮應力效應下前述中間體的相對形成能，我們推測二氧化碳還原反應(CO2RR)產出2C+的路徑將會被推動，其第一步關鍵中間體(_^*)COOH將比競爭反應水還原(HER)的中間體(_^*)H的形成能更增加約0.10 - 0.15eV，同時((_^*)CO→(_^*)CHO )的能障也增加約0.10-0.15eV，同時(_^*)CO的鍵結強度增加，表面上的(_^*)CO濃度因此升高。在適當應力區域，二碳中間體(_^*)OCCOH形成能會增加，並且在特定應力下此幅度大於一碳中間體(_^*)CHO約0.2eV，使得二碳產物的法拉第效率將有所提升。我們也藉由電子結構效應及立體結構效應解釋這些現象可能的原因。在本篇研究中，我們也發現在擠壓的應力下可能存在適於產生3C產物的表面銅原子排列，對後續材料的設計提供參考。

We have characterized the CO2 reduction capabilities of a series of nine transition-metal-chelated nitrogen-substituted carbon nanotube models (TM-4N2v-CNT), using density functional theory. Each of the chelated models consists of a four-N-substituted and one vacancy framework to mimic square planar homogeneous catalysts, and is coordinated to Fe, Ru, Os, Co, Rh, Ir, Ni, Pt or Cu. The results are further investigated to search for the possible electrochemical intermediates along the CO2 reduction pathway. We’ve found that all of the tested elements are predicted to favor the hydrogen evolution reaction over CO2 reduction energetically (with the exception of Cu), and that only Group 8 elements are predicted to bind CO effectively and other cases prefer HCOOH formation. The observed CO binding preference could be rationalized via ligand field theory based on the molecular orbitals of the square planar complexes. With a suitable applied voltage to stabilize all of the adsorbed CO intermediates, Ru and Os are predicted to produce CH4, whereas Fe is predicted to produce CH3OH. Increasing the curvature of the CNT could reduce the required potential in the potential-determining step substantially. However, the predicted catalytic sequence is subject to only the selection of a metal center.

We explored how the strain effect changing the selectivity of carbon dioxide reduction (CO2RR) on strained copper (100) surface. The most important intermediates in CO2RR pathway like *H, *COOH, *CO, *CHO and *OCCOH were considered. We chose 0-8% compressed on a-axis and 0-10% expanded on b-axis copper (100) surfaces to modeling the strain effect. The relative energies of intermediates in the strained situation were calculated. Under this strained environment, CO2RR would be accelerated over HER more than 0.1 eV. And this strain effect wouldn't increase the energy barrier of potential determining step((_^*)CO→(_^*)CHO) in CO2RR reaction path. The stabilization induced by strain effect on key intermediate of 2C products, *OCCOH were stronger than *CHO by 0.15eV. And it would cause the faradic efficiency(FE) of 2C products higher than the conventional one. And we also mention that the strained effect may occur in the “defective” surface like grain-boundary-rich one and oxide-derived one. Even the intermediates may affect each other by the strain effect induced by themselves.