異相電催化活化甲烷碳氫鍵生成硫酸甲酯

Abstract

甲烷至硫酸甲酯的直接氧化反應為一個實現甲烷增值及促進溫室氣體減排的有望方法。透過結合電催化甲烷氧化方法及硫酸根保護甲烷氧化方法,能解決直接甲烷氧化中因產物過氧化導致的低轉換率及低選擇性的問題。本論文研究磷酸釩奈米片在硫酸電解液中進行電催化甲烷直接氧化反應之效能及反應動力學。該系統能於相對於銀氯化銀電極(3M 氯化鉀)2.9 V 及 7 bar 之甲烷氣壓下達到 92.66 μA cm-2 之硫酸甲酯生成電流,優於當前以異相電催化甲烷生成硫酸甲酯之其他體系。以自製電化學反應槽進行控制甲烷氣壓、電極偏壓、溫度之多維度動力學實驗得出此系統存在電催化甲烷氧化及電催化硫酸分解兩競爭反應,並且硫酸鉀酯的生成會受限於電催化硫酸分解產生的氧氣造成的機制干擾。以臨場 X 光吸收光譜觀察釩原子在反應當下的動態變化後更進一步揭露在硫酸及電極偏壓施加的環境中,於催化劑表面會生成由硫酸根配位於釩原子之活性位點。此活性位點的存在能作為證據支持本系統可能為一個由電催化生成之硫酸根自由基活化甲烷碳氫鍵之機制而非由表面金屬氧官能基活化甲烷碳氫鍵之機制。本研究結合反應動力學及臨場光譜分析辨明反應之整體機制不僅能提供未來催化劑設計之根據,亦能為理論計算之基礎假設提供見解,協助未來更有效能的體系的開發並早日滿足實務的需求。

Direct oxidation of methane (CH4) to methyl bisulfate (CH3OSO3H) is a promising method to realize natural gas valorization and greenhouse gas emission mitigation. By integrating electrochemical methane oxidation with sulfuric acid-protected methane oxidation, problems hindering the practical use of direct methane oxidation processes such as low conversion rate and low selectivity caused by product overoxidation can be resolved. Here, we investigate the potential-dependent electrochemical methane oxidation behavior of a heterogeneous vanadium phosphate nanosheet catalyst in sulfuric acid solution for direct methane oxidation. This system achieves a maximum methane to methyl bisulfate (MBS) conversion current density of 92.66 μA cm-2 at the optimum anodic potential (2.9 V vs. Ag/AgCl reference electrode, 7 bar methane pressure), which is superior to the class of electrocatalytic heterogeneous materials for MBS production. In-situ X-ray absorption spectroscopy investigating the dynamic chemical features of vanadium atoms reveals the potential-driven formation of a metal-sulfate active site configuration, which is a crucial observation that supports the occurrence of the electrochemical sulfate radical-induced methane oxidation mechanism. Our research provides a general understanding of the sulfuric acid-protected electrochemical methane oxidation reaction mechanism on a heterogeneous surface and states the relevance of in-situ experiments to capture the relevant catalyst transformation behaviors that occurred during reaction conditions that provide new insights into the catalytic system’s mechanism for future material engineering and computational screenings.