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

Abstract

Iron carbide catalysts encapsulated in graphene-like carbon were synthesized via a facile method by pyrolysis of an 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 the structural and chemical properties of the prepared catalysts revealed a core–shell structure with iron carbides enwrapped by several graphene-like layers. The addition of a potassium promoter increased the amount of defects on graphene-like layers and facilitated the formation of iron carbides during the catalyst preparation. Fischer–Tropsch synthesis under typical reaction conditions (320 °C, 20 bar, H2/CO = 1, GHSV = 15\u2006000 ml gcat−1 h−1) was carried out in a fixed bed reactor. A higher light olefin selectivity was obtained than that on common iron catalysts, probably because of the electron-rich surfaces of the prepared catalysts that made it hard for hydrogen to hydrogenate the unsaturated intermediates. A volcano-like evolution of light olefin selectivity was observed on the catalysts with different contents of K, and the highest olefin selectivity reached 41.9% on the 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-like carbon layers that prevented the supported iron particles from migration and aggregation under harsh conditions in Fischer–Tropsch synthesis.