應用第一原理研究低維度金屬硫族化物與其缺陷結構於二氧化碳電催化還原反應之特性與機制

Abstract

首先，我們探討了2H-MoS2 鉬邊緣的催化行為。 在這裡，我們考慮了四種結構不同硫覆蓋率的鉬邊緣結構，為Mo edge 100%S、75%S、50%S與0%S。對於這些結構，我們分別計算其不同反應的自由能曲線，以釐清其反應路徑，並比較他們的反應能力。我們發現Mo edge 100%S、75%S和50%S的臨界電位(Limiting potential) 依序如下。 Mo edge100%S具有最大的臨界電位，75%S最小，而50%S則介於兩者之間。造成這些催化特性背後的原因，在本文中將會進行探討。此外透過理論計算，可以確認Mo edge 0%S為不穩定的結構，因而難以穩定存在。基於其他研究的結果，相比於鉬邊緣，硫邊緣雖被視為較不安定的結構，但仍能被觀察到。因此我們仍然研究了硫邊緣的催化特性。與鉬邊緣類似，我們也選擇了四種不同硫覆蓋率的結構，分別為S edge 100%S、75%S、50%S與0%S，進行其催化特性的探討。此外，我們也研究了2H-MoS2的平面硫空缺的催化機制。我們建立了三種大小的平面硫空缺結構，分別為單空缺 (1Vs)、雙空缺 (2Vs) 和三空缺 (3Vs)。我們發現這些平面硫空缺具有特殊的反應性，能使CO2分子自發在這些平面硫空缺上被分解，而使這些平面硫空缺展現了特殊的催化行為。而後，雖然對於單層MoS2而言，其最穩態為2H結構，但是1T-MoS2也能透過特殊的合成方式產生，因此我們也探討了1T-MoS2的催化機制。與前述之探討方式相似，我們也分別探討了1T-MoS2邊緣位置與平面硫空缺的反應行為。對於1T-MoS2邊緣，我們僅考慮1T-MoS2 50%S的結構，因為它在我們所選取的硫化勢範圍內，是唯一穩定的結構。而對於1T-MoS2的平面硫空缺的催化機制，我們也建立了單空缺 (1Vs)、雙空缺 (2Vs) 和三空缺 (3Vs)的結構，以觀察空缺尺寸對反應機制的影響，並分析造成這些反應行為差異的背後原因。最後，因為SnS2也是CO2電催化還原上常見的催化劑，我們也研究了1T-SnS2催化劑的催化行為。對於1T-SnS2邊緣，我們選擇了三種硫覆蓋率的結構:1T-SnS2 100%S、50%S和0%S進行CO2電催化還原的探討，發現這三種結構在不同的硫化學勢下都是穩定的。同時我們也研究了1T-SnS2平面單硫空缺(1Vs)的反應機制，並與邊緣位置的反應行為比較，以釐清在SnS2上的主要反應位置與產物。最後我們探討了產氫反應在這些催化劑上的反應性，並與二氧化碳電催化還原進行比較，以判斷兩者反應的選擇率。

Firstly of all, we explored the catalytic behavior of 2H-MoS2 Mo edges. In this study, we considered four structures of the Mo edge with various sulfur coverage: Mo edge 100%S, 75%S, 50%S, and 0%S. For these structures, we calculated the reaction free energies diagrams to clarify their reaction pathways and compared their catalytic abilities. We found that the order of limiting potentials for Mo edge 100%S, 75%S, and 50%S is as follows: Mo edge 100%S has the largest limiting potential, 75%S has the smallest, and 50%S falls in between. The reasons behind these catalytic characteristics will be discussed in this research. Additionally, Mo edge 0%S is confirmed to be an unstable structure by theoretical calculations, making it nearly impossible to be observed. Based on the results from other studies, the sulfur edges still are observable although they are considered more unstable than Mo edges. Therefore, we also studied the catalytic behaviors of sulfur edges. Similar to Mo edges, four structures with different sulfur coverage are chosen for the research on CO2 electroreduction reactions, corresponding to S edge 100%S, 75%S, 50%S, and 0%S. Furthermore, we investigated the catalytic mechanism of basal plane sulfur vacancies on 2H-MoS2. We constructed three sizes of the basal plane sulfur vacancies: mono-vacancy (1Vs), di-vacancies (2Vs), and tri-vacancies (3Vs). We found that these basal plane sulfur vacancies exhibit unique reactivity, leading to spontaneous CO2 dissociation on these vacancies and resulting in special catalytic behavior. Later on, despite the most stable structure for MoS2 monolayer is the 2H-phase, we also explored the catalytic mechanism of 1T-MoS2 because it has been synthesized in experiments. Similar to the previous discussion, we investigated the reaction behavior of 1T-MoS2 edges and basal plane sulfur vacancies. For 1T-MoS2 edges, we only considered the structure of 1T-MoS2 50%S, as it is the only stable structure within the chosen range of sulfur potential. For the research on the catalytic mechanism of planar sulfur vacancies on 1T-MoS2, we constructed structures with mono-vacancy (1Vs), di-vacancies (2Vs), and tri-vacancies (3Vs) to observe the impact of vacancy size on their catalytic behaviors and figure out the underlying reasons. Afterwards, SnS2 is also a widely used catalyst for CO2 electroreduction, so we studied the catalytic behaviors of 1T-SnS2 catalysts as well. For 1T-SnS2 edges, we selected three structures with different sulfur coverage: 1T-SnS2 100%S, 50%S, and 0%S, to discuss their catalytic mechanism. They all found to be stable at different sulfur potentials. Meanwhile, we investigated the catalytic behaviors of 1T-SnS2 basal plane mono-sulfur vacancy (1Vs) and compared it with that of the edges to confirm the main reaction sites and products for SnS2 catalysts. Finally, we explored the reactivity of the hydrogen evolution reactions on these catalysts and compared them with the CO2 electroreduction to determine the selectivity of the two reactions.