费托法
碳化物
化学
催化作用
无定形固体
化学稳定性
相(物质)
无定形碳
化学工程
碳纤维
反应性(心理学)
选择性
有机化学
材料科学
复合数
复合材料
病理
工程类
替代医学
医学
作者
Emiel de Smit,Fabrizio Cinquini,Andrew M. Beale,Оlga V. Safonova,Wouter van Beek,Philippe Sautet,Bert M. Weckhuysen
摘要
The stability and reactivity of ϵ, χ, and θ iron carbide phases in Fischer−Tropsch synthesis (FTS) catalysts as a function of relevant reaction conditions was investigated by a synergistic combination of experimental and theoretical methods. Combined in situ X-ray Absorption Fine Structure Spectroscopy/X-ray Diffraction/Raman Spectroscopy was applied to study Fe-based catalysts during pretreatment and, for the first time, at relevant high pressure Fischer−Tropsch synthesis conditions, while Density Functional Theory calculations formed a fundamental basis for understanding the influence of pretreatment and FTS conditions on the formation of bulk iron carbide phases. By combining theory and experiment, it was found that the formation of θ-Fe3C, χ-Fe5C2, and ϵ-carbides can be explained by their relative thermodynamic stability as imposed by gas phase composition and temperature. Furthermore, it was shown that a significant part of the Fe phases was present as amorphous carbide phases during high pressure FTS, sometimes in an equivalent amount to the crystalline iron carbide fraction. A catalyst containing mainly crystalline χ-Fe5C2 was highly susceptible to oxidation during FTS conditions, while a catalyst containing θ-Fe3C and amorphous carbide phases showed a lower activity and selectivity, mainly due to the buildup of carbonaceous deposits on the catalyst surface, suggesting that amorphous phases and the resulting textural properties play an important role in determining final catalyst performance. The findings further uncovered the thermodynamic and kinetic factors inducing the ϵ−χ−θ carbide transformation as a function of the carbon chemical potential μC.
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