Density functional theory (DFT) analysis is used to shed light on the intricate effects of the Co₂C and Co/Co₂C catalyst crystal facets on the selectivity of the C₂ oxygenate and hydrocarbon formation in Fischer–Tropsch synthesis. Three representative low-index Co₂C (101), (110), and (111) surfaces, varying in surface energy from low and medium to high, are model examples of different Co₂C exposed crystal facets. Since CH_x (x = 1–3), CO, and H species are the key intermediates critical to the C₂ oxygenate selectivity, all Fischer–Tropsch reactions related to CH_x (x = 1–3) species, including CO insertion into CH_x (x = 1–3) and CH_x + CH_y (x, y = 1–3) coupling to form C₂ species (C₂H_x and C₂H_xO), as well as the hydrogenation and dissociation of CH_x (x = 1–3) to form C₁ species (CH₄ and C), are used as examples examined at a typical FTS temperature of 493 K. On Co₂C (101) and (110) surfaces, CH and CH₂ species are dominant form of the CH_x species, CH self-coupling to C₂H₂ and CH coupling with CH₂ to CH₂CH is dominant C₂ species. However, on a Co₂C (111) surface, only CH monomer is a dominant CH_x (x = 1–3) species, and CO insertion into CH to form CHCO is a dominant C₂ species. CH₄ and C production on these three surfaces is impossible. These results show that C₂ species formation, rather than C₁ species, is a preferable pathway on different Co₂C crystal facets in FTS reactions. Moreover, the C₂ selectivity, quantitatively estimated from the effective barrier difference, is found to be sensitive to the Co₂C crystal facet. The Co/Co₂C (111) interface catalyst is more favorable for C₂ oxygenate formation in comparison to the pure Co₂C (111) catalyst, whereas the Co/Co₂C (110) and Co/Co₂C (101) interface catalysts are unfavorable for C₂ oxygenate formation in comparison to the pure Co₂C (110) and (101) catalysts. Therefore, for the FTS over Co₂C and Co/Co₂C catalysts, the Co₂C (111) crystal facet is found to have an unexpectedly high selectivity for C₂ oxygenates, whereas the Co₂C (101) and (110) crystal facets are found to have a high selectivity toward C₂ hydrocarbons. The results mean that controlling the crystal facets of Co₂C catalysts using well-defined preparation methods can be an effective tool to tune the FTS selectivity toward the most desirable products.

中文翻译：

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Co ₂ C和Co / Co ₂ C界面催化剂上费托合成的C ₂含氧化合物的合成：如何控制催化剂晶面以获得最佳选择性

密度泛函理论（DFT）分析用于阐明费托合成中Co ₂ C和Co / Co ₂ C催化剂晶面对C ₂含氧化合物的选择性和碳氢化合物形成的复杂影响。三个代表性的低折射率Co ₂ C（101），（110）和（111）表面的表面能从低，中到高变化，是不同的Co ₂ C暴露晶体面的模型示例。由于CH _x（x = 1-3），CO和H物种是对C ₂含氧化合物选择性至关重要的关键中间体，因此所有费托反应都与CH _x（x= 1-3）种，包括将CO插入CH _x（x = 1-3）和CH _x + CH _y（x，y = 1-3）耦合形成C ₂物种（C ₂ H _x和C ₂ H _x O）以及CH _x的氢化和解离（x = 1-3）形成C ₁物种（CH ₄和C），被用作在493 K的典型FTS温度下检测的实例。关于Co ₂在C（101）和（110）表面上，CH和CH ₂是CH _x的主要形式CH ₂与CH ₂ H _2的自偶联以及CH ₂与CH ₂ CH _2的CH偶联是主要的C ₂种类。但是，在Co ₂ C（111）表面上，只有CH单体是占主导地位的CH _x（x = 1-3）物种，CO插入CH形成CHCO的过程是占优势的C ₂物种。在这三个表面上无法生成CH ₄和C。这些结果表明，在FTS反应中，在不同Co ₂ C晶面上形成C ₂物种而不是C ₁物种是优选的途径。而且，C ₂从有效势垒差定量估计的选择性对Co ₂ C晶体晶面敏感。与纯Co ₂ C（111）催化剂相比，Co / Co ₂ C（111）界面催化剂更有利于C ₂含氧化合物的形成，而Co / Co ₂ C（110）和Co / Co ₂ C（101与纯Co ₂ C（110）和（101）催化剂相比，界面催化剂对C ₂含氧化合物的形成不利。因此，对于采用Co ₂ C和Co / Co ₂ C催化剂的FTS ，Co ₂发现C（111）晶面对C ₂含氧化合物具有出乎意料的高选择性，而发现Co ₂ C（101）和（110）晶面对C ₂烃具有高选择性。结果表明，使用定义明确的制备方法控制Co ₂ C催化剂的晶面可能是将FTS选择性调整为最理想产品的有效工具。