费托催化剂η-Fe2C（011）上CH4形成及C-C耦合机理研究

CH4 formation and C—C coupling mechanism on （011） surface of η-Fe2C Fischer-Tropsch catalyst

Fe2C是低温Fe基费托催化剂的主要活性相,研究其上费托反应机理具有十分重要的意义。从原子尺度上通过密度泛函理论(D FT)计算研究了Fe2C稳定晶相η-Fe2C的(011)表面上甲烷形成和C-C耦合的反应机理。计算结果表明,η-Fe2C(011)表面上甲烷形成的有效能垒为1.03eV,其低于CHi+CHj耦合反应的有效能垒(1.52~2.98eV),且最可能的C-C耦合反应路径为C+CH3。进一步比较研究了η-Fe2C(011)表面与其他Fe基费托催化剂表面之间的CH4和C2+选择性差异,发现选择性高度敏感于Fe基催化剂的表面与体相结构,其中η-Fe2C(011)表面具有较高的甲烷选择性。

Fe2C is the main active phase of low temperature Fe-based Fischer-Tropsch catalysts. It is of great significance to study the mechanism of Fischer-Tropsch reaction. Herein, spin-polarized density functional theory (DFT) calculations are performed to investigate the mechanism of CH4 formation and C-C-Coupling on the η-Fe2C (011) surface, where this crystal phase is thermodynamically stable. The calculated results show that the effective barrier of CH4 formation on such surface is 1.03 eV, lower than those of C-C coupling, and the C + CH3 is the most likely C-C-Coupling pathways. Subsequently, the effective barrier difference between the CH4 formation and C1-C1 coupling is used as a descriptor to compare the difference of the Ficher-Tropsch synthesis (FTS) selectivity between the η-Fe2C(011) surface and other Fe-based catalysts surfaces. The FTS selectivity is found to be highly sensitive to the crystal phases and surfaces of Fe-based catalysts, and the η-Fe2C(011) surface shows relatively high CH4 selectivity.