剖析單原子催化劑在電化學二氧化碳還原下的反應機制

Abstract

在後疫情時代，隨著交通運輸和各行業的復甦，溫室氣體排放量，特別是二氧化碳（CO₂）排放量逐漸上升，並在2022年超過了疫情前的水平。電化學二氧化碳還原反應（Electrochemical CO₂ Reduction Reaction, EC CO₂RR）提供了一種有前景的途徑，提供環境友好的方法將CO₂轉化為有價值的產物。雖然已有多種金屬催化劑被報導能有效地將CO₂轉化為氣態和液態產物，但其選擇性和活性仍然無法滿足日益增長的需求。因此，單原子催化劑（Single-atom catalysts, SACs）因其卓越的選擇性、穩定性和活性而受到廣泛關注。然而，SACs在CO₂RR中關於分子層級的反應機制尚未完全理解，需進一步研究並提供相關的實驗證據。 在本研究中，我們合成了不同中心金屬原子的單原子催化劑（M-SACs），這些SACs藉由氮和碳基底穩定材料結構。使用X射線吸收光譜（XAS）、X光繞射儀（XRD）和X射線光電子能譜儀（XPS）對其在CO₂RR中的行為及電子結構進行了研究。通過臨場氣相層析技術（GC）比較在中性條件下不同M-SACs的催化活性。GC結果顯示，鎳單原子催化劑（Ni-SACs）能夠產生高達90%以上的一氧化碳的法拉第效率（FE）。相比之下，Fe-SACs、Mn-SACs 和 Cr-SACs 表現出類似的趨勢，即在較低電位下有較高的 CO 產量。 臨場傅立葉轉換紅外光譜（in-situ FTIR）結果顯示，不同M-SACs的中間產物的起始電位和位置存在差異，並且展現出獨特的行為。在Mn-SACs和Cr-SACs的結果中，光譜中未觀察到反應中間體。我們推測可能因為中間體在生成後迅速脫附。而在Ni-SACs和Fe-SACs中，觀察到中間體吸附在表面上。儘管Fe-SACs表現出較強的介面電場，但其活性仍低於Ni-SACs。我們發現反應中間體在催化劑表面的吸附強度是影響催化劑活性與介面電場關係的另一個重要因素。Ni-SACs上反應中間體的吸附強度相比於 Fe-SACs較弱，這個發現與Ni-SACs表現出更好的活性互相呼應。我們的實驗結果提供了關於介面電場與分子偶極矩之間相互作用的證據，這種相互關係可通過外加電場大小和反應中間體的分子偶極矩變化觀察得知，突顯了介面電場及表面吸附中間體的電化學行為與電子結構在催化性能中的關鍵作用。

In the post-pandemic era, the transportation sector and various industries are experiencing a revival, leading to the increased greenhouse gas emissions, particularly carbon dioxide (CO2). Electrochemical CO2 reduction reaction (EC CO2RR) has been regarded as a promising approach to convert CO2 into value-added products. Although various metal-based catalysts have been used to efficiently convert the CO2 into gaseous and liquid products, the selectivity and activity remain insufficient to meet industrial requirements. As a result, single-atom catalysts (SACs) have attracted the attention, demonstrating their exceptional selectivity and stability. To develop the advanced SACs with enhanced electrochemical performance, the detailed mechanistic insights into the SACs are highly needed. In this work, various single-atom catalysts with different metal atoms (M-SACs) including Cr-SACs, Mn-SACs, Fe-SACs and Ni-SACs stabilized by four nitrogen and carbon-based support were synthesized. X-ray absorption spectroscopy (XAS), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) were used to investigate the geometric and electronic structure of M-SACs before and after reactions. Gas chromatography (GC) results showed the exceptional performance of Ni-SACs, maintaining a Faradaic efficiency (FE) of >90% for carbon monoxide (CO) production at -1 V (vs RHE) in 0.1 M KHCO3. In contrast, Fe-SACs, Mn-SACs and Cr-SACs exhibit a similar trend, achieving high FEs of CO production at lower over-potentials. In-situ FTIR results of M-SACs revealed the formation of surface-adsorbed intermediates during CO2RR. Unique electrochemical behavior of surface-adsorbed intermediates over Ni-SACs and Fe-SACs was observed. Although Fe-SACs exhibit a stronger interfacial electric field (IEF), the activity of Fe-SACs is lower than that of Ni-SACs. We found that the binding strength of surface-adsorbed intermediates influences the relationship between the activity and the IEF. The experimental findings provide valuable insights into the relationship between the interfacial electric field and the electrochemical behavior of surface-adsorbed intermediates. Thus, the electrochemical behavior and binding strength of surface-adsorbed intermediates play pivotal roles in the catalytic performance of M-SACs.