Effect of potassium promoter on Fischer-Tropsch synthesis to light olefins over iron carbide catalysts encapsulated in graphene-like carbon

Iron carbide catalysts encapsulated in graphene-like carbon were synthesized via a facile method by pyrolysis of iron-glucose precursor. Different amounts of potassium (0-5 wt%) were in situ doped into the catalyst simultaneously. Glucose played a role both as the precursor to form a carbon support and a reducing agent that reduced iron oxides to θ-Fe3C during the catalyst preparation. θ-Fe3C underwent a phase transformation to χ-Fe5C2 as the active phase in Fischer-Tropsch synthesis. Characterization of structural and chemical properties of the prepared catalysts revealed a core-shell structure with iron carbides enwrapped by several layers of graphene. The addition of potassium increased the amount of defects on graphene layers and facilitated the formation of iron carbides during the catalyst preparation. Fischer-Tropsch synthesis under typical reaction conditions (320 oC, 20 bar, H2/CO=1, GHSV=15000 ml·gcat-1·h-1) was carried out in a fixed bed reactor. Higher light olefins selectivity was obtained than that on the common iron catalysts, probably because of the electron-rich surfaces of the prepared catalysts that made hydrogen hard to hydrogenate the unsaturated intermediates. A volcano-like evolution of light olefins selectivity was observed on the catalysts with different contents of K, and the highest olefins selectivity reached 41.9% on 2K-Fe3C@C catalyst (i.e., doped with 2 wt% of K). The induction period of the catalyst was shortened by K addition. No drastic changes in the catalyst morphology and performance during 100 h time on stream can be ascribed to the protection of graphene layers that prevented the supported iron particles from migration and aggregation under harsh conditions in Fischer-Tropsch synthesis