臨場分析鐵單原子催化劑於尿素氧化反應之動態結構變化

Abstract

相對於工業所選擇的電解水反應（OER）來說，尿素氧化反應（UOR）被視為能應用在二氧化碳還原反應（CO2RR）電解器的替代陽極反應。然而，目前尚未明確建立尿素分子吸附取向、催化劑結構與產物選擇性之間的關係。本研究設計了兩種配位環境，分別為具有孤立活性位點的單原子鐵催化劑FeNC與相鄰活性位點的奈米氧化鐵FeO進行系統性的比較，以釐清尿素吸附取向與催化劑配位形式對產物選擇性的影響。結果顯示，FeNC不僅擁有最好的 UOR 催化活性，在1.33 V的陽極工作電位下即可達到10 mA cm-2。此外，FeNC對氮氣選擇性高達95%。透過臨場X光吸收光譜與拉曼光譜技術揭示了催化劑的動態結構以及反應中間體，說明 FeNC 能促進尿素分子內的氮原子進行分子內的 N-N 鍵的耦合，使氮氣產量上升，而FeO則主導了氮氧化物中間體 *NO 與 *NO3- 的生成。藉由分析尿素吸附在活性位點時尿素分子中化學鍵可移動的程度，建立了催化劑結構與產物選擇性之間的關係，並成功優化UOR催化劑的設計。

Urea oxidation reaction (UOR) has been considered as a promising alternative to anodic oxygen evolution reaction (OER) for carbon dioxide reduction (CO2RR) electrolyzer. However, the high selectivity of non-nitrogen species hinders its sustainable application. The relationship between catalyst structure and product selectivity has not been well-defined. Herein, this study systematically designed a single-atom iron catalyst and a nano iron oxide catalyst for urea mono-coordination and bi-coordination environments to clarify the impact of urea-catalyst coordination on product selectivity. The results showed that the single-atom iron catalyst exhibited the highest UOR catalytic activity with working potential of 1.33 V to deliver 10 mA cm–2, and 95% N2 selectivity. In situ X-ray absorption spectroscopy and Raman spectroscopy revealed the dynamic structure of the catalyst and reaction intermediates. FeNC could trigger a different route of the N-N coupling of urea molecules. In contract of FeO dominated the formation of the intermediates *NO and *NO3-. Based on the flexibility of the urea molecule in coordination with active sites, the structure-product selectivity relationship was established. These findings provided guidance for the design and optimization of catalyst for UOR performance.