Fischer–Tropsch synthesis (FTS) is one of the most attractive routes to convert syngas (CO + H₂) into liquid fuels and high value-added chemicals. However, FTS over Fe-based catalysts generates and emits large amounts of CO₂, which reduces the carbon atom economy and causes huge greenhouse gas emission. In this work, CO₂ formation mechanisms in FTS over Fe-based catalysts were systematically investigated by combining experiments and DFT calculations, aiming to provide atomic-scale insights into the CO₂ formation process. Our results indicate that the Boudouard mechanism, in which the surface O* species formed by CO* dissociation reacts with another CO* to form CO₂, plays a predominant role in CO₂ formation on the active χ-Fe₅C₂ phase, while the hydrogenation of surface O* species to form H₂O is hindered. The existence of the Fe₃O₄ phase is favorable for the reverse water-gas shift (RWGS) reaction, leading to the decrease of CO₂ selectivity and increase of the amount of generated H₂O. The modification by the potassium promoter does not alter the predominant reaction pathway for CO₂ formation over Fe-based FTS catalysts and the Boudouard mechanism still plays the dominant role. The potassium promoter can increase CO₂ selectivity and decrease the amount of H₂O mainly through the following two ways: (1) potassium largely increases the proportion of the χ-Fe₅C₂ phase and thus increases the amount of active sites for the Boudouard reaction; (2) potassium leads to the disappearance of the Fe₃O₄ phase and thus suppresses the RWGS reaction. The electronic structures were systematically analyzed to shed light on the nature of the potassium effect. On the one hand, the potassium promoter makes the d-band center of the χ-Fe₅C₂(510) surface atoms shift toward the Fermi level, facilitating the back-donation of electrons from the χ-Fe₅C₂(510) surface to the adsorbed CO* antibonding orbital; on the other hand, the direct interaction between K₂O and adsorbed CO* weakens the C–O bond by decreasing its electron density, which also contributes to the promoted CO dissociation.

中文翻译：

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铁基催化剂在费托合成中 CO _2的形成机理：组合的实验和理论研究†

费托合成（FTS）是将合成气（CO + H ₂）转化为液体燃料和高附加值化学品的最有吸引力的途径之一。然而，基于Fe的催化剂上的FTS产生并排放大量的CO ₂，这降低了碳原子的经济性并导致了巨大的温室气体排放。在这项工作中，通过结合实验和DFT计算，系统研究了铁基催化剂上FTS中CO _2的形成机理，旨在为CO ₂形成过程提供原子尺度的见解。我们的结果表明，由CO *离解形成的表面O *物种与另一种CO *反应形成CO ₂的Boudouard机理在CO中起主要作用₂形成在有源χ -铁₅ Ç ₂相，而表面的氢化的O *物种以形成ħ ₂ O的阻碍。Fe ₃ O ₄相的存在有利于水煤气逆反应（RWGS）反应，导致CO ₂选择性降低，H ₂ O生成量增加。改变了铁基FTS催化剂上CO ₂形成的主要反应途径，而Boudouard机理仍然起着主导作用。钾促进剂可以提高CO ₂选择性并减少H ₂的量ø主要通过以下两种方式：（1）氢很大程度上增加了χ-Fe的比例₅ Ç ₂相，因此增加了对鲍多尔德反应活性位点的数量; （2）钾导致Fe ₃ O ₄相消失，从而抑制了RWGS反应。对电子结构进行了系统分析，以阐明钾效应的性质。在一方面，所述钾助使χ-Fe的d带中心₅ Ç ₂（510）表面原子移向费米能级，便于从电子的背捐赠χ -铁₅ Ç ₂（510）表面吸附到CO *反键轨道上；另一方面，K ₂ O与被吸附的CO *之间的直接相互作用通过降低电子密度来削弱C–O键，这也促进了CO的离解。