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Breakthrough in mechanism of promoter in Fischer–Tropsch synthesis
Author: Huo Chunfang
Update time: 2011-10-21
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It is well known that nearly all iron-based Fischer–Tropsch catalysts need a definite quantity of alkali promoters to achieve their optimum catalytic activity and selectivity. However, due to the complexity of real catalytic systems, the microscopic understanding of alkali promotion effect is still an elusive and challenging subject. Recently, scientists of Institute of Coal Chemistry, Chinese Academy of Science and Synfuels of China Co., Ltd. for the first time reported the "morphology control" effect of potassium promoter on iron catalyst, which gives an important guidance in R&D the efficient catalysts. Their density functional theory (DFT) and experimental studies revealed that one crucial effect of potassium promoter (K2O) is modifying crystallographic orientation in favor of forming Fe crystallite with abundant high-active facets. DFT calculations indicated that potassium promoter can stabilize Fe(110), (100), (111), (211), (210), (321), (310) at different degree. This stabilizing effect changes the relative rate of crystal growth in different directions, and thus facilitates the formation of small particles with a large percentage of more active facets. For unprompted catalyst, the thermodynamically most stable Fe(110) facet has the largest contribution to the total surface area of Fe crystallite.As the loading of potassium promoter increase, the proportions of Fe(110) and (100) decrease, while the proportions of more active Fe(211) and (310) increase. This finding opens a new perspective for under­standing the promotion effect, and provides an innovative idea for designing efficient catalysts with controllable surface structures. This work is published on Angew Chem Int. Ed. 2011, 50, 7403-7406, which is a great breakthrough once again made by this group after their important finding that the reaction energy and effective barrier of CH4 formation on iron carbides (FexCy) have a linear relationship with the charge of the surface C atom and the d-band center of the surface, respectively (JACS, 2009, 131, 14713-14721).

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