As one of the oldest and most complicated heterogeneous catalytic reaction, the basic understand regarding the Fischer-Tropsch synthesis (FTS) in term of the catalyst active phase and reaction mechanism remain unclear and even in debates. In this report, pure phase iron-based catalysts including α-Fe, Fe5C2, Fe7C3 and Fe2C were prepared and used to identify their intrinsic activity. It was found that Fe5C2 is the most active catalyst while α-Fe underwent a gradual increase in activity in the induction period due to the structural transformation toward iron carbide. A transient experiment named high-pressure stepwise temperature programming surface reaction (STPSR), in-situ XRD and X-ray absorption fine structure (XAFS) experiments were conducted to follow the reaction dynamics over these catalysts and structural evolution under FTS condition. Coping with comprehensive density functional theory calculations, a vivid image about how the FTS processes are for the first time disclosed. It was found that though α-Fe was highly active for CO activation, the too strong binding with dissociated atomic carbon prevented subsequent C-C coupling and methanation. α-Fe catalyst, whose intrinsic activity for FTS was therefore rather low, tends to be carburized and transformed into thermodynamically more favorable iron carbide under FTS condition. For iron carbide, CO activation remained facile, but dissociated atomic carbon was largely destabilized. This facilitated greatly the formation of monomers and C-C coupling with preference of unsaturated hydrocarbon. Importantly, the effective barrier for C-C coupling was even lower than that of CO activation. The insights revealed are valuable for rationale of design for iron-based FTS catalysts.

Insight into iron-based Fischer-Tropsch synthesis reaction

251st National Meeting (2016)

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